RIDE/DIE
04-18-2005, 01:19 PM
Nutrition Basics: Dietary Nutrients
Carbohydrates & Sugars
Chemistry of Carbohydrates
Carbohydrates are made of carbon (C), oxygen (O), and hydrogen (H). Each of these atoms can have a specific number of chemical bonds (C - 4 bonds), (O - 2 bonds) and (H-1 bond).
Simple Sugars versus Complex Carbohydrates
Monosaccharides and disaccharides are sometimes called simple sugars; polysaccharides are called complex carbohydrates, starch or fiber.
Monosaccharides
The term monosaccharide means one sugar molecule. There are three monosaccharides:
Glucose
Fructose
Galactose
All three monosaccharides have the same chemical formula C6H12O6. All three monosaccharides have 6 carbons (hexoses), but they have different structures. The different structures affect their sweetness and absorption. For example, the structure of fructose makes it sweeter than glucose and galactose, but also decreases its absorption compared to glucose and galactose. In addition, fructose does not require insulin to enter body cells, but once converted to glucose, insulin is required.
Disaccharides
The term disaccharide means two sugar molecules. Two monosaccharides condense with the loss of a molecule of water to form a disaccharide. There are three disaccharides:
Maltose = Glucose + Glucose
Sucrose = Glucose + Fructose
Lactose = Glucose + Galactose
Polysaccharides
The term polysaccharide means many sugar molecules. Polysaccharides include:
Glycogen
Starch
Fiber
Glycogen
Glycogen is the storage form of glucose in animals. Glycogen is a long highly branched chain of glucose molecules linked together with alpha bonds. The branching provides many ends for rapid release of glucose. Glycogen is stored in the liver and muscle. Liver glycogen is broken down to release glucose if blood glucose levels fall too low, to provide glucose to the brain, nervous system, and developing red blood cells. Muscle glycogen provides rapid release of glucose to provide energy to muscle cells. The human body can only store enough energy as glycogen for about 1/2 - 2/3 of a day.
Starch
Starch is the storage form of glucose in plants. Starch is a long chain of glucose molecules some branched (amylopectin) and some not branched (amylose) linked together with alpha bonds.
Fiber
Fiber is a long chain of glucose molecules liked together with beta bonds. The human body does not have the enzymes to break beta bonds, as a result, fiber passes into the lower intestine undigested. In the lower intestine bacteria can ferment (breakdown) some fibers.
Digestion
When a person eats carbohydrates, enzymes hydrolyze (breakdown) the long chains to shorter chains, the shorter chains to disaccharides, and finally the disaccharides to monosaccharides.
Mouth: Salivary amylase begins carbohydrate digestion.
Stomach: HCl and pepsin deactivate salivary amylase.
Small Intestine: Pancreatic amylase breaks short polysaccharides down to disaccharides.
Intestinal Wall: Disaccharidases in the small intestine microvilli break disaccharides down to monosaccharides
Maltase breaks maltose down to glucose + glucose
Sucrase breaks sucrose down to glucose + fructose
Lactase breaks lactase down to glucose + lactose
Absorption
Glucose can be absorbed to some extent through the lining of the mouth, but for the most part, all nutrient absorption occurs in the small intestine. The monosaccharides cross the mucosal cells lining the small intestine microvilli by active transport and enter the capillaries of the intestinal villa.
Some fructose is converted to glucose in the intestinal mucosal cells. Fructose is not absorbed as rapidly as glucose; this may lead to a slower rise in blood glucose. Insulin is not needed for fructose to be taken up by body cells. The slower absorbance and rapid clearance may improve glycemic control. However, some people experience fructose malabsorption with a 20-50 gram fructose load. Children have a substantial fructose intake through sweetened drinks and juices. Children 6-18 months of age can exhibit carbohydrate malabsorption with ingestion of fruit juices (such as apple juice). Children who exhibit nonspecific diarrhea may benefit from a reduction in fructose intake.
The monosaccharides are carried to the liver via the portal vein. In the liver fructose and galactose are converted to other compounds, mostly glucose. Glucose is released from the liver into the blood and carried to the rest of the body. Once fructose is converted to glucose insulin is required for uptake by body cells.
Metabolism
Glucose plays the central role in carbohydrate metabolism.
Storage of Glucose as Glycogen
Glycogen is the storage form of glucose in animals. Glycogen is a long highly branched chain of glucose molecules linked together with alpha bonds. The branching provides many ends for rapid release of glucose. Glycogen is stored in the liver and muscle. Liver glycogen is broken down to release glucose if blood glucose levels fall too low, to provide glucose to the brain, nervous system, and developing red blood cells. Muscle glycogen provides rapid release of glucose to provide energy to muscle cells. The human body can only store enough energy as glycogen for about 1/2 - 2/3 of a day.
Glycogen holds a lot of water. If not enough carbohydrate is consumed to maintain blood glucose levels for the brain, nervous system, and developing red blood cells the breakdown of glycogen for glucose results in a loss of water, which many interpret as weight loss.
Using Glucose for Energy
The primary role of glucose in human nutrition is to supply energy to body cells. The brain, nervous system, and developing RBC can only use glucose for energy. Glucose (6-C) ? 2 Pyruvic acid 2(3-C) ? Acetyl CoA (2-C) ? TCA cycle (where energy is released as ATP get 38 molecules ATP/glucose)
Making Glucose from Protein
The body can convert body protein to glucose to some extent, but protein has jobs of its own other nutrients can't do. Body fat can't be converted to glucose to any significant extent, although fat breakdown can yield energy for body cells. However, the brain, nervous system, and developing RBC must use glucose for energy. Thus, if there is inadequate carbohydrate, proteins are broken down to make glucose to provide energy for these special cells. One of carbohydrates role is to spare body protein.
Using Fat for Energy
An inadequate supply of carbohydrate results in accelerated fat breakdown to provide energy. Triglycerides are the storage form of fat in adipose cells. Triglycerides are composed of a glycerol backbone and three fatty acids. Fatty acids are long chains of carbon, with hydrogen's and a hydroxyl group. Fatty acids are broken down into 2-C fragments which can be converted to acetyl CoA and enter the TCA cycle to provide energy.
The 2-C fat fragments can overload the TCA cycle, you get 2, 2-C units from glucose, you can get up to 9, 2-C units from an 18-C fatty acid. The fat fragments combine to form ketone bodies, which can accumulate in the blood and lead to a condition called ketosis (ketoacidosis). Ketosis disturbs the body's normal acid-base balance and can lead to coma and death. About 50-100 grams of carbohydrate daily are required to ensure the sparing of body protein and to prevent ketosis.
Conversion of Glucose to Fat
More glucose than what the body needs for energy or glycogen is converted to triglycerides in the liver and stored as a more permanent energy storage compound - body fat. Storing carbohydrate as fat is energy expensive. The body uses more energy to convert glucose to body fat than converting dietary fat to body fat.
Maintaining Consistency of Blood Glucose
To function normally, the body must maintain blood glucose levels within normal limits that permit the body cells to nourish themselves. If blood glucose levels go too low - it can cause a person to feel weak and dizzy. If blood glucose levels go too high - it can cause a person to feel confused and have difficulty breathing. Extremes in blood glucose levels, either too high or too low if left untreated can be fatal.
Regulating Hormones
If blood glucose levels go too high, insulin is released. Insulin signals body cells to uptake glucose for energy, stimulates the formation of glycogen, and stimulates the conversion of glucose to triglycerides to be stored as fat.
If blood glucose levels go too low, glucagon is released. Glucagon signals the liver to breakdown glycogen and release glucose into the blood.
Another hormone, epinephrine acts quickly stimulating release of glucose from glycogen into the blood and muscles, ensuring that all body cells have energy in an emergency.
Falling Out of Normal Range
The influence of food on blood glucose has lead to the over simplification that food governs blood glucose concentrations. Foods do not, the body does. In some people, blood glucose regulations fail. When this occurs two conditions can result; diabetes or hypoglycemia. Glucose may be modified as part of the treatment, but hormonal regulation or obesity (in the case of type 2 diabetes) is the cause not glucose.
Glycemic Index
The glycemic index describes the effect of food on blood glucose; how quickly glucose is absorbed, how high blood glucose rises, and how quickly it returns to normal. Different foods have different effects on blood glucose. For example, ice cream is high in sugar, but it has a lower glycemic index than a baked potato. A foods glycemic index differs depending on the food itself, fat content, fiber content, the form of the food, and whether it is eaten alone or with a meal.
Glycemic index of foods adjusted to a glycemic index of 100 for white bread
Maltose 152
Cornflakes 121
Glucose 138
White Bread 100
Honey 126
Shredded Wheat 97
Sucrose 83
Oatmeal 89
Fructose 26
White Rice 81
Corn 80
Ice Cream 69
Potatoes (mashed) 98
Yogurt 52
Potato chips 77
Whole Milk 44
Baked beans 70
Peas 50
Banana 84
Orange Juice 71
Apple 52
Types of Sweeteners
Caloric Sweeteners (provide calories)
Sugars
Caloric sweeteners include many regular sugars including; refined sugars, corn sweeteners, dextrose, HFCS, honey, syrups, crystalline fructose, lactose, invert sugars, glucose, maltose, and concentrated fruit juices
Intake of Added Sugar
According to the US Food Supply Data (JADA, 2002) American per capita consumption of added sugars went from 27 t (108 g)/person/day in 1970 to 32 t (128 g)/person/day in 1996. This increase was been driven by an increase in supply of high fructose corn syrup.
According to the Continuing Survey of Food Intake, from 1994 to 1996 (JADA, 2000) Americans age 2 and older consume equivalent to 82 g/day added sugar (20.5 t), which accounts for 16% of total calories. Adolescents age 12-17 had the highest intake averaging 20% of total calories from added sweeteners (adolescent females 97.7 g, adolescent males 141.8 g). The largest source of added sugar was regular soft drinks, which accounted for 2/3 of intake. Other sources were table sugars, syrups and sweets, sweetened grains, regular fruitades and milk products.
Guidelines for Sugar Intake
Dietary Guidelines for Americans 2000 advises to choose beverages and foods to moderate intake of sugars.
Dietary Goals 10% or less of total calories from sugar.
Food Guide Pyramid advises to use sugars "sparingly"
Total added sugars 6 t (24 g) 1600 calories
12 t (48 g) 2200 calories
18 t (72 g) 2800 calories
The average intake of added sweeteners for Americans age 2 and above is 20.5 t (82 g)/day for most Americans exceeds these recommendations.
Health Effects of Sugars
Nutritional Deficiencies
In excess sugar can contribute to nutritional deficiencies by supplying calories without providing nutrients. Bakery items, candies, and soft drinks provide calories with few nutrients. Whereas, grains, vegetables, fruits and dairy foods contain natural sugars and starches but also protein, fiber, vitamins and minerals. Sugar can contribute to nutrient deficiencies only by displacing nutrients.
For nutrition sake the appropriate attitude to take is not that sugars are "bad" and must be avoided, but that nutrient dense foods must come first. The goal is good nutrition and moderation. The amount of sugar a person can afford depends on how many calories are available beyond those needed for nutrients.
Honey does provide a few vitamins and minerals but the amounts are insignificant.
Tooth Decay
In excess both sugars and starches can contribute to tooth decay. Both sugars and starches begin breaking down to glucose in the mouth. Bacteria in the mouth ferment sugars and in the process produce an acid that can dissolve tooth enamel. Many factors are involved such as how long foods stay on the teeth, how often foods are eaten, and dental hygiene. Overall, the risk of dental caries increases with intake of nutritive sweeteners; however, sugars and carbohydrates do not work independently from other factors such as oral hygiene and fluoridation. For most people good oral hygiene will prevent dental caries.
Diabetes/Hypoglycemia
The influence of food on blood glucose has lead to the over simplification that food governs blood glucose concentrations. Foods do not, the body does. In some people, blood glucose regulations fail. When this occurs diabetes or hypoglycemia can occur. Glucose may be modified as part of the treatment, but hormonal regulation or obesity (in the case of type 2 diabetes) is the cause not glucose.
With people who have type 2 diabetes, attention is first given to the total amount of carbohydrate in the diet rather than the source. For people with diabetes sugars do not produce a greater glucose response (glycemic index) than complex carbohydrates. Intakes as high as 60 g sucrose or fructose may not adversely affect glycemic or lipid responses in persons with type 2 diabetes.
Hyperactivity or Misbehavior in Children
Controlled studies have failed to show an adverse relationship between sugar hyperactivity or misbehavior in children, even in children who by report are sensitive to sugar. The mechanism by which carbohydrate, including sugars, may affect mood is uncertain, but may involve the synthesis and release of serotonin in the brain. High carbohydrate intake stimulates the brain production of serotonin, which makes a person sleepy and sluggish.
Heart Disease
Usual intakes of sucrose and fructose do not elevate plasma triglycerides in most persons, provided calories are in balance. However, very high intakes of sucrose or fructose (2-3 times usual intake), or high carbohydrate diets (70-80% carbohydrate) can result in elevated plasma triglycerides which can increase heart disease risk. Remember carbohydrates stimulate release of insulin, which stimulates triglyceride formation, this is another rational for a balanced diet. A small percent of people are carbohydrate sensitive. These people respond to high doses of sugar or carbohydrate with abnormally high insulin secretion, which promotes triglyceride formation.
It is important to keep the effects of sugars in perspective. Other dietary factors such as total fat, saturated fat, and obesity have a much stronger association with heart disease than sugar intake.
Obesity
Obesity is a complex issue and cannot be attributed to one factor. Excess body fat arises from energy imbalance caused by taking in too many calories and by using too little. Because sugar adds calories to foods and beverages it has been hypothesized that sugar has a role in the etiology of obesity. Research does not support a direct connection between sugar and carbohydrate intake with obesity, unless excess intake of sugar containing foods leads to energy imbalance and weight gain.
Carbohydrates, including sugar suppresses appetite. Studies show an inverse relationship between sugar intake and obesity and direct a relationship between fat intake and obesity. However, foods high in sugar are often high in fat and excessive intake can contribute excess calories and fat. Food consumption surveys have shown that over the past two decades the total calorie intake of Americans has increased, and this increase has largely been in the form of carbohydrates, primarily in the form of soft drinks. Other studies have shown that children who are high soft drink consumers have higher calorie intakes than non-soft drink consumers. Studies have also shown that obese children and adults consumer a greater portion of their total calories from soft drinks than lean children.
Sugar Alcohols
Sugar alcohols provide calories and are thus caloric sweeteners, although they provide fewer calories than regular sugars because they are not completely absorbed. This allows products, which contain sugar alcohols to be labeled "sugar free" or "reduced calorie." Products may claim to be "sugar free" but this does not mean they care "calorie free."
Sugar Alcohol Comparative Sweetness Calories/gram
Isomalt 55% as sweet 2.0 calories/gram
Lactitol 40% as sweet 2.0 calories/gram
Maltitol 90% as sweet 3.0 calories/gram
Mannitol 70% as sweet 1.6 calories/gram
Sorbitol 50-70% as sweet 2.6 calories/gram
Xylitol 100% as sweet 2.4 calories/gram
Sugar alcohols occur naturally in fruits and vegetables. The body absorbs sugar alcohols slowly and incompletely. As a result they enter the blood stream slower than natural sugars. Because of the incomplete absorption sugar alcohols produce a lower glycemic response than sugars. However, side effects also occur because of the incomplete absorption. Greater than 50 g sorbitol or 20 g mannitol/day can cause gas, abdominal discomfort and diarrhea due to fermentation by intestinal bacteria (similar to lactose intolerance). For this reason, food products containing sugar alcohols carry a label "Excess consumption may have a laxative effect."
The real benefit of using sugar alcohols is that they do not contribute to dental caries. Bacteria in the mouth can't metabolize sugar alcohols as rapidly as sugar.
Non-Caloric Sweeteners (provide no or very little calories)
FDA Approved Non-Caloric Sweeteners
The FDA has approved four non-caloric sweeteners; saccharin, aspartame, acesulfame K, and sucralose. The body does not metabolize saccharin, acesulfame K, or sucralose, they pass through the kidneys unchanged. The body does digest aspartame, so technically it is a caloric sweetener, but the calories provided are insignificant.
Name Sweetness Commercial Name ADI
Saccharin 300X (200-700X) Sweet & Low 5 mg/kg
Aspartame 180X (160-220X) Nutrasweet/Equal 50 mg/kg
Acesulfame K 200X Sunette 15 mg/kg
Sucralose 600X Splenda 15 mg/kg
FDA Petitioned Non-Caloric Sweeteners
Three other non-caloric sweeteners have petitioned FDA and are awaiting approval; cyclamate, alitame, and neotame.
Accepted Daily Intake (ADI)
The safety limit of food additives is expressed as the acceptable daily intake (ADI). The ADI is the estimated amount/kg body weight that a person can safely consume everyday over a lifetime without risk. The ADI is a conservative level. It is usually 100 times less than the maximum level at which no observed adverse effects occur in animal studies.
Safety of Non-Caloric Sweeteners
There are questions as to the safety of non-caloric sweeteners. Considering that all compounds are toxic at some does, it is little surprise that large doses of non-caloric sweeteners or their by-products can have effects. The question to ask is whether their ingestion is safe in quantities people normally consume, and potentially abuse.
Saccharin Safety
Saccharin has been used for over 100 years in the US. Saccharin is not metabolized by the body and is rapidly excreted in the urine and does not accumulate in the body. Saccharin was originally included on the GRAS listing; however, questions on the safety of saccharin surfaced in 1977 when studies suggested large doses of saccharin (equivalent to hundreds of cans of diet soda daily for a lifetime) increased the risk for bladder cancer in rats. FDA proposed banning saccharin, but public out cry caused congress to impose a moratorium on the ban, which was repeatedly until 1991 when, the FDA withdrew its proposed ban on saccharin. Products containing saccharin must carry a warning label, "Use of this product may be hazardous to your health, this product contains saccharin, which has been determined to cause cancer in laboratory animals."
Studies of high saccharin users do not support an association between saccharin and cancer. The largest population study of saccharin, involving 9,000 men and women, overall showed saccharin did not increase the risk of cancer. However, in a subgroup of people (male heavy smokers and heavy saccharin users - six or more servings per day) the risk of bladder cancer was slightly increased. Common sense dictates that consuming large amounts of any substance is probably not wise, but at current, moderate intake levels, saccharin is assumed to be safe for most people.
Aspartame Safety
Aspartame is composed of two amino acids (phenylalanine and aspartic acid) and a methyl group (CH3). Alone they are not sweet, but combined they are 180 times sweeter than sucrose. In the digestive tract enzymes split aspartame apart. The body uses the amino acids just as if they had come from protein. Aspartame is one of the most studied of all food additives. Extensive animal and human studies document its safety. Long-term consumption of aspartame is not associated with any adverse health effects, except for a certain population subgroup with phenylketonuria (PKU).
Phenylketonuria
Phenylketonuria is an inherited disease that occurs in about 1 in 10,000-15,000. People with PKU are unable to dispose of excess phenylalanine. The accumulation of phenylalanime and it's by-products are toxic to the developing nervous system (by altering the synthesis of monoamine neurotransmitters), causing irreversible brain damage. For this reason all newborns in the US are screened for PKU. The treatment for PKU is a special diet that must strike a balance between providing enough phenylalanine for growth, but not too much to cause harm. For this reason products with aspartame must carry a warning label "Phenylketonurics contains phenylalanine."
In persons without PKU a bolus load of 50 mg/kg on repeat dose studies showed plasma phenylalanine levels near normal. Persons with PKU appear to tolerate the amount of phenylalanine in a diet soda sweetened with aspartame (104 mg/12 oz). Herterozygotes of PKU do not show changes after 12 weeks consumption.
Still there is compelling reason why children with PKU need to get all their pheylalanine from foods, and not from a non-caloric sweetener. The PKU diet excludes protein rich foods such as milk, meat, fish, poultry, cheese, eggs, nuts, legumes, and many bread products. Only with difficulty can these children obtain the many essential nutrients such as calcium, iron, zinc and B-vitamins. To suggest children with PKU should squander any of their limited phenylalanine allowance on aspartame, which contributes none of these nutrients, would open the way for poor nutrition.
Diketopiperzine
Aspartame breaks down to diketopiperzine (DKP) in liquid systems with heat exposure and loses its sweetness. Studies testing this product have eliminated it as a concern. Studies have shown that even if all the aspartame was converted to diketopiperzaine in beverages, the amount would be below the ADI of 3,000 mg/kg for diketopiperzine.
Methyl Group
During metabolism the methyl group momentarily becomes methyl alcohol (methanol) a potentially toxic compound. Enzymes convert methanol to formaldehyde, another toxic compound. Formaldehyde is then converted to carbon dioxide. The levels formed are modest, and fall below threshold levels at which they would cause harm. In fact tomato, citrus and grape juices yield higher amounts of methanol than is produced by the aspartame in a 12-oz diet soda.
Aspartic Acid
Aspartic acid also has the potential to cause brain damage at very high does, but under normal intake levels the brains mechanism for controlling aspartic acid levels ensures no adverse effect.
Other Accusations
Some people have tried to link aspartame to multiple sclerosis, lupus, seizures, brain tumors, and birth defects. There is no credible evidence to support a link between aspartame with multiple sclerosis or lupus. Human studies show aspartame neither causes nor enhances seizures. No evidence has been found that aspartame caused birth defects. The associated claim between aspartame and brain tumors has not been supported.
Some people claim to exhibit vague, but not dangerous symptoms due to unusual sensitivity to aspartame (headaches, dizziness, mood alterations, fatigue, vision problems). However, double blind studies have failed to reproduce reactions in controlled conditions. The CDC has reviewed 600 behavioral complains and concluded there was no association.
Estimated Aspartame Intake
The FDA estimates the general population consumer 6% of the ADI. Those 1-5 years consume 10.4% of the ADI. To reach the ADI a 150 pound adult would need to consume 97 packets of equal or 20 cans diet soda/day, a 40 pound child would need to consume 24 packets equal, 4 cans diet soda, or 9 (8oz) glasses of drink sweetened with aspartame.
Acesulfame K Safety
The FDA approved acesulfame K in 1988 after reviewing 90 safety studies conducted over 15 years. Some consumer groups believed acesulfame K caused tumors in rats. The FDA concluded that the tumors were not caused by the sweetener, but were typical of those commonly found in rat studies.
Sucralose Safety
The FDA approved sucralose in 1998 after reviewing 110 safety studies conducted over 20 years.
Alitame, Neotame, and Cyclamate Safety
FDA approval for alitame, neotame, and cyclamate is pending. To date no safety issue for alitame has been raised. Alitame is composed of L-aspartic acid, D-alamine, and a novel C-terminal amide moiety. It is 2,000 times sweeter than sucrose. In 1995 FDA concluded no carcinogenic effect and not reproductivity toxicity. However, recommendations on an ADI are waiting further research. The FDA petition is 0.34 mg/kg. No adverse effect was observed at 100 mg/kg.
Cyclamate has been battling safety issues for 50 years. Approved by the FDA in 1949 cyclamate was banned in 1969 principally based on one study indicating it caused bladder cancer in rats. The National Research Council has reviewed dozens of cyclamate studies and has concluded that neither cyclamate or its metabolites cause cancer. The Council did recommend further research on the risk of health and long term use. Although cyclamate does not cause cancer it may promote cancer once started.
Special Populations
Children
Because of their size and relatively high food and fluid intake compared to adults, children have the highest intake of non-caloric sweeteners. It has been suggested that children limit saccharin intake because data available for its use in young children is limited. Intakes of aspartame in children, ages 2-5 years is estimated at 8-17 mg/kg which is below the ADI of 50 mg/kg. Intakes of acesulfame-K in children are estimated at 3-9 mg/kg, which is below the ADI of 15 mg/kg.
Pregnant Women
It has been suggested that pregnant women consider careful use of saccharine. Saccharine can cross the placenta and may remain in fetal tissues because of low fetal clearance.
Aspartame intake during pregnancy does not change fetal expose to aspartic acid; however, fetal circulating levels of phenylalanine exceed maternal levels. A bolus of aspartame 34 mg/kg resulted in a peak plasma phenylalanine of 112 umole/L in normal subjects and 162 umoles/L in heterozygotes - still below the level that would cause neurological problems in the fetus (1,090 umole/L). Plasma response of methanol and formate were not significant after an aspartame load. Thus the use of aspartame within FDA guidelines appears to be safe for pregnant women.
Acesulfame K has been determined to be safe during pregnancy.
Source:
Whitney, E.N. & Rolfes, S.R. Understanding Nutrition, 8th ed. West/Wadsworth Publishing Co., Belmont, CA, 1999.
Carbohydrates & Sugars
Chemistry of Carbohydrates
Carbohydrates are made of carbon (C), oxygen (O), and hydrogen (H). Each of these atoms can have a specific number of chemical bonds (C - 4 bonds), (O - 2 bonds) and (H-1 bond).
Simple Sugars versus Complex Carbohydrates
Monosaccharides and disaccharides are sometimes called simple sugars; polysaccharides are called complex carbohydrates, starch or fiber.
Monosaccharides
The term monosaccharide means one sugar molecule. There are three monosaccharides:
Glucose
Fructose
Galactose
All three monosaccharides have the same chemical formula C6H12O6. All three monosaccharides have 6 carbons (hexoses), but they have different structures. The different structures affect their sweetness and absorption. For example, the structure of fructose makes it sweeter than glucose and galactose, but also decreases its absorption compared to glucose and galactose. In addition, fructose does not require insulin to enter body cells, but once converted to glucose, insulin is required.
Disaccharides
The term disaccharide means two sugar molecules. Two monosaccharides condense with the loss of a molecule of water to form a disaccharide. There are three disaccharides:
Maltose = Glucose + Glucose
Sucrose = Glucose + Fructose
Lactose = Glucose + Galactose
Polysaccharides
The term polysaccharide means many sugar molecules. Polysaccharides include:
Glycogen
Starch
Fiber
Glycogen
Glycogen is the storage form of glucose in animals. Glycogen is a long highly branched chain of glucose molecules linked together with alpha bonds. The branching provides many ends for rapid release of glucose. Glycogen is stored in the liver and muscle. Liver glycogen is broken down to release glucose if blood glucose levels fall too low, to provide glucose to the brain, nervous system, and developing red blood cells. Muscle glycogen provides rapid release of glucose to provide energy to muscle cells. The human body can only store enough energy as glycogen for about 1/2 - 2/3 of a day.
Starch
Starch is the storage form of glucose in plants. Starch is a long chain of glucose molecules some branched (amylopectin) and some not branched (amylose) linked together with alpha bonds.
Fiber
Fiber is a long chain of glucose molecules liked together with beta bonds. The human body does not have the enzymes to break beta bonds, as a result, fiber passes into the lower intestine undigested. In the lower intestine bacteria can ferment (breakdown) some fibers.
Digestion
When a person eats carbohydrates, enzymes hydrolyze (breakdown) the long chains to shorter chains, the shorter chains to disaccharides, and finally the disaccharides to monosaccharides.
Mouth: Salivary amylase begins carbohydrate digestion.
Stomach: HCl and pepsin deactivate salivary amylase.
Small Intestine: Pancreatic amylase breaks short polysaccharides down to disaccharides.
Intestinal Wall: Disaccharidases in the small intestine microvilli break disaccharides down to monosaccharides
Maltase breaks maltose down to glucose + glucose
Sucrase breaks sucrose down to glucose + fructose
Lactase breaks lactase down to glucose + lactose
Absorption
Glucose can be absorbed to some extent through the lining of the mouth, but for the most part, all nutrient absorption occurs in the small intestine. The monosaccharides cross the mucosal cells lining the small intestine microvilli by active transport and enter the capillaries of the intestinal villa.
Some fructose is converted to glucose in the intestinal mucosal cells. Fructose is not absorbed as rapidly as glucose; this may lead to a slower rise in blood glucose. Insulin is not needed for fructose to be taken up by body cells. The slower absorbance and rapid clearance may improve glycemic control. However, some people experience fructose malabsorption with a 20-50 gram fructose load. Children have a substantial fructose intake through sweetened drinks and juices. Children 6-18 months of age can exhibit carbohydrate malabsorption with ingestion of fruit juices (such as apple juice). Children who exhibit nonspecific diarrhea may benefit from a reduction in fructose intake.
The monosaccharides are carried to the liver via the portal vein. In the liver fructose and galactose are converted to other compounds, mostly glucose. Glucose is released from the liver into the blood and carried to the rest of the body. Once fructose is converted to glucose insulin is required for uptake by body cells.
Metabolism
Glucose plays the central role in carbohydrate metabolism.
Storage of Glucose as Glycogen
Glycogen is the storage form of glucose in animals. Glycogen is a long highly branched chain of glucose molecules linked together with alpha bonds. The branching provides many ends for rapid release of glucose. Glycogen is stored in the liver and muscle. Liver glycogen is broken down to release glucose if blood glucose levels fall too low, to provide glucose to the brain, nervous system, and developing red blood cells. Muscle glycogen provides rapid release of glucose to provide energy to muscle cells. The human body can only store enough energy as glycogen for about 1/2 - 2/3 of a day.
Glycogen holds a lot of water. If not enough carbohydrate is consumed to maintain blood glucose levels for the brain, nervous system, and developing red blood cells the breakdown of glycogen for glucose results in a loss of water, which many interpret as weight loss.
Using Glucose for Energy
The primary role of glucose in human nutrition is to supply energy to body cells. The brain, nervous system, and developing RBC can only use glucose for energy. Glucose (6-C) ? 2 Pyruvic acid 2(3-C) ? Acetyl CoA (2-C) ? TCA cycle (where energy is released as ATP get 38 molecules ATP/glucose)
Making Glucose from Protein
The body can convert body protein to glucose to some extent, but protein has jobs of its own other nutrients can't do. Body fat can't be converted to glucose to any significant extent, although fat breakdown can yield energy for body cells. However, the brain, nervous system, and developing RBC must use glucose for energy. Thus, if there is inadequate carbohydrate, proteins are broken down to make glucose to provide energy for these special cells. One of carbohydrates role is to spare body protein.
Using Fat for Energy
An inadequate supply of carbohydrate results in accelerated fat breakdown to provide energy. Triglycerides are the storage form of fat in adipose cells. Triglycerides are composed of a glycerol backbone and three fatty acids. Fatty acids are long chains of carbon, with hydrogen's and a hydroxyl group. Fatty acids are broken down into 2-C fragments which can be converted to acetyl CoA and enter the TCA cycle to provide energy.
The 2-C fat fragments can overload the TCA cycle, you get 2, 2-C units from glucose, you can get up to 9, 2-C units from an 18-C fatty acid. The fat fragments combine to form ketone bodies, which can accumulate in the blood and lead to a condition called ketosis (ketoacidosis). Ketosis disturbs the body's normal acid-base balance and can lead to coma and death. About 50-100 grams of carbohydrate daily are required to ensure the sparing of body protein and to prevent ketosis.
Conversion of Glucose to Fat
More glucose than what the body needs for energy or glycogen is converted to triglycerides in the liver and stored as a more permanent energy storage compound - body fat. Storing carbohydrate as fat is energy expensive. The body uses more energy to convert glucose to body fat than converting dietary fat to body fat.
Maintaining Consistency of Blood Glucose
To function normally, the body must maintain blood glucose levels within normal limits that permit the body cells to nourish themselves. If blood glucose levels go too low - it can cause a person to feel weak and dizzy. If blood glucose levels go too high - it can cause a person to feel confused and have difficulty breathing. Extremes in blood glucose levels, either too high or too low if left untreated can be fatal.
Regulating Hormones
If blood glucose levels go too high, insulin is released. Insulin signals body cells to uptake glucose for energy, stimulates the formation of glycogen, and stimulates the conversion of glucose to triglycerides to be stored as fat.
If blood glucose levels go too low, glucagon is released. Glucagon signals the liver to breakdown glycogen and release glucose into the blood.
Another hormone, epinephrine acts quickly stimulating release of glucose from glycogen into the blood and muscles, ensuring that all body cells have energy in an emergency.
Falling Out of Normal Range
The influence of food on blood glucose has lead to the over simplification that food governs blood glucose concentrations. Foods do not, the body does. In some people, blood glucose regulations fail. When this occurs two conditions can result; diabetes or hypoglycemia. Glucose may be modified as part of the treatment, but hormonal regulation or obesity (in the case of type 2 diabetes) is the cause not glucose.
Glycemic Index
The glycemic index describes the effect of food on blood glucose; how quickly glucose is absorbed, how high blood glucose rises, and how quickly it returns to normal. Different foods have different effects on blood glucose. For example, ice cream is high in sugar, but it has a lower glycemic index than a baked potato. A foods glycemic index differs depending on the food itself, fat content, fiber content, the form of the food, and whether it is eaten alone or with a meal.
Glycemic index of foods adjusted to a glycemic index of 100 for white bread
Maltose 152
Cornflakes 121
Glucose 138
White Bread 100
Honey 126
Shredded Wheat 97
Sucrose 83
Oatmeal 89
Fructose 26
White Rice 81
Corn 80
Ice Cream 69
Potatoes (mashed) 98
Yogurt 52
Potato chips 77
Whole Milk 44
Baked beans 70
Peas 50
Banana 84
Orange Juice 71
Apple 52
Types of Sweeteners
Caloric Sweeteners (provide calories)
Sugars
Caloric sweeteners include many regular sugars including; refined sugars, corn sweeteners, dextrose, HFCS, honey, syrups, crystalline fructose, lactose, invert sugars, glucose, maltose, and concentrated fruit juices
Intake of Added Sugar
According to the US Food Supply Data (JADA, 2002) American per capita consumption of added sugars went from 27 t (108 g)/person/day in 1970 to 32 t (128 g)/person/day in 1996. This increase was been driven by an increase in supply of high fructose corn syrup.
According to the Continuing Survey of Food Intake, from 1994 to 1996 (JADA, 2000) Americans age 2 and older consume equivalent to 82 g/day added sugar (20.5 t), which accounts for 16% of total calories. Adolescents age 12-17 had the highest intake averaging 20% of total calories from added sweeteners (adolescent females 97.7 g, adolescent males 141.8 g). The largest source of added sugar was regular soft drinks, which accounted for 2/3 of intake. Other sources were table sugars, syrups and sweets, sweetened grains, regular fruitades and milk products.
Guidelines for Sugar Intake
Dietary Guidelines for Americans 2000 advises to choose beverages and foods to moderate intake of sugars.
Dietary Goals 10% or less of total calories from sugar.
Food Guide Pyramid advises to use sugars "sparingly"
Total added sugars 6 t (24 g) 1600 calories
12 t (48 g) 2200 calories
18 t (72 g) 2800 calories
The average intake of added sweeteners for Americans age 2 and above is 20.5 t (82 g)/day for most Americans exceeds these recommendations.
Health Effects of Sugars
Nutritional Deficiencies
In excess sugar can contribute to nutritional deficiencies by supplying calories without providing nutrients. Bakery items, candies, and soft drinks provide calories with few nutrients. Whereas, grains, vegetables, fruits and dairy foods contain natural sugars and starches but also protein, fiber, vitamins and minerals. Sugar can contribute to nutrient deficiencies only by displacing nutrients.
For nutrition sake the appropriate attitude to take is not that sugars are "bad" and must be avoided, but that nutrient dense foods must come first. The goal is good nutrition and moderation. The amount of sugar a person can afford depends on how many calories are available beyond those needed for nutrients.
Honey does provide a few vitamins and minerals but the amounts are insignificant.
Tooth Decay
In excess both sugars and starches can contribute to tooth decay. Both sugars and starches begin breaking down to glucose in the mouth. Bacteria in the mouth ferment sugars and in the process produce an acid that can dissolve tooth enamel. Many factors are involved such as how long foods stay on the teeth, how often foods are eaten, and dental hygiene. Overall, the risk of dental caries increases with intake of nutritive sweeteners; however, sugars and carbohydrates do not work independently from other factors such as oral hygiene and fluoridation. For most people good oral hygiene will prevent dental caries.
Diabetes/Hypoglycemia
The influence of food on blood glucose has lead to the over simplification that food governs blood glucose concentrations. Foods do not, the body does. In some people, blood glucose regulations fail. When this occurs diabetes or hypoglycemia can occur. Glucose may be modified as part of the treatment, but hormonal regulation or obesity (in the case of type 2 diabetes) is the cause not glucose.
With people who have type 2 diabetes, attention is first given to the total amount of carbohydrate in the diet rather than the source. For people with diabetes sugars do not produce a greater glucose response (glycemic index) than complex carbohydrates. Intakes as high as 60 g sucrose or fructose may not adversely affect glycemic or lipid responses in persons with type 2 diabetes.
Hyperactivity or Misbehavior in Children
Controlled studies have failed to show an adverse relationship between sugar hyperactivity or misbehavior in children, even in children who by report are sensitive to sugar. The mechanism by which carbohydrate, including sugars, may affect mood is uncertain, but may involve the synthesis and release of serotonin in the brain. High carbohydrate intake stimulates the brain production of serotonin, which makes a person sleepy and sluggish.
Heart Disease
Usual intakes of sucrose and fructose do not elevate plasma triglycerides in most persons, provided calories are in balance. However, very high intakes of sucrose or fructose (2-3 times usual intake), or high carbohydrate diets (70-80% carbohydrate) can result in elevated plasma triglycerides which can increase heart disease risk. Remember carbohydrates stimulate release of insulin, which stimulates triglyceride formation, this is another rational for a balanced diet. A small percent of people are carbohydrate sensitive. These people respond to high doses of sugar or carbohydrate with abnormally high insulin secretion, which promotes triglyceride formation.
It is important to keep the effects of sugars in perspective. Other dietary factors such as total fat, saturated fat, and obesity have a much stronger association with heart disease than sugar intake.
Obesity
Obesity is a complex issue and cannot be attributed to one factor. Excess body fat arises from energy imbalance caused by taking in too many calories and by using too little. Because sugar adds calories to foods and beverages it has been hypothesized that sugar has a role in the etiology of obesity. Research does not support a direct connection between sugar and carbohydrate intake with obesity, unless excess intake of sugar containing foods leads to energy imbalance and weight gain.
Carbohydrates, including sugar suppresses appetite. Studies show an inverse relationship between sugar intake and obesity and direct a relationship between fat intake and obesity. However, foods high in sugar are often high in fat and excessive intake can contribute excess calories and fat. Food consumption surveys have shown that over the past two decades the total calorie intake of Americans has increased, and this increase has largely been in the form of carbohydrates, primarily in the form of soft drinks. Other studies have shown that children who are high soft drink consumers have higher calorie intakes than non-soft drink consumers. Studies have also shown that obese children and adults consumer a greater portion of their total calories from soft drinks than lean children.
Sugar Alcohols
Sugar alcohols provide calories and are thus caloric sweeteners, although they provide fewer calories than regular sugars because they are not completely absorbed. This allows products, which contain sugar alcohols to be labeled "sugar free" or "reduced calorie." Products may claim to be "sugar free" but this does not mean they care "calorie free."
Sugar Alcohol Comparative Sweetness Calories/gram
Isomalt 55% as sweet 2.0 calories/gram
Lactitol 40% as sweet 2.0 calories/gram
Maltitol 90% as sweet 3.0 calories/gram
Mannitol 70% as sweet 1.6 calories/gram
Sorbitol 50-70% as sweet 2.6 calories/gram
Xylitol 100% as sweet 2.4 calories/gram
Sugar alcohols occur naturally in fruits and vegetables. The body absorbs sugar alcohols slowly and incompletely. As a result they enter the blood stream slower than natural sugars. Because of the incomplete absorption sugar alcohols produce a lower glycemic response than sugars. However, side effects also occur because of the incomplete absorption. Greater than 50 g sorbitol or 20 g mannitol/day can cause gas, abdominal discomfort and diarrhea due to fermentation by intestinal bacteria (similar to lactose intolerance). For this reason, food products containing sugar alcohols carry a label "Excess consumption may have a laxative effect."
The real benefit of using sugar alcohols is that they do not contribute to dental caries. Bacteria in the mouth can't metabolize sugar alcohols as rapidly as sugar.
Non-Caloric Sweeteners (provide no or very little calories)
FDA Approved Non-Caloric Sweeteners
The FDA has approved four non-caloric sweeteners; saccharin, aspartame, acesulfame K, and sucralose. The body does not metabolize saccharin, acesulfame K, or sucralose, they pass through the kidneys unchanged. The body does digest aspartame, so technically it is a caloric sweetener, but the calories provided are insignificant.
Name Sweetness Commercial Name ADI
Saccharin 300X (200-700X) Sweet & Low 5 mg/kg
Aspartame 180X (160-220X) Nutrasweet/Equal 50 mg/kg
Acesulfame K 200X Sunette 15 mg/kg
Sucralose 600X Splenda 15 mg/kg
FDA Petitioned Non-Caloric Sweeteners
Three other non-caloric sweeteners have petitioned FDA and are awaiting approval; cyclamate, alitame, and neotame.
Accepted Daily Intake (ADI)
The safety limit of food additives is expressed as the acceptable daily intake (ADI). The ADI is the estimated amount/kg body weight that a person can safely consume everyday over a lifetime without risk. The ADI is a conservative level. It is usually 100 times less than the maximum level at which no observed adverse effects occur in animal studies.
Safety of Non-Caloric Sweeteners
There are questions as to the safety of non-caloric sweeteners. Considering that all compounds are toxic at some does, it is little surprise that large doses of non-caloric sweeteners or their by-products can have effects. The question to ask is whether their ingestion is safe in quantities people normally consume, and potentially abuse.
Saccharin Safety
Saccharin has been used for over 100 years in the US. Saccharin is not metabolized by the body and is rapidly excreted in the urine and does not accumulate in the body. Saccharin was originally included on the GRAS listing; however, questions on the safety of saccharin surfaced in 1977 when studies suggested large doses of saccharin (equivalent to hundreds of cans of diet soda daily for a lifetime) increased the risk for bladder cancer in rats. FDA proposed banning saccharin, but public out cry caused congress to impose a moratorium on the ban, which was repeatedly until 1991 when, the FDA withdrew its proposed ban on saccharin. Products containing saccharin must carry a warning label, "Use of this product may be hazardous to your health, this product contains saccharin, which has been determined to cause cancer in laboratory animals."
Studies of high saccharin users do not support an association between saccharin and cancer. The largest population study of saccharin, involving 9,000 men and women, overall showed saccharin did not increase the risk of cancer. However, in a subgroup of people (male heavy smokers and heavy saccharin users - six or more servings per day) the risk of bladder cancer was slightly increased. Common sense dictates that consuming large amounts of any substance is probably not wise, but at current, moderate intake levels, saccharin is assumed to be safe for most people.
Aspartame Safety
Aspartame is composed of two amino acids (phenylalanine and aspartic acid) and a methyl group (CH3). Alone they are not sweet, but combined they are 180 times sweeter than sucrose. In the digestive tract enzymes split aspartame apart. The body uses the amino acids just as if they had come from protein. Aspartame is one of the most studied of all food additives. Extensive animal and human studies document its safety. Long-term consumption of aspartame is not associated with any adverse health effects, except for a certain population subgroup with phenylketonuria (PKU).
Phenylketonuria
Phenylketonuria is an inherited disease that occurs in about 1 in 10,000-15,000. People with PKU are unable to dispose of excess phenylalanine. The accumulation of phenylalanime and it's by-products are toxic to the developing nervous system (by altering the synthesis of monoamine neurotransmitters), causing irreversible brain damage. For this reason all newborns in the US are screened for PKU. The treatment for PKU is a special diet that must strike a balance between providing enough phenylalanine for growth, but not too much to cause harm. For this reason products with aspartame must carry a warning label "Phenylketonurics contains phenylalanine."
In persons without PKU a bolus load of 50 mg/kg on repeat dose studies showed plasma phenylalanine levels near normal. Persons with PKU appear to tolerate the amount of phenylalanine in a diet soda sweetened with aspartame (104 mg/12 oz). Herterozygotes of PKU do not show changes after 12 weeks consumption.
Still there is compelling reason why children with PKU need to get all their pheylalanine from foods, and not from a non-caloric sweetener. The PKU diet excludes protein rich foods such as milk, meat, fish, poultry, cheese, eggs, nuts, legumes, and many bread products. Only with difficulty can these children obtain the many essential nutrients such as calcium, iron, zinc and B-vitamins. To suggest children with PKU should squander any of their limited phenylalanine allowance on aspartame, which contributes none of these nutrients, would open the way for poor nutrition.
Diketopiperzine
Aspartame breaks down to diketopiperzine (DKP) in liquid systems with heat exposure and loses its sweetness. Studies testing this product have eliminated it as a concern. Studies have shown that even if all the aspartame was converted to diketopiperzaine in beverages, the amount would be below the ADI of 3,000 mg/kg for diketopiperzine.
Methyl Group
During metabolism the methyl group momentarily becomes methyl alcohol (methanol) a potentially toxic compound. Enzymes convert methanol to formaldehyde, another toxic compound. Formaldehyde is then converted to carbon dioxide. The levels formed are modest, and fall below threshold levels at which they would cause harm. In fact tomato, citrus and grape juices yield higher amounts of methanol than is produced by the aspartame in a 12-oz diet soda.
Aspartic Acid
Aspartic acid also has the potential to cause brain damage at very high does, but under normal intake levels the brains mechanism for controlling aspartic acid levels ensures no adverse effect.
Other Accusations
Some people have tried to link aspartame to multiple sclerosis, lupus, seizures, brain tumors, and birth defects. There is no credible evidence to support a link between aspartame with multiple sclerosis or lupus. Human studies show aspartame neither causes nor enhances seizures. No evidence has been found that aspartame caused birth defects. The associated claim between aspartame and brain tumors has not been supported.
Some people claim to exhibit vague, but not dangerous symptoms due to unusual sensitivity to aspartame (headaches, dizziness, mood alterations, fatigue, vision problems). However, double blind studies have failed to reproduce reactions in controlled conditions. The CDC has reviewed 600 behavioral complains and concluded there was no association.
Estimated Aspartame Intake
The FDA estimates the general population consumer 6% of the ADI. Those 1-5 years consume 10.4% of the ADI. To reach the ADI a 150 pound adult would need to consume 97 packets of equal or 20 cans diet soda/day, a 40 pound child would need to consume 24 packets equal, 4 cans diet soda, or 9 (8oz) glasses of drink sweetened with aspartame.
Acesulfame K Safety
The FDA approved acesulfame K in 1988 after reviewing 90 safety studies conducted over 15 years. Some consumer groups believed acesulfame K caused tumors in rats. The FDA concluded that the tumors were not caused by the sweetener, but were typical of those commonly found in rat studies.
Sucralose Safety
The FDA approved sucralose in 1998 after reviewing 110 safety studies conducted over 20 years.
Alitame, Neotame, and Cyclamate Safety
FDA approval for alitame, neotame, and cyclamate is pending. To date no safety issue for alitame has been raised. Alitame is composed of L-aspartic acid, D-alamine, and a novel C-terminal amide moiety. It is 2,000 times sweeter than sucrose. In 1995 FDA concluded no carcinogenic effect and not reproductivity toxicity. However, recommendations on an ADI are waiting further research. The FDA petition is 0.34 mg/kg. No adverse effect was observed at 100 mg/kg.
Cyclamate has been battling safety issues for 50 years. Approved by the FDA in 1949 cyclamate was banned in 1969 principally based on one study indicating it caused bladder cancer in rats. The National Research Council has reviewed dozens of cyclamate studies and has concluded that neither cyclamate or its metabolites cause cancer. The Council did recommend further research on the risk of health and long term use. Although cyclamate does not cause cancer it may promote cancer once started.
Special Populations
Children
Because of their size and relatively high food and fluid intake compared to adults, children have the highest intake of non-caloric sweeteners. It has been suggested that children limit saccharin intake because data available for its use in young children is limited. Intakes of aspartame in children, ages 2-5 years is estimated at 8-17 mg/kg which is below the ADI of 50 mg/kg. Intakes of acesulfame-K in children are estimated at 3-9 mg/kg, which is below the ADI of 15 mg/kg.
Pregnant Women
It has been suggested that pregnant women consider careful use of saccharine. Saccharine can cross the placenta and may remain in fetal tissues because of low fetal clearance.
Aspartame intake during pregnancy does not change fetal expose to aspartic acid; however, fetal circulating levels of phenylalanine exceed maternal levels. A bolus of aspartame 34 mg/kg resulted in a peak plasma phenylalanine of 112 umole/L in normal subjects and 162 umoles/L in heterozygotes - still below the level that would cause neurological problems in the fetus (1,090 umole/L). Plasma response of methanol and formate were not significant after an aspartame load. Thus the use of aspartame within FDA guidelines appears to be safe for pregnant women.
Acesulfame K has been determined to be safe during pregnancy.
Source:
Whitney, E.N. & Rolfes, S.R. Understanding Nutrition, 8th ed. West/Wadsworth Publishing Co., Belmont, CA, 1999.