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).
Monosaccharides and disaccharides are sometimes called simple sugars while polysaccharides are called complex carbohydrates, starch or fiber.
The term monosaccharide means one sugar molecule. There are three monosaccharides: Glucose,Fructose and 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.
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
The term polysaccharide means many sugar molecules. Polysaccharides include: Glycogen,Starch and Fiber.
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 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 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.
When a person eats carbohydrates, enzymes breakdown the long chains to shorter chains, the shorter chains to disaccharides, and finally the disaccharides to monosaccharides.
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.
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.
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.
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
Health Effects of Sugars
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.
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.
