Carbohydrates (sugers and starches) provide energy to body cells, particularly the brain, which is the only carbohydrate-dependent organ in the body. Carbohydrates are classified according to the number of suger units they contain. The simplest carbohydrate is glucose. Glucose, also called blood sugar and dextrose, which contains one suger unit, flows in the bloodstream so that it is available to every cell in your body. Your cells absorb glucose and convert it into energy to drive the cell. Glucose is made up of carbon and water with a chemical formula of C₆H₁₂O₆ (6 carbon, 12 hydrogen, and 6 oxygen atoms), hence the name carbohydrate.

Maltose is made from two glucose atoms bonded together. Fructose and galactose have the same chemical formula as glucose but the atoms are arranged differently, the liver converts them to glucose. Sucrose is made of one glucose and one fructose molecule bonded together. Lactose is made of one glucose and one galactose molecule bonded together. Glucose, fructose and galactose are monosaccharides and are the only carbohydrates that can be absorbed into the bloodstream through the intestinal lining. Lactose, sucrose and maltose are disaccharides (they contain two monosaccharides) and are converted to monosaccharides by enzymes in the digestive tract. Monosaccharides and disaccharides are called simple carbohydrates. Suger alcohols, such as sorbitol and mannitol, are alcohol forms of glucose and fructose respectively. The term suger is usually used to describe monosaccharides and disaccharides.

Oligosaccharides contain 3 to 10 suger units are often breakdown product of polysaccharides which contain more than 10 suger units. Monosaccharide units linked together by glycosidic bonds are called polysaccharides, glycans or complex carbohydrates; there are three major polysaccharides that occur in food: starch, glycogen, and cellulose.

Starch is the principal food reserve in plants and the main source of carbohydrates in the human diet. It is a mixture of polyglucans; that is, homopolysaccharides made from glucose. There are two types of polyglucans in starch: amylose and amylopectin; amolose is digested slowly and amylopectin is digested quickly. Glycogen, the storage polysaccharide of animals, is present in all cells but is most prevalent in skeletal muscle and liver cells; its major components are triglycerides of fat. Cellulose, a non-starch polysaccharide, is the primary structural component of plant cell walls and is the major part of insoluble dietary fiber.

Dietary fiber is nondigestable carbohydrate and lignin that are part of plants. Soluble, or functional, fiber is beneficial and can create feelings of satiety, lower blood cholesterol levels, reduce postpandial blood glucose levels, and possibly improve cell insulin sensitivity. For more information on dietary fiber, see Cholesterol-lowering effects of dietary fiber. Lisa Brown, Bernard Rosner, Walter W. Willett and Frank M. Sacks. American Journal of Clinical Nutrition, Vol. 69, No. 1, pages 30-42, January 1999.

Most grains (wheat, corn, oats, rice) and vegetables are high in starch. Your digestive system breaks a complex carbohydrate back down into its component glucose molecules so that the glucose can enter your bloodstream. Simple carbohydrates enter the bloodstream at the rate of about 30 calories per minute, complex carbohydrates are digested more slowly and most enter the bloodstream at around 2 calories per minute. The rate of carbohydrate food digestion and glucose absorption into the bloodstream is indicated by its glycemic index (GI) and glycemic load (GL); low GI or GL indicates slow absorption.

Simple carbohydrates are converted quickly to blood glucose which causes the pancreas to secrete a large amount of insulin to lower blood glucose levels; this large amount of insulin tends to lower the amount of blood glucose to very low levels within 3 to 5 hours, which often leads to feelings of nervousness and irritability.

Insulin, a simple protein, controls how the body uses blood glucose. It enables glucose to enter cells, which use it for energy; it helps convert glucose into glycogen for storage in the liver and muscles; it facilitates the conversion of excess glucose into fat; it slows down the breakdown of fats and protein into glucose; it promotes the uptake of amino acids for making proteins. The action of insulin is opposed by glucagon and epinephrine.

Carbohydrate deficiency signs?

1. Decreased secretions from the mouth, nose, and eyes
2. Muscle weakness
3. Inability to concentrate
4. Easily startled
5. Difficulty swallowing
6. Voice affected by stress

If you suspect you have a carbohydrate deficiency, contact your doctor or healthcare professional immediately.

How much carbohydrate is needed?

The World Health Organization (WHO) recommends a minimum carbohydrate intake of 50g/day to avoid ketosis with the bulk coming from non-starch polysaccharides from a wide variety of appropriately processed and cooked cereals, vegetables, legumes, and fruits. I'm confused by this recommendation, non-starch polysaccharides are not digestable so they can't provide energy; the main source of energy for the body are carbohydrates that are digestable! I presume that the recommended intake of 50g/day are digestable carbohydrates. I'm led to conclude that they mean that we should eat carbohydrates that are high in dietary fibre with 50g/day available as digestable carbohydrates.

The recommended dietary allowance for carbohydrates is 130g/day based on the minimum amount of glucose needed by the brain. This minimum amount of glucose is usually execeeded because additional glucose is gained from fat and protein.

The WHO also recommend that between 55% to 75% of our daily calories come from carbohydrates with a low glycemic index. I personally find this recommendation at odds with the experiences of many type 2 diabetics, including myself, who have discovered that limiting their carbohydrate intake to below 55% is necessary to keep their blood glucose levels within an acceptable range. Higher levels of cabohydrates in the diet are also associated with higher triglyceride levels in the blood. For Type 2 Diabetics, I think that 55% should be the upper, not the lower, limit! I personally aim for around 40% of total calories from carbohydrates. The WHO also recommends that we should include 25 to 35 grams of dietary fibre/day (5-15g soluble) into our diet.

For more information on carbohydrates, see Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (Macronutrients), 2002, Chapter 6, pp207-264, Food and Nutrition Board (FNB), Institute of Medicine (IOM); and Carbohydrates in Human Nutrition, FAO/WHO Joint Report, April 1997. Also see my Protein and Fats pages.