Carbohydrates are the principal dietary source of energy. They are
comprised of simple sugars and complex carbohydrates. The most common
simple sugars, or monosaccharides (i.e., one sugar unit) include glucose,
fructose, galactose and mannose. Glucose is the main form in which
carbohydrate is absorbed, exchanged between tissues, and channeled into
metabolic pathways. One molecule of glucose contains 6 atoms of carbon, 6
atoms of oxygen and 12 atoms of hydrogen. The name carbohydrates (often
abbreviated as 'CHO') derives from the fact that these compounds are
primarily made up of carbon atoms and water. Disaccharides (i.e., two
sugar units) include sucrose (also called saccharose), maltose and
lactose. Oligosaccharides, such as raffinose and stachyose in legumes, are
composed of 3-10 sugars. Polysaccharides are substances in which multiple,
often hundreds of monosaccharides molecules, are linked together. The most
abundant edible forms are the starches present in grains, potatoes, etc.,
which are polymers of glucose. The two major forms of these complex
carbohydrates are amylose and amylopectin. In animals, the storage form of
carbohydrates is glycogen. Dextrins are fragments of starch produced by
dry heat, such as toasting.
Dietary fibers such as cellulose are polysaccharides in which glucose
or other monosaccharides are linked by bonds which cannot be hydrolysed by
the digestive enzymes produced in the intestine. Dietary fiber, dextrins
and some oligosaccharides are thus not digestible, except through the
action of enzymes produced by intestinal bacteria. Undigested dietary
fiber passes to the large bowel where it is fermented by bacteria to yield
lactic acid and the short-chain fatty acids acetic, propionic and butyric
acid. Humans derive measurable amounts of energy from the metabolism of
these products of bacterial fermentation. This process is very active in
herbivores which can thereby derive most of their energy from cellulose.
Other carbohydrate-related compounds include polyols (i.e., sugar alcohols
such as sorbitol and xylitol) and organic acids, such as ascorbic acid
(i.e., vitamin C), citric acid and malic acid. Although monosaccharides
yield slightly less energy than starches (3.7 vs. 4.2 kilocalories per g),
the energy value of dietary carbohydrates is generally considered to be an
even 4 kilocalories per gram.
Deficiency: There is no absolute requirement for
dietary carbohydrate, although the brain, the red blood cells and some
cells in the kidney use glucose as a preferred source of energy. The liver
has a considerable capacity to synthesize glucose from amino acids (from
dietary and body protein) and glycerol, released from lipid hydrolysis.
When intake of dietary sugars and starch is low or lacking, stored fat (triacyglyceride)
is mobilized, providing fatty acids to sustain energy generation in the
body. A carbohydrate-free diet is associated with increased protein
breakdown and ketone body formation in the liver, which may produce
dehydration and loss of cations, though not to the same dangerous extent
as in diabetic ketoacidosis. Starvation ketosis is greatly attenuated by
daily intakes of as little as 50-100 g of carbohydrate.
Diet recommendations: No formal diet recommendation
for carbohydrates exists as intakes are adequate in US diets, though
consumption of some 25 grams of dietary fiber (i.e. undigestible
polysaccharides) is recommended. The Daily Value for carbohydrates used in
nutritional labeling is 60% of total calories, or 300 g/day for a 2,000
Calorie diet. In 1985, the US average carbohydrate intake was 177 g in
women and 287 g in men. Approximately 23% came from fruits and vegetables
and 41% was from grain products. About half of the carbohydrate intake was
in the form of mono- and disaccharides and the reminder as complex
carbohydrates.
Food sources: Monosaccharides such as glucose and
fructose are found in fruits and honey. Sucrose (glucose linked to
fructose), or table sugar (produced from sugar cane or sugar beets) is the
most commonly consumed disaccharide, found also in molasses, maple syrup
and some fruits. Maltose (two glucose units) is a component of sprouting
grains, malted cereals and syrups. By converting the glucose in corn
syrups into fructose, isosyrups with increased sweetening power are
produced which are used instead of cane sugar, notably in confectionary
items and soft drinks. Lactose (a dissacharide of glucose and galactose)
is the sugar in milk. Amylose is the dominant starch in wheat and rice.
Amylopectin is found in corn, potatoes and tapioca.
Resistant starches are starches which are poorly digested in the human
gastrointestinal tract, either because the starch is in granular form
(e.g., uncooked) and therefore resistant to digestive enzymes, or in the
form of retrograded amylose, which may form after cooling starch that has
been gelatinized by moist heating. Up to 7-10% of starch from wheat, oats
and potatoes and 20% from baked beans can pass through to the colon.
Resistant starches which reach the colon are fermented there by the
microflora and converted into short-chain fatty acids, a process which
also yields small amounts of methane gas and hydrogen.
Recent research: Conversion of carbohydrate into fat
allows animals to build up fat reserves even when their feed contains very
little fat. It has therefore long been believed that excess carbohydrate
can readily be converted into stored body fat. However, it is now well
established that in man, fat synthesis is essentially negligible under
conditions of unrestricted access to food. On another front, it has been
shown recently that hyperactivity in children is not associated with sugar
intake. Of continued major interest are studies on the beneficial effect
of consuming more complex carbohydrates which have a relatively low
glycemic index (i.e., they cause a less abrupt increase in blood glucose
levels after meals) instead of the more rapidly absorbed sugars and
starches. This is particularly important for individuals affected by
insulin-dependent, as well as non-insulin dependent, diabetes mellitus and
because complex carbohydrates tend to exert favorable effects on blood
lipid levels in subjects at risk for cardiovascular disease. In addition
there is continuing research into the role of non-starch polysaccharides
(fiber) in the prevention of colonic cancer.
Toxicity: Ingestion of even large amounts of
carbohydrates has no adverse effects, except that lactose-containing milk
products can lead to diarrhea in lactase-deficient populations. In
individuals affected by certain inherited carbohydrate-metabolizing enzyme
deficiencies, carbohydrate intake may have to be carefully controlled to
avoid possibly severe adverse reactions.
