are widely distributed in plants and animals, where they fulfil
both structural and metabolic roles. In plants, glucose is synthesise
from carbon dioxide and water by photosynthesis and stored as starch
or is converted to the cellulose of the plant framework. Animals
can synthesise some carbohydrate from fat and protein, but the bulk
of animal carbohydrate is derived ultimately from plants.
Knowledge of the structure and properties of the carbohydrates of
physiologic significance is essential to understanding their fundamental
role in the economy of the mammalian organism. The sugar glucose
is the most important carbohydrate. It is as glucose that the bulk
of dietary carbohydrate is absorbed into the bloodstream or into
which it is converted in the liver, and it is from glucose that
all other carbohydrates in the body can be formed. Glucose is a
major fuel of the tissues of mammals (except ruminants) and a universal
fuel of the fetus. It is converted to other carbohydrates having
highly specific functions, eg., glycogen for storage; ribose in
nucleic acids; galactose in lactose of milk, in certain complex
lipids, and in combination with protein in glycoprotein and proteoglycans.
Most carbohydrates in the diet form glucose, galactose, or fructose
upon digestion. These are transported to the liver via the hepatic
portal vein. Galactose and fructose are readily converted to glucose
in the liver.
Glucose is specifically required by many tissues but does not have
to be provided as such in the diet, since other dietary carbohydrates
are readily converted to glucose, either during digestion (eg.,
starch) or subsequently in the liver (eg., fructose, galactose.
Glucose is also formed from the glycerol moiety of fats and from
glucogenic amino acids by gluconeogenesis. However, a minimum daily
intake of carbohydrate (50 g) is recommended in humans to prevent
ketosis and loss of muscle protein.
Diseases associated with carbohydrates include
diabetes mellitus, galactosemia, glycogen storage diseases, and