Foods supply carbohydrates in three forms: starch, sugar,
and cellulose (fiber). Starch and sugar are major and essential
sources of energy for humans. A lack of carbohydrates in the
diet would probably result in an insufficient number of calories
in the diet. Cellulose furnishes bulk in the diet.
Since the tissues of the body need glucose at all times, the
diet must contain substances such as carbohydrates or substances
which will yield glucose by digestion or metabolism. For the
majority of the people in the world, more than half of the diet
consists of carbohydrates from rice, wheat, bread, potatoes,
All life requires protein since it is the chief tissue builder
and part of every cell in the body. Among other functions, proteins
help to: make hemoglobin in the blood that carries oxygen to
the cells; form anti-bodies that fight infection; supply nitrogen
for DNA and RNA genetic material; and supply energy.
Proteins are necessary for nutrition because they contain
amino acids. Among the 20 or more amino acids, the human body
is unable to synthesize 8, therefore, these amino acids are called
essential amino acids. A food containing protein may be of poor
biological value if it is deficient in one or more of the 8 essential
amino acids: lysine, tryptophan, methionine, leucine, isoleucine,
phenylalanine, valine, and threonine. Proteins of animal origin
have the highest biological value because they contain a greater
amount of the essential amino acids. Foods with the best quality
protein are listed in diminishing quality order: whole eggs,
milk, soybeans, meats, vegetables, and grains.
Fats and Lipids:
Fats are concentrated sources of energy because they give
twice as much energy as either carbohydrates or protein on a
weight basis. The functions of fats are to: make up part of the
structure of cells, form a protective cushion and heat insulation
around vital organs, carry fat soluble vitamins, and provide
a reserve storage for energy.
Three unsaturated fatty acids which are essential include:
linoleic, linolinic, and arachidonic and have 2, 3, and 4 double
bonds respectively. Saturated fats, along with cholesterol, have
been implicated in arteriosclerosis, "hardening of the arteries".
For this reason, the diet should be decreased in saturated fats
(animal) and increased in unsaturated fat (vegetable).
a) MH + NAD+ ---> NADH + H+ + M + energy
b) ADP + P + energy ---> ATP + H2O