The process by which living organisms take in and utilize nutrients –
the substances or foodstuffs required for growth and the maintenance of life. Vital substances that cannot be synthesized
within the cell and must be present in the
food are termed essential nutrients. Organisms such as green plants can derive energy from sunlight and synthesize their nutritional requirements
from simple inorganic chemicals present in the soil and air by photosynthesis.
Animals, on the other hand, depend largely on previously synthesized organic
materials obtainable only by eating plants or other animals (see digestive
|Certain foods are important sources of carbohydrate (A), fat (B), protein (C), or roughage (D). Except for some refined products like cane sugar, most foods contain several different nutrients. Milk provides carbohydrate, fat, protein, calcium, and some vitamins. Even potatoes contain protein and vitamin C as well as energy-rich carbohydrate.
Human nutrition involves five main groups of nutrients: protein, fat, carbohydrate, vitamins, and minerals. Proteins, fats,
and carbohydrates are the body's sources of energy, and are required in
relatively large amounts. They yield this energy by oxidation in the body cells, and nutritionists measure it in heat units called food
calories (properly called kilocalories, each equal to 1,000 gram calories).
Carbohydrates (food starches and sugars)
normally form the most important energy source, contributing nearly half
the calories in a well-balanced diet. Cereal products and potatoes are rich
in starch; sucrose (table sugar) and lactose (present in milk) are two common sugars. Fats, which provide about 40% of
the calories requirement, include butter, edible oils, and shortening, and
are present in such foods as eggs, fish, meat, and nuts. Fats consist largely
of fatty acids, which divide into two
main classes: saturated and unsaturated. Certain fatty acids are essential
nutrients; but if there is too much saturated fatty acid in the diet, an
excess of cholesterol may accumulate
in the blood. Proteins supply the remaining energy needs, but their real
importance lies in the fact that the body tissues, which are largely composed
of protein, need certain essential
amino acids, found in protein foods, for growth and renewal. Protein-rich
foods include meat, fish, eggs, cereals,and beans. Too little protein in
the diet results in malnutritional diseases (see malnutrition)
such as kwashiorkor.
|Weight for weight pure fat supplies 2.25 times more energy than carbohydrate. Any protein surplus to the body's need for tissue growth and repair can also supply energy.
Minerals (inorganic elements) and vitamins (certain complex organic molecules)
provide no energy, but have numerous indispensable functions. Some minerals
are components of body structures. calcium and phosphorus, for example, are essential
to bones and teeth. Iron in the blood is vital for the transport of oxygen to the tissues; an iron deficiency
results in anemia. Milk and milk products
are good sources of calcium and phosphorus; liver, red meat, and egg yolk,
of iron. Other important minerals, normally well supplied in the Western
diet, include chlorine, iodine, magnesium, potassium, sodium, and sulfur.
Vitamins, which are present in small quantities in most foods, are intimately
associated with the action of enzymes in
the body cells, and particular vitamin deficiencies accordingly impair certain
of the body's synthetic or metabolic processes. A chronic lack of vitamin
A, for example, leads to a hardening and drying of the skin and can result
in irreversible damage to the conjunctiva and cornea of the eye. Beriberi is caused by a vitamin B1 deficiency, scurvy by a vitamin C deficiency, rickets by a lack of vitamin D.
|Simply to keep their metabolism going, children use far more energy per kilogram of their body weight than older people do; they also need much more protein in proportion to their size.