The food we eat consists chemically of a limited number of components (the nutrients) that can be used for building up or maintaining body tissues or for the vital functions of the body.
The main function of proteins, large molecules composed of many subunits called amino acids, is to provide the raw materials for growth. There are about 20 common amino acids found in animal and plant proteins. We take in these proteins in food, break them down into amino acids during digestion, and reassemble these, like building blocks, into our own body’s proteins.
Carbohydrates supply energy, and supply it quickly in a form readily usable by the body. They are eaten in the form of sugars and starches but are ultimately converted by the body to glucose, which can be either used directly as fuel or stored in the form of glycogen (animal starch) in the liver. Some glycogen is also stored in muscles, where it is readily available to supply energy for muscular movement. Some carbo-hydrates, such as cellulose (the structural material of plants), cannot be digested and absorbed by human beings, but are nevertheless useful. This dietary fibre (’roughage’) is found in bran, cereals, fruits and vegetables. It speeds the passage of food through the gut and assists easy, regular stool elimination. It also seems to protect against certain disorders; wide claims have been made that plenty of fibre reduces the incidence of diverticulosis, cancer of the colon and rec-tum, appendicitis, varicose veins and haemorrhoids, heart disease, gallstones and diabetes mellitus.
Like carbohydrates, fats (chemically lipids) also supply energy. They are eaten mainly in dairy produce, vegetable oils and meat. Fats are more concentrated as an energy source: they provide, gram for gram, twice as much energy as carbohydrates, but they require more chemical conversions and so are not such a ready source. Fats play two roles in the body. First, about one per cent of body weight is fat in cells, where it is a structural component of cell membranes and participates in the cell’s chemical reactions. Second, the remaining body fat is stored as tiny globules in cells that make up adipose tissue.
A very important dietary element is water, the medium in which life evolved; the human body is more than 60 per cent water, and needs a throughput of water for several reasons. The kidneys need to excrete water daily as a vehicle for removal of waste products, and through evaporation of sweat from the skin water also plays an important part in control of body temperature. You could live for several weeks without food, but for only a few days without water.
Minerals are chemical elements (single substances) such as calcium, iron and iodine. Some play well- established roles, others have less clear functions. Calcium is one of the former. Each of us needs about one gram a day of calcium, a mineral essential for healthy bones and teeth, blood clotting, muscular movement and the transmission of electrical signals along nerves. Iron, another well-known mineral, is needed in much smaller quantities to make some enzymes but its most important role is as a part of the oxygen-carrying haemoglobin molecule which is found inside red blood cells.
Vitamins are organic substances and are needed in only very small amounts; without them serious deficiency diseases occur. The body can make a few vitamins from raw materials but most of them must be present ready-made in our food. Lack of vitamin A, for example, causes night-blindness; lack of vitamin C causes scurvy. In the case of some vitamins (A and D, for example) an excess is harmful.
Your body is made up of various tissues and organs. These, in turn, are composed of microscopic building blocks called cells; and cells, in their turn, are made of building blocks that are chiefly proteins. Proteins are thus an essential part of all living matter, both animal and vegetable; every single living cell has a basic structure made up of protein and proteins also take part in metabolism – all the various chemical processes involved in living.