Dietary proteins are composed of amino acids and play a role in the manufacture of many structures and tissues, such as bone, muscle, skin and hair. They are essential for normal growth and development in children. In adults, proteins provide the raw materials needed for cell repair.
Cells use amino acids derived from the diet to make DNA and enzymes – molecules with key roles in maintaining healthy structure and function within the body. Of the 21 amino acids, many can be made in the body and therefore do not, strictly speaking, need to be eaten. However, eight cannot be made in the body and so must be provided in the diet. They are the “essential” amino acids.
It is said that adults need about 0.75–1g of protein for each kilogram of body weight per day.Meat, fish, eggs and dairy products are very good sources of protein, as are non-animal foods, such as beans, peas and nuts. Some grains, such as rice, wheat and maize, can also supply significant quantities of protein. In general, animal-derived proteins are more “complete” in terms of their component amino acids than vegetable sources. Vegetarians should eat a broad range of protein-containing foods, including beans,
pulses, nuts, seeds and grains, to ensure amino acid intake.
There is some evidence that too much protein can pose hazards. Excess protein is believed to cause the loss of calcium from bone, predisposing a person to osteoporosis (brittle bones; see p.38) and increased risk of fracture. In one study, women who ate more than 95g of protein a day were found to be 20 per cent more likely to have broken a wrist over a 12-year period when compared to women who ate an average amount of protein (less than 68g a day).