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Updated on March 24, 2012

Protein's main functions are to promote growth and to build and repair body tissue. Children need protein for growth in particular, and so do pregnant women with a "growing" baby. Adults need protein for repair and maintenance only. Proteins also assist the repair and replacement of different body tissues, such as bones, muscles, connective tissues, and the walls of hollow organs.

Important protein rich foods include meat, fish, eggs, legumes, nuts, potatoes and cereals.

The body cannot store protein and if more protein is eaten than is needed for growth and maintenance purposes the body burns it as fuel or stores it as fat. Conversely, if insufficient total kilojoules are eaten, the body uses protein for fuel instead of for growth or repair.
The recommended amount of protein for the human body's daily use has been dramatically reduced in the last decade or so as nutritionists have learned more and revised their thinking.

Back in the "dark ages" of nutrition — and that is only going as far back as the 1950s — meat was considered to be almost the be-all and end-all of protein because it contains all the essential protein factors in the right mix. However, modern nutritionists place far less emphasis on meat as a protein source and point out that a combination of legumes (soy beans, kidney beans, lentils) and grain can provide just as much protein in equally the right mix.

Is it a coincidence or one of nature's many miracles that this particular combination of foods has been the staple of many civilisations for centuries? Latin America's tortilla and beans and China's rice and bean curd are obvious examples.

In a recent survey conducted in UK almost 50% of the people knew that meat was a good source of protein, and 30% mentioned eggs and 20% ticked fish. But almost no one mentioned bread or potatoes. These figures would probably hold true for Australia. In fact, on a normal diet, most people obtain more protein from potatoes and bread than from fish. The point here is that although fish is a much richer source of protein than bread or potatoes, most people eat bread or potatoes once or twice a day, whereas they might only eat fish once every two or three weeks.

The recommended allowance for protein of 1 gram of protein per kilogram (2.2 pounds) of body weight or 70 grams for a man weighing 70 kilograms (154 pounds) represents a margin of safety of 50 percent above the actual adult requirement of about 45 grams of protein per day. Recent studies show convincing evidence to justify reducing the protein requirement still lower, which indicates that the recommended allowance of 70 grams of protein is a generous allowance in spite of the fact that it is common for the protein intakes in American diets to be considerably above this level.

The protein allowances for children must be high enough to provide for optimum growth. The allowance for infants is 3.5 times the allowance for adults or 3.5 grams protein per kilogram of body weight. In early childhood an allowance of 2.5 grams protein per kilogram suffices, and in late childhood 1.5 grams per kilogram of body weight is adequate. The quality of the protein must be insured at all times, and if milk is included daily in amounts adequate to meet the calcium needs, the chances are that the protein needs will be adequately taken care of from the standpoint of quality.

There are 21 amino acids that have been found in the proteins of foods. The amino acids present in individual food proteins vary greatly and the quality of protein present in a given food depends upon the particular combination of amino acids present. Of the 21 known amino acids in foods, only 10 have been found to be essential for the growth of the experimental animal, the albino rat. The remaining 11 amino acids can be synthesized by the body if the 10 essential amino acids are present in the diet in adequate amounts.

The 10 essential amino acids include the following: arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophane, and valine. Of these, only 8 have been found to be essential for the maintenance of nitrogen equilibrium in human adults.

Arginine is not essential, and histidine is only essential for growth in infants. Methionine and cystine contain sulfur in addition to carbon, hydrogen, oxygen, and nitrogen. But cystine is unessential if methionine is present in adequate amounts for growth. The requirements of the essential amino acids for the maintenance of nitrogen equilibrium in the human adult have been established. Also, the amounts of each of the essential amino acids occurring in a number of our foods have been determined. However, it is not necessary to calculate the amounts of individual essential amino acids in our foods because some foods naturally contain excellent combinations of the amino acids and when eaten in the right proportion in the diet will meet the protein requirement.

Also, some legumes and nuts furnish complete proteins; for example, soy beans, Brazil nuts, almonds, and peanuts. Cereals' and most legumes furnish incomplete proteins, but they can be counted on to furnish supplementary proteins when they are combined with foods in the diet furnishing complete proteins. For example, cereals and milk not only furnish an excellent combination of proteins but a combination that can be obtained at low cost. This is one of the reasons why cereals and milk are so important in low-cost diets.

Which foods have the best quality protein?

Foods which' provide the right proportion of amino acids for our needs are said to have a high biological value. Not surprisingly, human milk has the highest value - 100 per cent - but so have eggs. The next best in terms of quality are meat, fish and cow's milk, all of which are considered to have a 75 per cent biological value. What this means is that they can supply only 75 per cent of one of the amino acids we need (in this case sulphur-containing). However, all the other amino acids are present in large amounts.

But this only matters if you eat only the basic minimum of 1 1/2 ounces of pure protein a day - and in actual fact we eat far more. What is lacking in quality can be made up in quantity. Although bread has only 50 per cent biological value (it provides only 50 per cent of the amino acid lysine we need); by eating twice as much, we double the amount of lysine.

So if bread contains 10 per cent protein, to get your minimum daily allowance (1 1/2 ounces) you would, in theory, have to eat 15 oz of bread. But since bread only has 50 per cent biological value, you would actually have to eat 30oz of bread to get all the amino acids you need.

Fortunately, few people live on only one kind of food, and so what one food lacks may made up in another. For instance, meat lacks sulphur amino acids, but has a surplus of lysine; bread is short of lysine but has a lot of sulphur amino acids - so a meat sandwhich will provide the necessary intake of both.

Why are amino acids important?

Amino acids are known as the building blocks of life. There are 20 of them in all, composed of carbon, hydrogen, oxygen, nitrogen and sometimes sulphur. The different types can come together in all sorts of combinations to make an astonishing variety of proteins - just as the letters of the alphabet can form hundreds and thousands of words. So, although the basic material is protein, each tissue has quite a different structure.

To transform the protein in the food we eat into body tissue, it has to be broken down by the digestive system into its component amino acids. These are then sent round the body in the blood stream. Every tissue 'selects' the different amino acids it needs to form its own proteins.

Often the body can turn one type of amino acid into another which it needs, but there are eight amino acids which the body needs in exactly the right form; these are called the essential amino acids.


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