Which foods are the best sources of protein?
Meat, fish, eggs, cheese and milk are the richest sources of protein, but you probably get quite a lot of your protein from other foods as well, simply because you eat them in large quantities. Bread and flour contain about 10 per cent protein, while fried fish contains 20 per cent, but because we eat far more of the former than the latter, we get more than a quarter of our protein intake from bread and flour, and only one twentieth from fish.
Most foods contain some protein (only sugar or pure fats contain none at all). But fruit and vegetables contain only two or three per cent protein and don't make much of a contribution. On average you probably get about one third of your total protein intake from dairy products, and one third from meat of various kinds.
How much protein do we really need?
To keep going, you need a minimum of about 1 ounce (40 grams) of pure protein every day, but since no food is all protein, you need to eat quite a lot of food to get it - the equivalent of 10 oz of meat, or a pint-and-three quarters of milk, or 20oz of cereals. Of course, you don't actually make up your daily quota in one food alone, but from a number of different foods - say, 7 ounces of cereal, half a pint of milk and 3oz of meat - which between them provides you with the necessary l 1/2 ounces of protein.
But there are many different types of proteins and the usefulness of any one of these depends on how closely they resemble human tissues in their make-up; in other words, whether they contain the right amino acids.
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. However, all the other amino acids are present in large amounts.
But this only matters if you eat only the basic minimum of IV^oz 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 well be made up in another. For instance, meat lacks sulfur amino acids, but has a surplus of lysine; bread is short of lysine but has a lot of sulfur amino acids - so a meat sandwich 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 sulfur. 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.
Is 'extra' protein a good idea?
We tend to eat a lot of protein rich foods like eggs, cheese, fish and meat simply because we like the taste, and so many foods contain this nutrient that by the time you've eaten your fill (and taken in about 2,000 to 2,500 Calories a day) you've probably consumed about 3oz - about twice as much as you actually need.
Apart from repairing the body or for other body functions, protein is also used as a source of energy in the diet. Any excess will be used like the energy derived from fats or carbohydrates - either lost as heat or laid down as fat.
Do some people need more than others?
Protein is used for building new tissue so children obviously need protein to grow. But they actually need far less for growth than they do for general maintenance and repair. Very small babies who are growing fast in the first few months need a diet composed of 12 to 13 per cent protein - as opposed to an adult, who is no longer growing, and would need only six to seven per cent protein.
A young child needs only a little more protein (weight for weight) than an adult. Take an eight year old for instance. He is probably putting on about 15lb per year, which works out at only 7 or 8 grams a day. Since most of that weight is really water (muscle is three quarters water), he's only putting on about 2 or 3 grams of protein each day, whereas to maintain existing tissues he probably needs about 30 grams. Since the average child probably eats about 50 grams a day, shortage isn't a problem.
Surprisingly, pregnant women don't need to increase their protein intake at all. During pregnancy, the body is making much more efficient use of both energy and protein. So provided that your ordinary diet is adequate, you don't have to change your eating habits at all - the same diet will supply enough protein for the growing baby. If there is any doubt, an extra pint of milk a day is all you need to make absolutely certain.
The only other time when you might need more protein is during or after an illness when more demands are made on your system. Because protein breakdown is speeded up, your body may call for extra supplies, taking it from parts of the body which need it least - usually the muscles. You may have noticed how your legs look thinner after you have spent a few 'days in bed with 'flu; eating very little.
Afterwards, the lost muscle has to be replaced, but since this building up needs energy as well as protein, your diet should contain plenty of carbohydrates too.
Although the body can't 'store' proteins in the conventional way, the muscles are, in a sense, a protein store. So if you go short of food for a time, muscle protein is again used to restore the routine losses from the vital organs like the heart and liver which need it more.
Do athletes or sportsmen need more protein?
When you go into training or start playing tennis after a winter of idleness, you need extra protein to help your muscles grow. But the extra only amounts to a few grams every day -and that's easily supplied in your existing diet.
If you take extra exercise what you need is extra energy in the form of fats and carbohydrates. It's only if you aren't getting enough of these that you could risk running short of protein, because it is being called upon for fuel. But this is highly unlikely, and your appetite will almost certainly take care of that.