Proteins

Proteins are good (but expensive) sources of calories, but they have another great nutritional value not shared by carbohydrates or fats. Protein is the real living stuff of the body; an abundance of it is necessary for growth. Even in the adult the protein in the body does not stay still, it is continuously being broken down and built up again, and a little is lost along the way. The minimal protein loss in this way, which must be made up by protein in the diet, is not much more than an ounce a day for the average adult, but to be on the safe side it is advisable to eat twice this amount (as protein, not merely as high protein food).

All proteins are not equivalent for this purpose, and those from animal sources - from meat, eggs, milk, and fish--tend to be most efficient. However, a good mixture of vegetable proteins does very well, and you could certainly get along with no meat or fish at all if necessary. But in that case it is wise to use plenty of skim milk and cottage cheese. Very few adults in this country are protein starved and only rarely is there any need or excuse for buying special protein or amino acid supplements.

Carbohydrates

Traditionally, carbohydrates are the great source of calories and for many of the world's populations 60 to 75 per cent of the food calories are in this form. In the European and American dietary tradition, too, carbohydrates were formerly much more important than they are now. But at present in the United Kingdom less than 50 per cent of the total calories consumed are derived from carbohydrates because of the gradual replacement of carbohydrates by fats in the diet.

Among the carbohydrates in the British diet, the proportion of sugars to starches has increased in modern times; we eat more sugar and much less starch than formerly. We do not object to sugar within reason but this is scarcely a change for the better. Besides the suspicion that sugar promotes tooth decay (dental caries), sugar is a highly refined chemical which carries only calories, none of the protein, vitamins, and minerals that most other foods provide in some measure.

As for different kinds of sugars, there is little nutritional difference. Brown sugar provides a microscopic bit of minerals and tastes different because some caramel from processing persists. Glucose (dextrose) is no more quick to provide energy than ordinary household cane or beet sugar (sucrose), but it is less sweet.

Pure starches (for example, cornflour) are like sugar in providing only "empty" calories, but the "starchy" foods we eat - potatoes, breads, spaghetti - are by no means only calories. Potatoes provide significant amounts of vitamin C and of nicotinic acid, while the whole-grain cereals provide many water-soluble vitamins as well as proteins. The rather low opinion in which starchy foods are often held is not justified, we think, and the idea that starchy foods make people fat is a misconception. Some starchy foods are rather concentrated sources of calories - but far less so than fats - and getting fat is basically simply a matter of calories.

Fats

Fats are by far the most concentrated sources of calories and so are prized by men doing hard physical work in cold weather when there is a problem of sheer bulk of food to swallow in order to supply their high calorie need. Fats tend to be slowly digested so they have good "staying power", which is important if you are an explorer on an arctic trail. They also tend to be associated with fat-soluble vitamins as well as carrying "essential" fatty acids, but otherwise their nutritional virtue is simply as calories.

Many people like the taste of fats and the greasy sensation they make in the mouth. But perhaps the main reason most fats are eaten is that they soften foods and are very useful in cookery. These values can be achieved by intelligent cookery with much less fat than is customarily used, as will be seen in our menus and recipes. Different fats have different cooking properties, but we are now learning that oils can be used instead of solid fats with excellent results in most situations. Extremely low fat diets - less than 15 per cent calories from fats - are disagreeable for most people but can be fully satisfactory nutritionally.

Fortunately it is unnecessary to go to such extremes for blood cholesterol control if the fats are properly chosen. Around 25 per cent fat calories, with 30 as the upper limit, is suitable, provided that not more than about one-fourth of the fatty acids are of the saturated type (see Chapter 3). This means an allowance of 6 to 7 per cent of calories from saturated fatty acids or about 12 to 14 per cent of calories from the sum of meat and dairy fats plus margarine and cooking fat in the diet, since saturated fatty acids make up about 50 per cent of these fats.