Every newborn baby begins to search for the mother’s breast of its own accord and most need no encouragement to feed. Although this reflex disappears later, we never cease to eat of our own accord. Less self-evident is how to put together a balanced diet; this we have to learn. What is healthy food? Everyone agrees that we must consume proteins and vitamins, but opinions vary from time to time as to the type of food from which we should derive these nutrients. For example, meat was long considered to be an indispensable part of our diet. Recently, however, more and more vegetarians have been demonstrating that it is possible to have a healthy diet without meat. A great deal of uncertainty still exists about the exact relationship between nutrition and health. Scientists have been engaged for many years, for example, in investigating the role of fats in the occurrence of cardiovascular disorders. Whereas the consumption of cholesterol (derived from eggs and butter) was taboo at first, it is now thought that the main harmful factor is the quantity of saturated fatty acids contained in foods in the diet. The days when we ate solely to stay alive are long since past. We often eat for the pleasure of the company and because we find something appetizing; eating has become a social activity. For this reason, we tend to ignore our body’s signals that it has had enough.
Why we must eat
It goes without saying that a good diet is an essential prerequisite to health. Food is necessary for two reasons: it provides energy to power the body’s chemical reactions, and it furnishes the basic raw materials from which the body is made. The old adage that ‘you are what you eat’ has more truth than one might first imagine.
Building and maintenance
Above all else, it is the protein in the diet that is essential for growth, maintenance and repair of the body. Every living cell has protein in its structure. All of the enzymes, the biological catalysts that initiate and control body chemistry, and all hormones are proteins. Hair and nails are formed from the tough protein keratin. A newborn baby needs about twice as much daily protein per kilogram of body weight as an adult does. This reflects the rapid growth during infancy. Growth and repair, however, are taking place continuously even in the adult body. For example, the mucous membrane lining the small intestine is completely renewed every one to two days.
Source of energy
Generally, fats and carbohydrates in the diet supply most of our energy needs. Most people in the developed world eat more protein than they need for growth, however, and in these the excess protein is either converted to fat and stored or broken down and used to provide energy. Carbohydrates, taken into the body in sweet or starchy foods, are converted by the digestive process into the simple sugar glucose. Excess glucose is converted into glycogen, which is stored in the liver and muscles. Glucose is carried in the bloodstream as a source of energy, its concentration being controlled by the hormone insulin produced by special cells in the pancreas.
Only about 25 per cent of the energy in food can be converted into mechanical work, in the form of muscle-powered movement. Most of the rest is con-verted into heat. Even when a person is not doing physical work and is just sitting or lying down, he or she needs energy to maintain the internal body temperature and the activity of the internal organs. The minimal turnover of energy at complete rest is called the basal metabolic rate and is fairly standard for people of the same age and sex. The energy contained in food can be measured in several different units. The one familiar to most of us is the Calorie (Cal). A more modern unit of energy is the joule, and when such units were standardized internationally the Calorie was defined as 4.1868 kilo joules (kJ). Scientists can therefore determine the calorific value of foods by measuring the heat (energy) produced when they are totally burnt in a laboratory experiment, which gives a direct indication of how much energy they would supply when eaten and digested.
Of course, experiments do not always reflect the situation inside the body. Although burning may be complete in an experiment, in practice not all the food we eat is completely digested and absorbed. The American scientist W.O. Atwater (1844-1907) established that, of what we eat, only 92 per cent of proteins, 95 per cent of fats and 99 per cent of carbohydrates are normally absorbed. Atwater also calculated how much energy is released by ‘burning’ these components in the body: 1 gram of protein and 1 gram of carbohydrates each provide 4 Cal (17 kJ), 1 gram of fat 9 Cal (38 kJ).
Some components of the diet provide no energy, but they are nevertheless essential for health. If someone’s diet contained no minerals or vitamins, then he or she would not survive very long, because these substances are vital for the smooth running of the body’s chemistry. Most minerals, such as sodium (in common salt) and potassium, are present in the diet in amounts well in excess of daily needs; others, such as iron, calcium and iodine, may be in relatively short supply. Sodium and potassium play key roles in the action of nerves, iron is a component of the red blood pigment haemoglobin, calcium is necessary for healthy bones and teeth, and traces of iodine are essential for the correct function of the thyroid gland. Similarly, vitamins are a heterogenous group of sub-stances vital to life, even though most are needed only in minute quantities. For example, only 80 grams of vitamin A, needed for good vision and healthy skin, is a lifetime’s supply for one person. Several vitamins are harmful if taken in amounts surplus to needs.
A healthy diet must be a balanced diet. There are unlimited ways of achieving a well-balanced diet, and such a diet does not have to be tremendously varied (consider Eskimos, who traditionally eat almost exclusively meat and fish). In practice, however, variation is the best way of ensuring a supply of all the basic nutrients.
A good dietary balance includes guarding against surplus as well as deficiency. Almost anything eaten to excess can have harmful effects: one man died from overindulgence in carrot juice. Conversely, many diets are lacking in dietary fibre. Fibre, apart from stimulating the intestinal muscles, may protect against bowel disorders such as diverticular disease and cancer of the colon and rectum. The circumstances under which you eat your meals may be almost as important as what you eat. The digestive system will benefit from a relaxed, unhurried meal. For many households, there are family and social benefits in taking a leisurely meal together. Conversely, many elderly people are malnourished not because they cannot afford to eat healthily but because ‘it’s not worth bothering just for me’. The aged are one of the groups at risk of actual mal-nourishment; another are the anorectics, in whom excessive ‘dieting’ sometimes amounts to self-starvation. But for most people in developed countries the risks are not from undernourishment. While many in the Third World struggle to survive on inadequate diets, the problem in the West is one of overnourishment. Obesity, with its galaxy of attendant risks and disorders such as cardiovascular disease and hypertension, is rife in our society – mostly, the evidence suggests, because of unhealthy eating habits and overindulgence.