Diet is only one of the factors which influence health. Exercise, drugs like nicotine and alcohol, social and psychological factors all play their part. In practice, many people indulge in a diet which is imbalance  in favour of fat, sugar and alcohol, with too little fibre. Scientific research has implicated all of these in the incidence of diseases such as coronary heart disease, cancer and tooth decay. A high-fat, high-sugar diet combined with lack of exercise is likely to lead to excessive weight gain, then obesity, which with its related problems such as high blood pressure, diabetes and a raised blood cholesterol level increases the risk of heart disease.

Cholesterol in the blood is mainly manufactured by the body itself; only a proportion arrives via the diet. The amount of cholesterol in an average diet would not in itself raise blood cholesterol to dangerous levels. Saturated fats raise LDL (low-density lipoprotein) blood cholesterol levels; high levels cause cholesterol deposits which harden and block arteries. Blood containing high saturated fat levels clots more readily. Polyunsaturated fats, however, have the opposite effect.  There is some evidence that they, and still more so the monounsaturates, can repair damage done by saturated fats. Studies are not conclusive, however, and research continues.

Salt is found in most of the foods in a normal diet, processed and natural, so added salt is unnecessary for health, except when large amounts are lost from the body through sweating. On average we eat 10 times more salt than we need. Many middle-aged people have higher blood pressure than they should , which in turn increases the risk of heart attacks and strokes. A reduction in salt intake may produce a beneficial effect for these people, by reducing their blood pressure.

Sugar has been cited as another factor associated with heart disease, though the evidence is not as strong here as it is for fat. That sugar is harmful to teeth is indisputable. Bacteria feed on the sugar and produce acids which start the decay process. A healthy diet avoids sugar and all foods containing it; culprits are cakes, jam, fizzy drinks, canned soups and some breakfast cereals.

Fibre appears to be an important factor in preventing diseases of the large bowel such as appendicitis, cancer and diverticulitis. These diseases are rare in countries where the diet contains less fat, little sugar and more starch and fibre. Fibre may also slow the absorption of sugar and fat which may be beneficial in the management of some diseases such as diabetes and coronary heart disease.

Vitamin overdosing is a possible danger, particularly with fat-soluble vitamins such as A and D which are stored in the body, if taken in excessive amounts over long periods. Vitamin C and those of the B group are water-soluble and in general any excess is excreted without ill effects, though there may be a greater risk of kidney stones occurring in people who take massive doses of vitamin C tablets.