More and more perfectly ordinary people, particularly among the young, are giving up meat. The Vegetarian Society claims that 2,000 people every week are converted to the humanitarian, economic, spiritual, and physiological advantages of a meat-free diet.
Despite eating turkey at Christmas and black pudding when pregnant, for the past 20 years I have stuck to my (predominantly) meat-free and high-pulse diet because I believe it is healthier. But am I justified in this belief? Should parents worry or be pleased when their children follow the trend and turn against meat?
The scientific literature on the subject is difficult to interpret. People who do research into the putative benefits of vegetarianism often have firm ideological views before they start. In one study the vegetarian contingent was recruited largely or entirely from Seventh Day Adventists, most of whom hail from affluent parts of America and whose religious faith also forbids alcohol and tobacco.
One of the more credible research studies, conducted by a group of Oxford epidemiologists with no particular religious, moral or cultural axe to grind, recruited 6,000 vegetarians through the British Vegetarian Society and asked each of them to nominate a meat-eating friend or relative to act as a control. Twelve years later, the overall death rate in the vegetarians was 20 per cent lower than in the meat eaters, even after adjusting the figures for differences in gender, social class, smoking status and degree of obesity between the two groups. Deaths from cancer were 39 per cent lower, and those from coronary heart disease 28 per cent lower, in the vegetarians.
The Oxford study did not, however, account for differences in the amount of exercise taken. Vegetarian women in particular may take large amounts of exercise. In contrast, an American study of 5,000 young adults found vegetarians had significantly better cardio-respiratory fitness than meat-eaters. Differences in exercise may partly account for several other benefits seen in vegetarians and generally attributed to their diet: lower body weight, lower blood pressure, lower cholesterol levels, and a substantially reduced risk of diabetes.
However, studies in which habitual meat eaters changed to a vegetarian diet for a month or two have consistently demonstrated improvements in cholesterol levels. The changes reversed when the subjects returned to their usual diet. Unhealthy constituents of meat, particularly saturated fatty acids, and positive constituents of the vegetarian diet, particularly antioxidants (vitamins C and E and betacarotene), were thought to be jointly responsible.
Very few studies have compared a no-meat diet with a low lean-meat diet in terms of heart disease risk. In general, a low lean-meat diet seems to produce LDL cholesterol and blood pressure levels intermediate between those of strict vegetarianism and the standard Western diet.
The link between vegetarianism and reduced risk of cancer is complex. Lung cancer is commoner in meat eaters, partly (but not entirely) because of the higher prevalence of cigarette smoking in this group: ascorbic acid in fruit may protect against the condition. Cancer of the colon and rectum may be related to particular constituents of cooked meat, heterocyclic amines, which are carcinogenic at high dose in animals. But there is also evidence that folic acid, phytosterols and protease inhibitors, present in plant-based diets, protect against the condition.
But the results of two recently published large studies preclude any simplistic conclusion that "meat causes cancer". In one, the risk of colon cancer in American women rose steadily the more pork, beef and lamb they consumed, but eating skinless chicken or fish was associated with reduced risk of this condition.
In the other study, 1,900 strict or semi-vegetarians in Germany were followed up for 11 years. The overall death rate was 25 per cent higher, and the risk of cancer 50 per cent higher, in those who never touched meat or fish than in those who ate it occasionally. The authors concluded that strict vegetarianism may lead to malnutrition, which in turn might predispose to cancer.
Highly restrictive, faddy diets can cause nutritional deficiencies and serious health problems, particularly in vulnerable groups such as growing teenagers, pregnant women and people already diagnosed as having cancer. Deficiency of vitamin B12 follows inevitably from a strict vegan diet (no meat, fish or eggs); sufferers become anaemic and eventually develop nerve damage.
Deficiencies of other B vitamins, notably B6, have been found in some studies of strict vegetarians. Vegetarian children tend to be taller but leaner than their meat-eating contemporaries, but strictly vegan children may not meet their calorie or protein requirements and can become growth-retarded, while the exclusively breast-fed infants of vegan mothers can develop B12-related mental retardation. This is reversible once the deficiency is rectified.
A study just published in the medical journal "Thorax" showed that the incidence of tuberculosis in ethnic Asian people in south London was more than eight times higher in vegetarians than in regular meat eaters. Here the differences were attributed to dietary vitamin D deficiency, which is believed to weaken the immune system.
Iron deficiency is commoner in vegetarians, particularly in those who have only recently changed their diet. One study found that white teenage girls who had gone vegetarian were five times as likely to be iron-deficient as their meat-eating friends, while Asian vegetarian teenagers were 50 per cent less likely than Asian meat eaters to be iron deficient. This is in line with other evidence that nutritional deficiencies occur most frequently in people who describe their vegetarianism as an "exclusion diet" rather than as a traditional part of their culture.
It is well established that a high-carbohydrate, high-fibre, low-fat, low-meat diet containing a variety of fresh fruit and vegetables, and supplemented with oily fish or fish oil, has significant health benefits. But the evidence is more equivocal as to whether the prudent diet can be improved still further by removing all lean meat.
