The question of what to eat has become one of the most fraught questions of the day. Some people, tired of being bombarded by contradictory advice, may conclude that it is safer to give up eating altogether. That, however, is the only course of action whose results can be predicted with absolute certainty. Other people will, rightly, conclude that the safest thing to do is to maintain a healthy scepticism.

The Times reports today on one scientific study that claims that some vitamin supplements actually increase the risk of mortality, and on another that suggests a link between low-fat dairy products and female infertility. Both of these results may prove deeply irritating to people who have spent a great deal of money on supplements or made strenuous efforts of will to eat a low-fat diet. Quibbles about their accuracy remain, but both sets of findings were conducted by reputable teams using large sample groups. At the very least, it is hard to avoid the conclusion that moderation is a wise course.

It is one thing to be told that vitamin supplements have no proven effect. That leaves the buyer free to believe that a positive link will simply be found at a later date. It is quite another thing to find that the pricey antioxidants that they are taking for health purposes may be positively damaging. The research, published in The Journal of the American Medical Association, is substantial. It consists of a meta-analysis of 68 randomised trials involving more than 200,000 people. It concludes that taking regular doses of beta-carotene, vitamin A and vitamin E can actually increase the risk of mortality by 5 per cent.

No correlation is absolute proof of causation. Nor are meta-analyses entirely compelling, because they aggregate individual trials that have been conducted in different ways. Yet these findings should make consumers stop and think. The craze for vitamins has come at an odd time. Westerners no doubt have greater access to healthy and varied diets than at any other period of history. Many of those who take the most supplements are also enjoying the most nutritious fare. Nevertheless, it has become accepted that if a little of something is vital to the body, a lot of it must be even better. That is simply not proven.

The news that women wishing to get pregnant should eat more ice-cream may strike many readers as an altogether more palatable finding. But if it sounds too good to be true, it may be. The Nurses’ Health Study, which produced the findings, is a long-term and respected study of 116,000 people. Researchers claim to have found a link between a low-fat dairy diet and a failure to ovulate. But while intriguing and novel, this finding is not entirely robust because it depends quite heavily on the accuracy of women’s memories of what they had eaten over the previous year. Similar methodological problems may have bedevilled the finding by the same long-term study, many years ago, that vitamin supplements could reduce rates of cancer and heart disease by 13 per cent.

What, then, should we believe? The only truly reliable scientific trials are double-blind and placebo-controlled. There should be more such detailed research. In the meantime, it is common sense that women should not diet excessively, and that people who eat a balanced diet do not generally need supplements.