In nutrition, a plausible idea and a little bit of evidence are all that is needed to create a market. And such is the megaphone of marketing and the influence of countless “healthy eating” articles that these ideas, even the half-baked ones, can lodge very firmly in the national psyche.

The difficulty is that proper experiments are extremely difficult to carry out. The influence of a single nutrient in the diet is hard to measure. As a result, much of the evidence consists of comparisons between populations, one of the weakest forms of evidence.

For vitamin supplements, many studies in the 1980s and 1990s seemed to show that taking the pills reduced the risk of disease. But people who take vitamin pills are likely also to take care of their health in other ways: the pills may simply be a “marker” for health-conscious consumers.

Correcting for this is almost impossible. The only way to be sure is a randomised trial in which people are divided into two groups, one given vitamin pills and the other placebo, and followed up for as long as it takes to show a difference. When this kind of trial was done, the apparent benefit of several vitamin supplements disappeared. Indeed, the latest analysis — which combines the best of the trials to try to reach a definitive verdict — seems to show that they may do more harm than good.

Despite a growing weight of evidence, the supplement manufacturers continue to deny the findings. That is their right.

But what the public needs to know is that the best methods we presently have for establishing dietary truth have failed to show money spent on vitamin supplements is money wisely spent.

A sensible balanced diet is a better investment.

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