CONSUME more water and you will become much healthier, goes an old wives' tale. Drink a glass of water before meals and you will eat less, goes another. Such prescriptions seem sensible, but they have little rigorous science to back them up.

Until now, that is. A team led by Brenda Davy of Virginia Tech has run the first randomised controlled trial studying the link between water consumption and weight loss. A report on the 12-week trial, published earlier this year, suggested that drinking water before meals does lead to weight loss. At a meeting of the American Chemical Society in Boston this week, Dr Davy unveiled the results of a year-long follow-up study that confirms and expands that finding.

The researchers divided 48 inactive Americans, aged 55 to 75, into two groups. Members of one were told to drink half a litre of water (a bit more than an American pint) shortly before each of three daily meals. The others were given no instructions on what to drink. Before the trial, all participants had been consuming between 1,800 and 2,200 calories a day. When it began, the women's daily rations were slashed to 1,200 calories, while the men were allowed 1,500. After three months the group that drank water before meals had lost about 7kg (15½lb) each, while those in the thirsty group lost only 5kg.

Dr Davy confidently bats away some obvious doubts about the results. There is no selection bias, she observes, since this is a randomised trial. It is possible that the water displaced sugary drinks in the hydrated group, but this does not explain the weight loss because the calories associated with any fizzy drinks consumed by the other group had to fall within the daily limits. Moreover, the effect seems to be long-lasting. In the subsequent 12 months the participants have been allowed to eat and drink what they like. Those told to drink water during the trial have, however, stuck with the habit—apparently they like it. Strikingly, they have continued to lose weight (around 700g over the year), whereas the others have put it back on.

Why this works is obscure. But work it does. It's cheap. It's simple. And unlike so much dietary advice, it seems to be enjoyable too.