Ben Goldacre, 29 August 2009, The Guardian

Why would you listen to a government health message, or your GP practise nurse, when the Sunday Telegraph has much more exciting news? “Health warning: exercise makes you fat” is the kind of full-width headline you want to see across a broadsheet page: it’s affirmative, it’s reassuring, and it gives you clear permission to sit on your arse all day. “Re-programming body fat is the key to weight loss, not working out.” Praise be. “Is it possible that all that exercise is doing nothing to make us slimmer?” Please let the answer be yes.

The Telegraph produced three lines of research for this claim. Firstly, more people are spending more money on more exercise than before, but there is also more obesity around in the UK than before: explain that with your science. Then there was some speculative laboratory research about interfering with brown fat in animal models using stem cells and things: interesting to read, but distant from the headline claim, and not much use to you on a Sunday.

To properly examine whether exercise really will make you fat, they described two trials.

The first one, I can tell you right now, is cherry picked. The Cochrane Library is a non-profit collaboration of academics who produce unbiased, systematic reviews of the medical literature, and they have a systematic review of all the 43 trials that have been done on exercise for weight loss. This produces clear evidence that exercise is beneficial, albeit more modestly than you’d hope. “Exercise plus diet” was compared with “diet alone” in 14 trials : both groups lost weight, but 1.1 kg more in the exercise group. High intensity exercise was compared with low intensity in 4 trials, high intensity exercise came out better in all of them, with extra weight loss of 1.5 kg. There are also improvements in blood pressure, cholesterol, blood sugars, sense of well-being, and so on.

The Telegraph quoted one trial from Dr Timothy Church of Louisiana University, which compared three different levels of exercise with a personal trainer in overweight people. There were no significant differences between the weight lost in any of the groups, including the “control” group, who were not given a personal trainer at all. So it is true that exercise did indeed have no benefit, in this one single trial the Telegraph quoted, whilst ignoring the vast, overwhelming majority of published literature examing the same question. Dr Church speculates that the explanation for his finding is that people who exercised more also ate more. Fine.

Then there is the Telegraph’s second trial. “Another study due to be published next month in the journal of Public Health Nutrition by researchers at the University of Leeds draws similar conclusions. Professor John Blundell and his colleagues found that people asked to do supervised exercise to lose weight also increased the amount they ate and reduced their intake of fruit and vegetables.”

I have this trial in front of me. It’s simply not true that participants increased their food intake. Only 15% of all participants gained weight during the study, and these were the only people to increase their food intake, but in any case, the weight gained even by these people was lean tissue, and they lost fat tissue. In fact, what the Telegraph don’t tell you, bafflingly, is that overall, participants doing supervised exercise in this trial lost more weight. Much more weight. In fact, people doing exercise lost 3.2kg more weight, on average, over just 12 weeks.

Prof Blundell says: “the Telegraph article was a complete distortion of the facts of our investigation, which showed that exercise is very effective for weight loss. They completely reversed the outcome of our study.”

Misleading journalism like this is becoming a genuine public health problem. We’ve previously seen the evidence that people change their health behaviour in response to what they read in the media. To add to this, the World Cancer Research Fund recently commissioned a survey from YouGov. This was a proper survey, in a representative sample, from a reputable data collector, where anyone is allowed to see the questions and the results, not a secret PR survey to get free advertising in a newspaper.

Half of all respondents said they thought scientists and doctors were constantly changing their minds about healthy living advice, although in reality, healthy living advice hasn’t changed at all for at least a decade (don’t smoke, do some exercise, eat more fruit and veg). And a quarter of all respondents said that because scientists keep changing their minds, you might as well eat whatever you want, because it won’t make any difference anyway. Have another pastry and put the telly on.