Health Warning: Exercise Makes You Fat

August 29th, 2009 by Ben Goldacre in bad science, evidence, numerical context, telegraph | 81 Comments »

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.

imageThe 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.

If you like what I do, and you want me to do more, you can: buy my books Bad Science and Bad Pharma, give them to your friends, put them on your reading list, employ me to do a talk, or tweet this article to your friends. Thanks! ++++++++++++++++++++++++++++++++++++++++++

81 Responses

  1. gentleben said,

    September 2, 2009 at 11:54 am

    I had a theory that life expectancy was based around a finite number of heartbeats. The faster your heartbeats the quicker you die (like a mouse or a humming bird). Now i have never been fit enough to get to the stage where any significant bradicardia has occured, therefore by exercising all i am doing is using up my precious beats whilst doing something very boring (gym going, not proper fun sport)I used this to explain the significant number of rock and roll stars from the 70’s (when amphetamines were very popular) popping their clogs at an early age e.g Joe Strummer / all of the Ramones).
    I occasionally use this theroy to talk myself out of exercising especilally if it is going to be boring.
    I know my theory is bollocks but looking at this telegraph article i might get onto there science editor with my new ‘amazing discovery’

  2. beergut said,

    September 2, 2009 at 2:04 pm

    @Bogusman (post 45)

    In post 43 Irishaxeman said, “… xxx will shift fat fast”

    You said, “No it won’t. … Tried it … zero weight loss. ”

    I didn’t see Irishaxeman predict weight loss; I saw him predict a reduction in fat. As others have said earlier it is possible to be losing fat but not losing weight (as you are gaining muscle).

  3. muscleman said,

    September 2, 2009 at 5:49 pm


    The protein vs carbs for breakfast thing is too simplistic, which carbs? Cornflakes for eg are high GI, honey nut versions even more so. If you eat those for breakfast you will feel hungry again very quickly. Some of the mueslis and even some of the quick and convenient porridges are also not low GI. I eat muesli I make myself that is high in dried fruit and has a lot of seeds and nuts. There is no added sugar and I put no sugar on it. It is thus a low GI, moderate protein (from the nuts and the milk) breakfast that keeps me full well into lunchtime to the extent that if I don’t watch the time and rely on my stomach to tell me to eat it can be past 2pm before I feel the need to eat*, like today.

    *this is a problem for me since I run late afternoon/early evening and eating a late lunch can interfere with that in unpleasant ways. I’m just off for a run now in fact.

  4. Wilbert said,

    September 3, 2009 at 2:34 am

    I’m a postdoc working in metabolic research (obesity, diabetes etc; I did my PhD in Steve O’Rahilly’s department, mentioned above). The obesity problem is too complex to really discuss in a post here, but here are some rambling thoughts.

    – You will only gain body fat if your energy intake exceeds energy expenditure. This is obvious and has been stated above, but also involves a complex interplay of social and genetic factors. But even so…

    – Exercise does NOT make you fat (though it might make you gain weight from muscle growth, as mentioned several times above). This is ridiculous, and a dangerous seed to plant in the minds of lay readers.

    – Exercise has myriad benefits independent of weight loss. Exercise should also facilitate maintained weight loss because it will increase muscle mass and decrease fat mass; muscle is far more metabolically active than adipose tissue, therefore increased muscle mass will lead to increased basal metabolic rate.

    – Obese people have high serum levels of leptin (a hormone released by adipose tissue that signals to the brain to suppress appetite and stimulate basal energy expenditure). However obese people are leptin-resistant: they don’t respond to the elevated levels and so continue to eat (leptin didn’t evolve to make fat people feel full). The downside is that the neural circuits that control appetite may become adapted to abnormally high leptin levels, so that if an obese individual begins losing body fat by dieting, corresponding decreases in leptin result in increased hunger and decreased metabolic rate. Basically, the brain may think you are starving and so tells you that you’re hungry, whilst making the body conserve energy. Not great if you’re trying to lose body fat. This is one reason why losing weight by dieting alone is difficult. Far better to couple dieting with exercise (which will counter the reduced basal energy expenditure with increased physical activity).

    – The incidence of obesity within a given population roughly follows a bell curve because of the variability amongst that population of the genes and social factors that influence obesity susceptibility. Social/environmental changes over the last 30 years have seen this bell curve shift to the right (to higher BMI/body fat%/etc). The downside is that even a slight shift to higher average BMI/body fat% will cause a disproportionate increase in the number of people at the obese/morbidly obese end of the spectrum. The upside to this is that, on a population level, even a modest decrease in average body weight (say, 1.5kg) will have a big impact on decreasing the number of morbidly obese individuals. This is why I disagree with brookster’s comment, above, that a decrease of 1-1.5kg is “hardly clinically significant”. On a population level, such a decrease would be hugely significant.

    Although I work in research (where one general goal is to develop a drug to combat metabolic diseases) I’m more sympathetic to the sociological solutions described above by Alexa (#48, third paragraph). Even so, several studies show that lifestyle interventions aren’t very effective, hence the continued research in pharmacological approaches. I find it depressing that society is being crippled by what should be a readily preventable disease, but addressing the social and environmental causes of the problem is difficult to say the least.

    Apologies for the extended rambling…

  5. irishaxeman said,

    September 3, 2009 at 10:52 am

    One of the major obstacles to weight loss by exercise is the idea, implanted by the media, that you have to put in hours every day in a gym, on a bike or treadmill, carrying a designer waterbottle containing an expensive recovery drink. So people either don’t bother or give up, especially when faced by anorexic gym bunnies.

    The reality is that it takes half an hour a day and can cost relatively little IF someone who hasn’t got a vested interest in selling you a gym membership tells you how. A friend of mine who was the chief instructor for a major fitness group tells me she went mad trying to persuade women you could lose weight doing weight-training, and that low intensity aerobic didn’t work.

    So the problem is not cured but exploited by the leisure business and complicated by health and safety regulations. The equipment you really need for weight loss in a gym is no longer in most gyms – Olympic barbells, squat racks, and knowledgeable instructors.

    As the previous poster says, muscle metabolises faster!

  6. brookster said,

    September 3, 2009 at 12:10 pm

    “… muscle is far more metabolically active than adipose tissue, therefore increased muscle mass will lead to increased basal metabolic rate.”

    But the calorific differences per day between adipose and muscle tissue are tiny. I’ve seen figures quoted of 5-6 kcal/lb for muscle and 2 kcal/lb for fat.

    What interests me is the question of why diet-only and diet-plus-exercise cohorts lose similar amounts of weight in clinical trials?

  7. Wilbert said,

    September 3, 2009 at 1:19 pm


    but a very small difference in total energy expenditure, over a long period of time, has a significant impact on energy balance and therefore on your amount of body fat.

    I don’t have detailed knowledge of the clinical trials you mention, but I think others above have pointed out that diet-only may reduce body fat, whereas diet-plus-exercise will likely replace some of that fat with muscle and thereby negate weight loss of decreased body fat. What would be interesting would be an analysis of body fat % in these trials. And even if there were no difference (which I would find surprising), the diet-plus-exercise group would likely see improvements over the diet-only group in other more objective measures of metabolic health (serum triglycerides, fasting glucose levels, insulin sensitivity, etc).

    and I agree with irishaxeman. Many people think ‘exercise’ has to be a tedious chore that you pay for at the gym. Even if you don’t enjoy sport, you can still incorporate small lifestyle changes that will make a big difference over the long term (e.g. always using the stairs). Too many programs about weight loss (I’m thinking Trinny and Susannah-type crap) focus on dieting and treat exercise as some kind of hell that is better avoided. This is appalling. One of the many upsides of a more active lifestyle is that you can (within reason) eat what you like, which is far nicer (and healthier) than trying to stay ‘healthy’ by constantly fretting about your diet.

  8. brookster said,

    September 3, 2009 at 2:24 pm


    No, I don’t buy that explanation.

    Go back to the big meta analysis I referenced. The diet and diet-and-exercise groups both lost about 11kg on average.

    The diet-plus-exercise group were doing moderate exercise — walking, jogging and suchlike. Now, it’s debatable whether moderate exercise increases your muscle mass *at all* (someone find me a reference), but let’s be very generous and assume this group gained a kilo of muscle and lost 12kg of fat.

    This kilo of muscle weighs 22% more than the kilo of fat, so we’re only talking about 200g here. From the calorie figures I posted above, replacing about 1kg fat with 1kg muscle increases your resting energy expenditure by around 10 calories per day. Which is about a small bite of a single slice of white bread.

    Don’t get me wrong; I’m not advocating not doing any exercise. I’m just sceptical about its usefulness as a way of losing weight.

  9. Wilbert said,

    September 3, 2009 at 3:33 pm


    I haven’t actually looked at the meta analysis, so apologies if that undermines any statements I’m making! A quick look at pubmed finds this reference, showing that moderate exercise does increase lean mass:

    admittedly this is in post-menopausal women, and both loss in body fat (-0.33kg) and gain in lean mass (+0.31kg) are small, but still shows that moderate exercise has an effect.

    Also I’d refute your statement that a kilo of muscle weighs 22% more than a kilo of fat! But I’m with you on the effects that swapping a small amount of fat for a small amount of muscle will have on energy expenditure. The key is where that fat is lost from; losing small amounts of fat from the liver, heart or skeletal muscle will greatly benefit metabolic health, even if net weight loss is small or nothing (or even if you gain weight).

    Perhaps the main issue here is that people seek to lose weight (fat) for the sake of vanity rather than health.

  10. brookster said,

    September 3, 2009 at 4:28 pm


    “Also I’d refute your statement that a kilo of muscle weighs 22% more than a kilo of fat!”

    I know I shouldn’t use Google Answers as a source, but the author has referenced his figures — the quoted figure is that muscle tissue is approximately 18% denser than fat tissue.

  11. Alexa said,

    September 4, 2009 at 2:54 am

    @Bogusman: “Can you point me to the evidence base for these things that you are willing to bet about?”

    I base my hypothesis on sources such as the WHO who look at incidence of overweight and obesity worldwide. When countries develop to the point where work becomes more sedentary and convenience foods become more common, their populations become more obese. Details can be found here.

    As long as someone lives in a society that has created situations that make maintaining a healthy weight so difficult, individual diet and exercise interventions are swimming against the tide.

    I applaud the work of Wilbert and others who are seeking medical solutions to lifestyle diseases. But I do hope governments start waking up to the idea of more societal solutions.

  12. Suw said,

    September 4, 2009 at 8:17 am

    Interesting debate. I notice that so far no one has mentioned the effect of stress on appetite and, therefore, weight. When I was young, stress made me lose my appetite and I maintained a fairly skinny frame. Now it makes me hungry and I maintain a fairly… let’s say squishy frame, despite best efforts. And it’s not the kind of hunger you can easily ignore, either.

    So what’s the link, and how important is it? If stress can in some people increase appetite and preference for sweet foods (as some of the abstracts I’ve just looked at conclude), is that enough to counteract the effects of exercise? Does lowering stress improve weightloss?

    Willpower has to come into it too. Is willpower affected by stress (I’d guess it’s decreased)? If stress does decrease willpower and increases appetite, it would be harder for stressed people to stick to a diet and exercise regime. That would indicate that people looking at reducing their weight should address stress problems too.

  13. EleanorC said,

    September 4, 2009 at 10:53 am

    And today we have:

    1. study finds people with skinny thighs have more heart/cardiovasc problems;

    2. media reports this as “Having large thighs is good for you”;

    3. people everywhere go “Woohoo, pass the doughnuts! Aren’t scientists stupid with their mixed messages?”

  14. Bogusman said,

    September 4, 2009 at 11:01 am

    Alexa, the WHO page is heavy on associations and assumptions but a bit light on actual clinical evidence for cause and effect.

    Just a thought on use of BMI as an indicator which features on the page. BMI uses height squared as its denominator. Since we live in a 3-D universe there should really be a third power in there somewhere. The effect of not doing so is that average BMI will tend to increase with increasing average height in the population regardless of actual changes in bodily composition. Has the WHO corrected for this in assessing the extent of the obesity “epidemic”?

    Just for clarity – I do not doubt that obesity has increased in recent years but I do think that reliance on BMI as the main or only indicator has exaggerated the numbers somewhat.

  15. brookster said,

    September 4, 2009 at 1:35 pm

    @ Bogusman

    I think BMI is still useful for most people. It’s not very useful for people with very high muscle mass (such as sportsmen) or people with very low muscle mass (such as very old people). But those two groups are a small proportion of the total population.

    Its weight categories are also not universally accepted, but you’d need to ask an expert about that.

    I believe most health professionals now also use waist size as a criterion for weight-related risk factors.

  16. irishaxeman said,

    September 4, 2009 at 8:14 pm

    I find it fascinating that the accumulated experience of thousands, possibly millions, of sportspeople can be ignored because trials – albeit well-constructed – are done using protocols that are relatively ineffective. It’s not rocket science.

    I’ll quite happily issue a challenge to the ‘exercise doesn’t shift weight’ lobby. You maintain a monitored replacement calorie intake and do the old professional sprint circuit training – starting with 5 reps each of pressup, squat jump, ab curls and alternate leg shifts (leg shoots in a pressup position) – with only changeover rest. On day 2 you do 6 reps of each and so on daily until you get to 20 when you split to 3 sets of 10, then on to 3 sets of 20, 4 sets of 20 and so on building the volume. You could get to the 100 rep per set state like Alan Wells did prior to his Olympic 100m Gold in 1980. If you get to daily volumes of 10 sets of 20 (remember the monitored calorie intake?) and have not lost a kilo, then I’ll accept your contention. But until that kind of very gentle and gradual, but eventually brutal, progression is incorporated into studies, I stick with forty-plus years of doing weight manipulation and seeing the evidence in others.

  17. Bogusman said,

    September 5, 2009 at 1:08 am

    This sounds like a good component for a trial. You would need a decent sample size and also a control group that didn’t follow the exercise program. You would need to randomly allocate your volunteers to one group or the other and then monitor both groups. Obviously it would not be possible for the trial to be blind because subjects would be well aware whether they were following an exercise program or not but possibly the weighers and measurers could be blind.

    Others in these parts will know more about how to make such a trial statistically valid (I’m afraid I slept though most of my stats lectures 35 years ago).

    It would be really great to see it done though…..

  18. brookster said,

    September 5, 2009 at 9:55 am

    “I find it fascinating that the accumulated experience of thousands, possibly millions, of sportspeople can be ignored because trials – albeit well-constructed – are done using protocols that are relatively ineffective.”

    Erm, isn’t that exactly the logic that proponents of alternative medicine use?

  19. Suw said,

    September 5, 2009 at 10:47 am

    Regarding the athletes, are they fit because they are athletes, or are they athletes because they find it easier to be fit?

  20. lomasj said,

    September 6, 2009 at 7:09 am

    Its clearly not the Science Editor as fault – its a simple mistake on behalf of The Editor. He doesn’t know how to spell ‘fit’. The actual headline must surely have been ‘Exercise Makes You Fit’ – but clearly that would not have been a very interesting article!

  21. dabscience said,

    September 7, 2009 at 9:36 am

    Exercise may give you big thighs, but the good news is:
    “Large thighs protect against heart disease and early death”
    This headline apparently from a BMJ press release that quite rightly attracted criticism from Gary Schwitzer of

    The article to which the press release is referring is rather interesting and is accompanied by a well considered editorial that examines among other things the plausibility of such an association:

    Neither article nor editorial stray anywhere near making the sort of claim alluded to in the press release.

    Clearly such catchy headlines attract the attention of editors in the media, but BMJ are shooting themselves in the foot with this sort double standard.

  22. mikewhit said,

    September 7, 2009 at 5:07 pm

    In any case, not losing weight is not the same as getting fat – doesn’t muscle weigh as much (more than?) fat tissue ?

    Hence if you were ideally replacing fat with muscle, you would put on weight – no-one could say you were thereby “getting fat” – ask the Governator !

  23. evilphil said,

    September 8, 2009 at 9:19 am

    @68 – I don’t think that’s valid. Exercise trials are inherently difficult (compliance with regime and diet particularly problematic). Homeopathy and other CAM are actually easy to test in a double-blind randomised trial, so the trials easily trump the anecdotal evidence / wishful thinking. What Bogusman is pointing to is more like a (very) large number of individual case studies.

    Suw’s criticism is more valid though – an examination of athletes is self selecting.

  24. Bogusman said,

    September 8, 2009 at 10:39 am

    Quite right. I do appreciate that it’s hard and that’s probably why there is not much good quality data about.

    A lot of experience (including my own) is very much tied up with circumstances and environment and a myriad of confounding factors. At best it provides some information about what happens to a specific individual in a specific environment who is subject to a specific set of interventions. This is far from useless but equally it is far from telling a coherent and complete story.

    Clearly there is a relationship between energy intake and fat storage and energy expenditure. At the limiting case if your intake is reduced to zero you will eventually die and it is likely that you will become very thin first. However the idea that from this we can imply some sort of linear or even regular relationship between the variables seems to me to be extremely simplistic.

    As this is one of the most significant public health issues in the developed world it seems to me that we, as a community, should be devising ways to research the problem effectively rather than issuing “government guidelines” that are at best based on guesswork.

    I think that this is the most important take away from the Taubes book and I am very much looking froward to seeing David Colquhoun’s review.

  25. mudpaddle said,

    September 10, 2009 at 12:35 pm

    irishaxeman: Prof Blundell certainly has a history of exercise, he’s a runner and has completed several marathons

  26. samhealy said,

    October 25, 2009 at 4:16 pm

    In fact you don’t need the evidence of specific clinical trials to prove that more exercise — food intake and ambient temperature being constant — reduces weight.

    The human body gets its energy from food.

    An idealised perfect combustion system, running at 100% efficiency, will still lose mass as it burns energy according to the mass-energy equivalence relation. Human muscle tissue is fairly efficient (17-26%, Stephen Steiler) but nowhere near perfection. Exercise requires lots of mechanical energy. More fats and sugars will be burned by an exercising body than a sedentary one.

    Unless more food is eaten, environmental temperature changes or some other efficiency-altering parameter is tweaked, exercise will reduce the weight of a human body as certainly as driving a car will reduce the weight of the fuel in its tank.

    This is a simplified and sort of pedantic argument, but it at least shows that it’s a physical impossibility for an exercising body to lose less weight than a lazy one, if all other energy-related parameters are fixed.

  27. Bogusman said,

    November 3, 2009 at 3:16 pm

    No, I think that you do need to do the trials. I am always suspicious of statements of the “obvious” that are not backed by evidence. Personally I think it quite likely that one of the effects of exercise is to “tweak” some efficiency-altering parameter.

    Suppose for a moment that increased expenditure of mechanical energy caused the body to alter its biochemical processes such that the greater the mechanical energy output the less heat was produced. Obviously our subjective experience is that during exercise we get hot, but it’s entirely feasible that afterwards our temperature drops below the long-term average for long enough to offset the energy used during the exercise. In that way it is possible that exercise would not cause a net increase in chemical energy consumed from food.

    I have no clue whether this is a real phenomenon but I would like to know.

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  30. Jon Wade said,

    October 26, 2011 at 5:12 am

    I missed that headline, that was a good one. There was something equally stupid earlier this year about chocolate, something like “Dark chocolate may be as good for health as exercise” or some nonsense. So many people fell for it that the NHS stepped in and made an announcement:

    It is crazy really. I was talking to my mum a while ago, she was trying to remember why she had all these vitamins but could not. “I know I am taking them for something, but I cannot remember what. Maybe I will stop”. She did stop. Still healthy. Probably healthier if those recent reports on vitamins shortening lifespan are at all true.

  31. est said,

    March 14, 2015 at 8:43 am

    I was now listening the audio version of this article (I see this thread was a long time ago, BTW a very good audiobook) and I looked at the Cochrane review.
    Really the result are much more consistent with the notion that exercise has no beneficial effect for weight loss.
    1 kg – it is not clinically significant and might be caused for so many reasons, and might not even be fat weight.
    Heavily promoting exercise and claiming it is a cure for obesity based on so insignificant evidence is also very misleading and might be harming people and diverting resources from treatment and research of a really beneficial interventions.