Study reveals that women need to exercise for 77 hours to lose a kg of fat, and why knowing this can actually help maintain the motivation to exercise

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Although I am an advocate of exercise, I have more than once felt the urge to express the view that it’s not a particularly effective tool for weight loss. The view is partly based on my experience (the experience of a friend of mind that he recounted over dinner and you can read here is a prime example of this). But my view is really mainly founded in the science, which you can read about here.

One of the good things about this area of research is the fact that there’s quite a lot of very relevant research in it. And by ‘very relevant’ I really referring to intervention studies: studies where researchers ask people to take more exercise to see what effect this has. An example of such a study was published last week, and I want to share the results of it with you here.

In this study, 320 post-menopausal women whose weight ranged from normal to obese were randomised to either an additional exercise or no additional exercise group (the control group) [1]. Those in the exercise group were instructed to take 45 minutes worth of moderate-vigorous aerobic exercise, 5 times a week for a year. Both groups (the additional exercise and the control group) were instructed not to change their diets.

At the end of the year, it was found that the exercise group, compared to the control group, lost an average of 2 kg (4.4 lbs) of fat. I’d say that quite a lot of us would be glad to drop a couple of kgs of fat. But now I’d also like to focus on what these women had to do to achieve this loss.

While the exercise group were instructed to exercise 5 times a week for 45 minutes, what they actually did was exercise for an average of 3.6 days each week. Total exercise time averaged 178.5 mins per week. We can multiply this by 52 to get the total number of minutes exercise over the course of the year, and divide this by 60 to convert it into hours. Doing this, we get a total of just under 155 hours. That’s about 77 hours of exercise for each kg of fat lost.

Leaving aside any other health and wellbeing benefits for a moment, if fat loss is someone’s primary goal for taking aerobic exercise, then I believe its not a bad thing for them to be aware of such statistics. Some people hold the view that such messages discourage people from taking exercise. Maybe. But there’s another way of looking at it too…

I’ve known lots of people to start exercise regimes specifically with the goal of losing weight and specifically fat. Yet, so many of them fail to make much progress despite considerable effort. The usual interpretation is that ‘exercise doesn’t work for me’, and this often causes people just to give up.

But how about this: how about we let it be known that aerobic exercise is not generally very effective for weight loss, but perhaps encourage people to take exercise for a myriad of other good reasons. Should these individuals start to take exercise and find that their weight remains quite stable should not be surprised, because that’s what we expect. This can therefore reduce the risk of individuals believing that exercise ‘does not work for them’.

Paradoxically, knowing that aerobic exercise is not very effective for weight loss can, for some people, enhance the chances of them sticking with their exercise regime in in the longer term.


1. Friedenreich CM, et al. Adiposity changes after a 1-year aerobic exercise intervention among postmenopausal women: a randomized controlled trial. Int J Obes (Lond). 2010 Sep 7. [Epub ahead of print]

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