Some reasons why exercise may not translate into weight loss

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Earlier this month one of my posts focused on a study which tested the effect of exercise on weight loss in women. This year-long study found that, on average, the women needed to exercise for 77 hours to lose a kg of fat. Now, if I said I had pretty-much foolproof way of getting a couple of pounds of fat off you, but you’d have to do 77 hours of hours to achieve this, what would you say? I sometimes ask a similar question in real life and the answer is always the same.

Again, this is not to bash exercise. I believe exercise is good for all sorts of things, take exercise myself, and have literally just come home after my morning walk with my dog. But the fact remains that activity and aerobic exercises such as walking and running have generally little impact where weight loss is concerned. And as I pointed out in the post I’ve linked to above, I believe knowing this can actually help motivation for exercise. How? Because knowing it means some will be less likely to feel despondent when their new-found exercise habits do not register on the scales.

Here’s another thing – if exercise does not impact much on weight, then not exercising need not necessarily be a barrier to weight loss. Knowing this can help people who are regular exercisers who, for whatever reason, cease exercising for a period of time. You see when someone believes that exercise is a major component of weight control, they tend to believe that not exercising will inevitably cause them to pile on the pounds. This belief can cause many to ‘give up’ on all their healthy habits, including healthy eating.

So, it’s not uncommon to see regular exercisers, say, get injured, and as a result not only become sedentary, but also eat more rubbish. Not surprisingly, as a result, people can find the pounds really pile on when they stop exercising. The point is that this can have more to do with food than exercise.

This week saw the publication of another study that assessed the relationship between exercise and weight. This study, which focused on middle-aged men (average age 54), randomised subject to a 6-month exercise programme (walking, running or cycling) or no exercise [1]. The exercise was graded. It started at 30 minutes, 3 times a week, working up to 60 minutes, four times a week (and at higher intensity to).

At the end of the study, the men taking exercise lost an average of 1.8 kg in weight (4 lbs). The authors of this study then calculated how many additional calories were expended by the exercisers during exercise, and how this compared to the ‘energy’ in the form of lost weight. It turns out that only about 40 per cent of the exercise appeared to translate into the form of lost weight.

What this suggests that compensatory mechanisms are at play. The authors focus entirely on calories here (see below), but there’s more to weight than this. For example, exercise can boost levels of the stress hormone cortisol which can, at high levels, promote fat gain (and muscle loss).

Getting back to calories, what compensatory mechanisms may exist? One potential explanation is that when individuals take more exercise, they are less active in the rest of their lives. Actually, the authors of this study considered this in some depth, and quoted several studies that have found that, when individuals take more exercise, energy expenditure outside exercise drops due to reduced activity including through fidgeting. In their own study, however, there was no evidence of this sort of compensation.

So, the other major potential explanation for the fact that exercise does not translate to the expected weight loss is that individuals end up eating more. Food intake was not monitored in this study. However, they did monitor hormones two hormones that influence appetite: leptin and PYY. Exercise was associated with a lowering of leptin levels (24 per cent reduction). This is important because leptin suppresses appetite. In other words, there is indirect evidence here that exercise made the men more hungry.

Interestingly, the authors also cite evidence that lower levels of leptin are also associated with “an increase in reward-related behaviors”. Couple this with increased hunger and what can we get? Some reward eating or drinking which, for men in particular, can translate into a few beers and a curry.

Overall, this study should remind us of the fact that diet and exercise are not independent of each other, and highlights some of the reasons why adopting a purely calorie-based approach to weight loss rarely works.


1. Turner JE, et al. Nonprescribed physical activity energy expenditure is maintained with structured exercise and implicates a compensatory increase in energy intake. Am J Clin Nutr September 29, 2010 [epub before of print]

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