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May 15

Why exercise may not lead to weight loss

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Theres no doubt that exercise burns calories. So why has study after study found such modest average weight loss even after subjects follow relatively vigorous, well-designed exercise programs?

The usual answer is that you unwittingly eat more to compensate for your workout. Thats partly true, but it skims over a vital detail: Few of us are average. Break down the study results, and you find that exercise is highly effective at melting off pounds for some people, and ineffective for others. Scientists are now teasing out the factors that explain these different responses and poking holes in weight-loss plans that promise one-size-fits-all success.

Theres currently a strong interest in identifying behavioural phenotypes within the obese population so that treatments can be more specifically targeted, says Graham Finlayson, a biological psychologist at the University of Leeds. This is the case for exercise, food, diet, pharmacologic and surgical approaches.

The wide variability in response to exercise is shown clearly in the results of a 12-week program of supervised exercise, published in a review co-authored by Dr. Finlayson in the British Journal of Sports Medicine last month. Although the intensity and duration of each workout was the same for all 58 subjects, some lost more than 10 kilograms while others actually gained a small amount of weight opposite extremes from the average loss of 3.2 kilograms.

Dr. Finlayson and his colleagues suggest a long list of possible reasons for the variation. There are physiological possibilities, like the rate at which food leaves your gut; the production of appetite hormones like leptin and ghrelin; and the extent to which your body relies on fat versus carbohydrate for energy. All of these are affected by exercise and could influence appetite and food intake, though the evidence remains contradictory.

More recently, researchers have turned to the brain for clues. Another study by Dr. Finlayson and his colleagues, published last year in the Journal of Obesity, tested the food-reward responses of obese and sedentary subjects before and after a 12-week supervised exercise program. The food-reward test involved looking at pictures of different kinds of food before and after an exercise bout, and answering questions about their levels of desire in response to each picture.

After the 12-week exercise program was finished, the researchers divided the data into two groups based on how much weight theyd lost: the responders, whod lost an average of 5.2 kilograms; and the non-responders, with an average of 1.7 kilograms.

Sure enough, the difference between the two groups was clear even in the data collected before the exercise program had started. While the responders food-reward ratings had shown no change before and after a workout, the non-responders had shown a significant jump in desire for food, and particularly for high-fat and sweet food, immediately after working out.

Of the 34 subjects in the study, 14 (or 41 per cent) were classified as non-responders; but its not known how widely this response is found in the general population and nor is it clear whether the differences are innate or learned.

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Why exercise may not lead to weight loss

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