Does diet or exercise make you more hungry?

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Let’s say you spend three days burning 25 per cent more calories than you consume. You can do that by decreasing the amount of food you eat, or by increasing the amount of exercise you do. Is there any difference between the two approaches?

That’s what a new study in the American Journal of Clinical Nutrition, from researchers at the University of Ottawa, investigates. Specifically, it looks at how a diet-only or exercise-only caloric deficit affects markers of appetite. (The effects of exercise and diet differ in many other obvious ways: you can’t diet your way to a fast 10K, for example.)

In brief, the study had 10 volunteers who each (in randomised order) spent three-day periods in a tightly controlled caloric deficit through either eating less or exercising more. The daily exercise was roughly equivalent to walking for an hour at a brisk pace, so nothing crazy.

The overall picture that emerged from a series of appetite-related tests was that eating less affects your appetite more than exercise does. Under the diet condition, the subjects ate more food at a buffet lunch the day after the three-day experimental period, reported greater appetite and perception of food palatability, and had greater increase in the ability to detect a faint odor.

(That part was cool: the odor, n-butanol, isn’t even a food smell. Your nose simply works better when you’ve been in a sustained caloric deficit, and the effect is stronger when you’ve been dieting rather than exercising.)

The reason this result is surprising isn’t just that diet and exercise turn out to be different; it’s that the results are precisely the opposite of what most longer-term studies see. Exercise alone, at least in the modest amounts prescribed by this study, doesn’t tend to result in significant weight loss over periods of 6 to 12 months, whereas diet alone does (though the results don’t tend to last).

What’s the explanation for this seeming contradiction? I’m not sure, and neither are the authors, as far as I can tell. They suggest that the results argue against prescribing diet alone for weight loss, because people will initially feel greater hunger than if they’re also doing some exercise to achieve a similar caloric deficit. That fits with the fairly widespread advice that combining diet and exercise is the best way to go for weight loss (and for healthy living in general).

For me, the study was another reminder that “calories in, calories out” is a very incomplete picture of how the body works. That’s somehow easy to forget, but it shouldn’t be a surprise: running for an hour and forsaking dessert are clearly two very different challenges for body and mind. And of course, I know which one I prefer.

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