Won't exercise make me eat more and gain weight?

Sunday, July 31, 2011 • Chicago, IL 60657


In 2009, Time magazine ran a cover story called “why Exercise Won’t Make You Thin,” in which journalist John Cloud described his unsuccessful attempts to lose weight through exercise. Cloud’s central theme, supported by his own fondness for post-workout blueberry bars and a rather selective sampling of research, was that exercise actually makes you hungrier. As a result, he warned, “Fiery spurts of vigorous exercise could lead to weight gain.” The twisted logic that led to this conclusion was widely condemned by obesity researchers – but it did raise an important question. After all, it’s undeniably true that many people exercise diligently without losing weight. And exercise does make you hungrier.

            Some simple math illustrates that you’re up against. Let’s say you go out and bike six miles in about half an hour, then chug a typical recovery shake. You’ve burned about 280 calories and immediately downed 270 calories – so you haven’t accomplished much. The number of calories burned through casual exercise almost always corresponds to a surprisingly small chunk of food. That means dropping weight is not an easy process. But there’s no evidence to suggest that exercise actually causes you to gain weight.

            It’s true that increasing your physical activity levels can make you eel hungrier, but the same is true of eating less.  Your body will respond to any change that results in you taking in fewer calories than you burn with a series of physiological and behavioral tactics that conspire to keep you at  your current weight. That’s why almost none of the weight-loss interventions that have been tested in clinical trials achieve losses that the majority of participants sustain beyond a few years. It’s not just exercising to lose weight that’s hard – it’s losing weight by any means.

            Of course, there’s no debate that elite athletes drop pounds and keep them off through exercise. In fact, for long-distance runners, swimmers, and Tour de France cyclists, eating enough to meet their caloric needs is a constant challenge. So it’s clear that exercise really can help you lose weight – the only question is how much. A recent Harvard University study offers some clues. Researchers followed 34,000 middle aged women for 13 years, monitoring their diet, exercise and weight and reporting the results in the Journal of the American Medical Association in 2010. Just 13 percent of these women (who were eating “typical” American diets with no intervention) managed to avoid significant weight gain throughout the study, and these women averaged a full hour of moderate exercise every day. Anything less was unsuccessful. That’s a lot of exercise – unless you compare it to the daily lives of our ancestors who didn’t spend most of the day sitting at desks or in cars.

            In a sense, Cloud’s article was a wake-up call to anyone who thought that heading to the gym for half an hour a few times a week would, on its own, transform their bodies. You also have to pay attention to what you eat, both immediately after your workout and throughout the rest of the day. But the article’s most serious sin was underplaying the other benefits of exercise, from cardiovascular health to stress relief, that accumulate even if your weight isn’t changing. Exercise – and particularly “fiery spurts of vigorous exercise” – is the most powerful force for good health that we know of. And it won’t make you gain weight.