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At a public debate in May on the relative importance of exercise and diet in battling obesity, Yoni Freedhoff began his opening arguments with some basic physics.

"There's no debate about whether the laws of thermodynamics exist," said Dr. Freedhoff, the medical director of the Bariatric Medical Institute in Ottawa. Energy can't be created or destroyed, so weight loss ultimately depends on burning more calories than you consume. But which side of that equation should you focus on?

Dr. Freedhoff and Robert Ross, the director of the Centre for Obesity Research and Education at Queen's University, argued for diet and exercise, respectively, at the "forks v. feet" debate at the University of Ottawa (which you can watch at But this wasn't a clash between burger-and-beer scarfing exercise junkies and gym-phobic calorie counters. In fact, it's increasingly clear that the two factors can't be separated, as a new long-term study of more than 100,000 runners reveals.

The National Runners' Health Study was launched in 1991 by an epidemiologist named Paul Williams at the Lawrence Berkeley National Laboratory in California. Since then, Dr. Williams has tracked and analyzed the health habits and medical outcomes of his subjects using surveys. The latest results, which will appear in a forthcoming issue of the journal Medicine & Science in Sports & Exercise, examine the link between dietary patterns and weight in a cohort of 106,737 men and women.

It's well established that the more meat you eat, the heavier you tend to be; similarly, the more fruit you eat, the lighter you tend to be. This doesn't necessarily mean that meat is "bad" and fruit is "good," but they're convenient markers of general dietary patterns in Western countries.

As expected, Dr. Williams found a strong link between meat and fruit consumption in his subjects and their body mass index and body circumference (an indicator of dangerous visceral fat). But the study's key finding was that this link became weaker and weaker with each additional kilometre a day that the subjects reported running. The more they exercised, the less it seemed to matter what they ate.

For example, each additional serving of meat a day was associated with a 2.64-centimetre increase in waist circumference for women running less than two kilometres a day. The same serving of meat only added 0.81 cm for those running more than 8 km a day. The situation was reversed for fruit: Those running the least got the greatest benefit from each additional serving of fruit they ate.

The results aren't simply the result of running's calorie burn, Dr. Williams argues: "I believe that this has more to do with the regulation of body weight," he says.

Our bodies - when they're working properly - have several mechanisms that try to keep weight stable. For example, we tend to burn more fat rather than carbohydrate after a high-fat meal. But obese subjects tend to exhibit "metabolic inflexibility" - instead of adjusting to burn more fat, their bodies simply store the extra fat after a high-fat meal. Since aerobic exercise boosts your body's fat-burning abilities, it makes sense that those running more were able to handle an extra serving of meat without gaining as much weight.

Another possible explanation relates to appetite. At high levels of exercise, numerous studies have found that appetite tends to closely match energy requirements. But this relationship breaks down at lower levels of exercise. If you feed someone a 200-calorie snack early in the day, for example, heavy exercisers will unconsciously adjust their appetite to eat 200 fewer calories over the rest of the day. Sedentary subjects, on the other hand, will eat just as much as they normally would have.

This doesn't mean that exercise gives you immunity from your dietary choices. After all, Dr. Williams cautions, "we live in an environment that offers high-calorie foods that are convenient and satisfying," capable of ensnaring even dedicated exercisers.

Instead, the results suggest that, in the great "forks v. feet" debate, trying to fix one without considering the other simply doesn't make sense.

Alex Hutchinson blogs about research on exercise at His new book - Which Comes First, Cardio or Weights? - is now available.