PhotoSee all those people sweating in the health club? All that exercise is certainly good for their cardiovascular systems and overall health, but is it helping them lose weight? Not so much.

That's the conclusion of public health scientists Richard S. Cooper, MD and Amy Luke, PhD of Loyola University Chicago Stritch School of Medicine.

“Physical activity is crucially important for improving overall health and fitness levels, but there is limited evidence to suggest that it can blunt the surge in obesity,” Luke and Cooper wrote in the International Journal of Epidemiology.

The two scientists have been studying the link between physical activity and obesity for years. They say that when they started their research, their assumption was physical activity would prove very important to losing weight. But the more they investigated, they say the more their doubts grew. Now, they say the preponderance of evidence has shown their initial assumption was wrong.

Input and output

Losing weight, after all, is all about input and output. Your body burns a certain number of calories each day. If you consume more than that, you tend to gain weight. If you consume fewer, you tend to lose it.

So burning 300 calories at the gym allows you to consume the same number of calories you normally do and have a net 300 fewer on the consumption side. The problem is, we don't usually consume the same number of calories on days when we burn off 300 at the gym.

If you increase your activity, Cooper and Luke say, your appetite increases and you compensate by eating more food. So with or without increasing physical activity, calorie control remains key to losing or maintaining weight.

“This crucial part of the public health message is not appreciated in recommendations to be more active, walk up stairs and eat more fruits and vegetables,” the authors write. “The prescription needs to be precise: There is only one effective way to lose weight – eat fewer calories.”

Cooper is a professor and chair, and Luke is a professor and vice chair, of the Department of Public Health Sciences of Loyola University Chicago Stritch School of Medicine.

Food industry isn't helping

They contend the food and beverage industry has tried to divert attention from calorie consumption by promoting the theory that lack of physical exercise is a major cause of obesity. They point to a recent New York Times report that Coca-Cola, the world’s largest producer of sugary beverages, “is backing a new ‘science-based’ solution to the obesity crisis: To maintain a healthy weight, get more exercise and worry less about cutting calories.”

In their write up of their study, Cooper and Luke lay out evidence that physical activity alone is not key to losing weight.

A number of clinical trials have found that exercise plus cutting calories achieves virtually the same weight loss as calorie restriction alone. Observational studies find no association between energy expenditure and weight change.

Yes, it's true that some Americans exercise enough to influence body weight – a professional tennis player, for example – but the number who actually achieve that level of physical activity is minuscule.

While physical activity has many benefits, so does eating the right kinds of food. When embarking on a diet to reduce calories, just make sure you aren't depriving your body of needed nutrients.

The National Institute on Aging has reported on numerous studies showing a 30% reduction in calories promotes longer life. Eating a balanced diet, just smaller portions, is one way to achieve that.

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