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Exercise has lots of benefits, but it's not the key to weight loss, experts say.

Thirty minutes of moderate exercise, five times a week, is as effective as many prescription drugs for preventing and managing chronic disease, but “physical activity does not promote weight loss,” according to the authors of a recent editorial in the British Journal of Sports Medicine.

The article’s main author, London cardiologist Aseem Malhotra, is an outspoken critic of processed foods, especially those high in sugar. Malhotra argues that a diet high in carbohydrates and sugar, rather than a lack of exercise, is the main cause of excess weight.

“Let us bust the myth of physical inactivity and obesity. You cannot outrun a bad diet,” Malhotra and his co-authors wrote.

The editorial cites a Lancet report concluding that “poor diet now generates more disease than physical inactivity, alcohol and smoking combined.” Diseases associated with excess sugar intake include “obesity … hypertension, dyslipidaemia (high cholesterol), non-alcoholic fatty liver disease and cardiovascular disease.”

Weight loss studies show that diet is more important than exercise for weight loss. "As a rule of thumb, weight loss is generally 75 percent diet and 25 percent exercise,” Dr. Shawn M. Talbott of the University of Utah told the Huffington Post.

Talbott cited an analysis of 700 weight-loss studies that showed dieting without exercise is about four times as effective as exercise without dieting for weight loss.

Other medical experts warn against downplaying the importance of exercise. “It would be idiotic to rule out the importance of physical activity,” Dr. Mark Barker of the U.K.’s National Health Service told the BBC.

Exercise has health benefits apart from weight loss, including preventing chronic disease and improving mood, according to Catherine Collins of the British Dietetic Association.

Another benefit of exercise is that it helps those who have lost weight maintain their loss. The National Weight Control Registry, which keeps track of people in the U.S. who have lost at least 30 pounds and kept it off for a year or more, says that 90 percent of successful losers exercise.

“Diet is the main factor that can lower weight,” Dr. Mark Kelly of the American Council on Exercise told Fox News. “But it's exercise … that allows that lower weight to stick.”

mmaxwell@deseretnews.com.