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Athletic trainers used to advise that you could eat anything you want if you worked out five hours a week. Science has finally laid this misconception to rest.

Enthusiastic exercise won't cancel out the ill effects of bad diet, authors of a study in the British Journal of Sports Medicine say. "Let us bust the myth of physical inactivity and obesity," the authors write. "You cannot outrun a bad diet."

All over the Western world people are eating more calories without burning them off by doing more exercise. Calorie intake, particularly of refined sugar, has soared, while physical activity levels have remained flat. Not surprisingly, obesity is now a worldwide epidemic. While most people clearly need more exercise, the answer to the obesity epidemic is not necessarily eating more.

Exercise Is Not Without Its Benefits

It's clear that exercise has major health benefits. A report from the United Kingdom's UK's Academy of Medical Royal Colleges described "‘the miracle cure’ of performing 30 minjutes of moderate exercise, five times a week, as more powerful than many drugs administered for chronic disease prevention and management." 

There is no doubt that regular exercise reduces the risk of cardiovascular disease, Alzheimer's disease, certain kinds of cancer, and type 2 diabetes each by at least 30 percent. Regular exercise can sometimes (in conjunction with other treatments) even reverse cardiomyopathy, a potentially fatal condition. However, exercise is not a cure for obesity. 

The Problem Is Sugar

The reason working out is not enough to drop extra weight without proper diet is the peculiar role of sugar in human physiology. The human body does just fine (in most cases) with as much as 25 grams, or about 100 calories, worth of sugar a day in the diet. This can be either cane sugar or the fructose in fruit, berries, and high-fructose corn syrup, equivalent to two small pieces of fruit a day or about one-third of a common packaged snack. A small amount of fructose even helps the liver deal with glucose released from carbohydrate foods.

More than about 100 calories from sugar, however, or about 600 calories in total from all high-carbohydrate foods, overwhelms the body's ability to deal with sugar. Burning sugar releases free radicals. Free radicals are toxic. When the body has to release extra insulin to take sugar out of the bloodstream, it deactivates an enzyme called hormone-sensitive lipase. This keeps fat locked inside fat cells, even if you exercise. When this happens, your body can burn sugar (and that's not an altogether bad thing), but it cannot burn fat. Deactivating hormone-sensitive lipase also increases your appetite for sugar when you are under stress, since your body can't fuel itself to deal with the stress with fatty acids. You have to get your quick energy burst from sugar.

Exercise Plus Bad Diet Increases Weight Rather Than Decreasing It

Unfortunately, that's not all. At the same time the body is deactivating hormone-sensitive lipase, it is reactivating another hormone called lipoprotein lipase. This hormone powers fat cells to take fatty acids out of the bloodstream and store them as fat. Not only can fat not get out to be burned during exercise, any excess fatty acids in your bloodstream as a result of overeating will be quickly stored as even more fat. For this reason, very few people lose weight when they exercise. Most people gain.

Continue reading after recommendations

  • A Luke, RS Cooper. Physical activity does not influence obesity risk: time to clarify the public health message. Int J Epidemiol 2013. 42:1831–6. doi:10.1093/ije/dyt159.
  • A Malhotra, T Noakes, S Phinney. It is time to bust the myth of physical inactivity and obesity: you cannot outrun a bad diet. Br J Sports Med bjsports-2015-094911. Published Online First: 22 April 2015. doi:10.1136/bjsports-2015-094911.
  • Photo courtesy of skippyjon via Flickr:
  • Photo courtesy of skippyjon via Flickr:
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