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The Surprising Reason Fast Food Makes You Fat

You already know that the super-sized portions and addictive ingredients don't do your body any favors, but there's another, more shocking reason that eating the stuff can derail your weight loss efforts.

As it turns out, a diet rich in fast food kills off beneficial, calorie-burning gut bacteria, according to genetic epidemiology professor Tim Spector. While a diverse diet helps foster healthy microbes in the gut, eating a relatively small number of highly processed ingredients—like what you'd find in a McDonald's meal—is toxic to waist-whittling bacteria. In fact, many of the "good" microbes die off just days after starting a fast-food heavy diet.

To come to this finding, Spector had his 23-year-old son Tom adhere to a 10-day McDonald's diet comprised of McNuggets, fries, burgers and soda. "Before I started my father's fast food diet there were about 3,500 bacterial species in my gut, dominated by a type called firmicutes," the younger Spector, a genetics student, told The Australian. "Once on the diet I rapidly lost 1,300 species of bacteria and my gut was dominated by a different group called bacteroidetes. The implication is that the McDonald's diet killed 1,300 of my gut species," he explained.

The study findings suggest that many cases of obesity (and the related health implications) cannot be blamed on excess calories alone. The changes in gut bacteria also play a role. "It is clear that the more diverse your diet, the more diverse your microbes and the better your health at any age," Professor Spector explained. But it's not all bad news. Enjoying fast food once and while likely won't do much harm. (Just make sure you're not eating one of The 5 Worst "Healthy" Fast-Food Menu Items.) But to be extra cautious, consider counteracting the negative effects of the grub by including foods like dark chocolate, garlic and coffee in your diet the same day. All of these foods have been shown to increase the number of health-promoting, slimming microbes in the gut.

Dana Leigh Smith
Dana has written for Women's Health, Prevention, Reader's Digest, and countless other publications. Read more about Dana Leigh