Junk Food on the Weekend: Is It Really A Problem?

by Kristina C. - March 7, 2016

Photo Credit: by Kristina C
Photo Credit: by Kristina C

Many of us know someone who likes to think of herself as a healthy eater—someone who professes junk food is the exception, not the rule. It's just that she always treats herself to take-out pizza on Saturday night. And, more often than not, she has a few drinks after work on Friday with a big order of french fries. Come to think of it, there's always a bowl of jelly beans while watching movies on Sunday afternoon. Foods that seem like occasional treats have actually become habits, weekend in and weekend out.

Is this really a problem for health, though? Surely someone who eats nutritious foods like quinoa and kale the rest of the week doesn't have to worry about a little junk food now and then. Or does she?

A new scientific study aimed to find the answer to this question in rats. For a period of 16 weeks, Australian researchers gave healthy rats access to either a regular, balanced diet or a high-calorie 'cafeteria' diet with lots of sugar and fat. The rats on the cafeteria diet chowed down on junk food from the local (human) supermarket that included meat pie, cakes, and cookies.

A third group of rats, however, cycled between these two diets. For four days of the week they ate the balanced diet, and for the other three days of the week they ate the cafeteria diet. These were the rat versions of humans who eat junk on the weekends. Researchers measured different health parameters in the rats, as well as the species of bacteria living in their intestines.

Something odd happened with the appetites of the rats that cycled through the two diets. They gorged themselves when given access to the fatty, sugary foods, and consumed less than normal when given the regular diet. Something about the irregular diet led to irregular appetite.

The effects of the diets on the rodents’ weight were not surprising: rats on the normal diet were the leanest. Rats that cycled through both diets were 18% heavier, and the ones that always had the cafeteria diet were a whopping 136% heavier than those on the normal diet. The metabolic measurements of these groups paralleled their weight.

When it came to measurements of intestinal bacteria, however, the results were more interesting. Researchers found almost no differences between rats who had cycled between the diets and those who had been on the cafeteria diet the entire time. All of these rats showed a far different gut microbiome from the rats on a normal diet. A diverse collection of bacterial species is generally considered a marker of good health, but in these two groups the total number of bacterial species plummeted. Species in the Ruminococcus and Blautia groups (genera) were higher in these rats, making researchers wonder if the role of these bacteria in weight gain should be further investigated.

In other studies of both rats and humans, low diversity of gut bacteria has been linked to a lack of fiber in the diet. Fiber serves as food for bacteria in the colon, and microbes can go ‘extinct’, sometimes permanently, when rodents don’t consume enough of it. In this study it wasn’t clear exactly why the diet had made some of the bacterial species become scarce, but it could indeed have been the lack of fiber inherent in the high-fat, high-sugar cafeteria diet.

In one sense this study tells us what we already know: eating more junk food is correlated with weighing more. But in another sense it's surprising and sobering: even apart from weight gain, weekend junk food eating affects the gut microbiota in ways that could interfere with good health. This study hasn’t been done in humans yet, but the research seems to suggest that regularly relaxing your diet on the weekend might not be the best idea after all. If you switch up the jelly beans for carrot sticks, your gut bacteria may thank you.


Kristina Campbell is a freelance science writer specializing in the gut microbiota and digestive health.

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