The Science Behind Your Failed New Year's Diet

This article originally appeared in Tonic. 

It's a classic problem: You can lose weight, but keeping it off is another story. This is particularly difficult for people with obesity. About 80 percent of the time, people who lose weight gain it back. Often, they gain back more than they started with. Next comes a new diet, and so the yo-yo effect—cycles of weight gain and loss—is set in motion. A recent study in mice provides potential new insights about why previously obese people regain weight so easily. According to the study, published in Nature, the microscopic critters living in your digestive tract might contribute to a tendency toward gaining weight back after losing it.

In the past few years, a ton of research has come out about how the microbes in the gut contribute to our health and metabolism. Eran Segal, a computational biologist at the Weizmann Institute of Science in Israel, decided along with his colleagues to examine how gut microbes related to the yo-yo effect in mice. To simulate "weight cycling" in rodents, the researchers fed mice high-fat diets for about a month, followed by a period of "normal chow," followed by another period of the fatty stuff (yep, pretty much the equivalent of your pre- and post-new year's diet). They also looked at mice that ate all normal chow, all high-fat chow, and mice that ate normal chow for two of the three periods, then went on a high-fat diet for the first time when the weight-cycling mice were returning to the high-fat diet. Notably, the weight-cycling mice regained weight faster during that third stretch on the high-fat diet than the mice exposed to fatty foods for the first time: Like humans who lose weight and then seem to gain it back again in a hot minute, the mice that had once been fat gained back the weight more easily.

When the researchers looked at the bacteria present in the guts of mice from the different groups, they found that the weight-cycling mice—even after losing weight—had different gut microbes than mice that had never been fat. To test the hypothesis that the gut microbes in the weight-cycling mice led to their rapid secondary weight gain, the researchers transferred those microbes to mice that had no gut microbes of their own and exposed those mice to a high-fat diet. Lo and behold, the germ-free mice exposed to the weight-cycling microbes gained weight faster than those given microbes from mice on normal chow. That is to say, gut microbes from a previously obese mouse were enough to make a lean mouse get fat. Notably, this difference between the two groups only emerged when they were given a high-fat diet. When fed normal chow, neither group—not even the mice transplanted with obesity-associated microbes—gained weight. So it's not that obesity-associated microbes made mice get fat; they made them susceptible to weight gain on the more indulgent diet.

So what was it about the obesity-associated microbes that was promoting weight gain? The researchers compared compounds in the different groups of mouse stool and found that the previously obese mice had lower levels of flavonoids—plant compounds associated with metabolism and health in humans—in their stool. They also found evidence for more (or more active) flavonoid-degrading bacteria in the weight-cycling mice. The weight-cycling mice, with their lower flavonoid levels, also had lower calorie burn for their body weight than the lean mice. Based on these findings, the researchers think that weight cycling leads the microbiome to shift toward flavonoid-degrading bacteria, which lowers energy output and leads to weight gain. Sheesh.

Could flavonoid supplements reverse that effect? In mice they did, as it turns out. More research is needed in humans to determine whether flavonoid supplements could help us keep off weight, too. The study, plus previous observational data connecting flavonoids with human health benefits, "makes an interesting case for flavonoids—but again, more research is needed in humans," Segal says.

As you come down off your holiday bender and return to your normal chow, consider keeping this study in mind. If you return to junk-food mode come mid-February, you risk having a gut of bacteria that's primed you to gain weight—or so you would if you were a lab mouse.