In 2009, Danny Cahill won the eighth season of The Biggest Loser, a reality TV show in which contestants compete to lose the most weight. Over the program’s seven months, Cahill’s weight dropped from 430 pounds to just 191. But since then, he has regained 100. The same is true for most of the show’s contestants, several of whom are now heavier than they were before they took part.
Their story is all too common. Even when people successfully manage to lose weight, in the majority of cases, the vanished pounds return within a year—and often with reinforcements. For many people, weight loss isn’t just hard, it’s Sisyphean.
No one really understands the reasons behind this “weight cycling”, this so-called “yo-yo effect”. It seems to happen no matter your starting weight, or how much exercise you do. As my colleague Julie Beck noted earlier this year, the speed at which people lose weight might be important—but even that’s controversial. “There’s a lot of speculation but very little knowledge,” says Eran Elinav from the Weizmann Institute of Science in Israel.
Now, by studying mice, Elinav and his colleague Eran Segal have shown that the yo-yo effect might be at least partly driven by the microbiome—the huge community of bacteria and other microbes that share our bodies. These microbes help us to digest our food and extract energy from it. And if we change what we eat, our microbiome changes too, in terms of which species are abundant, and how they process our food. Some of these changes happen very quickly but, as Elinav and Segal discovered, others do not.