Yo-yo dieter?

It’s a familiar story to many. We lose a few kilograms but the weight comes back – and then some. Now, a new study published in Nature suggests that bacteria living in your gut contribute to this yo-yo effect. 

Researchers from Israel, led by Eran Elinav from the Weizmann Institute of Science, used mice to study why – when it comes to weight loss – it can be one step forward, two steps back.

They found cycling through high-fat and low-fat diets changes the balance of a mouse’s gut microbiota – their resident microbial populations – to favour weight gain.

Margaret Morris, a pharmacologist from the University of Sydney, Australia, who was not involved with the work, said such studies “show a lot of promise”, noting that “we’re just beginning to understand how important [gut microbes] may be”.

And with an estimated 44% of the global adult population overweight, the study could provide critical insight into how we might maintain a healthy weight after weight loss.

The link between the microbiota and weight has been well-established in mice. High-fat diets tip the gut microbiota towards “bad” bacteria. But few people had examined microbial populations after weight loss. Does the imbalance correct itself if a fat-fed mouse goes back to eating normal food and loses weight?

To find out, Elinav and his colleagues fed mice a high-fat diet. The mice, unsurprisingly, became obese.

Some of these animals were then fed a regular diet, and those mice returned to their original size. This cycle was repeated several times.

The researchers found oxygen consumption, energy expenditure and other telltale signs of obesity all returned to normal levels each time the high-fat/regular diet mice returned to their original weight.

But they put on more weight with each dieting cycle – so much so that the net weight gain of the mice cycling between high-fat and regular diets was higher than that of mice on a consistently high-fat diet.

So what was behind this increased weight gain? The answer may lie in compounds that interact with gut microbes known as flavonoids. 

Flavonoids play an important but complex part in nutrition. They are thought to influence the breakdown and storage of fats and have antioxidant properties.

When the researchers looked into the microbiota of obese mice, they found they processed fewer flavonoids. This was partly because they were absent from the fatty diet, but also because the microbiome of the mice had been shifted to destroy any beneficial flavonoids.

And the relationship between the dietary flavonoids and the microbiota wasn’t restored once the mice lost the weight and started eating normally again.

But, Elinav says, taking flavonoid dietary supplements “completely corrected the defect and abolished the exaggerated weight regain”.

So what does this mean for humans experiencing the dieting yo-yo effect?

The researchers developed an algorithm based only on the microbiota which could accurately predict how much weight mice regained.

“If this were applicable in humans, it would mean that – based on the microbes [in our gut] – we could tell a person who had successfully dieted, how [much] he or she would regain if they go back to their obesogenic dietary habits,” Elinav says.

Despite being conducted in mice, Morris notes the value of this study lies in the fact that “animals can’t cheat”, suggesting that the correlation found between the gut microbiota of mice and their increasing weight gain is a good foundation to look for potentially similar relationships in humans.

Next, the researchers hope to explore if that human microbiota shows the same patterns in response to yo-yo dieting.