Restricting our food intake can result in a range of health benefits, including reducing the risk of obesity. But when the dieting stops, the weight often piles back on, and a new study in mice may have identified why.

Scientists from the Shanghai Institute of Nutrition and the Chinese Academy of Sciences think they may have identified the bacteria responsible for the change in metabolism.

In trials involving mice being put through 10 different dieting protocols, species of Lactobacillus and their metabolites were shown to increase in the guts of the animals once their fasting ended and they were reintroduced to a less restricted diet. That microbiome change, the researchers discovered, assisted the intestinal tissues in absorbing more fat.

It's likely that the same process happens in the guts of humans, and periods of intermittent fasting or controlling calorie intake encourages the gut to increase its ability to extract fat from our diet, making it more likely for weight to be regained.

"Weight regain after dieting is still a big challenge, and the underlying mechanisms remain largely elusive," write the researchers in their published paper.

"Here we show that refeeding after various types of dieting induces quick fat accumulation in mice and enhanced intestinal lipid absorption contributes to post-dieting fat mass increase."

The precise chemical changes that coincided with increased levels of gut Lactobacillus included enhanced intestinal lipid (fat) absorption, increased lipid absorption in white adipose tissue, and decreased total lipid oxidation which has been linked to obesity in the past.

In the same study, the researchers identified a potential way of stopping the weight from returning after dieting: they fed the mice a variety of diets with differing levels of protein, finding that a high-protein diet restricted the growth of Lactobacillus, thus limiting the amount of fat that was accumulated.

By experimenting with the foods given to the mice post-diet, the team confirmed that the composition of the food they subsequently ate – that is, the level of protein in it – was more important than caloric intake in terms of suppressing fat increases.

"We demonstrate that feeding with a high-protein diet after dieting significantly prevents fat mass accumulation and even partially maintains the fat-loss effect induced by dieting, providing a potential practical way to prevent obesity after dieting," the researchers write.

The conclusion is that high-protein foods could help keep the weight off in humans after dieting as well, although more research is going to be needed to know for sure – studies carried out over a longer time period and involving actual people.

Knowing there's a possible increase in the intestinal tissues ability to absorb fat, apparently driven by the Lactobacillus bacteria, provides a target for researchers. The reason many people may struggle so much at keeping their weight loss may not just be a case of people eating more, or spending less time exercising after dieting, but a fundamental microbiome shift.

Another finding to emerge from this current study is that penicillin treatment was also able to restrict the growth of the Lactobacillus bacteria, potentially offering another antibiotic-based approach to preventing weight gain after dieting.

"Determining whether a high-protein diet has similar positive outcomes in individuals who seek to keep weight off will be the most clinically impactful next experiment," gastroenterologist Amir Zarrinpar from the University of California, San Diego, who wasn't involved in the research, writes in an accompanying commentary.

The research has been published in Nature Metabolism.