A few years ago, Liping Zhao, a microbiologist at Shanghai Jiao Tong University in China, put a man with a body mass index of 58.8 — classified as very severely obese — on a strict diet. Over the course of 6 months, the man shed more than 50 kg. In addition, a group of bacteria known asEnterobacter became undetectable in his stool samples, even though they had previously made up 35% of the microbes in his gut1.
Centre for Infections/Public Health England/Science Photo Library
Some of the roughly 1,000 bacterial species in the human gut help make us fat, while others keep us lean.
The decline and fall of a set of bacteria might seem incidental to the man's impressive weight loss, but Zhao and many other researchers say that the human gut microbiota — the assortment of 1,000 or so species of bacteria that inhabit our digestive tract — has an important role in regulating body weight.
“It's not calories alone,” Zhao says, that determine whether a person is obese. To keep the weight down, “You also need to take care of the nutritional needs of beneficial bacteria in the lower gut.” Similarly, some components of a healthy diet may curtail the growth of obesity-promoting bacterial strains.
Researchers are still unravelling the relationship between diet, gut microbes and body weight. “There are a lot of studies in humans, but those are only associations. There are a lot of studies of causation, but those are only in animals,” says Fredrik Bäckhed, a researcher at the University of Gothenburg in Sweden who investigates the gut microbiota using mouse models.
The task now, say Bäckhed and others, is to translate results from studies of lab mice into treatments for humans in the real world.
That is far from straightforward. Last year, Zhao conducted a clinical trial of the dietary regimen that caused the dramatic weight loss in his severely obese subject, including whole grains, traditional Chinese medicinal foods and 'prebiotics' — supplements that promote the growth of beneficial gut microbes. After 9 weeks, the nearly 100 study participants had improved markers of metabolic health and lower levels of potentially harmful bacteria, including Enterobacter, but they only achieved a modest weight loss of about 6 kg on average2.
But clinical trials into microbe-based interventions are just getting started, which is not surprising given the fact that serious research connecting gut microbes to obesity began scarcely a decade ago.
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