Bacteria growing naturally in the human gut could be playing a decisive role in determining whether someone becomes overweight or obese, according to a remarkable study involving laboratory mice fed with bacterial gut fauna from fat and thin people.
The results support earlier studies showing that gut bacteria, which are more numerous than the cells of the human body, probably play a significant role in boosting or cutting the risk of someone putting on weight.
In the latest study, scientists isolated gut bacteria from identical and non-identical twins, where one sibling was obese and the other was lean, and found that the bacteria caused mice to become lean or overweight depending on whether they received the bacteria from the lean or obese twin respectively.
The scientists also found which bacterial species are involved. The microbial group known as the Bacteroides, for example, was more prevalent in lean individuals and was also found to play a protective role against fat accumulation in mice fed on certain diets.
The mice in the study were specially bred "germ-free" animals lacking their own microbial gut fauna.
When fed a standard diet with the added microbes from the human gut, those receiving bacteria from obese individuals gained more fat than mice fed on bacteria from lean individuals, said Dr Jeffrey Gordon of Washington University School of Medicine.
"This wasn't attributable to differences in the amount of food they consumed, so there was something in the microbiota that was able to transmit this trait. Our question became: What were the components responsible?" Gordon said.
The transplanted gut microbes from humans led to metabolic changes in the mice that caused them to build up fat tissue in their bodies, which is also the key feature seen in people who are overweight or obese.
When the two sets of mice were put together in the same cage, the obese mice became lean. This indicated that they had shared their microbial fauna - mice eat each other's droppings - and that the "lean" bacteria were winning out in the battle to colonise the guts of the "obese" mice, Gordon said.
This suggests that recolonising the gut of obese people with healthy, lean-triggering bacteria may help them to lose weight, provided they also follow other advice on diet and exercise, he said.
"In the future, the nutritional value and the effects of food will involve significant consideration of our microbiota - and developing healthy, nutritious foods will be done from the inside-out, not just the outside-in."