They measured and found higher amounts of acetate in animals that had eaten a diet high in fats. Then they observed that when the animals were given acetate, whether by infusion or by direct injection of acetate into the brain, insulin was then released by the beta cells of the pancreas. "Acetate stimulates beta cells to secrete more insulin in response to glucose through a centrally mediated mechanism," explains Shulman. "It also stimulates secretion of the hormones gastrin and ghrelin, which lead to increased food intake."
"Taken together these experiments demonstrate a causal link between alterations in the gut microbiota in response to changes in the diet and increased acetate production," says Shulman. The increased acetate in turn leads to increased food intake, setting off a positive feedback loop that drives obesity and insulin resistance, he explained.
The study goes on to suggest that the feedback loop could be important during evolution. It would have prompted animals to gorge and build up body mass when they were fortuitous enough to encounter calorically dense food during times of food scarcity.
"Alterations in the gut microbiota are associated with obesity and the metabolic syndrome in both humans and rodents," Shulman continues. "In this study we provide a novel mechanism to explain this biological phenomenon in rodents, and we are now examining whether this mechanism translates to humans."
Sources: Nature: Parekh et al, Nature: Perry et al, Yale University