You may think you have your food all to yourself, but you're actually sharing it with a vast community of microbes waiting within your digestive tract. A new study from the University of North Carolina School of Medicine reveals some gut microbes increase the absorption of dietary fats, allowing the host organism to extract more calories from the same amount of food.

"This study is the first to demonstrate that microbes can promote the absorption of dietary fats in the intestine and their subsequent metabolism in the body," said senior study author John Rawls, PhD, associate professor in the Department of Cell and Molecular Physiology at UNC. "The results underscore the complex relationship between microbes, diet and host physiology."

Previous studies showed gut microbes aid in the breakdown of complex carbohydrates, but their role in dietary fat metabolism remained a mystery, until now. The research was published in the Sept 13, 2012 issue of the journal Cell Host & Microbe.

The study was carried out in zebrafish, which are optically transparent when young. By feeding the fish fatty acids tagged with fluorescent dye, the researchers were able to directly observe the absorption and transport of fats in the presence or absence of gut microbes.

The researchers pinpointed one group of bacteria -- Firmicutes -- as instrumental in increasing fat absorption. They also found the abundance of Firmicutes in the gut was influenced by diet: fish fed normally had more Firmicutes bacteria compared to fish that were denied food for several days. Other studies have linked a higher relative abundance of Firmicutes in the gut with obesity in humans.

"Our findings indicate that the gut microbiota can increase the host's ability to harvest calories from the diet by stimulating fat absorption," said the study's lead researcher, Ivana Semova, PhD, who was a graduate student at UNC at the time the study was conducted. "Another implication is that diet history could impact fat absorption by changing the abundance of certain microbes, such as Firmicutes, that promote fat absorption."

Although the study involved only fish, not humans, the researchers say it offers insights that could help inform new approaches to treating obesity and other disorders. For example, said Rawls, "If we can understand how specific gut bacteria are able to stimulate absorption of dietary fat, we may be able to use that information to develop new ways to reduce fat absorption in the context of obesity and associated metabolic diseases, and to enhance fat absorption in the context of malnutrition."

Study co-authors include Lantz Mackey of UNC, Juliana Carten and Steven Farber of the Carnegie Institution for Science, and Jesse Stombaugh and Rob Knight of the University of Colorado at Boulder.