Our guts are not entirely our own. Inside our intestines, human cells are at war with trillions of bacteria—a war over what happens to food as it moves through the body. Some microbes are beneficial, helping us extract energy from food; others lurk and wait for the chance to overrun our guts at the expense of our health.

A recent study adds nuance to scientists' evolving understanding of gut microbes. Ivana Semova and John Rawls of the University of North Carolina at Chapel Hill, along with their colleagues, studied the absorption of fatty acids in the intestines of tiny translucent zebra fish (Danio rerio). They found that the more the fish ate, the larger the population of a tribe of bacteria known as Firmicutes became in their guts and the more efficiently the fish's intestinal cells absorbed fat. The results mirror findings from studies with people and mice that have shown that high-calorie diets stimulate the growth of Firmicutes and that low-fat diets reduce their numbers. What remains unclear is whether Firmicutes are harmful or beneficial. Are they selfishly increasing their own numbers when the eating is good, forcing our cells to sweat to get the most out of our food? Or are they making digestion too easy, liberating so many calories from our food that we absorb far more than we need? Rawls suggests that the fish may recognize the presence of the bacteria and increase their own fatty acid absorption to compete with them. “It may not always be such a friendly arrangement,” he says.