Modern Native Americans may suffer from higher rates of diabetes thanks in part to the kinds of foods their ancestors ate, according to a new study that examined fossilized human feces.

On average, American Indian adults are 2.6 times more likely to have type 2 diabetes than Caucasian adults, according to the U.S. Centers for Disease Control.

In order to explain this trend, previous studies had hypothesized that prehistoric Native Americans' diet, alternating between feast and famine, favored individuals with thrifty genes that helped them to quickly store fat in times of plenty. Such genes may have been the key to success for hunter-gatherers, but in a modern world steeped in fatty foods and sugary snacks, they're a recipe for obesity and diabetes, according to the theory.

But University of Nebraska-Lincoln geneticist Karl Reinhard and his colleagues argue in a new paper published in the journal Current Anthropology that the composition of the ancient Native Americans' diet alone was enough to encourage the prominence of these thrifty genes.

The feast-or-famine scenario long hypothesized to be the pressure for 'thrifty genes' isn't necessary, given the dietary evidence we've found, Reinhard said in a statement Tuesday.

In order to get a better picture of what ancient American Indians were eating, Reinhard and his colleagues looked at coprolites - the scientific term for fossilized feces. Usually, we think of fossils as specimens like dinosaur bones or prehistoric plants where the organic matter is replaced by minerals.

But in this case the fossils are just really dried out bits of poop.  Coprolites usually form because an animal passed fecal matter in a dry environment; feces laid down in a moist area will soon be consumed by bacteria.

In the current study, Reinhard and his colleagues examined 20 human coprolites from a cavern in northern Arizona known to be home to various American Indian cultures. They found that the coprolites were made by people whose diet was very high in fiber and low in fat - mostly corn, sunflower seeds, wild grasses, and prickly pears.

Overall, the foods ancient Native Americans ate tended to be low on the glycemic index, meaning they tend to break down slower and release glucose more gradually than foods that are higher on the index, like white bread.

While lower glycemic index foods are a great diet choice in modern times, Reinhard and his colleagues think that generations of this kind of diet may have favored the development of the thrifty genes that helped ancient Native Americans retain what little fat they could glean from their food, but which make present-day Native Americans more susceptible to the deleterious effects of modern diets.

Since these high-fiber, low-fat foods weren't just eaten in lean times, the authors argue that the rise of the thrifty genes doesn't have to be pegged to a feast-or-famine explanation.

These were the foods eaten on a day-by-day basis during all seasons in both feast and famine, the authors wrote. They continued to be eaten even after agriculture was developed. Antelope Cave coprolites show that this high-fiber diet was eaten during the warmer seasons of food abundance.

SOURCE: Reinhard et al. Understanding the Pathoecological Relationship between Ancient Diet and Modern Diabetes through Coprolite Analysis. Current Anthropology 53: 506-512, August 2012.