The widely popular Paleo diet, based on the diets of our earliest ancestors (think Paleolithic era), is fiber-rich and consists mainly of foods that resemble those that can be hunted and gathered — fish, grass-fed pasture-raised meats, eggs, vegetables, fruit, fungi, roots, and nuts. The diet excludes processed foods and cultivated agricultural products. Celebrities like Uma Thurman and Jessica Biel are reportedly fans.

Western diets over the last 100 years have become high in fat and low in fiber — foods that move swiftly through our digestive tracts, leaving us hungry shortly after we've eaten.

Protein may be more important for appetite suppression than the breakdown of starch or fiber.

The idea behind the Paleo diet is that eating like a caveman will make you feel full more quickly. Our digestive systems, including the bacteria that colonize our gut, are designed for a diet that is low in fat and high in fiber — foods that linger in the gut, and, the theory goes, leave us feeling full, longer.

But the theory does not appear to hold up in reality, according to the findings of a new British study.

“Getting to the bottom of how our gut bacteria and diets interact to control appetites is vitally important for tackling the problem of obesity,” Glenn Gibson, a coauthor on the study, said in a statement.

The idea behind paleo-style diets is that the fermentation of plant fiber by bacteria in the colon produces short-chain fatty acids (SCFAs), which trigger production of appetite-suppressing hormones.

But when researchers from Imperial College London took fecal samples from three human vegetarian volunteers and three gelada baboons, the only modern primate to eat primarily grasses, and used them to establish gut bacteria cultures, they found that that the human and baboon cultures fed predigested starch produced more SCFAs that the human and baboon cultures fed predigested grass.

Based on these findings, it appears that protein may be more important for appetite suppression than the breakdown of starch or fiber, according to Timothy Barraclough, another coauthor of the study. A high fiber diet likely does not lead to increased SCFA production and increased appetite suppression.

You Still Need Fiber
The findings do not argue against the virtues of fiber; they just qualify them. The benefits of fiber cannot be denied, said Gibson, a professor of food and nutritional sciences. Fiber has positive influences on helping food move through the intestine, physiology, and bacterial composition.

It's just that the Paleo diet's insoluble fiber — in leafy greens, some fruit, and lots of vegetables — doesn’t send your brain the signal you're full the way soluble fiber does. Soluble fiber produces the SCFA compounds that tell the brain that the body has had enough.

The study was published online recently in mBio.