‘Paleo Diet’ May Not Suppress Hunger
The popular “Paleo diet,” oft described as an ancient diet dominated by grasses and modeled after diets from the Paleolithic period, may not quell hunger better than the modern “McDonald’s” diet.
This conclusion, as elucidated in a recent mBio study, was drawn after analysis of microbes in guts of vegetarian humans and baboons who eat grasses. It revealed starch may play a greater role in appetite suppression than fiber— and protein may play a greater role than both.
Microbes do not (always) rule men. And the experiment’s diets may not perfectly reflect real Paleolithic-era diets.
But the point— to show our gut microbes are highly adaptable, to the point where they may be trying to persuade us to not eat the bag of chips after eating just one — is giving nutritionists something to chew on.
“We believe that we have shown clear differences that demonstrate the flexibility of the modern man’s microbiota to adapt to different diets,” says the Imperial College London’s Gary Frost, senior author of the mBio study. “These extremes of diets do point to the generation of interesting molecules by the microbiota that may influence appetite regulation.”
While modern McDonald’s diets are low in fiber, high in energy, and high in fat, our digestive systems, and our gut bacterial colonies, for millennia were accustomed to low-energy, nutrient-poor, high-fiber diets. Many believe the current obesity epidemic exists in part because our appetite suppression systems, once associated with grassy Paleo diets, have been thrown off kilter by the mighty Big Mac.
For their study, Frost’s team examined the appetite-suppressing gut hormones peptide YY (PYY) and glucagon-like-peptide-1 (GLP-1), which can be triggered by short-chain fatty acids (SCFAs) in the colon. Fermentation of plant fiber in the colon by bacteria can produce these SCFAs. Frost’s team reasoned that digestion of a diet high in plant fiber may better quell appetite than our modern diets.
The team used fecal bacterial samples from three human vegetarian volunteers and three gelada baboons, which eat mainly grasses. The team fed bacteria two diets. One was a pre-digested, high-starch potato diet. The other was a pre-digested, high-fiber grass diet. The team counted alterations in the numbers and types of bacteria and metabolites arising from digestion.
The microbes found in human samples fed a potato diet generated the most SCFAs. Microbes from baboon samples fed potato diets generated fewer SCFAs than microbes from humans, but they generated more SCFAs than microbes found in grass-fed baboons. When the microbes were placed in a dish with mouse colon cells, the colon cells produced the PYY hormone. Those exposed to human microbes fed a potato diet produced the most PYY, followed by those exposed to baboon microbes fed potato diets.
The group’s conclusion: high-fiber, plant-based Paleo diets may not induce appetite suppression, as they do not result in increased SCFAs.
The possible evolutionary reason: Little to no appetite suppression may have been a survival adaptation, to keep early hominids grazing.
Some disagree. Loren Cordain is professor emeritus of the Department of Health and Exercise Science at Colorado State University, and author of The Paleo Diet, which describes a hunter-gatherer diet he says ranged far from grasses to include meat, seafood, vegetables, fruits and nuts. The diet— conceived of decades ago but popularized recently— is controversial, partly because it claims that our genes have changed little since the Palaeolithic era, so our diets shouldn’t. (We know far too little about both our ancient genes and ancient diets to draw such conclusions, critics say.) Cordain told Bioscience by email he thought Frost’s was “an interesting study.”
But, he said, it “has little or no relevance to ancestral hominin diets,” as nutrient digestion and absorption in vivo dramatically differs from that in vitro. “In humans, in vivo studies show potato starch is poorly absorbed without first cooking it,” Cordain says. And raw potatoes can produce glycoalkaloids (alpha chaconine and alpha solanine) which have occasionally proven lethal.
“The starch (potato) the authors employed simply could not have been consumed or assimilated to any degree by hominins living and evolving in Africa— to say nothing about potatoes, which are indigenous to S. America,” he says.
With the exception of the study’s Gelada baboon, Cordain contended, no other primates consume grass seeds, “because they have insufficient hind gut flora which contain the enzyme cellulase necessary to break down cellulose into short and medium chain triglycerides. If humans could digest grass, maybe we should not throw away our lawn clippings. We responded to [co-author] Matt Sponheimer's group's earlier publication in PNAS suggesting early hominins were grass consumers with a rebuttal letter in PNAS.”
A final point Cordain considers “crucial”: Early hominins increasingly consumed more animal food in their diets as they “concurrently evolved a large metabolically active brain. Animal protein is a rich source of the branch chain amino acids (leucine, isoleucine and valine). All have an appetite suppressive effect, particularly leucine. Multiple in vivo experiments in living humans have conclusively demonstrated this. Hence, randomized controlled trials (RCT) in living humans trump any supposed effects based on in vitro cultures of gut bacteria. Further in vivo RCT demonstrate animal protein to cause a much greater PYY release in the gut compared to either carbohydrate or fat [see the work of the University College London’s Rachel Batterham].”
This is why, Cordain concludes, RCTs of high protein diets “show them to be the most effective strategy to get weight off and keep it off (see the DIOGENES trial.)”
In response, Frost noted Cordain’s reply is “really interesting.” However, he said, “Like any experiments, we accept there are limitations. The methods employed in our paper were in vitro, to try and understand mechanisms involved in the digestion of foods that did make up early hominin diets." He noted again that he believes the study illustrates the ability of modern man’s microbiota to adapt to different diets.
Frost added his crew does not argue that “hominin’s diets became more complex, and that RTC data is more powerful. However, the aim of this study was to investigate impact on the mictobiota, metabolome, and the production of PYY and GLP-1. This is not possible to do in humans at this time.”
He said his study did address increasing complexity of diets when it offered the finding that, in all of the potato- or grass-consuming bacterial cultures, levels of PYY rose with levels of the amino acids isoleucine and valine. This supports the increasingly accepted idea that protein plays a greater role in appetite suppression than either starch or fiber.