How does the Paleo diet impact gut health?

By Jeffrey Letourneau

Whose diet is healthier: yours, or your hunter-gatherer ancestors’ a million years ago? It’s a tricky and somewhat contentious question. On the one hand, modern humans – particularly those living in developed nations – have continual access to food, and access to a greater diversity of food than ever before experienced in human history. However, with this bounty come things like ice cream, hamburgers, endless permutations of cheese and carbs… foods we know are not good for us, but for which the short-term satisfaction often wins out against any guilt we feel. But even if we had the willpower to make the choices, just how clear is it what those choices are? Would you eat as your ancestors did, or are there aspects of the modern diet (beyond access to food) that you would keep?

Proponents of the Paleo (short for Paleolithic) diet advocate eating as our hunter-gatherer ancestors did by consuming meat, eggs, nuts, fruits, and vegetables while avoiding processed foods. While products like bread and baked goods are obviously excluded, this diet also advocates omitting grains, legumes, and dairy, which were all products of the Agricultural Revolution. The Paleo diet, which was developed in the 1970s, stems from the notion that humans did not evolve to eat much of what comprises the modern diet. One alleged health benefit commonly attributed to the Paleo diet is improved gut health. As a gut microbiome researcher, I was curious to understand just how this diet affects the trillions of bacteria that live in the gut.

This summer, a study on this topic was published, led by Dr. Angela Genoni at Edith Cowan University in Australia. Dr. Genoni and her colleagues examined the gut microbiomes of those following a Paleo diet and compared them to individuals on a non-Paleo healthy diet (control group) in order to test how exactly this diet impacts the gut. Ninety adult participants participated in the study, and participants following the Paleo diet were required to have followed the diet for at least a year. Since there is variation in how people interpret and practice the Paleo diet, the researchers divided Paleo participants into two distinct groups. Participants that considered themselves Paleo but consumed greater than one serving of grains per day were allocated to the “Pseudo-Paleolithic” group, while those who consumed fewer than one serving of grains were deemed “Strict-Paleolithic”. Part of what was unique about this study was its long-term nature, since all participants had made no major changes to their diet in the past year. This allowed the authors to examine how long-term eating habits can change the microbiome.

One of the most striking findings was that blood samples from Strict-Paleolithic participants had levels of trimethylamine-N-oxide (TMAO) more than twice as high as the control group. TMAO is associated with increased risk of cardiovascular disease and the buildup of plaque in the arteries. The Pseudo-Paleolithic group also had higher TMAO, but not as high as the Strict-Paleolithic group, and the difference with the control group was not statistically significant. A study published earlier this year found that eating high amounts of red meat (but not white meat) for four weeks increased the levels of TMAO in urine and plasma. Since Paleo participants in this study tended to eat more red meat than the control group, this finding makes sense. Additionally, the less whole grains one consumed, the higher their TMAO tended to be, so it remains unclear whether TMAO is driven by increased red meat consumption, decreased whole grain consumption, or both.

To examine differences in the types of bacteria present in the guts of participants, the researchers sequenced stool samples and found three groups of bacteria that differed: Hungatella, Bifidobacterium, and Roseburia. Hungatella is known to play a role in the production of TMAO, and was higher in Paleo participants. In general, there was more Hungatella in people who ate more protein. Paleo participants also had less Bifidobacterium and Roseburia than the control group. Both of these groups of bacteria are generally considered to be beneficial, as they are involved in the breakdown of complex carbohydrates and the production of compounds called short-chain fatty acids.

Taken together, these findings fail to provide any evidence that the Paleo diet is beneficial for gut health. In fact, if anything, they suggest just the opposite. That said, one should not draw such sweeping conclusions from this study alone. In particular, it’s worth noting that the Paleo diet has been shown to have some benefits, such as reduced insulin secretion, over a typical American diet. By contrast, this study compared Paleo participants to a control group that had a “healthy” diet. Overall, it seems likely that the Paleo diet may be an improvement over the typical Western diet, but still not optimal.

So where does that leave us? Dietary guidelines such as those published by the World Health Organization (WHO) continue to recommend the inclusion of diverse sources of fiber, including whole grains, as part of a healthy diet. Earlier this year, a report commissioned by the WHO analyzed data from hundreds of previous studies, totaling nearly 5000 adult participants. The authors found that eating more fiber or whole grains specifically were both associated with reduced risk of diseases such as heart disease andcolorectal cancer. This effect was so strong that increased fiber intake was linked to lower risk of death overall. As far as red meat, Americans should probably be eating less of it. Recent reports suggest that people in North America eat, on average, over six times the recommended daily amount.

Ultimately, the Paleo diet gets some things right, such as removing heavily processed foods, while perhaps going too far by excluding whole grains and legumes. So based on current evidence, there’s no need to ditch the beans and quinoa, but maybe go easy on the Count Chocula this year.

 

Peer-reviewed by Dominika Trzilova

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