The "paleolithic diet" is based on the hypothesis that eating the same food groups as our Stone Age ancestors - such as cultivated plants and unprocessed meats - suppresses appetite. But new research led by Imperial College London in the UK finds this may not be the case.
According to the background of the study, the diets of ancestral human populations incorporated higher levels of indigestible plant material, compared with modern-day diets, which tend to be high in fat and sugar.
Past research has suggested that one of the reasons there has been such an increase in obesity is that the mechanisms behind appetite suppression in humans have evolved to adapt to a higher intake of plant material than what is found in present-day diets.
The researchers explain that when plant fibers are fermented by gut bacteria, this produces short-chain fatty acids (SCFAs). These SCFAs trigger appetite-suppressing hormones, known as peptide YY (PYY) and glucagon-like-peptide-1 (GLP-1).
This process has led to the hypothesis that a diet high in plant fiber, such as the paleo diet, could promote better appetite suppression. But the research team set out to test this theory.
"Understanding how a paleo-like diet impacts the colon's microbiota and the signals those bacteria produce to release hormones that reduce appetite may give us new insight that we can adapt in the modern world," explains study leader Gary Frost.
Idea of a high-fiber, plant-based diet for appetite suppression is 'flawed'
For their study, recently published in the journal mBio, the researchers collected fecal samples of three humans who followed a vegetarian diet and fecal samples from three gelada baboons. The team notes that baboons are the only modern primates that mainly eat grasses.
Researchers say the theory that a paleo diet suppresses appetite is "flawed."
Gut bacteria cultures were created from each fecal sample. In a flask, the cultures were exposed to two different diets - either a high-starch diet from predigested potato or a high-fiber diet from predigested grass.
The team monitored any changes in the types and numbers of bacteria and measured the metabolites produced by digestion.
The researchers were surprised to find that human gut bacteria cultures exposed to the high-starch diet produced the highest SCFA levels. In addition, they found that baboon gut bacteria cultures exposed to the high-starch diet produced more SCFAs than baboon cultures exposed to the high-fiber diet.
The team then tested some of the human and baboon cultures that had already been exposed to a high-fiber or high-starch diet on mouse colon cells. This caused the cells to release PYY.
The researchers found that the colon cells exposed to human cultures on a high-starch diet released the highest levels of PYY, followed by colon cells that were exposed to baboon cultures on a high-starch diet.
According to the investigators, these findings suggest the idea that a paleo diet suppresses appetite is "flawed." In essence, they believe high-fiber, plant-based diets do not increase SCFA production or reduce appetite.
In fact, the researchers believe that a diet high in protein may be an effective appetite suppressor. Further investigation of the gut bacteria cultures revealed that as proteinogenic amino acids - isoleucine and valine - increased during digestion, the levels of PYY increased.
Commenting on the findings, the researchers say:
"We found that diet does play a major role in affecting gut bacteria and the production of a hormone that suppresses appetite but not in the direction predicted by the ancestral diet hypothesis. Also, bacterial products were correlated with hormone release that were different from those normally thought to play this role."
"By comparing microbiota and diets outside the natural range for modern humans," they continue, "we found a relationship between diet and appetite pathways that was more complex than previously hypothesized on the basis of more-controlled studies of the effects of single compounds."
The researchers point out that there was an important limitation to their study. They were not able to account for the role of gut cells, which also absorb and release metabolites.
Earlier this year, Medical News Today reported on a study from Louisiana State University, which suggested gut bacteria play a role in why dark chocolate is good for us.