The Mediterranean diet is characterised by high intake of fruit, vegetables, legumes, nuts and cereals; moderately high intake of fish; regular but moderate alcohol consumption; and low intake of saturated fat, red meat, and dairy products. Most (88%) of the vegans, almost two thirds of the vegetarians (65%), and around a third (30%) of the omnivores consumed a diet with a high adherence to the traditional Mediterranean diet.
The researchers found that levels of SCFAs were strongly associated with the quantity of fruit, vegetables, legumes, and fibre habitually consumed, irrespective of whether the person was vegan, vegetarian or omnivorous. SCFAs have been linked to health promoting effects, including a reduced risk of inflammatory diseases, diabetes, and cardiovascular disease.
Vegetarian and vegans were found to have gut bacterial compositions associated with long-term fibre intake. Specifically, Prevotella and Lachnospira, known as good fibre-degrading organisms leading to the production of SCFA, were more linked to plant-based foods, which may explain the higher levels of SCFA found in vegans, vegetarians and in individuals with high-level adherence to the Mediterranean diet.
On the other hand, levels of trimethylamine oxide (TMAO)—a compound that has been linked to cardiovascular disease—were significantly lower in the urine samples of vegetarians and vegans than they were in those of the omnivores. However the analysis showed that the more omnivores followed a Mediterranean diet, the lower were their TMAO levels.
TMAO levels were also linked to the prevalence of microbes associated with the intake of animal proteins and fat, including L-Ruminococcus (from the Lachnospiraceae family). Eggs, beef, pork and fish are the primary sources of carnitine and choline, compounds that are converted by gut microbes into trimethylamine, which is then processed by the liver and released into the circulation as TMAO.