From Meat to Microbes to Main street: is it time to trade in your George Foreman Grill? - Human Food Project To explore the possibility that plasma TMAO levels might be associated with specific gut microbial taxa, the researchers collected stool samples from 23 of the 26 vegans and vegetarians and 30 of the 51 omnivores (at baseline). Sequencing the gene encoding for bacterial 16S rRNA of the stool samples, the researchers found some interesting patterns. As diet can significantly shape the composition of your gut microbiota – and differences between vegetarians have been noted in previous studies – the resulting microbial differences were not completely unexpected. However, one particular group of bacteria stood out – Prevotella. When the researchers grouped all 53 people analyzed into so-called enterotypes – regardless of diet – they found that individuals that had gut microbiota enriched with Prevotella, had higher levels of TMAO in their blood. Conversely, those with gut microbiota enriched with Bacteroides showed much lower levels of TMAO. While the concept of enterotypes is a matter of debate, Prevotella-enrichment is still evident for those individuals with higher levels of TMAO. Interestingly, previous studies have suggested that enriched levels of Prevotella are associated with a higher carbohydrate diet – that is, less meat. However, three of the four individuals (see plot) in the Prevotella-enriched group were meat eaters, and the fourth was a vegan/vegetarian. On the flipside, the 49 that clustered together – more Bacteroides, less Prevotella – was a mix of omnivores and vegans and vegetarians. These findings suggest that previous dietary habits can drive microbial composition in the gut that in turn effect the ability of your microbial ability to synthesize TMA/TMAO from dietary carnitine. The potential role of Prevotella as a key player in this saga is made more interesting when you consider the results of feeding d3-carnintine to mice. In a series of experiments, the researchers spiked the chow of a set of mice and compared the gut microbiota to mice receiving the same chow but without the d3-carnitine. You guessed it, the mice receiving the d3-carnitine had significantly higher levels of Prevotella and unclassified Prevotellaceae. Since the chow was the same between the two groups of mice, the carnitine seems to have the unique ability to nudge along the growth of Prevotella as well. Or more specifically, Prevotella spp. appear to harbor the genes necessary to synthesize carnitine. Though the microbiota between mice and humans are not directly comparable, it is interesting. If we assume for the moment that TMAO can accelerate atherosclerosis, then the observation that enriched levels of Prevotella are associated with higher levels in of TMAO become more interesting – especially as enrichment of this group of bacteria seems to occur in vegans and vegetarians as well as omnivores. So what causes enrichment of Prevotella? In a paper published in 2010, Italian researchers found that rural African kids in Burkina Faso had super high levels of Prevotella, while a similar aged group of kids in suburban Italy had none. The striking dietary difference between the kids in Burkina Faso and the Italian kids was whole grain consumption. As with many rural African communities, grains – in this case millet and sorghum – are processed manually by stone grinding. Minus any winnowing afterwards, these whole grains made up >50% of the daily calories in this village. In addition, they did eat some legumes (black-eyed peas), some mango fruit, some butter and very limited amount of vegetables. All in all, this is a very monotonous diet dominated by whole grains and very little diversity of anything else. In contrast, the Italian kids got ~25% of their daily calories from highly processed bread, biscuits, pasta, and rice. The Italian kids also consumed legumes, a range of vegetables, milk, chicken, beef, fish, code, sole, eggs, extra-virgin olive oil, butter, yogurt, cheese, snacks, and a diversity of fruits. All in all, a much more diverse diet, though apparently devoid of whole grains but high in processed foods. As mentioned, the Italian kids had no traceable amounts of Prevotella in their stool samples. Below is a figure showing seven of the kids from Burkina Faso plotted against me and two other individuals (Mr. A & B) recently sequenced as part of the American Gut project. While the sequencing data between American Gut and the African study are not directly comparable (different lab protocols etc), they are informative for this comparison. While all of the kids from Burkina Faso showed very high levels of Prevotella (~>50%), the seven in the graph are the highest. In fact, the Prevotella levels among these Burkina Faso kids are among some of the highest reported for a cohort anywhere in the world. On left you see my Prevotella levels are 1.5%, Mr. A is at 0% and Mr. B at 32%. Interestingly, neither myself or Mr. A eating much bread, pasta or any other grain-based foods. In my house, we get an artisan loaf of bread every few weeks to make sandwiches, occasionally eat bread when dining out, but can’t think of the last time I had pasta at home. Likewise, after chatting with Mr. A about his diet, he avoids all processed foods and eats a lot of fish – occasionally eating beef. In short, he follows a Paleo diet. I, too, follow more or less a Paleo diet but not for ancestral reasons per se – but more due to the fact I have a type 1 diabetic daughter so I am hyper aware to the insulin spiking effects of processed foods. But I do love the taste and smell of freshly baked bread! As for Mr. B, an email exchange reveals a very diverse diet of plants and animals, but a taste for whole wheat and whole grain foods in general – including whole oats for breakfast. Mr. B cooks often with whole wheat and whole grain ingredients. All three of us are average BMI and physically active. In the Burkina Faso paper, the researchers attributed the high levels of Prevotella to grain-based carbohydrates and specifically dietary fiber intake. However, while I personally do not eat grains on a regular basis, I do eat an extraordinary diversity of plants and thus have high fiber intake – yet low to no Prevotella. I would say some weeks I average 40-80g a day of dietary fiber from a range of foods. I go out of my way to try and consume a diversity of dietary fiber sources. Mr. A also eats lots of dietary fiber, but not from grain sources. That said, it doesn’t appear that that dietary fiber in general is driving enrichment of Prevotella. If it were, both myself and Mr. A would have higher levels of Prevotella. So, it appears some how specific to the starch in grains or the dietary fiber in grains. Prevotella, a gram-negative bacteria, are a common member of rumen and also present in high numbers in pigs, poultry and evidently some humans. A recent genome analysis of two common Prevotella (P. ruminicola and P. bryantii) reveals they posses an extensive repertoire of genes targeted towards the degradation of non-cellulosic polysaccharides such as hemicelluose and pectin – which is present in the cell walls of grasses and cereals (a.k.a. grains). While they can also utilize starches and simple sugars, they have carved out a specific ecological niche when it comes to chomping down on the cell wall (think bran) of grains. Interestingly, in a study of 51 obese Finnish adults fed either a fiber-rich rye bread (whole grain) or a highly refined wheat bread for 12 weeks, those consuming the highly processed bread (no whole grain) showed a 37% decrease in the phylum Bacteroidetes, which includes genus Prevotella. Suggesting further that something particular to the whole grain – presumably the bran in the intact grain – is potentially modulating levels of Prevotella. However, another study by the folks at the University of Nebraska didn’t show much movement (up or down) in the abundance of Prevotella in 28 healthy individuals eating either whole-grain barley or brown rice. This same research group also fed resistant starch (RS2 and RS4) in the form of crackers to ten people and saw nothing exciting with the abundance of Prevotella – suggesting at least in this small study, resistant starch (like that found in grains) effects levels of Prevotella. It’s also worth noting that we see higher levels (15-30% of bacteria) of Prevotella among San Bushmen in Namibia who have shifted from hunting and gathering to more westernized foods, such as porridge made from goats milk and whole-grain maize meal (unpublished data from our work in Africa). Colleagues working among Amerindians in South America have noted elevated levels of Prevotella among more traditional groups eating lots of cassava. The contribution of other agricultural products is less clear, but overall, suggests some interesting patterns with this widely used root food. If enriched levels of Prevotella are in fact associated with higher levels of circulating TMAO – and you buy into the current thesis that TMAO accelerates atherosclerosis – then it’s worth paying attention to. And, if elevated levels of Prevotella might be associated with whole-grain based diets (but not highly processed carbs), then the genus Prevotella has come of age since the advent of agriculture (last 0-10,000 years more or less – depending on where you ancestors hail). A potentially interesting question then might be, is there any deleterious effects associated with the blooming of this genus in our recent evolutionary past? If humans (and hominids in general) have lived for millions of years with low to no levels of Prevotella, one might suspect that if all of sudden they constituted 30, 50, or even 90% of the total gut bacteria at any given time in some individuals, then there might be some downside. But nobody knows for sure – as nobody has asked the question. Though some have suggested they are reservoirs for antibiotic resistant genes – but not all antibiotics genes it appears. Humans have been eating meat and thus carnitine for a long time. The question is, are the genes necessary for metabolizing carnitine a recent phenomenon, or have they always been with us? If there is some specific association between Prevotella and the genes capable of metabolizing dietary carnitine, then it appears – possibly – to have been ushered in with the agricultural revolution and has no precedent throughout much of our evolutionary past. How ironic would it be if the microbial ability to metabolize dietary carnitine from red meat is linked to whole-grain consumption! As for my hardcore Paleo eating brothers and sisters, this is the place in the story where you smile. Even if Prevotella is currently serving as a reservoir today, what’s to stop other bacteria from taking up the genetic call of duty to metabolize carnitine if Prevotella were to get depressed at a population level?