While chewing, do you ever stop to think about how the food you are consuming will affect your body? Its probably more likely that you notice its aroma, flavor or texture. Perhaps after the meal you register that you feel satiated, full, or even uncomfortably full. I have always loved to eat -- growing up dinner was a time to connect with my family. As an adult, I enjoy exploring new foods, for the sake of savoring the flavor of the food, and also how it makes me feel both physically and emotionally. After becoming aware of the trillions of bacteria that live in my gut, I added a new dimension to my thoughts on eating -- will my microbes enjoy the food that I give them? This realization led me to question how my food choices affect my microbial populations. Do they change after every meal? Or after a dramatic shift in dietary habits? The answer, it turns out, is a combination of both of these options.
As I discussed in my last post, our guts are dominated by two main groups of bacteria - Bacteriodetes and Firmicutes, with smaller populations of Actinobacteria and Proteobacteria. These larger populations of bacteria tend to vary little in a person over time. For instance, my gut contains 71.5% Bacteriodes (from the phylum Bacteriodetes) and it is unlikely that any other group of bacteria will be able to outcompete these dominant microbes and take over. Therefore researchers tend to refer to these abundant bacteria as our core microbiota, as they are common in many different people.
At roughly the age of 2, our gut microbial population resembles what it will look like for the rest of our lives in that we obtain our core microbes. The makeup of our microbiota is defined by a number of factors - including our genes, environment, and diet. The factor that we possibly have the most control over is the food we put in our mouths. Data is collecting that our microbial counterparts affect our health in both positive and negative manners. Therefore when we think about “healthy” eating we should really expand our view to include how our food will affect our gut microbiota.
A number of studies have shown that our core microbial populations are relatively stable over time despite changes in diet and season (a small sampling: 1-4). For example, Wu et al asked participants about their short term (24 hour) and long term (1 year) diets to assess their average nutrient intake and then correlated this information with their measured populations of gut microbes (1). They found that long term diet was associated with the type of core bacteria present: people who consumed more animal products had higher levels of Bacteriodes (phylum Bacteriodetes), whereas people ate more carbohydrates contained enriched Prevotella (phylum Bacteriodetes). Another study followed the Hutterites, a traditional community, and how their gut microbiome changed with the seasons over the course of one year (2). Davenport et al chose this population to remove the environment as a variable, as the Hutterites live on communal farms and share meals. The authors found that the gut microbiome varied more between individuals than in a specific individual over time, suggesting that a person’s microbial population is relatively stable despite seasonal differences in food consumption. These studies suggest that our long term dietary habits, for example how much fiber and fat we consume, are primarily responsible for shaping our personal microbial ecology.
Despite our long term gut microbiome stability, changes in dietary habits can result in rapid (albeit not drastic) effects on our microbial populations. For instance, the Hutterite population mentioned above had a more diverse microbiota in the winter than in the summer. This result is surprising as the main difference in the diet between these seasons was the consumption of more fresh produce in the summer, and the availability of fresh fruits and vegetables would lead to a more diverse diet and logically a more diverse microbiota. However, these authors observed the opposite effect. They postulated that the carbohydrate fermenting bacteria might dominate the gut ecosystem when the community consumes the summer diet.
Wu et al also assessed the short term effects of diet on the gut microbiota, albeit on a very small sample size. They isolated 10 people in a hospital and gave them either a high fat/low fiber or low fat/high fiber diet for 10 days, monitoring their gut microbes along the way. After 24 hours, all of the subjects’ microbiomes changed regardless of which diet they consumed! However, the changes observed were significant within an individual, but not large enough to say change one person’s microbiota into another’s.
Another study tested how shifting from an omnivore to a strictly plant- or animal-based diet affected an individual’s gut microbial population (5). David et al found that the switch to a wholly animal-based diet more dramatically changed the gut microbes than ingesting only plant-based foods. Interestingly, this change occurred within the first 24 hours of consuming only animal products. This same study also determined whether bacteria associated with plant- and animal-based foods are transferred to the human gut. The researchers found bacteria associated with fermented foods (cheese and salami) or with genes derived from chloroplasts (plant organelles) in the feces of both the animal- and plant-eating subjects, with the type of genes observed corresponding to the respective dietary source. This finding suggests that you can obtain probiotics (or their genes) from fermented foods. However, this study did not address whether these bacteria will permanently colonize the human gut.
Even though the types of core bacteria might not change in your gut much over the long term, what they are doing can be directly modified by the food you eat. Bacteria are highly adaptable and can swap genes with each other. For example, a recent study showed that the gut bacteria of Japanese individuals gained the ability to breakdown seaweed. The genes responsible for this activity were transmitted from marine bacteria that live on seaweed (6) (a benefit of eating raw food!). In other words, the gut microbes of these people adapted to a diet that includes raw seaweed by sharing genes with the organisms that consume seaweed in the ocean. So by consuming more raw and fermented foods, you may be transferring genes to your gut microbes to help you break down the ingredients of the respective fermented food. So the next time you eat sauerkraut, think about how you may be enabling your gut microbes to more efficiently break down cabbage.
So what is the take home message from all of these studies? Your diet affects which microbes colonize your gut and these microbes have the potential to modulate your health. In the short term, the microbial populations are constantly shifting in response to your daily food intake. Changing your microbes in the long term, however, would probably require a lifestyle change - eg becoming a carnivore, vegetarian or vegan. So next time you decide what to eat for dinner, stop and think about how your food might affect your microbes. What do they like to eat, you ask? Stay tuned for the next post….
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