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The Trillions of Mouths You Feed Each Day

Are your gut microbes more famished than you realize?

Pascal Gagneux, used with permission
A staple food of the Hadza hunter-gatherers from Tanzania are fibrous tubers.
Source: Pascal Gagneux, used with permission

Our intestine is home to approximately 100 trillion bacteria. There are more microbes in a teaspoon of intestinal contents than there are stars in our Milky Way galaxy. It’s a humbling experience to realize that humans, with our highly evolved, complex brain that can build towering skyscrapers and compose fine works of art, are, in essence, a bacteria-filled tube. But the reality is that we are housing galaxies of microbes within our gut, and all of those microbes play a key role in regulating and maintaining our overall health. But when they are not staving off disease, what are all these microbes doing there? Eating.

A major function of the gut microbiota is to consume carbohydrates. But not just any type of carbohydrates, a specific type called microbiota-accessible carbohydrates or MACs. MACs are complex carbohydrates, the types found in fruits, vegetables, whole grains, and legumes. When our microbiota consumes MACs they release compounds into our gut that helps our body regulate its immune system, keeps pathogenic or bad bacteria at bay, and even contribute to whether we are lean or obese. What happens if you haven’t eaten any MACs? Does your microbiota lay in waiting, famished, hopeful that you will feed it again soon? Not exactly. When your diet doesn’t contain enough MACs, your microbes are forced to rely on the only other carbohydrate source it has left, you. Your intestine secretes a slimy coat of carbohydrates that lines your intestinal wall called mucus. This mucus lining is a rich source of carbohydrates that starving microbes can feast on when dietary pickings are slim. In this way many of our gut microbes can have a bit of a Jekyll and Hyde type of personality. Provide gut microbes with plenty of sustenance in the form of MACs and they will happily convert them into molecules our body needs to be healthy. Starve them of dietary MACs and they will munch on your mucus lining inching ever closer to your intestinal wall. This scenario could alert your immune system that a microbe is getting dangerously close to penetrating the protective wall your body has constructed to keep a safe distance between them and us. The long-term ramifications of this situation could be an immune system that’s on a hair trigger, impacting not only the health of your gut but your entire body.

While MACs are not denoted on a food’s nutritional label or ingredient list, there is a proxy that is labeled. Dietary fiber is the closest approximation we have for MACs. Consuming foods that are high in dietary fiber can provide needed nutrition for our microbial partners. But it is clear that Americans are not getting enough dietary fiber. The average American consumes a measly 15 grams of dietary fiber per day. This falls far short of the 29-38 grams recommended by the FDA and is even further still from the 100-150 grams of fiber consumed by modern day hunter-gatherers. Presently, a major source of scientific inquiry is how dietary fiber consumption relates to the health of the microbiota throughout life and over generations.

A growing number of studies have revealed that the average Western microbiota has far fewer microbial species living in their gut relative to people living a lifestyle and eating a diet more similar to our early agrarian or hunter-gatherer ancestors (1-4). It appears that as our consumption of dietary fiber has decreased, so has the number of different types of bacteria living in our gut, stars in our internal galaxies flaming out. Scientists don’t yet know what the long-term ramifications of this gut extinction event might be. But the simultaneous stratospheric rise in diseases such as obesity, diabetes, heart disease, autoimmune diseases, and even depression in our society points to a potential common thread underlying all these conditions. While these highly complex diseases are likely to be the result of several insults, a death by a thousand cuts, more scientists are starting to view a diseased Western microbiota as a major knife wielder.

How can you keep your microbiota healthy? While there are several factors that affect the microbiota, diet appears to be a major lever we can control. Eating a diet filled with dietary fiber, a “Big MAC diet”, can help your microbiota focus on consuming food and not you. In practice this means each meal needs a healthy portion of fruits, vegetables, beans, or whole grains so that you are consuming at least the 29-38 grams of dietary fiber per day recommended by the FDA. For example, the day could start with a bowl of steel cut oatmeal with berries, then a kale salad sprinkled with nuts, seeds, and dried fruit for lunch, and finally a dinner comprised of a veggie filled Mediterranean bean soup. This type of diet ensures that our microbes have plenty to eat so that they can maintain a robust and thriving community within our gut. So the question is, what have you fed your microbiota today?


1. De Filippo, C., et al. “Impact of Diet in 
Shaping Gut Microbiota Revealed by a Comparative Study in Children from Europe and Rural Africa.” Proc Natl Acad Sci U S A 107.33 (2010): 14691–6. Print.

2. Yatsunenko, T., et al. “Human Gut 
Microbiome Viewed across Age and Geography.” Nature 486.7402 (2012): 
222–7. Print.

3. Schnorr, S. L., et al. “Gut Microbiome of 
the Hadza Hunter-Gatherers.” Nat Commun 5 (2014): 3654. Print.

4. Martinez, I., et al. “The Gut Microbiota of Rural Papua New Guineans: Composition, Diversity Patterns, and Ecological Processes.” Cell Reports (2015): In Press.