top of page

The Trillions of Mouths You Feed Each Day Erica Sonnenburg, PhD

Updated: Nov 14, 2019

Are your gut microbes more famished than you realize? Our intestine is home to approximately 100 trillion bacteria. There are more microbes in a single teaspoon of intestinal content than there are stars in our Milky Way galaxy. It’s a humbling experience to realize that humans—with our highly evolved, complex brains that can build towering skyscrapers and compose fine works of art—are, in essence, bacteria-filled tubes. 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. When they are not staving off disease, what are all these microbes doing there? Eating. Your microbiota and you The gut microbiota, also referred to as the microbiome, is the collection of microorganisms that call your intestines home. A major function of this community of bacteria 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 gut microbiota consumes MACs it releases compounds into our gut that help our body regulate its immune system, keep 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 lie in wait, 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 line 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.

Microbes within the colon located in the top left corner separated from colon cells (bottom right corner) by a layer of mucus (diagonal green across the image). Credit: Kristen Earle, Gabriel Billings, KC Huang, Justin Sonnenburg

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. The immune system is put on alert 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, they have a proxy that is labeled: dietary fiber. It’s the closest approximation we have for MACs. Consuming foods that are high in dietary fiber helps ensure the best nutrition for our microbial partners. Unfortunately, the evidence points out 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 30-38 grams recommended by the FDA, and it’s woefully short of the 100-150 grams of fiber consumed by modern-day hunter-gatherers. Much of the current scientific inquiry is looking at 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 Westerner has a microbiota with 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. 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 microbial extinction 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—death by a thousand cuts—more scientists are starting to view a diseased Western microbiota as a major knife-wielder.

The Big MAC diet How can you keep your microbiota healthy? While several factors 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 30-38 grams of dietary fiber per day recommended by the FDA.

For an example of what this would look like, 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.

Five ways to boost gut health

  • Feed your microbes lots of high-fiber foods (for example, beans, artichokes, berries, avocados, and whole grains)

  • Eat bacteria through probiotics or fermented foods

  • Don’t over-sanitize; regular soap and water is plenty

  • Avoid unnecessary antibiotics

  • Spend time outside to expose yourself to nature’s microbes

So, what have you fed your microbiota today?


  • 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.

  • Martinez, I., et al. “The Gut Microbiota of Rural Papua New Guineans: Composition, Diversity Patterns and Ecological Processes.” Cell Reports(2015): 527–38. Print.

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

  • Smits, S.A., et al. “Seasonal Cycling in the Gut Microbiome of the Hadza Hunter-Gatherers of Tanzania.” Science 357.6353 (2017): 802–6. Print.

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

  • This Week in Health: Inside your Microbiome by Harvard Public Health

Erica Sonnenburg, PhD

Erica Sonnenburg, PhD, is a senior research scientist at the Stanford University School of Medicine in the Department of Microbiology and Immunology, where she studies the role of diet on the human intestinal microbiota. She has published her groundbreaking findings related to the microbiota in prestigious journals such as The Proceedings of the National Academy of Sciences, Cell, and Nature.

She is the co-author, along with her husband, Justin Sonnenburg, of the book The Good Gut: Taking Control of Your Weight, Your Mood, and Your Long-Term Health.

To subscribe to the SOULFUL Insights health and wellness newsletter click here.

1,985 views0 comments


bottom of page