Dr. Will Bulsiewicz on the Microbiome, Part I: Top 8 Takeaways

You can’t talk about health without talking about the microbiome. The Rogue Scientist offers the Top 8 takeaways from gut expert Dr. Will Bulsiewicz.
The gut

I’m a scientist with a huge interest in health. I’m also someone who has dealt with food sensitivities. Both of these facts mean I have a strong motivation to learn more about the gut microbiome.

I’ve written some articles on the microbiome already, focusing somewhat on tyramine intolerance, including microbiome basics, probiotics, and neurotransmitters. But after listening to Rich Roll’s fascinating and information-rich interview Dr. Will Bulsiewicz, I decided I would discuss the gut microbiome in broader terms.

What is the microbiome?

Why is it so important?

What can you do to improve yours?

Who is Will Bulsiewicz?

Dr. Will is a board-certified gastroenterologist and the New York Times bestselling author of Fiber Fueled and The Fiber Fueled Cookbook. He’s a gut expert and passionate on the topic, and so I definitely recommend listening to the entire podcast (see links at the bottom).

But until then, here are the Top 8 takeaways from the interview:

#1: Your Microbiome is a World of Its Own

Microorganisms are everywhere. They’re all over us humans, too — this is known as the microbiome — but they’re most concentrated in the gut.

We have many diverse species of microorganisms — also known as microbiota — living within us. According to Dr. Will, like humans they have cliques/friends, different skill sets, and different dietary needs.

Also, everyone’s gut microbiome is unique, including those of identical twins. In fact, we share more microbes with our spouses than with our siblings.

Our microbiome is involved in many important tasks: digestion (giving us access to nutrients), metabolism, immune system (70% of your immune system is in your gut), weight, hormones, mood, brain health, cognition, and genetic expression (i.e. epigenetics, which you can learn more about here).

Finally, the gut and brain have a close relationship. Many neurotransmitters we associate with the brain are actually produced in the gut; in fact, most serotonin gets produced here. Some even call the gut the “second brain.”

#2: Your Microbiome Plays a Crucial Role in Your Health

The microbiome doesn’t explain everything health-wise, but more and more evidence has stacked up showing it plays a strong role in health and disease, including metabolic disorders such as high blood pressure, obesity, and Type 2 diabetes.

Dr. Will provides a few examples from the research literature:

  • In one study, they found they could predict a subject’s blood glucose reaction to a particular food based on their microbiome alone.
  • The microbiomes of obese humans are different than the non-obese. In fact, in at least one study, they found they could predict a person’s fat percentage from their microbiome.
  • Dr. Will asserts that depression is an inflammatory disorder, stemming from the gut. This is a somewhat new (and controversial) idea, but given how much serotonin is made in the gut, it’s worth considering.
  • Lower diversity in the microbiome predicts lower likelihood of survival with certain cancers. For example, in a melanoma treatment study, they found that those who’d just taken antibiotics didn’t take to the immunotherapy as well, whereas those who had diverse microbiomes or got stool transplants got better results. Also, those who ate more fiber fared better.
  • Another study showed increased survival for non-metastatic colon cancer if patients increased fiber intake after diagnosis.

Dr. Will offers a list of these studies and others he mentions throughout this interview, but you have to go to his website and sign up for his email list to get them.

#3: Diversity is KEY

Dr. Will talks a lot about “diversity.” Think about your investment portfolio, or an ecosystem on planet Earth. In both cases, diversity is the way to go. Why is this?

In a diverse ecosystem, each species of animal, plant, fungus, etc. performs a different function. If one struggles in a given year, the others can fill in the gaps and keep the system functioning well. Likewise, if your tech stocks take a dump, the other stocks or investments that perform well can mitigate your losses.

Your gut works the same way. Every type of microorganism in your gut has a different function and offers unique benefits. The more diverse your gut microbiome in terms of microbiota species, the better your body can do its job and keep you healthy.

Many things determine the diversity of our microbiomes. Our diet is a big one, obviously. Our lifestyles matter too. Dr. Will states that antibiotics and stress are two factors that definitely decrease diversity. 

So what increases microbiome diversity?

For starters:

  • Eating more plants and more fiber (I cover these in the sections that follow)
  • Eating a big variety of plants (Dr. Will says diversity on the plate = diversity in the gut)
  • Being around other people
  • Spending time outdoors
  • Exercise, which increases butyrate-producing microbes (fiber does as well)

He also recommends cutting ultra-processed foods.


If you want a healthy gut and better health in general, eat more fiber. Why?

  • Fiber feeds your microbiota, which keeps them healthy
  • Fiber produces compounds called short-chain fatty acids, which are beneficial to health
  • Fiber increases gut microbiota diversity

Women need 25 grams of fiber per day, and men need 38 grams. Most Americans get nowhere near this number. I eat a pretty plant-forward diet and often struggle to reach 25 grams in a day.

When it comes to fiber content, on average:

Legumes (beans, lentils) > Whole Grains > Veggies/Fruits

In my experience, if you want to get enough fiber, salads and fruit aren’t enough. You’ll need whole grains and legumes too.

#5: Eat More Plants

Dr. Will clearly states the importance of eating a plant-heavy diet. Keep in mind that “plants” includes veggies, fruits, nuts, seeds, whole grains, and legumes (beans and lentils).

Plants contain fiber, crucial to good health, as mentioned in #4. They’re also loaded with gut-beneficial polyphenols: e.g. flavonoids, capsaicin, lignans, quercetin, and resveratrol, to name a few.

Dr. Will doesn’t push veganism, but provides strong evidence that a diet with lots of plants, and many varieties of plants, is key to health. I’m finding more and more experts agree with this.

Bonus: above and beyond the immense health benefits of a plant-forward diet, such diets are FAR better for our stressed planet and often cheaper too.

#6: Eat More Fermented Foods

Research has begun to show that fermented foods such as sauerkraut, kimchi, yogurt, miso, tempeh, kombucha, and kefir have a beneficial effect on the gut.

Why? For one thing, during the fermentation process these bacteria produce a variety of compounds that can be good for health.

According to a landmark study from Stanford University, regular consumption of fermented foods increases microbiota diversity and reduces signs of inflammation. If you want to learn more about this, check out Andrew Huberman’s fantastic interview with Dr. Justin Sonnenberg.

NOTE: If you have tyramine or histamine intolerance, fermented foods are off the table. I address this in Part II.

#7: Consistency Over Perfection

Dr. Will talks about the importance of consistency when it comes to diet and gut health. There are no tricks, hacks, or special cleanses to fix your gut.

Nor do you need to go overboard when starting out.

Instead, gut health (and gut repair for those who have gut issues) is a slow process. You make a small change, then another, then another.

Maybe you add more veggies to your plate at dinner. Or you try a small amount of fermented food per day. Whatever you do, keep at it.

You don’t have to eat perfectly. But consistent consumption of plants and fiber will yield long-term benefits.

#8: Poop Matters

Dr. Will talks a lot about poop, a topic most experts fail to address, despite its importance. He goes into detail about poop consistency and frequency, constipation, gas, and gut problems such as Inflammatory Bowel Disease.

Worth a listen if you have poop concerns.

One thing he mentions to help with constipation: magnesium. Magnesium citrate, oxide, or sulfate work best, but you want to start small and then increase if you don’t get results.

That’s Not All…

Dr. Will discusses something almost no gut or health expert ever talks about: food intolerance. He even talks about histamine intolerance, something most docs know nothing about! I will cover this in Part II.

Dr. Will Bulsiewicz Resources

If you want to listen to the podcast or learn more about Dr. Will’s books, here is that information:



Apple Podcasts

Rich Roll’s site

Dr. Will’s Books

According to the podcast, when it comes to gut health, Fiber Fueled is the “why,” and the newer The Fiber Fueled Cookbook is the “how.”

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The Rogue Scientist

Christie Hartman is a writer and scientist specializing in science-based health. A biology major as an undergrad, she completed her PhD in behavioral genetics at the University of Colorado Boulder. Before starting her writing career, she worked as a scientist and professor at CU’s School of Medicine, where she studied the genetic contributions to substance abuse and antisocial behavior.

2 thoughts on “Dr. Will Bulsiewicz on the Microbiome, Part I: Top 8 Takeaways”

  1. I’m 77 this year and I have had migraines for years, a lot of giddiness, tiredness, visual disturbance but not always with headaches. They often knock me off my perch for some days, when I’m only fit to blob on sofa and doze. I take Zolmitriptan and anti-nausea tablets with paracetamol for them. I do try to eat healthily, though generally find cooking and food the most boring subjects on earth (I’d be happy if I didn’t have to eat at all) and there’s a lot I either can’t eat because it upsets my stomach ( I have no gall bladder) or don’t like. It’s got to be quick and easy! In an effort to eat more that’s ‘good for you’ last year I got some frozen fruit and veg (my husband eats virtually no fruit and only peas in the way of veg so fresh stuff gets wasted). Beans are supposed to be good so, though I never really liked broad beans, I decided to get some baby ones and try again. Every time I had portion a migraine followed. I thought this very odd, considering these are supposed to be ‘healthy’, so put the question on Google – and up came tyramine and your very informative book. Well, that explained lot! Out went the raspberries, red cherries and a whole raft of ‘healthy’ food items. Now I see that the microbiome is all the rage and have just enrolled with Zoë to see what it can do for me as I have recently been told I’m pre-diabetic. No good relying on the doctor, they don’t have time or expertise for such personal attention. Anyway, I’d like to thank you for the book and your Rogue Scientist ongoing information. I still would never have heard of tyramine without it – or the broad beans (still don’t like ‘em anyway!)

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