The gut doctor: why your microbiome matters with Dr Will Bulsiewicz

Have you ever thought about the impact of your gut health on your overall health?

Here’s a summary of the podcast of Steven, from the diary of a CEO, with the well-recognised gut doctor– Dr Will Bulsiewicz.

It’s a crucial discussion that has ways to transform your life. So we’ve decided to make a summary of it.


Right now, we’re in the midst of a gut health crisis—it’s everywhere, and it impacts more than just digestion.

Having a healthy gut microbiome is essential if we aim to be truly healthy, which should be one of our top priorities. 

The benefits of checking out this Avea article? Just a few minutes of your time, and you’ll gain insights that could be pivotal in boosting your longevity.

Who is the gut doctor, Dr. Bulsiewicz?

Dr Builsiewicz graduated with a chemistry degree from Vanderbilt University, then moved on to Georgetown, a leading medical school. 

He completed his residency in Internal Medicine at Northwestern, where he received the highest award available. 

He also served as chief medical resident and spent four years focusing on gastroenterology and clinical research. 

Currently, he’s the US medical director for Zoe, where he’s deeply involved in ongoing clinical research. Gastroenterologist, or as he likes to say, a specialist in “guts and butts,” is what he calls himself. 

The conversation about gut health has really taken off since around 2006. Dr Builsiewicz believes that, despite his extensive background, the prominence of the gut microbiome in discussions is a relatively new phenomenon for him too.

A common misconception people have is that when they look in the mirror and don’t like what they see, or they feel unwell and bloated, they think it’s caused by something superficial or immediate. But, I believe many of these issues trace back to the gut microbiome. 

Dr Will Bulsiewicz

We used to think if you just ate well, you’d be healthy. But we missed a crucial part: these choices significantly impact your gut microbes, which in turn can transform your body’s physiology.

Gut microbes, his nerdy term for the microorganisms you can’t see but are there, cover our entire bodies. If you hold up your thumb, there are as many microbes on it as there are people in the UK—truly fascinating!

What are gut microbes?

Gut microbes are essentially microscopic, living organisms that cover every external part of your body, particularly thriving in the colon—your large intestine. 

In this prime location, we find a staggering 38 trillion microbes, mostly bacteria, along with fungi, parasites, and viruses.

How did they get there? 

It starts in the mother’s womb. Emerging evidence suggests that initial contact with microbes begins before birth, but significant exposure occurs when the water breaks. 

At this point, as a baby passes through the birth canal, he or she is introduced to these microbes in what can be seen as nature’s welcoming committee. 

This exposure marks the beginning of a lifelong partnership with these microbes, vital for health due to co-evolution over billions of years.

Here’s the most intriguing point. 

Humans might have existed for a few million years, but these microbes were the first life forms on Earth. 

All life, including our immune systems, evolved with these microbes. So you better not think of yourself as just one organism, but rather a superorganism, composed of trillions of these tiny life forms.

Dr Builsiewicz stresses that if these microbes were absent, humans would face severe health consequences. These microbes are essential for digesting food and extracting energy, training our immune system, and influencing our metabolism, mood, and even hormonal balance. 

Research, including his work at Zoe, shows that microbes play a critical role in managing our responses to food, affecting everything from blood sugar to fat levels after meals.

In essence, gut microbes are not just passengers; they are integral to our physiology and health, influencing everything from our brain function to how we feel daily.

Everyone’s microbes are different

Each person has a unique microbial landscape. If you compare the genetic codes of two individuals, they are 99.99% identical. Yet, when it comes to the gut microbiome—the 38 trillion microbes residing inside us—the diversity is astonishing.

For instance, imagine you have an identical twin brother. Despite sharing the same genes, upbringing, and often diet, research indicates that only about 25% of your gut microbes would be the same. This means that even in such closely related individuals, the microbiome could be 75% different.

The variation can be even more pronounced between two unrelated individuals. Even if their diets are quite similar, their microbiomes would likely be more different than similar. 

This vast diversity within our microbiomes begs the question: how much does this affect our health?

As a gastroenterologist, Dr Builsiewicz became deeply interested in the gut microbiome because he believed that many, if not all, of his patients’ conditions could be traced back to issues with their gut microbes. 

Conditions like irritable bowel syndrome, Crohn’s disease, ulcerative colitis, and even acid reflux are often linked to microbial imbalances.

But this goes beyond just gastrointestinal disorders. A range of conditions are linked to an unhealthy gut microbiome—hyperlipidemia, high blood pressure, type 2 diabetes, major depression, and autoimmune diseases. 

In fact, more and more findings suggest that our gut microbiome impacts far more than just our digestive health, affecting everything from our metabolism and immune system to our mental health.

There’s often no need for a stool test to confirm gut microbiome issues; the signs are evident in the patient’s broader health profile. This understanding showcases the profound influence our gut microbes have on our overall health and well-being.

70% of the immune system is actually located in the gut, not, as is commonly thought, in the bone marrow. Although immune cells originate in the bone marrow, they migrate to reside within the walls of the intestine. 

This is crucial from an evolutionary perspective, as the gut is our primary point of contact with the external world. Unlike the skin, which acts as a protective barrier, the gut actively decides what to absorb into the body and what to expel.

To help this selective process, there’s a need for a physical barrier. This is where the epithelial layer comes in—a modest, yet critical, single layer of cells held together by ‘tight junctions’. 

On one side of this layer is 70% of your immune system, and on the other side, the 38 trillion microbes.

When this barrier is compromised, substances from the intestines can enter the bloodstream, potentially causing infections and prompting an immune response. 

A common issue in the modern word is chronic inflammation. It is often a result of this barrier breaking down, allowing elements that should be excluded to enter the body. This keeps the immune system in a constant state of alert, trying to fend off these intruders.

How do we repair this barrier? The answer lies with the microbes. 

There are microbes that are essential in maintaining the gut barrier, which is replenished every 3–4 days. This turnover means the barrier you have today wasn’t there a few days ago. So, the integrity of this new barrier relies heavily on these microbes.

Whilst human cells of the gut barrier renew around every 3 days, the microbes undergo much faster turnover, replicating approximately every 20 minutes. 

This rapid cycle shows the dynamic nature of our gut environment and the incredible potential for change based on our dietary choices. Any microbe can exponentially increase its population in just a day, if given its preferred condition

Foods to improve gut health

Here’s an eye-opening fact from the introduction of his book, ‘’Fiber Fueled’’: the average person eats about 1.3 kg of food each day, which adds up to roughly 475 kg annually. 

Over a lifetime, that’s about 36,300 kg of food. Compared to the tiny amounts of medication we might take, it’s clear that food is the real medicine we give our body daily.

Before the transformative 2014 study by Warren David at Duke University, it wasn’t known, or at least emphasised, how human gut microbes were directly influenced by our diet. The findings revealed that switching diets could lead to visible changes in the gut microbiome within just 24 hours, proving the gut’s resilience.

But whilst the gut can recover, it’s important not to overload it with poor food choices. Instead, balancing the diet with predominantly high-quality foods boosts gut microbes.

Think of feeding your gut microbes like watering plants—each type thrives on different nutrients. The choices you make can promote the growth of specific microbial families. 

Dietary diversity, aka a wide range of plants, is beneficial for your gut health. According to the American Gut Project, incorporating about 30 different types of plants each week is ideal. 

But listen. You don’t have to do drastic, unsustainable changes. Slowly increasing the number and celebrating small victories can help ensure lasting changes.

Recent studies, like one from Stanford University, show that incorporating fermented foods can significantly increase gut microbial diversity. 

See every meal as a chance to improve your gut health through smart, diverse food choices, turning even a sparse microbial environment into a thriving ecosystem.

Fermented and prebiotic foods

Every living thing on Earth has a microbiome. 

A single apple could carry about 100 million microbes. These microbes aren’t just passengers; they’re integral to the apple’s lifecycle, helping its growth from flower to fruit and eventually contributing to its decomposition. 

We should all find comfort in the natural decay of food. It’s simply part of Earth reclaiming its resources.

Fermentation is like harnessing this natural process through controlled decay, allowing certain microbes to preserve and transform the food. 

For instance, turning cabbage into sauerkraut involves nothing more than chopping it, packing it into a jar with a saltwater brine, and letting it sit in a cool place. After a week, the cabbage transforms into sauerkraut, which is tangy and acidic, highly different to the bland canned version. This transformation is facilitated by bacteria and yeasts that not only preserve the food but enhance its nutritional profile.

Discover 20 foods to improve your gut health.

During the fermentation process, the microbes consume the food, grow stronger, and multiply, turning simple ingredients into a powerhouse of probiotics. 

Microbes can also help unlock prebiotics, the components of our food that feed the microbes within us. Moreover, during fermentation, microbes release chemicals known as postbiotics, which can have various health benefits.

Prebiotics are essentially the food for microbes, probiotics are the beneficial microbes themselves, and postbiotics are the compounds produced by these microbes that benefit our health. 

Discover the difference between prebiotics and probiotics.

Gut health and metabolism 

Dr Builsiewicz explains that metabolism is not just about how fast or slow food is processed, which often correlates with body size, but fundamentally about how our bodies manage energy. 

A Zoe Predict One study showed how someone’s gut microbiome can predict responses to food, including variations in blood sugar and fat levels. This insight is crucial for anyone wanting to manage their metabolic health effectively.

Short-chain fatty acids produced by gut microbes are pivotal in activating receptors that enhance insulin sensitivity, reduce fat storage, and increase fat burning.

This plays a key role in regulating blood sugar, cholesterol, and fat distribution around the abdomen. 

What does your poop say about your health

Did you know that about 60% of faecal matter is bacteria?

Examining one’s stool is similar to a cardiologist checking a patient’s pulse for cardiovascular health. As a gastroenterologist, he can gauge a person’s gut health by the appearance and characteristics of their stool, as 60% of it is microbial in origin. 

This isn’t just expelled food; it’s a direct insight into one’s gut health. In Zoe’s “blue poop” study, participants consumed blue-dyed muffins, and the time it took for the dye to appear in their stool (gut transit time) was measured. This simple test could predict gut microbiome health and related factors like cardiovascular risk and visceral fat levels.

Gut transit time can be a personal health metric of one’s digestive system. Clinical validation shows that transit times under 14 hours are rapid, while over 58 hours are slow, with 24 to 48 hours being the norm for most. 

This variation in transit times correlates with differences in gut microbiome and dietary habits, particularly fibre intake. 

High fibre consumers generally exhibit normal transit times, whilst those with low fibre intake may experience extremes. 

A balanced fibre diet to regulate bowel movements is essential for a transit time close to 24 hours to maintain optimal gut health. 

What should your poop look like?


There’s undoubtedly a connection between your stool appearance and your gut health.

For reference, the Bristol Stool Scale, a diagnostic tool, categorises stool based on its form— from type 1 (hard pellets, indicative of constipation) to type 7 (liquid, showing diarrhoea).

The consistency and shape of stool, influenced heavily by microbial activity in the gut, can indicate dietary adjustments needed for optimal health.  

Besides, large-scale studies, like the “Big Poo Review,” involved 142,000 UK participants and aimed to better understand bowel movements nationwide. I

This study and others demonstrate that consistent stool types (ideally type 3, 4, or 5, which indicate normal digestive health) correlate with a balanced gut microbiome and adequate fibre intake.

A diet rich in legumes, whole grains, fruits, and vegetables is linked to this optimal stool type. Conversely, those with diets low in fibre, or high in animal products and fats, tend to experience more extreme stool types, such as Type 1 (hard pellets, indicating constipation) or Type 7 (liquid, indicating diarrhoea).

What is a healthy poop colour?

Stool colour can be a significant indicator of health conditions. Brown stool, the norm, results from bile produced by the liver helping fat digestion. 

White stool suggests a bile blockage, whilst yellow stool may indicate poor fat digestion, linked to pancreatic issues. 

Green stool could be due to consuming large amounts of green vegetables or an infection like giardiasis.

The most important is to not ignore red or black stool, which can indicate serious conditions. 

Red stool may suggest bleeding in the lower gastrointestinal tract, possibly from haemorrhoids or polyps, and could be an early sign of colorectal cancer. 

This type of cancer is increasingly prevalent amongst younger people, potentially due to dietary shifts and microbiome changes. 

Black stool might be caused by benign factors such as bismuth medication or more seriously by an upper gastrointestinal bleed, which darkens the stool due to the duration the blood spends in the intestines. 

A foul odour in black stool often indicates bleeding. He strongly advises medical consultation if these colours appear, particularly red, due to the potential severity of underlying issues.

Fibres and gut health relationship

Fibre is broken down by gut bacteria into short-chain fatty acids (SCFAs), which act as signalling molecules. 

These molecules communicate with the body’s cells, influencing genetic expression and modulating immune responses. 

This is particularly relevant in autoimmune conditions, where increased SCFAs can enhance T-regulatory cells that help suppress the immune system.

There has been a dramatic rise in autoimmune diseases, which have surged by 500% in the last 50 years. This has nothing to do with genetic changes, but rather the changes in diet and lifestyle that affect our microbiome. 

Increasing dietary fibre isn’t as straightforward as one might hope, particularly for those who are not accustomed to high-fibre diets. Many struggle initially because the human body rely entirely on gut microbes to digest fibre; we lack the necessary enzymes ourselves. 

If someone’s microbiome is compromised and unaccustomed to fibre, introducing high amounts suddenly can cause discomfort and digestive issues. 

A gradual approach to increasing fibre intake is the best—start with small amounts and slowly building up over weeks or months, allowing the gut microbes to adapt and improve their fibre-digesting capabilities.

Your gut is like a muscle. It adapts and becomes more capable of handling diverse foods without discomfort, over time. 

Remember, not all fibre is the same; it varies significantly between plants. 

  • Soluble fibre, which dissolves in liquids, predominantly feeds gut microbes and acts as prebiotic fibre. 
  • Insoluble fibre helps in bowel transit and has other health benefits like improving cholesterol levels. 

By consuming a variety of plants, different fibres support a diverse range of gut microbes, enhancing overall gut health.

Why are faecal treatments getting famous?

Faecal transplants have been used successfully for over a decade to treat infections like C. diff, but their effectiveness in treating conditions such as ulcerative colitis has been less conclusive. 

Intriguing results from smaller studies suggest that certain donors’ faecal matter might induce remission in diseases like ulcerative colitis, hinting at a future where faecal transplants could be tailored to treat specific gut-related conditions. 

Faecal transplants are administered primarily through two methods. Traditionally, during a colonoscopy while the patient is asleep, the transplant material—liquefied stool—is introduced directly into the colon. 

Recent advancements have allowed for the dehydration of stool, which is then encapsulated and taken orally. This new method could potentially allow individuals to undergo faecal therapy daily, drastically simplifying the process compared to the invasive nature of a colonoscopy.

Should you count calories?

Dr Builsiewicz challenges the conventional wisdom of ‘calories in, calories out’ as the sole determinant of weight loss. 

Whilst acknowledging that reducing caloric intake can lead to weight loss, he points out that this approach is overly simplistic and often unsustainable. 

The body compensates by slowing metabolism, which can lead to increased hunger signals and, ultimately, weight regain. This yo-yo effect is detrimental, as lost weight often includes muscle mass, which is not regained in the same way fat is, potentially leaving a person less healthy than before.

It is more important to consider your dietary quality over mere calorie counting. High-quality, prebiotic-rich diets can improve the health of the microbiome, enhancing overall health and longevity. 

Such diets activate gut hormones like GLP-1, which naturally increase feelings of fullness and help regulate appetite. Thus, you eat until they are satisfied without overconsumption. 

On the other hand, an ultra-processed diet tends to increase cravings and fail to trigger fullness signals, leading to overeating and subsequent metabolic imbalance. 

Is your gut microbiome inherited?

Much of your gut microbiome is inherited from your parents. There’s a profound influence that mothers have on their newborns through natural birthing methods and breastfeeding. 

This initial microbial transfer is crucial for the child’s health. Disruptions to these natural processes, such as caesarean births or bottle feeding, can increase the likelihood of conditions like obesity, allergies, asthma, and autoimmune diseases in children.

Even crazier is the fact that people who share living spaces tend to have healthier gut microbiomes and share more microbes than those who live alone. 

Interestingly, the quality of these relationships also affects microbial sharing; closer, more connected relationships lead to greater microbial exchange. 

This suggests the significant role of human interaction in shaping our microbiome health. Despite the distractions of modern life pulling us away from direct human contact, maintaining strong personal connections can enhance gut health.

Gut health and stress

Stress can have significant physiological effects, including on our gut microbiome. Stress, whether from public speaking or deeper, chronic sources like childhood trauma, manifests in the gut, potentially leading to severe cramps or other digestive issues. 

Early-life stress has been shown to alter gut microbiome, stress response, and brain function, as evidenced by functional MRI scans.

This connection suggests that unresolved emotional issues, including trauma, can impede the healing of gut-related health issues, despite optimal lifestyle habits like proper diet, sleep, and exercise. 

Dr Builsiewicz shares his clinical experiences where patients only begin to see substantial health improvements after addressing and healing from past emotional traumas, highlighting the integral role of psychological well-being in overall physical health. 

Reducing stress often leads to rapid health improvements, which he finds incredibly rewarding in his practice, even more so than dietary changes like increasing fibre intake.

Alcohol and gut health

Much like rubbing alcohol is used for cleaning to kill microbes, drinking alcohol can damage your gut microbiome. 

The after-effects of heavy drinking, such as hangovers, are not just about dehydration but significant damage to the microbiome, leading to prolonged recovery times. 

Beyond liver disease, there are various health conditions linked to alcohol consumption. So, the commonly accepted thresholds for “moderate” drinking might just in fact be excessively high.

A pivotal study, monitoring levels of bacterial endotoxin, or lipopolysaccharide (LPS), (which should not be present in the bloodstream) was quite revealing to Dr Builsiewicz. 

Its presence indicates damage to the gut barrier and results in inflammation, potentially leading to severe illness. 

The study found a direct correlation between blood alcohol levels and LPS levels, rising and falling together, showing the immediate inflammatory impact of alcohol consumption on gut health. 

Even minimal amounts of alcohol could harm your gut microbiome.

The brain gut connection

Now, you can’t talk about the gut without talking about the brain. Our gut and brain are in constant communication, a connection that is fundamental to our overall health. 

Studies reveal that 95% of the body’s serotonin, often called the happy hormone, is produced in the gut, not the brain as commonly assumed. 

This serotonin affects mood, energy levels, and focus, and communicates with the brain via the vagus nerve, which acts as a direct communication line between the gut and the brain.

50% of dopamine, known as the reward hormone, is also produced in the gut, alongside over 30 other neurotransmitters. 

The gut also produces metabolites like postbiotics which can cross the blood-brain barrier and influence brain functions such as mood and focus. 

A study of increased short-chain fatty acids in children’s diets, showed significant improvement in their focus. This shows the importance of dietary fibre in supporting mental and cognitive health through its production of beneficial gut metabolites.

The best diet for gut health

Dr Builsiewicz’s trick? He suggests focusing on what he terms “F goals” to frame his dietary advice. 

Each letter represents a key food category beneficial for gut health: 

F for fruits, which he believes are unjustly criticised yet crucial for preventing diabetes and helping weight loss, and fermented foods to enhance microbial diversity.

G for greens and unrefined grains rich in fibre and nutrients. 

O for Omega-3-rich seeds like chia and flax, as well as walnuts. 

A for aromatics such as onions and garlic, which support heart health and cancer prevention. 

L for legumes, which he labels the ultimate superfood due to their extensive health benefits, including reduced risk of heart disease and cancer.

Last but not least, gut health and sex life

Did you know that a healthy gut microbiome directly influences libido and hormonal balance?

Gut bacteria impact hormone regulation, with specific microbes like the estrobolome managing estrogen cycles in women and similar bacteria affecting testosterone in men. 

Conditions such as erectile dysfunction and polycystic ovary syndrome are also connected to gut health, suggesting that gut microbes play a pivotal role in managing and potentially rectifying these issues.

Gut microbes could also influence interpersonal attractions, affecting pheromones and potentially guiding social interactions and mate selection. 

Dr Builsiewicz suggests that compatibility might even extend to microbial compatibility, impacting how individuals perceive each other’s scents and overall attractiveness. 

This theory is supported by studies indicating that people with a healthy microbiome might appear more physically attractive, as good health often enhances one’s outward appearance.