113. Dr. Karan Rajan: Why Gut Health Shapes Your Longevity

Nov 18, 2025

Dr. Karan Rajan, a GI surgeon and gut health expert with a following of over five million on social media, delves into the intricacies of the gut microbiome. He explains how gut health is fundamental to digestion, immunity, and even skin health. Topics include the significance of fiber diversity, the dangers of low-fiber diets, and the impact of fermented foods. Dr. Rajan also discusses microbiome testing, the influence of circadian rhythms, and practical strategies to rebuild gut health after antibiotics. Tune in for actionable insights on optimizing your microbiome!

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ANECDOTE

Surgeon Turned Microbiome Educator

Dr. Karan Rajan trained as a GI surgeon and later developed a large social media presence translating microbiome science.
He combined clinical work with research and media to educate millions on gut health.

INSIGHT

Microbiome Lives In The Distal Colon

The colon is the main home of the microbiome, concentrated in the distal colon and rectum.
Modern diets and many fiber supplements mostly feed the proximal colon, missing key bacteria at the distal end.

INSIGHT

Regional Bacteria Produce Different Beneficial Molecules

Different bacteria live in different colon regions and produce different short-chain fatty acids with distinct benefits.
Butyrate from distal bacteria nourishes the colon lining while early-colon bacteria produce acetate and propionate that cross-feed downstream species.

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You can eat healthy, exercise daily, and still struggle with fatigue, inflammation, or brain fog. Why? Because every system in your body is connected. The gut, brain, and immune system are constantly communicating, and most health plans ignore that network.

In this episode of Extend, I sit down with Dr. Karan Rajan to explore how the gut microbiome shapes everything from digestion and immunity to mood, skin, and longevity. We unpack the science behind leaky gut, fiber diversity, fermented foods, and the connection between microbiome imbalance and chronic inflammation.

We also dive into practical steps such as how to feed all three layers of your colon, the right way to use fiber supplements, the truth about probiotics, and how daily habits like sleep timing, stress, and even phone use in the bathroom affect gut health.

Dr. Karan Rajan is a surgeon, educator, and gut health expert with more than five million followers across social media. His research in microbiome science and human health has made him one of the most trusted voices translating complex medicine into actionable steps for modern life.

What's Discussed:

(00:00) From Surgery to the Science of the Microbiome
(05:18) How Fiber Diversity Fuels a Healthy Gut
(12:14) The Link Between Leaky Gut, Skin, and Inflammation
(19:49) What Destroys the Microbiome and How to Rebuild It
(27:56) Poop, Circadian Rhythms, and Real Gut Health Markers
(40:13) Fiber Supplements, Fermented Foods, and Smart Probiotics
(54:05) Inside the Future of Gut Health and the Loam Project

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Learn more about Dr. Shah:

Website: https://www.drshah.com/

Clinic: https://www.next-health.com/

Instagram:@darshanshahmd

Learn more about Dr. Karan Rajan:

Instagram: drkaranrajan

YouTube: Dr Karan

Linkedin: Dr Karan Rajan