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Dietitian Sophie Medlin16 March 2023

Heal Your Brain: How Your Gut Secretly Controls Your Brain Health - Dietitian Sophie Medlin

6Frameworks
15Insights

Episode overview

Dietitian Sophie Medlin explains how digestion, the microbiome, immune signalling and serotonin production connect gut health with mental and physical health. She offers practical guidance on dietary variety, fibre, hydration and balanced eating while challenging restrictive diets, detoxes and nutrition misinformation. The conversation also covers body image, supplements, probiotics and her path from NHS and academic work to City Dietitians and television.

Key ideas

  • Gut microbes communicate with the brain through chemical, hormonal and immune pathways.
  • Eating roughly 30 different plants a week can support microbial diversity.
  • Dietary variety and adequate fibre matter more than repeating a narrow set of supposedly healthy meals.
  • Highly restrictive ketogenic, carnivore and one-meal-a-day diets can deprive gut bacteria of needed substrates.
  • Nutrition advice should account for symptoms, lifestyle, sustainability and the individual's broader health.
  • Detoxes, juice cleanses and colonic irrigation lack health benefits and can cause harm.
  • Social-media diet culture can amplify unsupported claims about dairy, gluten, coconut oil and body ideals.
  • Probiotic benefits depend on researched strains, viable formulation and delivery to the colon.

Transcript available · source text is retained privately and is not published

Frameworks in this episode

Mindset4 steps

80% Then Optimize

Get most of the way with easy basics before chasing fine details

Read →
Mindset4 steps

Add, Don't Subtract

Improve your diet by adding good foods that crowd out the bad

Read →
Peak Performance5 steps

The 30 Plants a Week Rule

Feed gut-microbiome diversity by eating 30 different plants every week

Read →
Peak Performance5 steps

The Brain-Nutrition Protocol

Fuel mood and focus with omega-3s, B vitamins, anthocyanins and vitamin D

Read →
Self-Mastery4 steps

The Mindful Eating Check-In

Eat a small portion attentively, then pause and ask if you want more

Read →
Self-Mastery4 steps

Zoom Out on Your Diet

Skip strict rules; periodically review the week and fill the gaps

Read →

People & resources mentioned

Attributed to the moment in the episode. Timestamps are approximate.

People · 12

  • Sophie MedlinMentionsA dietitian and gut-health expert who chaired the British Dietetic Association for London, lectured at King's College London, and founded City Dietitians.

    “her name is Sophie Medlin and she is a dietitian”

    ▶ Watch at ~0:00approx.
  • Ali AbdaalMentionsthe ongoing podcast where every week I get the chance to sit down with inspiring authors, entrepreneurs, creators, and other inspiring people.

    “what happens Ali as you know is when people are busy and they're working too hard”

    ▶ Watch at ~21:00approx.
  • John CryanRecommendsA researcher whose work on psychobiotics and the gut-brain connection is recommended by Sophie Medlin.

    “the amazing work of John cryen is a really good place to to start looking at this stuff”

    ▶ Watch at ~24:00approx.
  • Liver KingMentionsDescribed as a carnivore-diet influencer who promoted an ancestral lifestyle while secretly using steroids.

    “the liver King liver King oh yeah this dude was eating like raw brain and testicles”

    ▶ Watch at ~1:01:00approx.
  • Jennifer LopezMentionsmentioned

    “Jennifer Lopez with a tiny bit of cellulite on the beach circled”

    ▶ Watch at ~1:04:30approx.
  • Kate MossMentionsmentioned

    “Kate Moss generation of nothing tastes as good as skinny feels”

    ▶ Watch at ~1:04:30approx.
  • Deliciously EllaMentionsAn influencer whom Sophie Medlin says promoted plant-based eating and avoiding dairy.

    “deliciously Ella came along and said everyone should be plant-based and not drink Dairy”

    ▶ Watch at ~1:07:00approx.
  • Joe WicksMentionsAn influencer whom Sophie Medlin says encouraged people to consume coconut oil.

    “along comes Joe Wicks and tells everyone to eat coconut oil”

    ▶ Watch at ~1:07:30approx.
  • Rachel KentMentionsA King's College researcher studying how health technology affects wellbeing and everyday life.

    “I've got a very good friend Dr Rachel Kent who is um she's an amazing researcher at King's College”

    ▶ Watch at ~1:35:00approx.
  • Gwyneth PaltrowMentionsmentioned

    “thanks to people like Gwyneth Paltrow and other people who are promoting gluten-free diets as some sort of Health elixir”

    ▶ Watch at ~1:39:00approx.
  • LisaCoinedOne of Sophie's friends who runs The Gut Stuff and helped pitch its television show.

    “my lovely friends Alana and Lisa who run a company called the gut stuff had been pitching this show”

    ▶ Watch at ~2:19:30approx.
  • AlanaCoinedOne of Sophie's friends who runs The Gut Stuff and helped pitch its television show.

    “my lovely friends Alana and Lisa who run a company called the gut stuff had been pitching this show”

    ▶ Watch at ~2:19:30approx.

Resources · 27

  • British Dietetic AssociationMentionscompany

    “she's also the chair of the British dietetic Association for London”

    ▶ Watch at ~0:00approx.
  • Channel 4Mentionscompany

    “is actually also on TV these days as an expert for channels 4 show”

    ▶ Watch at ~0:30approx.
  • King's College LondonMentionsplace

    “she works as a lecturer at King's College London”

    ▶ Watch at ~0:30approx.
  • YouTubeUseswebsite · Google

    “at the moment according to the YouTube analytics 81 of you who are”

    ▶ Watch at ~1:00approx.
  • NHSMentionscompany

    “I was in the NHS for about eight years”

    ▶ Watch at ~3:00approx.
  • City DietitiansCoinedcompany · Sophie Medlin

    “I've been running my clinic business called City dietitians for about four years now”

    ▶ Watch at ~6:00approx.
  • Heights Smart ProbioticCoinedproduct · Sophie Medlin and Heights

    “the ones that we're particularly interested in when we develop the height smart probiotic for example”

    ▶ Watch at ~24:00approx.
  • HuelUsesproduct · Julian Hearn

    “I have been a pain customer of huel since 2017”

    ▶ Watch at ~27:00approx.
  • NutriBulletUsesproduct · Capital Brands

    “I put them into my NutriBullet blender type thing”

    ▶ Watch at ~27:30approx.
  • Trading 212Recommendssoftware · Trading 212

    “it's a great app that lets you trade stocks and funds”

    ▶ Watch at ~28:30approx.
  • World Cancer Research FundMentionscompany

    “the world cancer research fund says we should only have three portions of red meat per week”

    ▶ Watch at ~31:30approx.
  • BrilliantRecommendswebsite

    “brilliant is a fantastic resource for anyone who wants to develop new skills”

    ▶ Watch at ~55:00approx.
  • WeWorkRecommendscompany

    “if you're looking for a co-working space for you or your team then I'd 100 recommend wework”

    ▶ Watch at ~58:00approx.
  • Daily MailMentionswebsite

    “sidebar of shame in the Daily Mail in women's magazines”

    ▶ Watch at ~1:04:30approx.
  • InstagramMentionssoftware · Meta Platforms

    “and then we had the social media and we had Instagram”

    ▶ Watch at ~1:05:00approx.
  • NetflixMentionscompany

    “if you watch Netflix for example you'll see men who don't apparently go”

    ▶ Watch at ~1:12:30approx.
  • Men's HealthMentionsother

    “men who've been on the front covers of like Men's Health”

    ▶ Watch at ~1:13:00approx.
  • The One ShowMentionstv show

    “on the one show where they go let's go and talk to our expert Sophie”

    ▶ Watch at ~2:19:30approx.
  • How to Lose Weight WellMentionstv show

    “I've done kind of inside the factory and uh how to lose weight well”

    ▶ Watch at ~2:19:30approx.
  • Inside the FactoryMentionstv show

    “I've done kind of inside the factory and uh how to lose weight well”

    ▶ Watch at ~2:19:30approx.
  • The Gut StuffCoinedcompany · Alana and Lisa

    “my lovely friends Alana and Lisa who run a company called the gut stuff”

    ▶ Watch at ~2:19:30approx.
  • MonkeyMentionscompany

    “they finally got commissioned by the production company monkey and channel 4 took it”

    ▶ Watch at ~2:20:00approx.
  • Love IslandMentionstv show

    “what I'm doing is not I'm not on love Island right”

    ▶ Watch at ~2:21:00approx.
  • City DietitiansRecommendscompany · Sophie Medlin

    “we've got loads of blogs on our website which is City dietitians.co.uk”

    ▶ Watch at ~2:21:30approx.
  • The Psychobiotic RevolutionRecommendsbook · John Cryan

    “psychobotic Revolution John cryan is an amazing place to start it's really accessible and so super super interesting”

    ▶ Watch at ~2:21:30approx.
  • The Gut StuffRecommendscompany · Alana and Lisa

    “the gut stuff who are part of the TV show they um have amazing resources on their website”

    ▶ Watch at ~2:21:30approx.
  • Deep DiveCoinedpodcast · Ali Abdaal

    “that's it for this week's episode of Deep dive”

    ▶ Watch at ~2:22:00approx.

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Insights & moments

The myth-busts, hot takes, explainers, and tools worth keeping.

Myth Buster· 7

Myth Buster06:30

Dietitian vs nutritionist: why the difference matters

Sophie explains that dietitians are the medically trained, regulated nutrition professionals, the only ones allowed to work in NHS hospitals, and accountable to a regulator like doctors and nurses. 'Nutritionist' is an unregulated title anyone can use after a cheap online course, so credentials matter. Nutritionists still do valuable public-health and research work; they just do a different job.

  • Dietitians are medically trained and regulated (HCPC), like physiotherapists vs massage therapists
  • Only dietitians can work in NHS hospitals
  • 'Nutritionist' is an unregulated title; anyone can use it
  • 'Nutritional therapist' sits more on the alternative-therapy spectrum
  • Check credentials before taking nutrition advice

“anyone can call themselves a nutritionist so in this country term nutritionist isn't regulated”

Sophie Medlin · 07:00
#dietitian#nutritionist#credentials#regulation
Myth Buster34:00

Intermittent fasting: no proven benefit beyond weight control

Sophie says there's no evidence of benefits from fasting beyond weight control, and emerging data suggests people who skip breakfast often overcompensate by eating more in the evening, which is metabolically worse. With a 'biological night' around 9pm slowing processing, front-loading calories earlier tends to be better. The real rule: listen to your body, eat when you're hungry.

  • No evidence of fasting benefits beyond weight control
  • Skipping breakfast can lead to overeating late, which is metabolically worse
  • A 'biological night' from ~9pm slows how we process food
  • Front-loading calories earlier can reduce later intake
  • If you wake hungry, eat; circadian rhythms differ between people

“there's no evidence of any benefits of fasting Beyond weight control”

Sophie Medlin · 34:30
#intermittent fasting#breakfast#circadian rhythm#metabolism
Myth Buster52:30

Why low-carb, keto and carnivore diets wreck your gut

Cutting carbs removes the whole grains that feed crucial probiotic species, starving your 'colonic friends' and allowing negative bacteria to overpopulate. Sophie sees patients on keto or very low-carb diets develop bad IBS, constipation and piles. Keto was designed for epilepsy, and no qualified dietitian recommends the carnivore diet, which she calls horrendous for gut health.

  • Low-carb diets starve key probiotic bacteria that live on whole grains
  • Result can be IBS, constipation, piles and overgrowth of bad bacteria
  • Ketogenic diets exist clinically only for epilepsy
  • Very low-carb keto is almost always unsustainable, fuelling failure cycles
  • The carnivore diet is 'horrendous' for the microbiome

“when you go on a low carb diet you're starving off some of your really important friends in your colon”

Sophie Medlin · 53:00
#low carb#keto#carnivore#gut bacteria
Myth Buster38:30

Gluten-free is pointless unless you're coeliac

Only about 1% of the UK population has coeliac disease, where gluten is genuinely toxic and must be avoided completely. That rate hasn't risen, yet gluten-free dieting has exploded thanks to wellness influencers. Sophie says going gluten-free is often just a low-carb diet by the back door, with no benefit for most people, and it can make restaurants take real coeliac needs less seriously.

  • ~1% of the UK has coeliac disease; that rate hasn't increased
  • For coeliacs, even a crumb of gluten causes real harm
  • Gluten-free for weight loss is really just eating fewer carbs
  • No benefit to cutting gluten if you don't have coeliac disease
  • Fad gluten-free dieting makes eateries complacent about true allergies

“about one percent of the UK population has celiac disease ... the rate of celiac disease hasn't increased over the last few years at all…”

Sophie Medlin · 1:38:30
#gluten-free#coeliac#fad diets#wellness
Myth Buster1:42:00

Dairy, plant milk and the bone-health trap

Sophie is clear that ethical or religious reasons to avoid dairy are entirely valid, but for health reasons there's no data that plant milks are better, and vegans have higher fracture rates. You can only mineralise bone until about age 30 (peak bone density), after which bones act as a mineral store. Dairy also supplies iodine; cutting it is driving iodine deficiency and thyroid problems in young people.

  • No health data shows plant milks beat dairy for humans
  • Peak bone density is reached by ~age 30, then it's downhill
  • Vegans have higher rates of bone fracture
  • Dairy is a key source of iodine for thyroid function
  • Vegans should fortify with calcium, iodine and B vitamins

“there's absolutely no data to show that plant milks are better for you as a human than dairy milk”

Sophie Medlin · 1:42:00
#dairy#plant milk#bone health#iodine#vegan
Myth Buster1:35:00

Juice cleanses and 'detoxes' are dangerous nonsense

Sophie calls juice cleanses and detoxes a terrible idea with no health benefits, sharing how a researcher friend ended up in intensive care with sepsis and kidney stones after following online juicing advice. Depriving your body of nutrients makes your liver and kidneys work harder, producing more oxidative stress. A juice for extra vitamins is fine; a cleanse or 'detox' is not.

  • Juice cleanses and detoxes have no proven health benefits
  • A researcher friend landed in intensive care with sepsis and kidney stones
  • Depriving the body of nutrients makes the liver and kidneys work harder
  • This creates more oxidative stress, harming you
  • A juice for extra vitamins is fine; a 'cleanse' is not

“anything that kind of involves let's say the word detoxing it's just nonsense”

Sophie Medlin · 1:35:30
#juice cleanse#detox#oxidative stress#wellness myths
Myth Buster1:27:00

Why at-home microbiome stool tests can do more harm than good

Commercial microbiome and personalised-nutrition tests are academically fascinating but limited, Sophie says. There's no reference data for what's 'normal' in people with gut conditions, so results can be frightening and raise anxiety, which itself worsens gut symptoms. In practice a clinician can often infer what's going on from your symptoms, and for most people the resulting advice, eat more plants, oily fish, is the same anyway.

  • No reference data exists for 'normal' microbiomes in people with gut conditions
  • Results can be alarming and increase anxiety, worsening symptoms
  • Clinicians can often infer the picture from symptoms alone
  • For most people the dietary advice ends up broadly the same
  • Best avoided if you already live with any gut symptoms

“if you're going to do it on your own the chances are you're going to end up with a load of information that you don't…”

Sophie Medlin · 1:28:00
#stool testing#microbiome tests#personalised nutrition#anxiety

Hot Take· 2

Hot Take30:30

The 3 nutrition mistakes personal trainers keep making

Sophie flags three recurring errors she sees in personal-trainer meal plans: putting clients on the same foods on repeat (starving protective gut bacteria), unintentionally low fibre (often ~16g vs the 30g needed) from cutting carbs, and too much red or animal protein. She stresses it's not the trainers' fault, but nutrition advice should come from a qualified professional.

  • Same foods on repeat starves gut bacteria that help protect against obesity
  • Low-carb plans often land around 16g fibre, half of the ~30g needed
  • Excess red meat raises bowel-cancer risk (limit 350-500g/week)
  • It's outside a trainer's remit; see a dietitian for tailored plans

“you're starving off your essential gut bacteria and some of those really important gut bacteria are actually protecting you from obesity”

Sophie Medlin · 31:00
#personal trainers#fibre#gut bacteria#diet plans
Hot Take1:13:30

The hidden cost of chasing visible abs

Sophie argues visible-abs culture is quietly damaging. Most women need 22-25% body fat for a regular menstrual cycle, yet getting a six-pack often means dropping to 17-19%, stopping their period and harming bone density. For men, very low body fat lowers testosterone and energy and raises injury risk; the ripped physiques on screen are often unrealistic or achieved in miserable, unsustainable ways.

  • Most women need 22-25% body fat for a regular menstrual cycle
  • A six-pack often requires 17-19% body fat, risking loss of periods and bone density
  • Low body fat in men lowers testosterone and raises injury risk
  • Magazine-cover shreds are often achieved feeling depressed and dehydrated
  • Screen physiques set an unrealistic, often unattainable standard

“most women need a body fat percentage of at least 22 to 25 to have a regular menstrual cycle”

Sophie Medlin · 1:14:00
#body fat#abs#menstrual cycle#body image#testosterone

Explainer· 5

Explainer11:30

How your gut secretly controls your whole body

Sophie opens the gut 'black box': every part of you is physically built from what you absorb, and the colon houses an ecosystem of bacteria that ferment plant fibre into metabolites like butyrate that control inflammation. 70% of immune cells live in the bowel and 95% of the body's serotonin is made in the gut, so the gut is constantly, chemically and hormonally, communicating with the rest of the body.

  • Every tissue is physically made from nutrients absorbed in the gut
  • Colon bacteria ferment indigestible fibre into metabolites like butyrate
  • Butyrate helps control inflammation in the body
  • 70% of immune cells live in the bowel
  • 95% of the body's serotonin is produced in the gut

“we also produce 95 of the body's serotonin in the gut which links to our brain health our mental health”

Sophie Medlin · 12:30
#microbiome#butyrate#serotonin#immune system#inflammation
Explainer15:30

What's normal poo and the signs you should get help

Because no one talks about poo, people put up with symptoms they don't have to. Sophie lays out the baseline: pooing anywhere from three times a day to three times a week is normal, aiming for a smooth sausage shape and a medium-brown colour. Red flags include a consistent change in bowel habit over two weeks, urgency, hard painful stools, very foul smell, or any abdominal pain.

  • Normal frequency is three times a day to three times a week
  • A consistent change in your bowel habit over ~2 weeks warrants help
  • Aim for a smooth, sausage-shaped stool that holds together
  • Medium-brown is normal; occasional colour shifts from food are fine
  • Urgency, hard/painful stools, very foul smell or any abdominal pain are worth checking

“passing stall anything between three times a day and three times a week is normal”

Sophie Medlin · 15:30
#bowel health#symptoms#stool#when to see a dietitian
Explainer31:30

How much red and processed meat is actually safe

The World Cancer Research Fund guidance is a maximum of about 350-500g of red meat per week, and little to no processed meat (sausages, bacon, cured meats) for bowel-cancer protection. Sophie explains that excess animal protein your body can't absorb gets fermented by 'bad' bacteria producing methane and sulphur, so very foul-smelling gas is a sign you're overfeeding them. Fish and eggs are gentler; plant proteins feed good bacteria.

  • Cap red meat at ~350-500g per week
  • Little to no processed meat (sausages, bacon, cured meats)
  • Foul-smelling gas can signal too much animal protein feeding bad bacteria
  • Fish is easier on the bowel than meat and good for the brain
  • Gut bacteria also like plant proteins like soya and tofu

“the world cancer research fund says we should only have three portions of red meat per week and that's 350 to 500 grams in total”

Sophie Medlin · 31:30
#red meat#processed meat#bowel cancer#protein
Explainer1:15:30

Can you be overweight and healthy? TOFI vs FOTI

Yes, Sophie says: some people are naturally heavier, and if they eat well, exercise and have little visceral fat, they're healthy 'overweight' people, sometimes denied treatment because of BMI. She contrasts TOFI (thin outside, fat inside) with FOTI (fat outside, thin inside). A naturally thin smoker who never exercises may be far unhealthier than a heavier person who looks after their body.

  • Some people are genetically predisposed to be heavier and still healthy
  • Visceral fat, not overall weight, drives much metabolic risk
  • TOFI = thin outside but fat (visceral) inside
  • FOTI = heavier outside but metabolically healthy inside
  • BMI alone can wrongly gate medical treatment

“you can absolutely be heavier than society would like you to be and be a very healthy person”

Sophie Medlin · 1:16:00
#bmi#visceral fat#tofi#weight and health
Explainer1:22:00

The research: probiotics, cortisol and the stress feedback loop

Sophie walks through controlled studies where probiotics significantly reduced psychological stress, cortisol and depression scores versus placebo, objective chemical markers, not just self-report. In one study benefits lasted eight weeks after stopping. She frames anxiety as the 'anti-focus' hormone: reducing cortisol improves concentration, and probiotics can break the gut-stress feedback loop that keeps a 'stressed microbiome' releasing more cortisol.

  • Probiotics significantly cut cortisol and depression scores vs placebo
  • Effects in one study lasted eight weeks after stopping
  • Anxiety is the 'anti-focus' hormone; lowering cortisol aids concentration
  • A stressed environment creates a 'stressed microbiome' releasing more cortisol
  • Specific strains (cerebiome) can break that feedback loop

“once you have the probiotic have a significant reduction of psychological stress significant reduction in cortisol”

Sophie Medlin · 1:22:00
#probiotics#cortisol#stress#anxiety#focus

Tool· 1

Tool45:30

How to spot an ultra-processed food

Sophie gives simple rules of thumb: if it's been through a factory and comes in a packet with a long ingredients list full of preservatives, emulsifiers, colourings and e-numbers you wouldn't recognise from your kitchen, it's ultra-processed. Emulsifiers may disrupt the gut lining and the protective mucosal layer. She adds an important caveat: don't demonise affordable staples for people in financial hardship.

  • Long ingredient lists with unrecognisable additives signal ultra-processed foods
  • Watch for emulsifiers, preservatives, colourings and e-numbers
  • Emulsifiers may disrupt the gut lining and mucosal layer
  • Even 'health halo' vegan products can be ultra-processed
  • Don't demonise cheap staples for those who can't afford alternatives

“if it comes in a packet if it's been through a factory and it's come in a packet it's going to be processed”

Sophie Medlin · 45:30
#ultra-processed food#emulsifiers#labels#gut lining