Exercise & Leaky Gut: What Every Athlete Should Know

When it comes to exercise, there is definitely a happy medium that provides health benefits. Too little activity is associated with detrimental health effects, including a compromised immune system, decreased resistance to stress, and decreased resilience to circadian rhythms.

However, too much (too strenuous, too intense) activity also negatively impacts health, causing dysregulated cortisol, increased susceptibility to immune-related diseases and infection, and a leaky gut.

Leaky Gut

It actually shouldn’t come as a surprise that intense and strenuous activities cause gut problems. Up to half of all long-distance runners experience something called runner’s diarrhea (colloquially referred to as “runner’s runs,” “runner’s trots,” or “the gingerbread man”).

The symptoms include dizziness, nausea, stomach or intestinal cramps, vomiting, and diarrhea, which occur mainly while running. All these are symptoms of something more insidious happening in the gut.

gut issues

While not all endurance athletes suffer overt symptoms, strenuous exercise does appear to increase intestinal permeability in everyone who indulges in exhaustive exercise, albeit to varying degrees. A variety of studies have documented increased intestinal permeability in athletes who reported no gastrointestinal symptoms. And one study showed that well-trained athletes who suffered from exercise-induced gastrointestinal symptoms experienced significantly more intestinal permeability after exercise than asymptomatic athletes.

Cortisol (Oh and CRH too) & Gut Health

The way that exercise increases intestinal permeability is multi-faceted. First, intense activity is a stress on the body and activates the Hypothalamus-Pituitary-Adrenal (HPA) Axis, i.e., the Stress Axis or the Fight-or-Flight Response. This causes the release of two important hormones that directly affect gut health.

The better known of these hormones is cortisol, the master stress hormone. This secretion of cortisol is largely responsible for the negative impact of intense activity on the immune system, however it can also directly affect the integrity of the gut barrier as well. Very high cortisol alters tight junction assembly, making the gut more permeable to low molecular weight substances and less permeable to high molecular weight substances. High cortisol further, decreases gut motility, decreases mucus production in the gut (by goblet cells), inhibits digestion (by reducing pancreatic enzyme secretion and gallbladder function), decreases intestinal blood flow and may even directly alter the gut microbiome.

Yep, cortisol is not your gut’s friend. But when it comes to activating the HPA Axis, there’s another upstream hormone that has an even more profound impact on the health of the gut: corticotropin-releasing hormone, or CRH. CRH is released by the hypothalamus in response to stress (regardless of whether the stress is a lion chasing you, a traffic jam when you’re late for an important meeting, or a 3-hour long marathon training run).

CRH signals to the pituitary gland to release another hormone which then signals to the adrenal glands to release everything they release, including cortisol. CRH not only increases the permeability of the intestinal barrier (which causes a leaky gut), but it also increases permeability of the lung barrier, the skin barrier, and the blood-brain barrier. It does this two ways:

  1. First, it activates mast cells (an immune system cell type that resides in most tissues and is a major producer of histamine, for example during allergic reactions) which then secrete a variety of substances (histamine, heparin, and cytokines) that trigger the opening of tight junctions.
  2. Second, CRH has a direct effect on the tight junctions by increasing the incorporation of a type of protein into the junction that causes those junctions to be more open (the tight junction protein is called claudin-2, and when claudin-2 is incorporated into tight junctions, epithelial barriers are more leaky, or permeable).

However, the stress response isn’t the only way that exercise causes a leaky gut.

Blood Flow & Leaky Gut


In order to prioritise blood flow to the heart and skeletal muscles during exercise, blood flow is diverted away from the gastrointestinal tract and other visceral organs (like the liver and spleen). This lack of sufficient blood flow results in what is called ischemic injury (injury that results from inadequate blood supply) to the gut, which disrupts the intestinal barrier and thus increases intestinal permeability (aka, the dreaded leaky gut).

Participating in strenuous and exhaustive exercise further stimulates the production of a class of proteins called heat shock proteins (so called because the first of this family of proteins was discovered to be induced by fevers). Heat shock proteins have a direct effect on tight junctions (by affecting the levels of two integral protein families called occludin and claudin), opening them up and causing a leaky gut. In fact, understanding how exercise directly impacts the formation of gut epithelial tight junctions is a robust field of study. If you want to get into geeky details, this review article is a great place to start: http://bit.ly/2zyVCSz.

Perhaps given all this, it’s no surprise that vigorous exercise is also associated with a condition called food-dependent, exercise-induced anaphylaxis, in which the exercise-induced increase in intestinal permeability facilitates the absorption of allergens from the gastrointestinal tract.

Sports Drinks & Ibuprofen Are Not Your Gut’s Friend

Sports Drinks

A few conditions can aggravate the increased intestinal permeability caused by strenuous exercise. One study showed that the use of ibuprofen, a nonsteroidal anti-inflammatory drug (NSAID), significantly exacerbated both intestinal permeability and intestinal damage caused by strenuous exercise in well-trained athletes (ironically, popping ibuprofen is a common practice for endurance athletes).

There is also a strong correlation between both food intake and the consumption of carbohydrate-dense, electrolyte-enhanced beverages and gastrointestinal symptoms in endurance athletes. Strenuous exercise inhibits gastric emptying (the movement of food from the stomach to the small intestine), which is then further inhibited as the concentration of carbohydrates and salt in the stomach increases, so sugary sports drinks can actually make the problem worse.

Of course, it goes without saying that dehydration also causes heightened symptoms. It remains unknown whether food and overly concentrated sport drinks actually increase intestinal permeability or simply magnify the symptoms felt by the athlete. Watch this space.

Heat & Exercise = Further Inflammation

Environmental conditions also have an impact. One study showed that a sixty-minute run in both hot (91°F or 33°C) and cool (72°F or 22°C) conditions caused increased intestinal permeability, but that the amount of endotoxin (bacterial protein from Gram-negative bacteria) detectable in the blood was much greater after strenuous exercise performed in hot conditions but not in cool conditions (http://bit.ly/2yxeVI5). This implies that strenuous exercise is more inflammatory if performed in the heat.

Probiotic treatment may also help protect the gut from increased permeability caused by strenuous exercise. One study showed that probiotic supplementation reduced the amount of proinflammatory cytokines in the blood after strenuous exercise in male athletes and decreased the amount of zonulin (a protein that opens up tight junctions between the gut epithelial cells, implicated in a few autoimmune diseases) detectable in the feces (http://bit.ly/2zEINpv).

probiotic supplement

Runners, cyclists, and triathletes have been studied for exercise-induced intestinal-barrier dysfunction. Although there have been no definitive studies on the connection between resistance training and intestinal permeability, it probably depends on the style of workout and the amount of rest time between sets. Certainly, high-intensity, short-rest workouts have been shown to increase cortisol secretion more than traditional resistance training.

Low Intensity V Higher Intensity Exercise

By contrast, regular exercise at a relatively low intensity may protect the gastrointestinal tract from becoming diseased. There is evidence that physical activity reduces the risk of colon cancer, gallstones, diverticulosis, and inflammatory bowel disease, which is yet another argument for increasing physical activity while avoiding strenuous exercise. Again, it’s the happy medium thing.

The Take Aways

So there you have it, it’s not all doom and gloom. Taking the stress out of training will go a long way in protecting the integrity of your mucosal lining and gastrointestinal health in general. This looks different for everybody but too many long and strenuous workouts crammed into an already busy schedule are going to place a strain and likely compromise overall gut health integrity. Limiting the amount of intensive workouts in hot weather, staying away from NSAIDS and ditching those gels and sports drinks will go a long way in protecting you from developing a leaky gut. As will taking a quality probiotic, eating for gut health (fermentable fibres), removing as many toxins as possible from your environment and treating any detected intestinal pathogens.

If you suspect a leaky gut may be the root cause of your digestive issues and performance issues then I thoroughly recommend a Poop test to dig a little deeper ;)

The 'Life-Juggler's' Guide to a Sub 3 Marathon continued....

London Marathon Race Week

Nursery Flu.


One can read as much of the scientific and lay press as one likes - and hence apply logic to circumstance - but when it actually happens to you, you never fail to get this - why me why now face!

I upped the mileage in a pre-race mild panic that i wouldn't be able to cover the distance despite having run my fastest ever half marathon in Bath — didn’t down the other stresses and immersed in the viral milieu of babyhood - i got ill, as did everyone else in the environs. 

I don’t do ill very well - as i am sure not many of you readers do either. Taking to bed - admitting that you are not bulletproof is a toughie. I threw everything at it.


exercise pill
  • Lemon/Ginger/Manuka honey/Apple Cider Vinegar combo on waking
  • Oregano and garlic oil capsule three times a day
  • Doubled my probiotic powder (Had just started Dr Grace Liu’s BifidoMaximus)
  • Put turmeric and black pepper on everything
  • Drank rosehip tea like it was vintage pinot
  • Vintage Pinot ;)

3 days of night sweats, drenching the bed and the Friday before the marathon - after a short 20 min ‘test’ run - I followed this by lots of food and an afternoon in bed.. bod went into conservative mode as i battled with the voice in my head saying I really was not well enough to tow the line at a marathon. 

I know the medical director of the London Marathon - Prof Sanjay Sharma and asked his advice. 

The words - viral endocarditis - were mentioned and i was told to be sensible. If you experience a heavy colds with fever in the lead in to a race - medics will advise you not to race. The risk of the virus - even if feel better for race day - there will likely still be virus in the body - travelling to the heart and causing a potentially life threatening condition is real.

Now i am not sensible - never have been - although i have become progressively more and more responsible since having Sophia.
Medical words of caution heeded - but still - these are things that happen to other people?You read about them - but its rare… right?

Not as rare as you think. My friend and prior training buddy Parys Edwards experienced this after toing the line at the Ironman 70.3 Champs in Vegas a few years ago. She was hospitalised and given high dose anti-biotics through a drip for days. She was lucky but it took a long time to recover. 

Still I managed to convince myself that Parys and me were different. I have trained with many people over the years - but aside from Jodie Swallow and Declan Doyle - I have never known anyone who can over-ride their central governor and push themselves past exhaustion - like Parys.

I have never been able to do that - always cautionary - always holding out for that sprint finish. Brett Sutton always told me that this was my downfall - my continual self-analysis - ‘just shut that brain up and race’ .. ‘suicide pace’ always alluded me. Perhaps this is a good thing. 

Saturday morning before the Sunday race.

Still not decided. Eat - go back to bed ( not so easy when you have a 1 year old toddler climbing all over you!)

running hippo

4pm - Go to London and see how you feel.

'You can always run with a hippopotamus'

A Tweet i received which cheered my dour countenance  - you have a get out plan - always have a get out plan even if it does involve a slog with a hippo.

Slept on the train and arrived at my friend Kiri’s place - where a bounty of healthy foods awaited me and my race pack. Thanks chick! A positive sign for my recovery came out of the fact i was constantly hungry in these last two days and allowed myself to eat freely and increased the amount of carbs in my diet.

Although i am well fat adapted - as in i can burn fat to a relatively high heart rate - i still do have carbohydrates to maintain some metabolic flexibility. I just need less off them to maintain steady energy levels in an event.

 I think i ate almost whole loaf of home-made Deliciously Ella Seed Bread (gluten fee) with Avocado mash, amongst other delicacies.

More of said supplements above, a himalayan salt bath and bed.

Never sleep well before a race - even if I have low expectations - and I honestly did at this point. Not particularly helped by the fact Kiri lives a stones’ throw from the famous Fabric nightclub.

6.45am Race Day

I will run. 

There is a reason London Marathon is said to be one of the best marathons in the world.

It lives up to the hype. I was in the Club Championships (CC) entry having got a qualifying 1/2 mara time so was lined up metres behind the row of Kenyan Cheetahs. 

tamsin lewis london marathon

For many a common complaint is that it is difficult to run the first few miles of the mara at all as you are barging past aforementioned hippo’s et al. Thankfully being in the CC start meant it was full pace from the start.

I found myself next to Hywel Davies - someone who I usually think of as an endurance nutter - i then reminded myself that to many I myself am an endurance nutter. 

He was aiming for sub 2.30 and blazed into the distance. Steady and Easy were my mantras. I rarely run with heart rate but the cautionary tale from Prof Sharma was ringing in my ears so i watched my heart rate like a hawk. It formed part of my decision to tow the line. Resting HR and HRV normalised in the 48hrs pre race - muscle aching persisted but its common to feel heavy pre race as extra glycogen in the muscles store water alongside. See how you feel. There is always the Hippo.

I generally felt ok during the first miles - i knew my legs were heavy - no real bounce - but i tried to stay in the moment - take in the crowds, keep my leg turnover high and not get carried away running threshold pace like many around me. Pacing is one thing I have always been good at. I prob under-pace rather than anything. It was quite cold at this point and i felt silly for ditching my gloves at the start. Body fine, peripheries not so. I noticed there were many discarded gloves at the side of the road, so i stopped and put some on. Bit cheeky but it took my mind of my cold hands - solvable discomfort - we like that.

At Tower Bridge we approached the half marathon mark and i was still feeling good - and really did draw energy from the crowds at this juncture. I reminded myself running past my medical school Kings College - that running a marathon at that time to me (2003/4) had seemed like an impossible task. Here i was trying to do it at a pace which i could at that time not even run for 5 minutes. 

There were some dark times at medical school - a severe head injury after a skiing accident, binge eating/starving/over-boozing - a story for another time. Suffice to say i felt a mixture of sadness and pride crossing that bridge.

At 13 miles i stopped for a quick pee… then noticed less felt heavy on starting again. Hang on - they didnt feel like this at the end of the Bath Half and i still had over half way to go….hmmm … turned to the guy running next to me - ‘Do your quads hurt?’   (misery loves company)…

“No.. The real pain doesn’t start until mile 22”.. Damn that is 9miles away.. i don’t think my legs will last. 


I am stubborn - often very stubborn - and like the majority of you reading this i do not like to quit. 

When the pain hit - I asked myself

london marathon tamsin

Why am i in pain ? 

Too little mileage ? But i never get quad pain? You’ve only run 27k as your longest run… Shut up coach said didn’t need to run long - must be the post-viral inflammation… muscle contraction less effective. 

Will the pain improve or worsen?

At Ironman UK the last (and only) time I ran a marathon I took paracetamol at the start of the run. Studies shows it can reduced perceived effort - especially in the heat. It wasn’t hot - but at this point i was thinking perhaps i should have considered having some on board. Considered asking a random member of the crowd - maybe a boozehound had some prophylaxis in his pocket. Decided was silly thought. Note I NEVER recommend pain-killers other than paracetamol in endurance events as studies have repeatedly shown they can cause kidney and even heart damage. Chronic use even worse. One of my criticisms of former coach Brett Sutton - used to dish out Voltaren like smarties. 


As a side note - NSAIDS like this can quite easily burn a hole in your stomach esp. when taken whilst exercising which generally increases gut ‘leakiness’ and acidity - As Multiple Ironman Champ Lucy Gossage will tell you from her personal experience of NSAID induced gut bleeding and subsequent anaemia (before she turned pro) - it is NOT worth the risk. Some say - 'well i’ll just take an acid-blocker alongside the painkiller' - we have acid in our stomachs for a reason - it aids digestion and prevents harmful bacteria/viruses/parasites from getting in. Do not recommend. GI Upset in endurance sport is obviously a blog post in its own right. I use activated charcoal - £3 on Amazon.. superb for gas/bloating - and also for mopping up the toxins from booze on a night out :)


The next 13miles went from being cheery/waving and - ‘isn’t this incredible, i am finally running the London marathon’ … to ‘bugger… i may not make this.. the pain in my quads was amplifying.

Looked at my watch - i had averaged 6.30 mile pace for the first half and was capable of talking all this time - felt easy .. then the leg pain - so CV discomfort (why would there be i was well below threshold).. energy felt constant.

Mile 19 - Do you legs hurt more now than they did at mile 15?

Not really.

Great … Its therefore not pathological .. they will not give way..  

Heart rate?  Steady not going over 150bpm.

pace had dropped to 4.30 pace for a couple of miles - i couldn't seem to keep my cadence up. Coming back past Tower Bridge I had another word myself. Its now or never.. you may not be in top shape but you have come here to run a sub 3hr marathon and if you want to do that - then its time to up your game. 

You play these games with yourself which in reflection seem a bit silly.

For me i was picturing my daughter Sophia (just 14months) and the tenacity she now shows lifting and walking with things.. what would she say to Mummy? You didn’t make your goal because you couldn't stand the pain? I pictured my Mum and Sophia watching the marathon on TV willing me on - they weren’t - they were on Bournemouth beach - but it is what i chose to believe at that time and it worked. Why do you run? 

I had my second gel - Torq Rhubarb/Custard and sipped it slowly - all the time practicing the psychological tip of externalising. I thought of my summers with my beloved Grandpa - eating freshly rhubarb - topped with steaming homemade custard. 

The Tunnel - at this point you only have a few miles left - a friend had told me the BlackFriars tunnel was a lonely place - but i welcomed the sensory change and felt somewhat energised by heavy based music. 

The Embankment - I saw a man collapsed just ahead. He looked in a bad way. I had to resist urges to go over and help as medical aid had just arrived. I learnt later he died. RIP David Seath.  You just never know when you card is up. This thought pressed me for a few minutes. Prof Sharma had told me that is should not race if i was ill in race week, yet i felt confident enough in my self awareness/physiology that i could at least start.

Heart Rate 152.

Those final miles were painful. I had imagined trotting along the embankment smiling at the crowds and taking in the atmosphere but all i was looking at was the clock fast approaching 3 hours. Now or never its just a bit of pain. I had to do a bit of elbowing past folks in the final mile as from somewhere i found some leg speed and finished with a sub 6.20mile. Always like a sprint finish me. 2.58 (which in my head is infinitely better than 2.59). Semantics.


marathon tamsin

I did my own personal high five and then had a few tears. Boxed ticked… you can be a little bit proud of this one. 

Yes my predicted time based on half mara performance was around 5 minutes faster - but all things considered - i am happy - and i’m ok. Heart rate dropped into normal range relatively quickly - and i didn’t feel unwell. I’d like to say i put my feet up with some of my wonderful friends who came to watch me (Kiri, Arianna - thank you for the fuel and massage) - I did - but only for an hour - as back to the mommy role… race to get home to see Sophia before she went to bed. I’ll pretend she is proud of me for now.. and maybe one day we can run a marathon together.

Key Take Away Points.


1.” It Doesn’t Have to be fecking Hard - But it Does have to be Consistent - which in turn makes it fecking hard. “    Thanks Sutto.

2. Long Slow Steady Runs in my opinion are not the pinnacle of marathon training.

3. Functional Strength training pays dividends.

4. Mental Resilience cannot be under-estimated.

5. Knowing that your Blood biomarkers are optimised is important - esp. if you are not feeling as ship-shape as you once have done. 


My (Historical) Issues


  1. Low Iron (Ferritin <20)
  2. Inappropriate Vitamin B12/Folate metabolism (MTHFR mutation - 23andMe > Genetic Genie)
  3. Low Progesterone ( Agnus Cactus and appropriately timed natural progesterone helped with this).
  4. Low free/available Testosterone (from years of being on the contraceptive pill which increases binding protein)

5. Low Magnesium

6. Too much Omega 6 compared to Omega 3 - measured as red cell Omega 3: 6 ratio.

7. Low Thyroid

My GP gave me ferrous sulphate which upset my gut (causes inflammation which as an athlete already predisposed to leaky gut is the last thing you need)

Floradix 2 full capfuls/day is my preferred effective option - although ferrous gluconate/bisglycinate with vit C is effective & tolerated well.

The other tests are not available through the NHS so we did them through my company www.curoseven.com - alongside a host of other integrative health testing (dutchtest.com, Metabolic/Organic Acids Testing, and Gut Function Testing.

Its only in the last few weeks after a few tweaks to my supplement regime based on the results of the above and through the help of incredible and infinitely humble Dr Tommy Wood that I have started to feel like a million dollars.  

Despite juggling of all the plates commonly known to fit, full time working, mums.

My house is not tidy, however.  I’m looking to out-source this one   :)

I recently presented with pal Ben Greenfield at The Biohacker Summitt and at The Public Health Collaboration event. 

Exciting, Busy Times.

I recently heard a term coined - "productivity addict" - anyone else like to put their hand up to this one? ;) Must Forest Bathe more.

Next Up I will be part of the Expert Panel with Dr Kelly Brogan and Dr Rangan Chatterjee (BBC Doctor in The House) at

This Event  in July.

Sure will be interesting. 


ben greenfield tamsin lewis