Why Does Exercise Sometimes Upset Your Gut?
Ever finished a long run, HYROX session or hard workout and wondered why your stomach feels like THAT?
Bloating. Cramping. Nausea. An urgent dash to the bathroom.
We often blame what we ate beforehand, but sometimes, the exercise itself may be part of the problem.
What happens to your gut during intense exercise?
When you're exercising hard, your body prioritises blood flow to your working muscles and skin to help regulate temperature. This means less blood flow reaches your digestive system.
As your workout continues, particularly during prolonged or intense exercise and in hot conditions, your core temperature also rises.
Your gut doesn't love either of these things.
The cells lining your intestine are normally held closely together by structures called tight junctions, which act like little seals. They allow nutrients through while helping keep unwanted substances inside the gut.
But under significant exercise stress, these seals can temporarily become more permeable.
In simple terms: your gut can become a little more “leaky”.
Research has found a strong relationship between rises in core temperature during exercise and increases in intestinal permeability. In one systematic review, a core temperature above 39°C was consistently associated with increased permeability.
Why does this matter?
One concern is LPS (lipopolysaccharide), a component of certain gut bacteria.
When the gut barrier becomes more permeable, substances like LPS have greater opportunity to cross into circulation and interact with the immune system. This may contribute to the inflammation and gastrointestinal symptoms some people experience during or after demanding exercise.
Think:
Hard exercise → reduced gut blood flow + increased body temperature → temporarily increased gut permeability → greater exposure to bacterial components → potential GI symptoms.
This could help explain why some people experience bloating, nausea, cramping or diarrhoea after a marathon, long run, HYROX or particularly intense training session.
Can we support the gut barrier?
This is where the research gets interesting.
One small human study found that zinc carnosine and bovine colostrum reduced the exercise-induced increase in intestinal permeability. After 14 days, the combination reduced the increase by 85% compared with placebo.
Other nutrients, including glutamine and curcumin, have also been investigated for their role in supporting intestinal barrier function.
The takeaway
If your gut is regularly getting in the way of your training, book a consultation for an individualised approach to understanding your symptoms and supporting your gut health.