Why Lack of Blood Flow Happens & Why It Causes Bathroom Issues When Running
The digestive system needs blood flow to function efficiently, but as soon as you start running your body diverts blood from your digestive system to fuel your working muscles, heart, and lungs.
In fact, research has shown that blood flow decreases by 20% within just 10 minutes of running, and by a dramatic 80% after 1 hour of running at 70% of your VO2max.
This isn’t necessarily a bad thing in the short term, it’s how your cardiovascular system prioritizes survival during physical exertion.
But it does create problems for your gut.
Without this blood flow, your gut doesn’t have the oxygen and nutrients it needs to function efficiently and remove waste products.
- This reduced efficiency impairs digestion and actually accelerates the movement of food through the intestines.
- This, combined with the physical jostling of running, is what leads to cramping, diarrhea, and a sudden urge to use the bathroom.
How Heat Makes Runner’s Trots Worse
The longer you run, the more internal heat you produce. This is compounded when you train and race in hot conditions.
As a result, your body continues to shift more blood flow to the skin to help keep you cool. This leaves even less blood in your stomach, especially late in races when you need to digest and absorb gels.
Not only does this further slow the digestive system but scientific studies have shown that when your core temperature reaches 38.5°C (101°F), intestinal permeability increases.
This is often called “leaky gut” and is estimated to affect 70% of endurance athletes.
What is Leaky Gut and Why it Causes Runner’s Gut
In layman’s terms, leaky gut is like developing tiny holes in your gut lining that allow endotoxins to pass through that normally shouldn’t.
Studies have shown that when these endotoxins leak into your bloodstream, they trigger an acute-phase inflammatory response.
This inflammatory response triggers the urgent bathroom needs you experience during runs.
Does Caffeine Make Runner’s Stomach Worse?
Caffeine is one of the most underappreciated triggers of exercise-induced GI distress, and it directly compounds the gut stress that running already creates through blood flow diversion.
Research has shown that caffeinated coffee stimulates colonic motor activity to a level 60% as powerful as eating a full meal, with the urge to defecate occurring in susceptible individuals within minutes of consumption.
Caffeine binds to receptors in the colon that trigger muscle contractions, which push intestinal contents toward the rectum faster than normal digestion would.
Your gut is already deprived of blood and oxygen the moment you start running. Adding a direct colon stimulant on top of that creates two simultaneous forces pushing your body toward urgency.
What Else Causes Runner’s Stomach?
Blood flow diversion and heat are the primary causes of runner’s trots. Three additional factors can trigger stomach issues on their own or make existing symptoms significantly worse.
Does Dehydration Make Runner’s Trots Worse?
Dehydration reduces blood flow to the gut through the same pathway that running does, which means starting a run even mildly dehydrated doubles the trigger for needing to use the bathroom.
Research has found that fluid losses of just 2% of body weight measurably reduce blood flow to the gut during exercise, compounding the drop that running alone already causes and increasing the urgency to use the bathroom.
This happens because dehydration lowers your total blood volume, leaving even less blood available for the gut once your muscles start pulling it away.
Less blood flow to the gut also speeds up leaky gut, allowing bacteria to enter the bloodstream and triggering a stronger urge to go than running alone would produce.
Can Race-Day Anxiety Cause Stomach Problems When Running?
Most runners need to use the bathroom more urgently on race day than during training runs at the same pace. The gut-brain axis is why.
Research has shown that stress hormones released before a race directly stimulate the colon and speed up how fast food moves through your intestines, triggering the urge to poop before you even start running.
Your nervous system treats race-day pressure as a physical threat. It floods your body with cortisol and adrenaline, both of which act directly on the gut.
Cortisol increases leaky gut. Adrenaline speeds up how fast food moves through your intestines. The result is the same stomach urgency that running creates, activated before you have taken a single step.
Does Taking Ibuprofen Before a Run Cause GI Issues?
Taking ibuprofen before a race is one of the most common pre-race habits among runners and one of the most damaging things you can do to your gut before exercise.
A study found that runners who took ibuprofen before a 160-km race had significantly more leaky gut and gut inflammation in their bloodstream than runners who did not, leading to worse stomach problems during and after the race.
NSAIDs block the enzymes your body uses to maintain the protective lining of the gut wall. Without that lining intact, the gut barrier weakens before you even start running.
Exercise then accelerates leaky gut on a wall that has already been chemically damaged, leading to worse stomach issues than running alone would cause.
Taking ibuprofen the night before creates the same problem. The protective gut lining takes 24 hours or longer to recover, so pre-race ibuprofen for sore muscles damages your gut before the race even starts.
Why changing your diet likely won’t work to prevent runner’s trots
Changing your diet is unlikely to fix issues with having to go to the bathroom when you run because it’s not the culprit.
Eating less fats, fibers, and proteins or not eating 1-2 hours before your run doesn’t change the fact that…
- Running increases the rate at which food moves through the GI tract
- Your body will direct blood away from the stomach and increase the chances of leaky gut
- The up and down motion of running jostles the organs and irritates the stomach and
This has been confirmed in the medical literature, with studies concluding that “there’s no evidence to recommend gluten-free, low-FODMAP, lactose-free, or fiber/fat/protein restriction diets to runners who suffer from runners’ diarrhea” [1] [2]
Some runners who are eating very high protein or high fiber diets may find some relief by reducing these foods 8-12 hours before a run, but it’s not a guarantee.
But you also can’t completely eliminate these foods from your diet. You need fiber, protein and carbohydrates to fuel and recover from your training.
Instead of focusing on your diet, you should aim to address the actual issues causing your runner’s trots.
Psyllium Seed Husks Can Help you go to the bathroom before you start running
Starting your runs with an empty gut will help eliminate many of the digestive problems that result from reduced blood flow.
Psyllium seed husks are a source of soluble fiber. They also help add bulk to your stool and make bowel movements easier and more consistent.
In fact, 96% of runners who used psyllium husks the night before running experienced complete relief from runner’s trots.
Probiotics That Reduce GI issues in Runners
Probiotics are a combination of live beneficial bacteria that naturally live in your body. They help digest food and create a healthier environment in the gut.
But not all probiotics will help with the specific bathroom issues that plague runners.
Specifically, the strain bifidobacteria has been shown in research to significantly lower the incidence and severity of GI symptoms in marathon runners.
A 2019 study found that bifidobacteria supplementation reduced GI issues in marathon runners by 57%.
“Probiotics supplementation was associated with a lower incidence and severity of GI symptoms in marathon runners. Reducing GI symptoms during marathon running may help maintain running pace during the latter stages of racing.”
Jamie N. Pugh, Ph.D. — European Journal of Applied Physiology, 2019
Protect Your Stomach From Heat Stress
Blackcurrant extract is rich in anthocyanins and has been shown in research to improve the body’s ability to regulate its temperature and mitigate stress on the gut caused by running in the heat.
In one study, participants who supplemented with blackcurrant for 7 days and then raced in hot conditions experienced a 40% reduction in heat-induced cell damage and “leaky gut”.
In a separate study, blackcurrant was shown to reduce total gastric distress symptoms in runners by 73%.
| Cause | Why It Happens | Solution |
|---|---|---|
| Reduced Blood Flow | Running diverts up to 80% of blood away from the gut to working muscles, slowing digestion and accelerating intestinal transit. | Bifidobacteria strains in MAS Flush reduced GI symptoms in marathon runners by 50–57% by strengthening the gut lining. |
| Heat-Induced Leaky Gut | Core temps above 38.5°C (101°F) create microscopic holes in the gut lining, allowing endotoxins into the bloodstream and triggering inflammation. | Black Currant Extract in MAS Flush reduces heat-induced gut cell damage by 40% and total GI distress by 73%. |
| Undigested Food & Jostling | The up-and-down motion of running physically irritates a gut that hasn’t fully digested its last meal, compounding urgency. | Digestive enzymes in MAS Flush break down proteins, fats, and carbs more efficiently. Avoid solid meals within 2–3 hours of running. |
| Caffeine | Caffeine stimulates colon contractions at 60% the power of a full meal, accelerating transit before and during a run. | Eliminate pre-run coffee and caffeinated gels. Use MAS Flush psyllium husk the night before to empty the gut proactively. |
| NSAID Use (Ibuprofen) | NSAIDs block the enzymes that maintain the gut’s protective lining, weakening it before exercise begins. | Avoid ibuprofen for at least 24 hours before racing. MAS Flush probiotics help rebuild baseline gut lining strength over time. |
