3 Reasons You Have to Go to the Bathroom When You Start Running

513+ Happy Runners

3 Reasons You Get Runner's Trots (and the Simple, Science-backed Solution)

If you never start a run without toilet paper tucked into your shorts or you know where every porta potty is on all your running routes we dig into the science on why it happens and how to fix it for good

513+ Happy Runners

3 Reasons You Have to Go to the Bathroom Everytime You Start Running (And the Simple, Science-backed Solution)

The digestive system needs blood flow to function efficiently, but as soon as you start running your body diverts blood rom your digestive system to fuel your working muscles, heart, and lungs.

In fact, research has shown that…

Blood Flow Decreases

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With 10 minutes of starting to run

Further Decreases By

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After 1 hour of running

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.

The second major mechanism involves heat stress, and this is where things get really interesting from a scientific perspective.

As your body heats up, both from the running you’re doing and the outside temperature, blood is further diverted away from the digestive system and allocated to cooling itself.

Heat & humidity divert blood away from the stomach to cool your body

Your body generates heat the longer you run, further diverting blood

So, both increased duration and increased temperatures create a compounding effect on your digestive system.

Moreover, recent research has identified that when your core temperature reaches 38.5°C (101°F), intestinal permeability increases.

This often called “leaky gut” or, more scientifically, “exercise-induced gastrointestinal syndrome”, and is estimated to affect 70% of endurance athletes.

You may have heard the term “leaky gut” thrown around before, but never really knew what it meant.

Basically what happens is that stress in the gut (from heat, lack of blood flow, and jostling) causes intestinal damage and increases intestinal permeability.

In layman’s terms, this is like developing tiny holes in your gut lining that allow things to pass through that normally shouldn’t.

This includes bacterial lipopolysaccharides, also known as endotoxins.

When these endotoxins leak into your bloodstream, they trigger an acute-phase inflammatory response.

Specifically, the release of interleukin-6 in response to an increasingly endotoxic environment.

This inflammatory response is part of what causes the cramping, nausea, and urgent bathroom needs you experience during runs.

So, How Do You Prevent These Issues

Use psyllium seed husks so you always Start your runs with an empty gut

Starting your runs with an empty gut will help eliminate many of the digestive problems that result from reduced blood flow.

Since you’re reading this, I can probably assume you’ve tried changing your pre-run and even your night-before nutrition to eat foods that will be digested quickly and are easy on the stomach.

This is definitely a good step, but as you’ve likely noticed, it doesn’t always work.

Luckily, there is one relatively new ingredient we’ve seen make a huge difference: Psyllium seed husks.

Psyllium seed husks are a source of soluble fiber and have been shown in research to help make bowel movements easier and more consistent.

Taking psyllium in the evening helps ensure you can evacuate completely before your morning run.

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of runners reported immediate relief from runner’s trots

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Experienced less stomach pain during workouts and races

Add Probiotics & Digestive Enzymes to Improve Digestion

Bloating and digestive issues occur more often in endurance athletes because the fuel we need to train and recover often comes from difficult-to-digest foods such as plant fibers, complex carbohydrates, and proteins.

Probiotics are a combination of live beneficial bacteria and/or yeasts that produce enzymes that aid in breaking down difficult to digest foods.

This means your stomach is more efficient and does not need to work as hard to digest the foods you eat, dramatically reducing the stress placed on it while running.

But, not all probiotics are created equal. In the case of runners dealing with stomach issues, make sure your probiotic contains these:

Bifidobacteria, which has been shown in literature to reduce the severity of GI symptoms during training as well post-race in marathon runners.

Digestive enzymes (specifically Protease, Amylase, Lipase, Cellulase) which are the enzymes that help break down and digest the foods you need to fuel your training

Helps produce enzymes that breakdown difficult to digest foods

Reduces the stress placed on the stomach so there is less chance of GI distress

Protect Your Gut From Heat-Related GI Issues

Outside temperatures aren’t the only time your stomach is susceptible to heat-related stress. It also happens as our internal body temperature rises with increased effort.

Luckily, there is an innovative natural ingredient that can mitigate this: New Zealand blackcurrant.

New Zealand blackcurrant extract is rich in anthocyanins and has been shown in research to strengthen the gut lining, mitigating heat stress and improving thermoregulation.

In one study, participants who supplemented with New Zealand 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%.

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reduction in leaky gut when racing in hot conditions

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total reduction in GI distress symptoms in runners

Increase Absorption of Gels When Racing to Avoid Upset Stomach

Recent research has demonstrated that specific probiotic strains (bifidobacteria) can increase glycogen utilization by up to 15%.

The studies show an increase mean total carbohydrate oxidation from 1.87 grams per minute to 2.20 grams per minute during 2-hour endurance sessions.

This enhanced utilization occurs because this combination of probiotics increases both the abundance and activity of sodium-glucose transporter 1 (SGLT1), a protein that actively transports glucose into cells

The data has also demonstrated that the enhanced carbohydrate absorption allows the body to rely more heavily on ingested fuel rather than depleting precious muscle glycogen stores.

Increases glycogen utilization by up to 15%

Prioritizes ingested carbs to help spare muscle glycogen

Ready for an all-in-one solution?

Introducing mas flush

The first supplement created specifically for runners to stop bathroom issues before they start

Clinically effective ingredients scientifically shown to improve gut health and reduce gastric distress to ensure you never need to carry toilet paper again.

Makes bowel movements more consistent so you never start your runs needing to go
Practically eliminates gastric distress during workouts and races
Significantly improves digestion of the foods you need to fuel your training
Reduces ‘leaky gut’ by up to 40%

Hundreds Have Experienced Real Relief, Peace of Mind and Results

I honestly can’t believe this product worked so well. Before using flush, I’d be scrambling back home to go to the bathroom at least 3 days per week. It made me scared to go to group runs or even try out new trails for fear of not having a bathroom nearby. I’ve been using for 3 weeks now and not once have I had to go to the bathroom once I started running. This is such a relief!

I tried this product during the beta launch because I really struggled with stomach pain when racing and even after hard workouts. There were times after hard workouts that it would take hours for my stomach to feel normal. MAS Flush changed everything. I’ve been using it for a month and I haven’t had any stomach pain after workouts. I’m so glad I found this.

I’ve tried countless gut products, but this is the one that actually worked. For the first time in years, my digestion feels normal. No more discomfort, bathroom urgency, or unpredictable bloating. It’s been such a relief. This formula actually works.

I honestly thought needing to go to the bathroom within the first few miles of a run were just something all runners needed to deal with. I was given MAS Flush as a gift during a running retreat and I was definitely skeptical, but I didn’t need to go to the bathroom during the 3 runs we did at the retreat, which is unusual, so I kept using. It’s been working every day for the last 3 weeks. I can’t believe I didn’t look into something like this sooner.

I honestly thought needing to go to the bathroom within the first few miles of a run were just something all runners needed to deal with. I was given MAS Flush as a gift during a running retreat and I was definitely skeptical but I didn’t need to go to the bathroom during the 3 runs we did at the retreat, which is unusual, so I kept using. It’s been working every day for the last 3 weeks. I can’t believe I didn’t look into something like this sooner.

I’ve tried countless gut products, but this is the one that actually worked. For the first time in years, my digestion feels normal. No more discomfort, bathroom urgency, or unpredictable bloating. It's been such a relief—this formula actually works. I love how gentle it is but still powerful enough to make a real difference day-to-day.

Non GMO

Clinically Tested

Vegan

Research-Backed

Non GMO

Clinically Tested

Vegan

Research-Backed

Non GMO

Clinically Tested

Vegan

Research-Backed

Non GMO

Clinically Tested

Vegan

Research-Backed

Non GMO

Clinically Tested

Vegan

Research-Backed

Non GMO

Clinically Tested

Vegan

Research-Backed

Non GMO

Clinically Tested

Vegan

Research-Backed

Non GMO

Clinically Tested

Vegan

Research-Backed

Revolutionary Ingredients Backed by Science

MAS Flush is more than just a probiotic. We’ve carefully chosen the specific strains of bacteria and yeasts proven to help the most for athletes. Then we combined them with cutting-edge ingredients that help improve all the unique aspects of gut health runners and endurance athletes face.

Psyllium Husk Powder [Plantago ovata] (1500mg)

Psyllium, derived from the plants known as ispaghula, is a primarily soluble, bulk-forming fiber supplement shown in research to help improve digestive and metabolic health.

Compared to similar fibers, psyllium is less fermentable in the gut, which means it’s less likely to produce flatulence, stomach pain, and nausea.

Research has demonstrated that psyllium makes bowel movements easier and more consistent which can help alleviate the issue of runner’s trots by ensuring you’re able to go to the bathroom before your run¹.

Psyllium has also been shown in research can help add bulk to stool, which helps relieve the feeling of needing to go to the bathroom. This is especially helpful due to the jostling that occurs in the digestive tract during running.

1. Jalanka J, Major G, Murray K, Singh G, Nowak A, Kurtz C, Silos-Santiago I, Johnston JM, de Vos WM, Spiller R. The Effect of Psyllium Husk on Intestinal Microbiota in Constipated Patients and Healthy Controls. Int J Mol Sci. 2019 Jan 20;20(2):433. doi: 10.3390/ijms20020433. PMID: 30669509; PMCID: PMC6358997.

Bifidobacterium Longum Powder [100B CFU/G] (100mg)

Bifidobacteria are a type of probiotics, which are healthy bacteria found in your intestines that help digest fiber, prevent infections, and produce important compounds, such as B vitamins and healthy fatty acids.

One of the main functions of bifidobacteria is to digest fiber and other complex carbs your body can’t digest on its own, which can lead to bathroom issues.

Specifically, bifidobacteria supplementation has been shown in research to significantly lower the incidence and severity of GI symptoms in marathon runners¹.

Research has also shown that runners who supplemented with bifidobacteria saw a reduction in the severity of GI symptoms during training² as well as a reduction in the duration of GI symptoms post-race³.

1. Kekkonen RA, Vasankari TJ, Vuorimaa T, Haahtela T, Julkunen I, Korpela R. The effect of probiotics on respiratory infections and gastrointestinal symptoms during training in marathon runners. Int J Sport Nutr Exerc Metab. 2007 Aug;17(4):352-63. doi: 10.1123/ijsnem.17.4.352. PMID: 17962710.

2. Roberts JD, Suckling CA, Peedle GY, Murphy JA, Dawkins TG, Roberts MG. An Exploratory Investigation of Endotoxin Levels in Novice Long Distance Triathletes, and the Effects of a Multi-Strain Probiotic/Prebiotic, Antioxidant Intervention. Nutrients. 2016 Nov 17;8(11):733. doi: 10.3390/nu8110733. PMID: 27869661; PMCID: PMC5133117.

3. Pugh JN, Sparks AS, Doran DA, Fleming SC, Langan-Evans C, Kirk B, Fearn R, Morton JP, Close GL. Four weeks of probiotic supplementation reduces GI symptoms during a marathon race. Eur J Appl Physiol. 2019 Jul;119(7):1491-1501. doi: 10.1007/s00421-019-04136-3. Epub 2019 Apr 13. PMID: 30982100; PMCID: PMC6570661.

Black Currant Extract 4:1 [Ribes nigrum] [Berry] (240mg)

One of the most common reasons runners experience bathroom issues later into runs and races is due to internal and exertional heat stress. This is because blood is redistributed away from the digestive system to help with thermal regulation.

As a result, lack of blood supply stresses the gut, which results in bathroom and stomach issues.

Recent research has shown that New Zealand blackcurrant…

Reduces total gastric distress symptoms by 73% in runners¹.
Reduces ‘leaky gut’ by up to 40%²

1. Willems MET, Flood TR, Kuennen MR, Lee BJ. Anthocyanin-rich New Zealand blackcurrant extract reduces treadmill running-induced gastrointestinal symptoms in the heat: pilot observations. Res Sports Med. 2025 Jul-Aug;33(4):440-447. doi: 10.1080/15438627.2025.2471380. Epub 2025 Feb 25. PMID: 39996414.

2. Lee BJ, Flood TR, Hiles AM, Walker EF, Wheeler LEV, Ashdown KM, Willems MET, Costello R, Greisler LD, Romano PA, Hill GW, Kuennen MR. Anthocyanin-Rich Blackcurrant Extract Preserves Gastrointestinal Barrier Permeability and Reduces Enterocyte Damage but Has No Effect on Microbial Translocation and Inflammation After Exertional Heat Stress. Int J Sport Nutr Exerc Metab. 2022 Mar 14;32(4):265-274. doi: 10.1123/ijsnem.2021-0330. PMID: 35287112.

Endurance-Specific Digestive Enzymes

Fueling your training properly means you have to digest and absorb the foods you eat, but this can be difficult for a number of reasons.

First, the foods that fuel our training are often difficult to digest, such as plant fibers, starches, and proteins.

Second, after hard training blood is still being shifted away from the stomach to aid in muscle recovery, which leaves fewer enzymes available to break down your meals. This is why you’re often not hungry for hours after hard workouts.

Our enzyme blend is specifically designed to target the foods endurance athletes need the most and are most difficult to break down.

Protease 500,000 HUT/g – helps break down proteins into absorbable amino acids

Amylase 100,000 DU/g – splits starches into usable glucose

Lipase 15,000 FIP/g – breaks fats into fatty acids for sustained energy

Cellulase 150,000 CU/g – shreds tough plant fibers your body can’t process alone.

The result is faster, more complete digestion that delivers more building blocks to recovering muscles while keeping your gut calm and race-ready.

Lactobacillus Acidophilus [10B CFUs]

Hard running diverts blood away from your gut, loosening the tight seals between intestinal cells and letting bacteria leak into your bloodstream.

Your immune system responds with inflammation, which triggers cramping, bloating, and nausea.

Lactobacillus acidophilus reinforces those cellular seals before they fail.

This specific probiotic strain crowds out harmful bacteria, tightens the gaps in your intestinal lining, and quiets the inflammatory response.

Research shows that supplementing daily for at least four weeks reduces GI symptom severity during long efforts and cuts down on emergency bathroom stops mid-race.

Bifidobacterium animalis [50B CFUs]

Hard training temporarily weakens your gut lining, allowing inflammatory compounds to leak into your bloodstream and suppress your immune system.

This is why runners tend to catch the cold and flu more when training hard.

Bifidobacterium animalis helps seal these gaps and strengthen the gut lining to prevent these inflammatory compounds from entering the blood stream.

It also helps cultivate and prime virus fighting cells so your body is more capable of fending off the pathogens that cause getting sick.

Consistent supplementation helps you spend less time recovering from illness and more time building fitness.

MAS Flush Vs. Generic Probiotics

While almost any probiotic will help improve your gut health, you have unique challenges and needs as a runner that generic probiotics just don’t address.


Helps alleviate needing to go to the bathroom as soon as you start running
Formulated to target endotoxins to eliminate stomach pain during races
Designed to reduce heat stress, a major cause of GI distress when racing and working out hard
Specific ingredients to improve carbohydrate absorption and metabolism while running
Advanced digestive enzymes to naturally digest fiber, complex carbs, and other difficult to digest foods

Ready to Never Let Stomach and Bathroom Issues Ruin Your Runs Again?

Why You’ll Love It

As we the latest scientific research on gut health continues to evolve, the more we understand just how important a healthy gut it is, not just to our overall health, but our athletic performance.

The problem is that most probiotics don’t address the specific needs of endurance athletes.

For example, if you…

Consistently struggle with needing to use the bathroom soon as you start running or within the first few miles
Suffer from stomach churning, pain and cramps that cause you to slow down when racing
Have trouble taking or absorbing gels and other carbohydrates when racing and either getting an upset stomach or bonking all together
Have trouble taking or absorbing gels and other carbohydrates when racing and either getting an upset stomach or bonking all together
Feel so nauseas after hard workouts or training in the heat that you can’t eat for hours or even the rest of the day

Then MAS Flush is the probiotic specifically designed for you.

Not only does it help you improve your gut health and create a health environment for better digestion like most probiotics do, it also helps with the unique stomach issues you face as a runner.

MAS Flush is more than just a probiotic. It’s the endurance athlete’s probiotic.

Ingredients

Psyllium Husk Powder [Plantago ovata] (1500mg)

Research has demonstrated that psyllium makes bowel movements easier and more consistent and can help add bulk to stool.

This helps ensure you never have to go to the bathroom as soon as you start running ever again.

Bifidobacterium Longum Powder ]100B CFU/G] (100mg)

Helps digest fiber and other complex carbs your body can’t digest on its own

Research has also shown that runners saw a reduction in the severity of GI symptoms during training as well as a reduction in the duration of GI symptoms post-race.