It’s finally time to experience training & racing without stomach issues
The first supplement created specifically for runners to stop stomach, bathroom & nausea issues before they start
Whether it be stomach churning during workouts and races, nausea from gels and fluids, or needing to go to the bathroom during runs, MAS Flush is the first probiotic supplement created specifically for the unique needs of runners
It’s finally time to experience training & racing without stomach issues.
Clinically Tested
Vegan
Research-Backed
Non GMOClinically TestedVeganResearch-BackedNon GMOClinically TestedVeganResearch-Backed
Olivia H.
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!
Teddy Adler
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.
Emma M.
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.
Marko V.
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.
Every runner fears starting a run without nearby bathrooms.
Frequent needs to go to the bathroom early in a run are usually caused by not being able to quickly digest the foods you ate for dinner or when your digestive system isn’t able to keep up and you have inconsistent bathroom times.
MAS Flush contains Psyllium seed husks, which when taken in the evening helps make bowel movements easier and more consistent.
It can also 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.
Taking just 4 MAS Flush capsules after dinner will make it easy to go to the bathroom as soon as you wake up so you can virtually eliminate needing to go to the bathroom during your runs.
Put an end to stomach pain and cramping during and after hard workouts that lead to you slowing down and not getting the most from your performance.
Stomach issues during races and hard workouts often happen for two reasons.
First, endotoxins (bacterial toxins) are released during strenuous, long-duration exercise which lead to cytokine release, oxidative stress, and altered gastrointestinal function.
Second, internal and exertional heat stress causes blood to be 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.
MAS Flush’s combination of Bifidobacterium Longum and Blackcurrant have been shown in research to
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.
Usually, this is caused by not producing enough of the enzymes that help break down these foods.
Our endurance-focused digestive enzyme blend is specifically designed to help your body naturally digest the foods you need to fuel your training while improving your gut microbiota to create a more favorable environment in your stomach.
The enzymes in MAS Flush have been shown in research to…
One of the most difficult things about racing half, full and ultra marathons is taking in enough fuel to keep your glycogen levels topped off without causing stomach issues.
All that simple sugar is difficult for your body to process quickly, which is a main contributor to why many runners suffer to fuel properly during races.
The included Bifidobacterium in MAS flush has been shown in research to improve carbohydrate absorption and metabolism during endurance races, potentially sparing muscle glycogen during prolonged exercise.
Additionally, Bifidobacterium may help maintain gut barrier integrity, reduce gastrointestinal distress, and improve nutrient absorption.
There’s a reason we often refer to hard workouts as “gut-busting”. That’s because they often leave us with stomach discomfort after the workout.
This happens because after a hard training run, blood is still being shifted away from the stomach to aid in muscle recovery.
This leaves fewer enzymes available to break down your meals.
It’s also why you’re often not hungry for hours after hard workouts.
The digestive enzymes in MAS Flush have been shown in research to reduce post-workout stomach discomfort so you can jump start your recovery process with the nutrients your muscles need.
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.
The probiotics in MAS Flush help 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 has been shown in research to help you spend less time recovering from illness and more time building fitness.
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…
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 EnzymesFueling 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.
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.
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| 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 |
Experience the benefits of a healthy gut, better digestion and no more bathroom fears
MAS Flush works best if you take it every day.
In addition to alleviating bathroom issues before and during running, the probiotics in MAS Flush help you more efficiently digest foods, absorb nutrients, and create a favorable environment for the growth of “good microflora” in the large intestine.
That said, if you only wanted to use on the days you run for the improvement of bowel movements, it will still work great.
Like almost any product, you should definitely use MAS Flush a few times during your training cycle before using it during a race.
The most significant results and changes often occur during the first few uses as your gut and digestive system experience ingredients for the first time.
However, we did specifically formulate the product to help with many of the digestive challenges we face during the race so it should be a significant aid during your most important performances.
Yes, MAS Flush is safe to take with any other probiotic or gut health product you may be taking.
The current medical literature has found that it’s difficult to overdose on probiotics or experience dangerous side effects from a large dose of probiotics.
You should not notice any side effects when taking MAS Flush.
That said, the most significant results and changes often occur during the first few uses as your gut and digestive system experience ingredients for the first time.
This means your bowel movements the first evening and the morning after you’re your first few doses may be more noticeable than they will be as you continue to use the product.
MAS Flush should be taken in the evening, ideally after your last meal.
That’s because one of the main ingredients, psyllium husks, takes at least a few hours take effect in the bowels.
This means that when you take it in the evening, it should make it easier to go to the bathroom as soon as you wake up. In addition, it helps improve bowel movements so you evacuate more during this first bathroom visit, making sure you don’t need to go as you start your run or during.
Probiotics and the ingredients in MAS Flush are generally considered safe, but if you have any concerns you should definitely contact your healthcare professional first.
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…
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.
Research has demonstrated that psyllium makes bowel movements easier and more consistent and can help add bulk to stool.
Helps digest fiber and other complex carbs your body can’t digest on its own
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.
Has been shown in research to effectively protect and strengthen the gut lining.
Running diverts blood away from your gut, loosening the tight seals between intestinal cells and letting bacteria leak into your bloodstream.
Hard training temporarily weakens your gut lining, allowing inflammatory compounds to leak into your bloodstream and suppress your immune system.
The probiotic bacteria in ProbioSEB Duo create a favorable environment for the growth of “good microflora” in the large intestine, supporting normal functioning such as absorption of water and nutrients and passage of waste products.
Take 4 pills a few hours before bed or after your last meal.
We suggest taking in the evening because one of the main ingredients, psyllium husks, takes at least a few hours take effect in the bowels.
This means that when you take it in the evening, it should make it easier to go to the bathroom as soon as you wake up.
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84 doctors and dietitians recommend Flush to improve gut health without compensation.
19 Customer(s) recommended this item
I began taking this before my long runs. I have always had a sensitive stomach and particular trouble on intense efforts like speed sessions and long runs. Specifically, I have always had trouble on efforts of two hours or more with my stomach. I found that a lower dosage of two capsules worked better for me than the full four capsules. The support team at MAS was very helpful in finding that right balance. I definitely see a difference when I take it before my long runs versus when I do not. I also found it very helpful in my most recent marathon. There were no porta potty emergencies.
As a long distance runner, I experience unscheduled biological emergencies mid run and was hopeful MAS Flush would help eliminate this condition. So far, it seems to be helping, but I am still occasionally experiencing the condition. It’s only been a week so perhaps I need more time for MAS Flush to fully take effect.
I was just about to call it quits on marathon training. I kept seeing MAS Flush ads pop up and thought it was just another too-good-to-be-true supplement. But I decided I had nothing to lose. Before even the second week I began to notice things changing. The frequency, and urgency, of bathroom stops was decreasing. My only regret is that I didn’t purchase it sooner!
This is a game-changer for me! Urgent, unplanned trips to the PortaPotty or woods had become my “new normal” once I started increasing my volume for marathon training. Every long run. Every intense tempo season. And, yes, my last BQ effort that ended at the mile 19 toilet. I was skeptical about the reviews for MAS Flush, but figured I had nothing to lose. Within 2 weeks, I could run right past the PottaPotty on my long runs and my everyday habits seemed to be stabilizing. But the real test came last weekend at 5 weeks into taking the supplements. BQ attempt number 2 and I nailed that it! NO GI distress. NO frequent nervous bathroom lines prior to the start. And NO detour to the many toilets along the course. After two years of experimenting with pre-run diet, mid-race fueling and every powder or pill I could find, this actually did the trick. I absolutely would recommend this if you struggle like me. Believe me, I am a customer for life!
For 20 years, I planned every run around where I would have a bathroom. For group runs, I would arrive 20 minutes early to be sure I could do the group run without stopping. If, on a run, I couldn’t find a bathroom within 1 – 1.5 miles from the start, I would have “an issue.”Terrible. Embarrassing and uncomfortable.Then, I started using MAS Flush. After about 3 days of using the supplement, I would wake up, use the bathroom and the runs would go off with no interruption. No problem.OMG. This was a game changer for me. Now, I take the supplement at night and in the morning, I have no angst about intestinal issues. THANK YOU. 5 STARS.
I’m a 74 year old runner. I noticed that over the last several months every run brought about some horrible feelings with my digestive system with unfortunate results.I realized my stomach issues occurred after running about three miles. I read about MAS Flush used it and have had amazing results. This product has had some positive effects on my recovery period.I have used several different brands but never had the results that this one has had. As a result I had very little confidence in any product. Now I do trust in this company and product.
I was a little skeptical because there’s so much hype in advertising and it can be hard to trust that anecdotal endorsements are real. But I was desperate so decided to try it. I had tried Probiotics and soluable fiber before but they did not improve running GI events. Taking a few capsules at bedtime seems to be really effective in preventing my emergency GI issues while running. I’m pretty excited about that! Planning run routes around bathroom availability, stopping in corn fields or phoning for emergency pickup has been my run training history for years. Almost quit the sport. I’ve only been using it for a short time but so far so good!
Honestly, I wasn’t expecting much. The first few days I think I may have even felt worse. Then one morning it felt like everything kicked in and I had more energy, my bathroom visits have been surprisingly regular, and I feel much better after my runs. I didn’t even realize having an queasy stomach wasn’t normal after long runs. Thank you so much!
The biggest surprise for me was how much calmer my stomach feels. I used to deal with post-run stomach pain, almost to the point where I couldn’t eat for hours after a hard run. Now everything feels much more stable and predictable. That alone makes it a win.
My morning bathroom routine is back. No more guessing games, no more discomfort—just relief daily.
When I run, I normally, after 10-15 minutes, have to make a pit stop. When I do long runs I, many times, have to stop 2-3 times. Since starting to use, MAS Flush, I still schedule in the stop after 10-15 minutes but then I am fine: 100%. About half the time, I don’t have to stop because I was able to empty out before the run.
Just wanted to check in and let you know that it’s been 1 month since I’ve started taking MAS FLUSH and during the 18 runs which I’ve done since, I have not had the urge of a bowel movement while running !Keep in mind that this is coming from someone who, in my 9 years of running, I don’t think that I’ve ever gone 3 (maybe 4) consecutive runs without having to stop to poop.Truly, a game changer !!As I mentioned before, I had kind of resigned myself to living with this unfortunate & uncomfortable condition and I really can’t believe that I have found a solution !A huge thanks to you & MAS FLUSH !
This supplement transformed my running! I know that sounds over the top but it really did. Without fail my stomach would always start churning late in races and slow me down and I almost always had bathroom issues after really hard workouts. It took a week or two to really kick in but all those are virtually gone now. I’ve only raced once but I never got that stomach churning feeling and I was right on my PR despite being in marathon training. Thank you so much, this is awesome!
I must say that I was skeptical at first. The first day or two I didn’t feel much. However, I continued to use this product and felt less indigestion with a feeling of being less bloated as well. I recommend flush to those who have a sensitive stomach like mine and looking to settle their stomach during hard workouts. 5 stars!
For an unknown reason, for the last months I needed bathroom stops too often during my early morning runs. This was a new thing and I could not explain it as nothing changed as far as I know. I came across MAS Flush through a running related newsletter and decided to give it a go as nothing else I tried had helped. I was so surprised and impressed that my issues faded after just a few days of taking MAS Flush! 95% of the time I don’t have emergency stops now. I just need to make sure to give myself about an hour as it causes 2-3 bowel movements.
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 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 love how regular this makes my morning bathroom routine. I used to fear the mornings I’d wake up and not need the bathroom since I knew it was going to happen on the run. Since starting Flush I am so much more regular in the morning and it’s such a relief
I struggled for years with bloating and sluggish digestion. After 3 weeks on this product, my digestion is smooth, and I feel lighter every day! It’s like hitting the reset button on my gut health. I’m hooked!
The short answers, with the research behind them.
Running pulls blood away from your gut to your working muscles, and the bouncing speeds food through your intestines. Heat makes it worse: in a 2025 study, 11 of 12 men reported stomach symptoms during an hour of running at 34°C (93°F).
Give it 3 to 4 weeks of daily use. In a 2019 study of marathon runners taking a daily probiotic, stomach symptoms dropped in weeks 3 and 4, and runners on a placebo saw no drop.
In the same study, runners on the probiotic slowed down 7.9% over the last third of the marathon. Runners on the placebo slowed down 14.2%. That means runners on the probiotic lost about half as much pace late in the race, although finish times were about the same.
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If you order any of our supplements for the first time and don’t like it, you can keep it. Just notify us and we’ll give you a full refund. No complicated forms and no return necessary.
To protect ourselves from fraud, the It’s-On-Us guarantee is only valid for first-time purchases of a product, and redeemable up to 365 days after purchase. If you bought more than one supplement and weren’t happy with your purchase, in some instances, we may ask that you send the products back to us for a refund. Don’t worry though, you’ll never have to pay for return shipping.
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