Many runners who start taking a new probiotic find themselves feeling more gassy, bloated and experiencing bathroom issues than they did before they started.
This seems counter-intuitive since probiotics are designed to alleviate these issues.
So, why does this happen and will it eventually get better?
That’s the research we’re going to dig into in this article, including:
- What a probiotic changes in your gut during the first 2 weeks
- Why those changes hit runners harder
- And how to time the start so those 2 weeks happen during easy training, well before race week.
What Changes in Your Gut During the First 2 Weeks?
A probiotic works by adding new bacteria to your gut that eventually helps eliminate harmful bacteria and improves the existing microbiome.
However, fighting this harmful bacteria causes side effects. In addition, the new bacteria start breaking down some of the fiber and carbohydrate you eat.
This is what leads to the initial changes you might experience, including…
1. The Balance of Bacteria in Your Gut Shifts
Probiotic bacteria attach to your gut lining, which leaves less room for harmful bacteria to attach.
They also make lactic acid and bacteriocins (natural antibacterial compounds) that slow the growth of harmful bacteria and change the balance of the bacteria around them.
As that balance shifts, so do the acids and other compounds your gut bacteria produce.
That shift is part of why people taking any probiotic, not just runners, can have more stomach trouble in the first couple of weeks.
2. More Gas and Bloating
When probiotic bacteria break down fiber, they make acids called lactate and acetate.
Other bacteria already living in your colon feed on those acids and turn them into butyrate.
Lab studies found this process always produces gas as well.
That means a new probiotic can leave you gassier or more bloated than usual at first.
3. Needing the Bathroom Sooner and More Often
Acetate and butyrate also speed up how fast food moves through your colon.
In animal studies, these acids trigger cells in the gut lining to release serotonin, which signals the muscles in your colon wall to contract and push food along.
Researchers have seen the same speed-up in people taking one of the most common probiotic species.
A 2022 review measured what the probiotic species Bifidobacterium animalis does to bathroom habits in healthy adults.
Bifidobacterium animalis increased how often people had a bowel movement, and in trials of 14 days or less it shortened the time food took to move through the colon.
That means in the first 2 weeks you may need the bathroom sooner and more often than you’re used to, including 3 miles (5 km) into a morning run.
Why Does the First Week on a Probiotic Hit Runners Harder?
Most regular people don’t face the same challenges to their stomach that runners do.
For example, when you run hard or long, blood moves away from your gut to your working muscles. In fact, a 2012 review found that blood flow to your gut can drop by 80% or more.
Without this blood flow, your gut doesn’t have the oxygen and nutrients it needs to remove waste products.
This means a runner is more likely to experience changes to their gut microbiome since their stomach is working harder than a sedentary person.
Running also adds additional challenges to the stomach most other sports don’t face.
When running, every stride jostles your gut, which means many of the ways a probiotic is working to clean up your gut and make it healthier also make it more sensitive to the jostling and other demands running places on your stomach.
Runners already have more frequent, looser and more urgent bowel movements than people who don’t run, and a 1994 survey found the urgent need to go was the most common reason runners stopped during a run.
In the first 2 weeks, a probiotic adds extra gas and moves food through your colon faster, on top of the same pressures that cause runner’s trots.
So plan your start so those 2 weeks fall on easy runs close to home.
On top of that, most probiotics aren’t designed with any of this in mind.
They’re formulated for general digestive health, but the benefits of a probiotic depend on the specific strain and dose.
A strain that helps someone who sits at a desk all day may never have been tested in a gut that loses 80% of its blood flow during a long run.
That’s why the strains in your probiotic matter as much as when you start it.
Will Your Stomach Get Better?
The stomach trouble from a new probiotic usually eases after the first 2 weeks.
A 2019 study found that marathon runners who took a probiotic for 28 days before their race had fewer moderate stomach symptoms in weeks 3 and 4 than in weeks 1 and 2.
The runners taking the placebo saw no change.
If your race is less than 4 weeks away, here is how long a probiotic takes to work before a race.
Researchers see the same thing when they look across the full length of a trial.
A 2016 analysis compared the side effects reported by people taking a probiotic with people taking a placebo across 6 trials.
Across 1,909 people, those taking a probiotic reported no more stomach or digestive side effects than those taking a placebo.
Even better, the same changes that cause trouble in the first 2 weeks are what make your gut work better once it adjusts.
A Healthier Balance of Gut Bacteria
As the probiotic bacteria crowd out harmful bacteria, the symptoms from the first 2 weeks fade.
In a 2023 study, healthy adults who took a probiotic for 12 weeks reported bloating, stomach rumbling and stomach pain less often than people taking a placebo, and the mix of bacteria in their gut changed too.
A Stronger Gut Lining
Butyrate, the compound your gut bacteria make from the probiotic’s acids, helps the cells of your gut lining close the gaps between them.
That matters for runners, because those gaps are how endotoxins get into your blood when blood flow to your gut drops on a long run.
In a 2012 study, trained men who took a probiotic for 14 weeks had lower levels of zonulin, a marker of a leaky gut lining, than men taking a placebo, and their levels dropped back into the normal range.
That means their gut lining was more tightly sealed than the placebo group’s, which is what keeps endotoxins out of your blood.
Absorbing More of What You Eat
A healthier gut also gets more out of the food and fuel you give it.
In a 2020 study, trained cyclists who took a probiotic for 4 weeks used the carbohydrate from their sports drink up to 18% faster during a 2-hour ride.
That means more of each gel becomes fuel for your muscles instead of sitting in your stomach.
The benefit reaches everyday meals too, and in a 2017 study young women taking Lactobacillus plantarum 299v absorbed 22.4% of the iron in a meal instead of 17.4%.
More Regular Bathroom Trips
In the same 2023 study, people taking a probiotic also had fewer slow bowel movements and fewer that felt incomplete than people taking a placebo.
For a runner, a more regular schedule means you can go before your run instead of during it.
That means the first 2 weeks are your gut adjusting to the changes that make it stronger, so give a probiotic at least 4 weeks before you decide whether it’s working.
How to Start a Probiotic Without Upsetting Your Stomach
You can’t know in advance how your gut will respond in those first 2 weeks, so the goal is to choose when they happen and keep everything else the same while they do.
- Start at least 4 weeks before a goal race. The marathon runners in that 2019 study took their probiotic for 28 days before race day, and their stomach symptoms settled after the second week, so a 4-week lead puts the adjustment well behind you.
- Start in an easy week. Begin on a week with no long run over 90 minutes and no race-pace workout, so a sooner-than-usual bathroom urge means a detour instead of a missed session.
- Take it in the evening, after dinner. Swallow it at 8 pm and you have about 11 hours before a 7 am run, so if it leaves you gassy, that happens on the couch instead of on the road.
- Keep the rest of your diet steady and don’t start a new gel, a new fiber supplement or a new pre-run breakfast in the same 2 weeks, because you won’t be able to tell which change caused the gas.
- Drink a full glass of water with it, about 8 oz (240 ml), because fiber such as psyllium needs water to form the gel that firms up your stool.
- Give it 3 weeks before you judge it. Symptoms that are still getting worse after 3 weeks, or that are severe at any point, are a reason to stop and talk to your doctor.
If that list is more than you want to manage, the simplest version is to start on the Monday of an easy week, at least 4 weeks before your next race.
What Should a Runner’s Probiotic Include?
Getting through the first 2 weeks only pays off if the probiotic is built for the stress running puts on your gut.
A 2019 position stand from the International Society of Sports Nutrition concluded that the benefits of a probiotic depend on the specific strain and dose.
A runner’s gut faces unique challenges due to the stresses of training that normal probiotics don’t address.
That’s why we formulated MAS Flush around probiotic species tested in marathon runners, plus the ingredients a probiotic alone can’t replace.
2 of its 3 probiotic species, Lactobacillus acidophilus and Bifidobacterium animalis, were part of the formula that gave marathon runners 57% fewer stomach symptoms than a placebo in the last third of the race in that 2019 study.
That means those species have been tested in the part of a race where stomach trouble usually hits.
We added 1,500 mg of psyllium husk to firm up loose stools before you run, and digestive enzymes to help you digest the foods you need to support your training.
The 600 mg of blackcurrant extract is there for hot races, because it cut heat-related damage to the gut lining by 40% in runners.
The Probiotic Formulated Specifically for Runners
GET MAS FLUSH
-
Less stomach distress during and after workouts and races. -
Process gels faster because it increases how much carbohydrate you burn when racing. -
Better digestion of the foods you need to fuel your training. -
40% less damage to your gut lining when training and racing in hot weather.
You take 4 capsules every evening, which already matches the evening timing in the list above.
Your First 4 Weeks on a Probiotic
Here’s what to expect in each part of your first month on a probiotic, and what to do about it.
| When | What to expect | What to do |
|---|---|---|
| 4+ weeks before your race | Nothing yet. This is your start date. | Begin on the Monday of an easy training week with no long run or workout. |
| Weeks 1 and 2 on a probiotic | Bowel movements can come sooner and more often, sometimes with extra gas. | Take it in the evening, keep your diet and fueling unchanged, and run routes near a bathroom. |
| Weeks 3 and 4 on a probiotic | Stomach symptoms settle, as they did for marathon runners in a 2019 study. | Return to long runs and workouts, and rehearse your race-day fueling. |
| Race week | Your gut has had 4 weeks to adjust. | Keep taking it at the same time each evening and change nothing. |
