1. MAS Flush
MAS Flush earns the top spot because it's built around probiotic species that were tested in marathon runners. Plus, it adds the 3 things a probiotic alone can't do by itself: help prevent runner's trots, digest the food you train on, and protect your gut lining in the heat.
This is the probiotic built to keep a runner's gut healthy.
Most probiotics are formulated for everyday bloating and regularity. But, running introduces challenges to the gut that most general probiotics don't address.
For example, on long or hard runs, 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 is what causes runner's trots and stomach issues late in races.
But, specific probiotic strains have been proven in research to help protect your stomach from these issues. And, it's the inclusion of these specific strains that MAS Flush is ranked first.
A 2019 study found that runners who took the probiotic strains found in MAS Flush (Lactobacillus acidophilus and Bifidobacterium) had 57% fewer GI symptoms in the final third of the marathon than runners who took a placebo.
It showed up in their pace, too. Studies show that these strains slowed the build-up of creatine kinase, inflammation and oxidative stress and increased clearance of lactate
The end result is that the runners who used these probiotic strains slowed down 44% less than the group that used a placebo.
Further research found that runners on the probiotic absorbed 18% more of the carbohydrates they took in during a run, which is the difference between a gel that works and a gel that sits in your stomach.
Then MAS Flush adds 3 additional ingredients to help tackle the unique challenges a runner's stomach faces that a probiotic can't address.
Psyllium husk, which helps add bulk to stool to relieve the feeling of needing to go to the bathroom when you start running.
In MAS's own survey of its customers, 96% of runners reported immediate relief from runner's trots and 74% had less stomach pain.
It also adds blackcurrant extract, which protects the gut lining when your core temperature climbs on a hot run. In a 2025 study, runners who took blackcurrant had 73% fewer GI distress symptoms.
Plus, 4 digestive enzymes (amylase, protease, lipase and cellulase) to help break down the carbohydrate, protein, fat and fiber you eat around training, when less blood reaches your gut and digestion slows.
At $39.95 for 30 days it costs more than the drugstore options and less than Seed or Nella. The 3-bottle bundle brings it to $33.95 a bottle, which is $1.13 a day.
What it does well
- Carries the specific species that cut late-race stomach symptoms and improved performance in marathon runners, at 70 billion CFU a day
- Psyllium so you start runs with an empty gut: 96% of runners in MAS's survey reported immediate relief from runner's trots
- Blackcurrant for the gut lining in heat, and 4 digestive enzymes for the food you train on
- Same 4 capsules every evening on training days and race day, with no separate race protocol
Where it falls short
- 4 capsules a day, more than any other product here
- $39.95 a bottle, more than the drugstore options
- Needs 4 weeks of daily use before race day. It isn't a same-day fix
- Not formulated for general gut or other stomach conditions
