Why the same probiotic works for some people and not others

Two friends start the same probiotic on the same Monday. By week six, one says her afternoon bloating has noticeably eased. The other can't tell any difference and wonders what she's doing wrong.

Probably nothing. Researchers have identified several reasons one probiotic can produce very different results in different people. Here's what the studies show.

Your gut helps decide whether a probiotic can settle in

Researchers at the Weizmann Institute in Israel took samples from along the gut lining of volunteers before and during a course of an 11-strain probiotic. The probiotic bacteria survived the trip through the digestive tract. But whether they settled into the gut lining followed patterns that were specific to each person, to each region of the gut and to each strain (Zmora et al., 2018).

Those patterns could be predicted from features of each person's gut microbes and gut tissue before they started. Stool tests, however, couldn't tell who was colonized and who wasn't. Overall, the probiotic's effect on the gut community was temporary and individual (Zmora et al., 2018).

That last detail has a practical side. Finding a probiotic strain in a stool sample shows that it passed through, not that it settled in where it might do its work.

What's already living there matters

Another study followed a single strain, Bifidobacterium longum AH1206. After people took it, the strain stayed in the gut of 30% of participants for at least 6 months, without causing digestive symptoms or changing the rest of their gut community. In everyone else, it didn't persist (Maldonado-Gómez et al., 2016).

The people in whom it stuck tended to start out with low levels of their own B. longum, and their gut microbes were missing certain genes for breaking down carbohydrates (Maldonado-Gómez et al., 2016). In other words, a probiotic may have a better chance of settling in when there's an open niche for it, and your starting microbiome helps decide whether that niche exists.

It's worth adding that the same researchers describe colonization by live bacteria as mostly transient in general (Maldonado-Gómez et al., 2016). So a probiotic that doesn't stick around isn't necessarily useless. It may simply work only while you keep taking it.

Bloating has more than one cause

A probiotic can only help with the part of a problem it can reach. A clinical review on chronic bloating and abdominal distension lists several possible drivers: food intolerances, a past infection that disrupted the gut microbes, a gut that's more sensitive to normal sensations, slow transit, and an abnormal reflex involving the muscles of the abdominal wall. The authors note that no treatment approach has been consistently successful (Lacy et al., 2021).

If your friend's bloating was mostly about slow transit and yours is mostly about food intolerance or gut sensitivity, the same capsule may simply be working on a different problem. After 40, shifting hormones can add another layer. Here's what changes in the gut in this decade.

Expectations and timing play a role

Digestive symptoms respond strongly to placebo, which is exactly why controlled trials matter. A meta-analysis of 73 drug trials in IBS found that 37.5% of the 8,364 patients given a placebo reported improvement. Placebo response rates were higher in trials with shorter treatment periods and in trials where doctors, rather than patients, reported the outcomes (Ford and Moayyedi, 2010).

That doesn't make anyone's improvement fake. It does mean a friend's success story, or your own good first week, isn't proof that a probiotic is working. Symptoms can also rise and fall on their own, which is why tracking over several weeks is more reliable than a quick impression.

"The same probiotic" often isn't

Finally, two products that look alike may not be alike. In a review of the field, researchers pointed out that clinical results conflict for many probiotic strains and formulations, and they called for better quality evidence, more transparency and stronger regulation (Suez et al., 2019).

A friend's product may use a different strain, dose or formulation than yours, even if the front labels look similar. Only the strain codes and amounts on the back tell you whether you're actually taking the same thing.

What to do with this

• Compare strain codes, not brand names. Make sure you're taking what worked for someone else, or what was studied.
• Run your own trial. One product, taken daily for 8 to 12 weeks, with a simple 0 to 10 bloating score at the same time of day.
• Change one thing at a time. Otherwise you won't know what made the difference.
• Accept a "no." If a well-chosen strain doesn't help after a fair trial, your gut may simply not be a match for it. Trying a different well-studied strain, or a different approach, is reasonable.

And if your bloating is new, happening most days, or comes with blood in your stool, weight loss, anemia or pelvic pain, see a doctor before experimenting further.

Sources

1. Zmora N, Zilberman-Schapira G, Suez J, et al. Personalized gut mucosal colonization resistance to empiric probiotics is associated with unique host and microbiome features. Cell. 2018;174(6):1388-1405.e21. PMID 30193112
2. Maldonado-Gómez MX, Martínez I, Bottacini F, et al. Stable engraftment of Bifidobacterium longum AH1206 in the human gut depends on individualized features of the resident microbiome. Cell Host Microbe. 2016;20(4):515-526. PMID 27693307
3. Lacy BE, Cangemi D, Vazquez-Roque M. Management of chronic abdominal distension and bloating. Clin Gastroenterol Hepatol. 2021;19(2):219-231.e1. PMID 32246999
4. Ford AC, Moayyedi P. Meta-analysis: factors affecting placebo response rate in the irritable bowel syndrome. Aliment Pharmacol Ther. 2010;32(2):144-58. PMID 20412064
5. Suez J, Zmora N, Segal E, Elinav E. The pros, cons, and many unknowns of probiotics. Nat Med. 2019;25(5):716-729. PMID 31061539

This article is for general information and isn't medical advice. Talk to your healthcare provider about digestive symptoms that are new, persistent or worrying, and before starting a supplement if you take prescription medication.