What the Evidence Really Says About Probiotics

Probiotic pills seem to be available almost everywhere: on supermarket shelves, in pharmacies and at local health-food shops.

Countless products loudly advertise their probiotic content. Nearly anything marketed as even remotely health-focused is likely to mention probiotics somewhere on its packaging.

Across every platform, millions of viral videos urge people to increase their probiotic intake for benefits ranging from improved gut health to stronger immune defences.

Why are probiotics so widely recommended?

It is a worthwhile question, since the evidence supporting these pills is far less convincing than you might expect.

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The term probiotics generally refers to several species of bacteria that inhabit the gut and are linked with good health.

For instance, greater levels of bifidobacteria in the gut are associated with lower obesity rates, while lower levels of lactobacillus are linked to a greater risk of heart attacks.

The thinking, then, is to give people pills or supplements that contain these potentially helpful bacteria. If we can change the gut microbiome, perhaps we can make people healthier and lower their risk of a wide range of serious health problems.

The problem with that proposition is that the gut is exceptionally complex, whereas introducing one bacterial strain into the diet is exceedingly simple.

"I once went to a presentation where the researcher described probiotics as going to the middle of the Amazon and dropping 10,000 of one tree into the forest and assuming that would fix any problems." – Dr Gideon Meyerowitz-Katz

I attended a presentation at which the researcher compared probiotics to travelling into the centre of the Amazon, dropping 10,000 trees of one type into the forest, and presuming that would solve every problem.

They might be useful in some circumstances, but they could also bring a range of unintended consequences.

The evidence for probiotic supplements

There is also an enormous body of research on probiotics. Although some trials have reported positive results, many are of very low quality, and do not really answer whether probiotics genuinely work.

Probiotic supplements are also among the areas most affected by academic misconduct. Many dozens of randomised trials investigating probiotics’ effects have been retracted for a variety of concerning reasons.

Conversely, the larger trials generally fail to identify benefits from probiotics.

The largest study to assess probiotics for preventing common colds found no difference in the primary outcome of cold severity between probiotics and a matched placebo in 2021.

There appear to be no probiotic benefits for eczema. Nor did the largest randomised trial of probiotics in premature babies, involving 618 infants, find any benefit.

(troyanphotos/Canva)

Even in a setting where probiotics might most clearly be expected to help - preventing infections that cause diarrhoea - no improvements are seen.

In 2020, a large systematic review of trials on probiotics for preventing infectious diarrhoea included 82 studies and more than 12,000 participants. It found no benefit from probiotic pills.

More concerningly, probiotics may carry potential harms. A 2021 Cochrane Collaboration review examining probiotics to prevent gestational diabetes - high blood sugar occurring during pregnancy - found little or no evidence of benefits, but concluded they probably increased the risk of pre-eclampsia.

Strangely, a Cochrane review of probiotics for women already diagnosed with gestational diabetes identified some possible benefits. However, that may be because most trials included in the review came from a research group that has had more than a dozen papers retracted over serious integrity concerns.

Why “probiotics” is an imprecise term

Part of the difficulty with this research is that 'probiotics' is itself a somewhat meaningless label.

It is applied to all sorts of bacterial pills, most of which are not standardised, and many of which could have different effects.

Perhaps the ratio of bifidobacteria to lactobacillus has to be precisely right to produce the hoped-for benefit. Or perhaps probiotics must be highly personalised to work as intended. At present, we simply do not know.

There are also an enormous number of health problems for which some type of probiotic has been proposed as a treatment.

I have covered nearly a dozen so far, but there are many more. There could be medical areas in which a particular probiotic works for a particular condition that I have simply not reached in researching this article.

Even so, I do not think the overall evidence for probiotics is particularly positive. The findings from the biggest randomised trials - and there are quite a few of them - are unpromising.

Related: Scientists Have Found an Important Reason You Might Want to Reconsider Taking Probiotics

They may help with issues directly connected to gut health, such as preventing antibiotic-associated diarrhoea, but for everything else the evidence does not look particularly strong.

Many foods contain beneficial bacteria. If you enjoy kimchi, yoghurt or sauerkraut, there is probably no bad reason to continue eating them.

At the very least, they can offer your taste buds a wonderful adventure, and it is plausible that they improve your health.

But what about the many probiotic pills, powders and other concoctions?

I think the evidence makes it reasonably clear that they are unlikely to provide major benefits to your health.

Gideon Meyerowitz-Katz is an epidemiologist working in chronic disease in Sydney, Australia. He writes a regular health blog covering science communication, public health, and what that new study you have read about actually means. He has previously helped ScienceAlert break down the real science behind the most viral papers.

This article was fact-checked and edited by Michael Irving. While we take pride in our process, we are only human. If you spot an error, please let us know.