It’s cold and flu season. Should you take a probiotic?

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It’s been cold and windy in Melbourne these past few weeks, and I’ve had a niggling virus – the sort where the worst symptoms go away after a few days, but the blocked nose lingers and lingers … not fun!

As the weather closes in and the microbes start to circulate, some of my friends are turning to a class of supplements heavily marketed for their cold-busting properties: probiotics.

A study by Federation University’s Hafiz Ahmad and colleagues crossed Examine’s desk a few weeks ago: they randomised 118 kids across Victorian childcare centres to get either a probiotic or a placebo over six months.

They found no difference in respiratory tract infection rates – but a significant drop in dreaded gastro infections over the last three months of the study.

Interesting! But that’s one study – and, as we’ll see, there is voluminous literature on probiotics. What’s the bigger picture?

Bread and wine

Humans have long sought out bacteria – to ferment milk into yoghurt, wheat into bread, grapes into wine. The Romans recorded barbarians using soured milk to treat diarrhoea; the Turks used yoghurt (a Turkish word deriving from yoğuşmak, to curdle) to treat gastrointestinal problems.

Around the turn of the 19th century, scientists started to isolate bacteria in the human gut, and the field of probiotics was born.

Unfortunately, and as is often the case, the supplement industry has run far ahead of the science, selling probiotics in their billions to consumers long before any high-quality research had been done. “Snake oil for the new millennium,” spat the the president of the American Society of Microbiology in 1999; based on the limited evidence at the time, he noted, it wasn’t clear that eating fermented foods – let alone supplements – was good for you.

Since then, the amount of research being done on probiotics, and the microbiome in general, has dramatically increased.

We know animal microbiomes can be arranged in ways that make us more or less susceptible to chronic disease; we know in humans certain diseases seem to have microbiome signatures. And we know the lining of the digestive system is our largest immune-related organ – a region of constant cross-talk between the immune system and the microbes we harbour.

The mechanism, at least, seems sound.

So … do probiotics work?

The first thing to understand about a healthy microbiome is: one healthy microbiome tends to look really different from another.

To study whether an intervention makes someone better, you typically need to compare them with a healthy control volunteer. But when scientists study microbiomes from healthy people, they all tend to look very different. We still can’t explain that variation.

We can study microbiomes in unhealthy people – but the microbiome responds to the environment. Is the microbiome making them sick, or is the sickness altering the microbiome?

This lack of agreement on what a healthy or unhealthy microbiome looks like hinders our ability to show whether probiotics actually make a positive difference.

“At present, the analysis of an individual gut microbiome cannot provide a meaningful index of health or a prediction of disease presence or risk,” reads the International Scientific Association for Probiotics and Prebiotics’ consensus statement.

Then there’s the problem of shifting the microbiome via probiotics. A meta-analysis of 22 studies published in January found probiotics made no difference to the diversity of gut flora. “What you’re taking in the probiotics is very little – perhaps millions of microbes. It’s not a powerful force,” says Associate Professor Gal Winter, a microbiome researcher based at the University of New England.

There is one telling exception to this rule: a powerful treatment for C. difficile infection that really does seem to recolonise the gut with helpful bacteria. Unfortunately, it is in the form of a poo transplant – not something palatable for most consumers.

What about consumer-level probiotics?

It can be difficult to tell what the evidence says because there is now so much of it: one Australian-based probiotics company quotes a systematic review and two randomised controlled trials to back their pill.

“You’ll find tens of thousands if not millions of papers studying it. But despite that, our understanding is still very limited,” says Winter. “As humans, we always search for the magic cure: give me this probiotic that will fix me. There is no such thing – or if there is, we haven’t found it.”

So how do we pick good studies from bad?

One option is to look for expert-level consensus guidelines.

The American Gastroenterological Society declines to recommend probiotics for most conditions – even Irritable Bowel Syndrome (IBS).

“A lot of people are being cheated. As a scientist, I see such a big unregulated market,” says Professor Rajaraman Eri, who has studied IBS and the gut-brain axis at RMIT. “For many people, it is a waste of money.”

What about for general health, or preventing respiratory infections?

The International Scientific Association for Probiotics and Prebiotics concluded there isn’t robust enough evidence to suggest healthy people should take them – a statement we should take seriously, considering the association is funded by probiotic manufacturers.

On the other hand, Cochrane – the doyens of gold-standard clinical evidence – found probiotics are likely better than a placebo at preventing common colds, though the evidence is weak.

So that’s probiotics. But we shouldn’t miss the forest for the trees. As Winter notes, a typical probiotic might contain millions of bacteria, while the microbiome comprises more than 100 trillion microbes.

Thus, the more important determinant of microbiome condition is not probiotics, but what we feed our existing microbes.

“Changing what we eat is probably the biggest intervention we can do,” says Winter.

Microbes feed on fibre. The more fibre we eat, and the more diverse it is, the more diverse our microbiome becomes, says Winter.

Dietary fibre consumption reduces the risk of diverticulitis and colon cancer, and may improve the gut barrier. Plant-based and Mediterranean diets – both rich in fibre – have been shown to lower levels of circulating inflammatory molecules.

Both Winter and Eri recommend fermented foods, such as sauerkraut or kimchi: not only do you get the probiotics, but you get the fibre they feed on and any beneficial products they may have made. “It’s so simple,” says Winter. “And it’s been practised for thousands of years.”

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