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

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?

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 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 c