Colds

What Gut Bacteria Have to Do With Catching a Cold

The evidence here is moderate, resting on a proper Cochrane review rather than scattered small studies.

A cold is a mild infection of the nose and throat, usually caused by a virus called rhinovirus.

Gut bacteria don't cause or cure colds, but a Cochrane review found they measurably shift the odds of catching one and how the body handles it.

A Cochrane review found a real, if modest, protective effect

A Cochrane review pooling 12 randomised trials and 3,720 people found probiotics nearly halved the odds of experiencing at least one cold-type illness compared with placebo.

People taking probiotics also had shorter episodes when they did get sick, by nearly two days on average, and were 35% less likely to be prescribed antibiotics for it.

The review authors rated the evidence quality as low, mostly because the individual trials were small and varied in which probiotic strain they tested. That's an honest limitation worth stating plainly rather than glossing over.

Source: Probiotics for preventing acute upper respiratory tract infections

A cold itself reshapes the bacteria living in your throat

Separately from the gut, researchers deliberately infected six healthy volunteers with rhinovirus and tracked the bacteria living in their throats before, during, and after infection.

Two genera, Haemophilus parainfluenzae and Neisseria subflava, both increased noticeably during the infection itself.

Both of these are known to be involved in secondary bacterial infections that sometimes follow a cold, such as ear infections and sinusitis. This may help explain why a cold that starts as a simple virus sometimes turns into something needing antibiotics.

Source: Changes in microbiota during experimental human Rhinovirus infection

A daily fermented milk drink cut infections nearly in half in a trial

A randomised controlled trial gave 96 healthy office workers either a fermented milk drink containing the probiotic strain Lactobacillus casei Shirota, or a plain control milk, once a day for 12 weeks over winter.

The group drinking the probiotic milk had a cold-type infection rate of 22.4%, compared with 53.2% in the control group, more than double the difference.

As with any probiotic, this only works if the bacteria are fed properly afterwards. A strain added through fermented milk still needs fibre and plant compounds in the rest of the diet to keep the wider gut community balanced enough to support it.

Source: Daily intake of fermented milk with Lactobacillus casei strain Shirota reduces the incidence and duration of upper respiratory tract infections in healthy middle-aged office workers

What this means for you

A cold that lasts more than ten days, comes with a high fever, or brings breathing difficulty needs a doctor's assessment rather than home management. In addition, the research above shows probiotics measurably lower the odds of catching a cold and shorten it when it happens, while the infection itself reshapes bacteria in ways linked to secondary infections. A varied, plant-rich diet supports the same gut bacteria this research ties to fewer and shorter colds. That's the same idea behind GutLab, everyday gut health starts with regularly feeding the microbiome a broad range of plant-rich ingredients.