“Without bacteria, viruses, fungi, and other microorganisms, there wouldn’t be life on earth, and we wouldn’t be here,” says Lita Proctor. “Every kind of living thing is dependent on communities of microbes we call microbiomes, and that includes humans.”

Can certain diets help you lose weight by changing your gut microbes, as some people claim? When researchers randomly assigned 609 overweight or obese people to healthy low-fat or low-carb diets for a year, both groups lost the same amount of weight.

“People think that probiotics are all effective for the same health issues,” says University of Washington probiotics researcher Lynne McFarland. “That’s not the case.” But you’ll never hear that from some probiotic manufacturers, who are happy to have you believe that their products are good for whatever ails you.

Probiotics sellers can make vague claims like "supports digestive health," but does that mean you can expect relief from your GI problem? A single strain may not cover all the bases. Here’s a snapshot of the evidence on which probiotic strains may help with which digestive issues.

“Is your gut microbiome the key to health and happiness?” ran The Guardian headline in 2017. “Germs in your gut are talking to your brain,” declared the New York Times in 2019.

A new field of research suggests that the gut microbiome may influence mood, mental health, and the nervous system's vitality. But is the supplement market way ahead of the evidence?