As interest in gut health continues to grow, an age-old method of preparing food has returned to the spotlight: fermentation. Recent U.S. government dietary guidelines are now encouraging people to eat more fermented food.

Humans have relied on fermentation for thousands of years — long before refrigerators existed — as a way to keep food from spoiling. Many cultures have developed their own traditional versions, from yogurt, kimchi and sauerkraut to the South Indian dishes idli and dosa.

Fermentation is a process in which naturally occurring microbes, including bacteria and yeast, break down and preserve food. Those same microbes can also help keep the gut in balance, competing with other, less beneficial bacteria already living there. "We've been doing this for ages and we just found out more recently that it's actually helped our gut health," said Dr. Lisa Ganjhu, a specialist who treats the stomach and intestines.

Being fermented, however, does not automatically make a food healthy. Beer and wine are fermented too, yet they tend to affect the microbiome negatively. Experts also warn against sugary or heavily processed products that market themselves as "gut healthy," advising consumers to look instead for plain foods labeled "live cultures."

So which fermented food is best? Experts say there is no proven ranking, so the wisest approach is to eat many different kinds regularly. Ideally, they should be just one part of a balanced diet — one that also includes fiber, which feeds the helpful bacteria in the gut.