The science of fermented foods

NEW YORK – Breweries sell craft kombucha on tap. Fruit vinegars and cultured chilli pastes are featured on five-star restaurant menus. Walmart and Target stock kimchi and kefir.

Fermented foods have been popular in the US for decades, but, more recently, they have been thrust into the national spotlight.

In January, the Trump administration released new dietary guidelines that recommended, for the first time, that Americans eat sauerkraut, kimchi, kefir and miso for better gut health. And Health Secretary Robert F. Kennedy Jr often promotes the benefits of sauerkraut and yogurt, saying they helped him lose weight and sharpen his thinking.

Humans have been fermenting foods for thousands of years. What does the science show now about how they may affect our health? We asked experts to explain.

What are fermented foods?

They are defined as any foods or drinks that have been transformed by microbes such as yeast, bacteria and fungi in ways that preserve and infuse them with tangy, savoury, fruity or funky flavours, said Robert Hutkins, an emeritus professor of food science at the University of Nebraska-Lincoln.

Milk can turn into yogurt and kefir; vegetables become sauerkraut and kimchi; fruit is transformed into vinegar and wine; wheat is fermented into sourdough and beer.

Even chocolate and some coffee are fermented, said Maria Marco, a professor of food science at the University of California, Davis.

Are fermented foods good for us?

Research on their health effects is limited, Marco said. But some studies suggest that the foods – excluding alcohol – may offer a few benefits.

In a trial published in 2021, for instance, researchers split 36 healthy adults into two groups: one that ate a lot of fermented foods like yogurt, kimchi and kombucha, and the other that ate plenty of fibre-rich foods like legumes, whole grains, fruits and vegetables.

After 10 weeks, those in the fermented food group had significantly lower levels of inflammatory markers in their blood – and more diverse gut microbes – than they did at the start of the study. Both measures are associated with lower risks of chronic disease. Those in the fibre group had no changes in those measures.

Other research has found associations between fermented food consumption and less risk of eczema – as well as kimchi consumption and lower rates of obesity; yogurt consumption and reduced risks of Type 2 diabetes and weight gain; and sauerkraut consumption and fewer irritable bowel syndrome symptoms.

In one study of more than 46,000 US adults published in 2023, researchers linked fermented food consumption to small reductions in blood pressure, body weight, waist size and blood insulin and triglyceride levels.

The existing studies on the health benefits are promising, Hutkins said. But many are limited by their size, duration or observational nature – meaning they cannot prove cause and effect. More research is needed to see whether eating fermented foods can directly improve people’s health, he said.

And not all research has been positive.

Some studies have found higher rates of stomach and oesophageal cancers in people in East Asia who ate a lot of kimchi and other fermented vegetables, said Suzanne Devkota, director of the Cedars-Sinai Human Microbiome Research Institute. The evidence for the cancer link is weak, though, she said, and many other factors could have influenced that result.

Why might fermented foods be good for us?

Many fermented foods are healthful in their own right, Marco said. A serving of kimchi or sauerkraut counts as a serving of vegetables, for instance, and yogurt and kefir provide calcium, potassium and protein.

The microbes that ferment foods can also help with digestion and the absorption of nutrients, and even create nutrients of their own.

The lactose in milk and the gluten in wheat, for example, are broken down during fermentation, making yogurt, kefir and sourdough bread easier for those with lactose or gluten intolerance to digest. They also help the body absorb minerals such as calcium, iron, magnesium and zinc, and create new vitamins like folate, riboflavin, vitamin B12 and vitamin K during the fermentation process.

But researchers are just beginning to probe other unique attributes of fermented foods that may influence our health.

There is emerging evidence that some molecules that microbes make during the fermentation process can reduce inflammation, regulate blood sugar and help us feel full, among other benefits. But those effects have mostly been demonstrated in studies on rodents and in petri dishes, said Sean Spencer, a gastroenterologist and microbiome scientist at Stanford.

Some researchers hypothesise that exposure to the “friendly” microbes in some fermented foods can reduce inflammation by training the immune system to tolerate harmless microbes or potential allergens, yet be ready to mount a robust response to “bad” microbes in the future, Marco said.

What’s the takeaway?

There is enough evidence to support incorporating more fermented foods into your diet, Spencer said – such as having yogurt for breakfast, adding kimchi to eggs or sandwiches, or making salad dressings with apple cider vinegar.

People who are immunocompromised, however, should be cautious with fermented foods that contain live microbes, such as those labelled raw, unpasteurised or as containing live cultures (typically found in refrigerated sections). Such people may not be able to handle the “bacterial load” present in fermented foods, even if the bacteria are “friendly”, Devkota said.

Some fermented foods, including sauerkraut, cheese and wine, can contain large amounts of histamines, natural molecules that may trigger symptoms such as diarrhoea, hives or headaches in those who are sensitive, Marco said.

And people with high blood pressure should be mindful of fermented foods that are typically high in sodium, like sauerkraut, kimchi, miso and soya sauce, Hutkins said.

While beer, wine and other types of alcohol are technically fermented foods, it is best to limit them, Marco said, given their established health risks. NYTIMES