What are the benefits of fermented foods?
Fermented foods show real benefits for antioxidant activity and digestibility, and there are indications that they may support resistance to infection. For claims around mental health and ADHD, the research is still too limited to draw any meaningful conclusions.
Fermentation increases the antioxidant activity of milk, grains, fruit and vegetables. Fermented milk and grains have also been found to contain substances that can lower blood pressure. In addition, fermentation improves the digestibility of grains, legumes and tubers, and reduces substances that normally block the absorption of nutrients. The latter is particularly important for people with limited access to a varied diet.
Kefir is the most extensively studied fermented drink. Several studies suggest possible benefits in lowering blood fat levels and blood pressure, and in supporting the treatment of a stomach bacterium (Helicobacter pylori). Most of this evidence comes from small studies with a high risk of bias, however, and whether it is safe to use over many years has barely been examined. Kefir appears safe for healthy people, but firm recommendations are still premature.
A specific fermented dairy drink containing a probiotic bacterial strain reduced the likelihood of catching a common infectious illness and the average number of infections per person. Two of the six researchers work for the manufacturer of the product, so bear that in mind when weighing this result.
Fermented foods and probiotics are also being studied for effects on the brain and mental health. For depression there is some early indication of a beneficial effect, but the evidence is not yet strong. For ADHD, schizophrenia and bipolar disorder the picture is unclear. A small study in children with ADHD found no improvement in ADHD symptoms. A study in healthy women found preliminary signs of changes in the brain after eight weeks of probiotic yoghurt, but those results did not hold up under more rigorous statistical analysis. The gut-brain connection is biologically plausible, but solid evidence in people is still lacking.
This answer is based on several combined study overviews and a handful of studies in people. The overviews cover broad categories of fermented food, but the quality of the underlying studies varies considerably: for kefir, 12 of the 18 publications examined had a high risk of bias. The effects on the brain come from a very small study (40 women) whose results did not hold up after correction. In the infection study, two of the six authors have ties to the manufacturer. The researchers themselves rate the evidence for kefir as limited and not strong enough for firm recommendations. The effects on antioxidant activity and digestibility are the best documented.
Which health interventions have the greatest impact on living longer?
Exercise, a Mediterranean diet, adequate sleep, social contact and quitting smoking each have a measurable influence on how long you live in good health. Combining several healthy habits brings the greatest benefit.