Which fermented foods are the healthiest?
Yoghurt and kefir have the most human evidence, so if you want to make one choice, start there. For kimchi, kombucha and miso, human evidence is not yet available, although they show promise in the laboratory.
Yoghurt and fermented dairy products have the strongest footing in the research. Large population studies show that yoghurt is associated with a lower risk of type 2 diabetes, and that fermented dairy as a whole is linked to a lower risk of stroke and cardiovascular disease. Cheese, surprisingly, does not raise cholesterol the way you would expect based on its fat content. These are associations, not proven causes, but they recur across multiple large studies.
Kefir is the most studied individual fermented product. There is at least one comparative study showing beneficial effects in lactose intolerance and in the treatment of the stomach bacterium Helicobacter pylori. Kefir is also associated with lower cholesterol, better blood glucose regulation and lower blood pressure, but the vast majority of those findings come from laboratory and animal research. Artisanal kefir also differs considerably from industrial kefir in composition and potential effect, so the brand or preparation method matters.
Tempeh is an affordable, protein-rich option for which fermentation demonstrably improves the properties of soy. However, clinical research in humans is virtually absent. Kimchi, miso and kombucha are widely praised, but for all three, comparative human studies on effects on gut health are lacking, which is precisely what the claims are about. Popularity has clearly run ahead of the evidence here.
The microorganisms in fermented foods demonstrably reach the gastrointestinal tract, but whether they permanently alter the gut microbiome remains uncertain. Fermented foods also supply bioactive peptides, vitamins and minerals through fermentation, but some studies find no measurable health benefit. In short: yoghurt and kefir have the most human evidence, though it concerns associations rather than proven effects. For kimchi, kombucha and miso, this has not yet been studied in humans.
This answer draws on multiple reviews and two systematic analyses, one of which includes pooled data. The strongest findings come from large population studies on yoghurt and fermented dairy. For kefir, there is one comparative study in humans. The remaining evidence for specific products (tempeh, kimchi, kombucha, miso) comes largely from laboratory and animal research. The researchers themselves emphasise that large-scale clinical research in humans is lacking for most products.
Which diet is best supported by evidence for a long, healthy life?
For anyone looking to use diet to invest in a long and healthy life, the Mediterranean diet is currently the best-supported choice. Large human studies consistently show positive effects on both cardiovascular health and cognitive health.
Is eating organic really healthier?
Eating organic probably does not give you better nutritional value, but it does lower your exposure to pesticide residues and to antibiotic-resistant bacteria through meat. For pregnant women and young children, that is the strongest concrete argument.
Is raw milk healthier than pasteurised milk?
Raw milk offers no proven health benefits that outweigh the risk of infection. If you are concerned about vitamin intake, pasteurised milk is simply a safe choice.
Is fibre-maxxing (eating as much fibre as possible) healthy?
Eating more fibre is a good idea for most people in the West, but fibre-maxxing as an extreme principle offers no additional benefit beyond meeting the recommended 25 to 30 grams per day through ordinary foods such as legumes, vegetables and wholegrains.
Are smoothies and fresh juice just as healthy as whole fruit?
Whole fruit beats fruit juice: more fibre, greater satiety and fewer free sugars. Smoothies fall somewhere in between, but are not equivalent to whole fruit either.
Are there dietary patterns that have been proven to reduce your risk of cancer?
The Mediterranean diet and sufficient vegetables and fruit are consistently associated with a lower cancer risk, although the effects are modest and the findings are associations. The evidence is strongest for colorectal cancer; ideally combine diet with other healthy habits for the greatest effect.