Does eating flaxseed help your gut?
Through its fibre and lignans, flaxseed has a plausible positive effect on the gut, but for the gut as a specific target the evidence in humans is still limited. If you want to take concrete action, opt for at least 30 grams of ground flaxseed per day.
Flaxseed contains two types of active components for the gut: fibre and lignans. Both serve as food for gut bacteria. Laboratory models and animal studies have shown that flaxseed stimulates the production of short-chain fatty acids, including butyrate. Butyrate is an important fuel for the cells lining the gut wall and has a beneficial effect on its integrity.
The lignans in flaxseed, in particular a compound called secoisolariciresinol, are converted by gut bacteria into two other compounds: enterodiol and enterolactone. This process is well documented in laboratory and animal research. There is an interesting interplay involved in this conversion: you need healthy gut bacteria to activate the lignans, and the lignans in turn feed those bacteria.
The effect on the composition of the gut microbiome is mixed. In a laboratory model, overall bacterial diversity actually decreased, while in rat studies beneficial bacterial strains increased. How this plays out in humans over the long term has not yet been sufficiently studied. So far there is little direct clinical evidence from well-designed studies in humans that look specifically at gut health as an outcome.
What is well supported in humans are the cardiometabolic effects. At a dose of at least 30 grams of ground flaxseed per day, for at least twelve weeks, blood pressure, blood lipids and inflammatory markers decreased measurably. Part of this effect runs through the gut microbiome, which means it also indirectly says something about the gut mechanisms. That provides a concrete takeaway: ground flaxseed works better than whole seeds, because the fibre and lignans are then released more easily.
Evidence comes primarily from in-vitro fermentation models, animal studies (including a rat study in rheumatism), and reviews. Direct clinical evidence for gut health in humans is limited. The cardiometabolic evidence (blood pressure, blood lipids) is most strongly supported by human studies and meta-analyses.
Does eating garlic help your gut health?
There are preliminary indications that garlic is beneficial for gut health, with the strongest evidence being a randomised study that demonstrated improved gut transit at 1600 mg of garlic powder per day. Regular garlic in your daily meals likely also has a modest positive effect, although evidence for an ideal dose or form is lacking.
Does intermittent fasting help your gut health?
For healthy people and people with metabolic syndrome there is reasonably consistent evidence that intermittent fasting has a beneficial effect on gut bacteria and inflammation markers. If you have a chronic bowel disease, discuss fasting with your doctor first, as evidence in that group is still lacking.
Do probiotic pills actually help your gut?
Probiotics genuinely help, but not for everything and not equally with every pill. The evidence is strongest for antibiotic-associated diarrhoea and irritable bowel syndrome; for Crohn's disease, they demonstrably do not work.
Are fermented foods and probiotics good for your gut?
Fermented foods, particularly yoghurt and kefir, show positive indications for gut health; for most other products such as kimchi and kombucha, this has not yet been demonstrated in humans.
Are plant-based diets better for your gut than mixed diets?
A plant-based diet appears to be beneficial for your gut bacteria, but fermented foods perform at least as well in a direct comparison. Whether healthy people also notice a meaningful long-term benefit has not yet been demonstrated.
Does eating a varied diet help your gut bacteria?
Fermented products have the strongest evidence base for increasing gut bacterial diversity. Eating broadly varied meals also helps, but above all avoid heavily processed products containing emulsifiers such as CMC.