Probiotics

Probiotics are capsules, powders or drinks of live bacteria sold to improve gut health, and increasingly to slow aging. Specific strains have shown specific, modest benefits in trials, but the pooled effects on cholesterol in adults, and on muscle, inflammation and thinking in older adults, are small, fragile or absent, and the products run well ahead of their data.

Don't take a probiotic as a general anti-aging supplement. Feeding the bacteria you already have with fibre and a varied, plant-rich diet is the better-supported lever. If you are seriously ill or in hospital, don't start one without asking your doctor.

Benefits are tied to a strain and an endpoint

A probiotic is not one thing. Trials test a particular strain, or a mix, for a particular outcome, and a result for one says little about another. That is why the honest summary of the field is "strain- and endpoint-specific": a product that helped one measure in one trial is not shown to help anything else.

Cholesterol: real but tiny

Pooled across 155 randomised trials in about 9,900 adults, probiotics lowered LDL ("bad") cholesterol by only about 0.15 mmol/L, and the authors call the size and consistency of the effect uncertain.^[1]

Muscle and walking speed: small and fragile

In trials of adults with an average age of 50 or more, probiotics added about 1.9 kg of grip strength and 0.08 metres per second of walking speed, though most studies carried a high risk of bias.^[2] In another pooled analysis, of four probiotic trials in older adults, walking speed came out about 0.1 metres per second faster, but an apparent grip-strength gain did not hold up when the analysis was checked for robustness; evidence for prebiotics (fibres that feed gut bacteria) and synbiotics (the two combined) is still thin.^[3]

Inflammation and thinking: mostly null

A pooled analysis of 20 trials in older adults found no significant effect of probiotics on either inflammation or thinking.^[4] A single-strain trial in 66 healthy adults over 70, partly run by the strain's maker, did not significantly lower blood C-reactive protein, a standard marker of inflammation, though it lowered a stool marker of gut inflammation.^[5]

The more positive result comes from a broader review that pooled probiotic, prebiotic and synbiotic trials in adults aged 60 and over. Across the six trials that used the 30-point Mini-Mental State Examination, a standard memory-and-thinking test, it found a gain of just over 1 point, with low-to-moderate certainty, results that varied widely between trials, and benefits seen more consistently in people without dementia.^[6] Because the trials were mixed, that gain cannot be credited to probiotics alone.

Not risk-free

In critically ill patients in intensive care, the bacteria from a probiotic capsule have been traced into the bloodstream, matching the capsule's own strain.^[7] The study covered only intensive-care patients, so the risk for healthy people is unknown but has not been shown; it is the reason not to add a probiotic during serious illness without medical advice.

What to do

Skip probiotics as an anti-aging supplement. No trial shows they slow aging, and the pooled effects on the outcomes that have been tested are small or absent.
If you want a probiotic for a specific problem, choose the strain that was tested for that problem, and judge it on whether that problem improves.
Put the effort into diet instead: fibre and a plant-rich, Mediterranean-style pattern feed the microbes you already have. Fermented foods are a reasonable extra; see fermented foods.
During serious illness or a hospital stay, ask before starting one.

The wider picture of how the gut changes with age is on Dysbiosis.