Also known as: Lactobacillus, Bifidobacterium, Live cultures, CFU, Multi-strain probiotic
FoundationalGood evidence
Live bacteria taken to influence the gut microbiota. Effects are strain-specific rather than general, which means the strain names on the label matter far more than the headline billions figure.
Typical dose
1.0–50.0 CFU per day
Daily and consistently; with or just before food
What it is
Probiotics are live microorganisms which, in adequate amounts, are intended to confer a benefit. The great majority of commercial products use species from two genera: Lactobacillus and Bifidobacterium.
The most important thing to understand is that effects are strain-specific, not species-specific and certainly not general. Lactobacillus rhamnosus GG and Lactobacillus rhamnosus GR-1 are the same species with different research records and different effects. A product listing only genus and species, without the strain designation, is telling you almost nothing.
Potency is stated in colony forming units, or CFU. Products advertise anything from one billion to a hundred billion, and higher is not automatically better — several well-researched strains are effective at a few billion. What matters is whether that count is guaranteed at the end of shelf life rather than at the point of manufacture, which is a meaningful difference many labels obscure.
How it works
Probiotics work through several routes rather than by permanently colonising the gut, which most do not do. They compete with less desirable organisms for space and nutrients, and produce compounds that inhibit them.
They strengthen the gut barrier by supporting tight junctions between intestinal cells, and interact with the immune tissue that lines the gut, which is where a large share of the body's immune activity sits.
They also ferment fibre into short-chain fatty acids, including butyrate, which fuels the cells lining the colon. Because most strains are transient rather than permanent residents, benefits generally persist only while you keep taking them.
What the evidence shows
The evidence base is large but fragmented, precisely because it is strain-specific. Pooling trials of different strains together, as many reviews do, tends to produce muddy conclusions that understate what specific strains have shown.
The strongest findings concern digestive disturbance associated with taking certain medications, and travel-related digestive upset, where specific strains have reasonably good support.
There is growing research on immune outcomes in athletes, with some trials reporting fewer upper respiratory symptoms during heavy training blocks. Evidence for general wellbeing in healthy people with no digestive complaints is much weaker, and this is the use most products are actually sold for.
Dosing
Effective doses vary enormously by strain, from around one billion CFU to fifty billion or more. Match the dose to what the research on that specific strain used rather than assuming more is better.
Buy on strain names, not on the billions figure. A product listing three named, researched strains at ten billion is a better buy than an unnamed blend at a hundred billion.
Check whether the CFU count is guaranteed through to the expiry date. Take consistently, since most strains are transient and benefits fade after stopping. Timing relative to food is less important than consistency, though many products suggest taking with or just before a meal to buffer stomach acid.
Most strains are transient; benefits generally persist only while taking them
Timing
Daily and consistently; with or just before food
Competition status
Permitted — not on the WADA Prohibited List
Category
Foundational
Safety and side effects
Probiotics are well tolerated by most healthy people. Bloating, gas and changes in bowel habit are common in the first week or so and usually settle as things adjust.
The meaningful caution concerns anyone whose immune system is compromised, who has a central venous line, or who is seriously unwell. Introducing live organisms in those situations is not a decision to make from a supplement label, and medical advice should come first.
Some products contain dairy, soy or other allergens as part of the growth medium, so allergen declaration matters. Speak to your doctor before use if you are under medical care.
Who should avoid it
Speak to your doctor before use if your immune system is compromised, if you have a long-term intravenous line, or if you are seriously unwell — live organisms are not a label decision in those situations. Check allergen declarations, since growth media may contain dairy or soy. Expect some bloating in the first week.
This entry is provided for information only. Food supplements should not be used as a substitute for a varied and balanced diet and a healthy lifestyle. Consult a qualified healthcare professional before use if you are pregnant, breastfeeding, taking medication or under medical supervision.
