Do Probiotics Work? What the Evidence Actually Says

Where the evidence is reasonable

The clearest evidence is for diarrhoea linked to antibiotics. Antibiotics disturb the gut's bacteria, and diarrhoea is a common side effect. The ODS fact sheet summarises several meta-analyses (studies that pool the results of many trials):

Probiotic	What the pooled trials found
Lacticaseibacillus rhamnosus GG (Szajewska and Kołodziej, 2015)	The risk of diarrhoea fell from 22.4% to 12.3% in people taking antibiotics
Saccharomyces boulardii, a yeast (Szajewska and Kołodziej, 2015)	In adults, the risk fell from 17.4% to 8.2%

Even here there are limits. Another meta-analysis (Jafarnejad and colleagues, 2016) found a benefit in adults aged 18 to 64 but not in those aged 65 and older, and results for one strain do not carry over to another.

Irritable bowel syndrome (IBS) is the other common reason people try them. In a 2014 meta-analysis of 23 trials (Ford and colleagues), probiotics cut the risk that IBS symptoms would persist or not improve by 21%. A British Dietetic Association review (McKenzie and colleagues, 2016) noted that most studies showing a real drop in symptoms used products containing several strains. The ODS concludes that probiotics "might reduce some symptoms of IBS" and that better trials are needed. If you have IBS, it is worth working with your GP or a dietitian rather than trying products at random.

Where the evidence is weak

General "gut health" in healthy people. The ODS states that "there are currently no formal recommendations for or against the use of probiotics in healthy people". It notes that probiotic bacteria may settle in the gut only for a while, in patterns that vary a lot from person to person, and that much of the research linking gut bacteria to how the body uses energy comes from mice. Whether those effects apply to people is not known.

Weight and fat loss. Probiotics are often sold as a fat-loss aid. The research does not back that up:

• A 2019 meta-analysis by Suzumura and colleagues found that probiotics or synbiotics "reduced waist circumference slightly (by 0.82 cm) but had no effect on body weight or body mass index".
• A 2018 meta-analysis by Borgeraas and colleagues found the effects on weight were "small and of questionable clinical significance".

Less than a centimetre off your waist is not something you would see in the mirror. A calorie deficit, enough protein and daily steps will do far more; how to lose fat without losing muscle covers how.

"More billions are better." The ODS notes that products with higher counts are not necessarily more effective. What matters is whether that strain, in that amount, helped people in studies.

Probiotics for lifters and athletes

Probiotics are often marketed to people who train, for "gut health", recovery or staying well during hard training. The International Society of Sports Nutrition reviewed this in its 2019 position stand on probiotics (Jäger and colleagues, Journal of the International Society of Sports Nutrition). Its main points:

• The benefits are "strain- and dose-dependent", so a finding for one product does not transfer to another.
• Some strains may support immune health in athletes, and "specific probiotic strains can reduce the number of episodes, severity and duration of upper respiratory tract infections".
• Some strains may help the gut barrier during long, hard exercise in the heat, and some may help recovery, but much of the newer research, including on body composition and stress hormones, is early and still needs proper testing in people.

In other words: there may be a case for a specific, tested strain for a specific problem, such as frequent colds in a hard training block. There is no strong case for a general "gut health" capsule.

For lifters, the more common gut problems usually have a simpler cause:

• A big jump in fibre on a new diet, rather than a gradual increase.
• Lactose in whey concentrate, milk-based shakes or large amounts of dairy.
• Sugar alcohols such as maltitol, sorbitol or erythritol in protein bars and "low sugar" snacks.
• Sheer volume of food on a bulk, eaten in a few large meals.

Fix those first. If you still want to try a probiotic for a specific problem, such as while taking a course of antibiotics, check with your doctor or pharmacist, choose a product that names its strains, and give it a fixed trial.

Are probiotics safe?

For most healthy people, yes. The ODS fact sheet says probiotics "are unlikely to cause harm in healthy people, and side effects are usually minor", and the NHS says that if you have a healthy immune system they "shouldn't cause any unpleasant side effects". The NHS also advises anyone with an existing health condition or a weakened immune system to talk to a doctor before taking any probiotic supplement.

The picture is different for some groups:

• People who are seriously ill or have a weakened immune system. The World Gastroenterology Organisation advises restricting probiotics in people with poor immune function to the strains and uses that have proven benefits. In a 2019 study of more than 22,000 intensive care patients at a Boston hospital, those given L. rhamnosus GG had a markedly higher risk of that bacterium getting into their bloodstream.
• Babies born early. In 2023, the US Food and Drug Administration (FDA) warned that giving probiotics to preterm infants can cause serious, potentially fatal infections.

They are not medicines. As of October 2026, probiotics in the US are mostly sold as dietary supplements, and the ODS notes that the FDA has not evaluated them through its approval process for medical uses. In the UK, the NHS says probiotics are "generally classed as food rather than medicine", so they "don't go through the rigorous testing medicines do", and you cannot always be sure a product contains the bacteria on the label, enough of them, or that they survive to reach your gut. The ODS also points out that US labels only have to give the total weight of microorganisms, which includes dead cells, so look for the number of colony-forming units (CFU) guaranteed at the end of the shelf life instead.

What to try before a probiotic

Before spending money on capsules, the food-based basics have better support for most people:

• Fibre. UK guidelines recommend 30 g a day for adults, and most people eat well under that. Raise it gradually, with plenty of water.
• Variety. In the American Gut Project (McDonald and colleagues, mSystems, 2018), people who ate 30 or more different types of plant a week had more varied gut bacteria than those who ate 10 or fewer.
• Fermented foods. In a 10-week study at Stanford (Wastyk and colleagues, Cell, 2021), people who ate more fermented foods such as yoghurt, kefir, kimchi and sauerkraut ended up with more varied gut bacteria and lower markers of inflammation.

Gut health for lifters covers each of these in more detail.

When to see a doctor

See a doctor rather than trying supplements if you have:

• blood in your stool, or black, tarry stools;
• unexplained weight loss;
• a lasting change in your bowel habits, especially if you are over 50;
• severe or persistent pain in your tummy;
• diarrhoea that does not settle, or signs of dehydration;
• a family history of bowel cancer, coeliac disease or inflammatory bowel disease.

Speak to your doctor or GP before taking a probiotic if you are seriously ill, have a weakened immune system, or want to give one to a baby born early. If you take regular medicines or have a long-term condition, ask your doctor or pharmacist first too.

How coaching helps

Gut problems are easier to sort out when someone can see the pattern. With one-to-one coaching from CoachAk, your weekly check-in takes about three minutes and includes a space for digestion notes alongside your weight, training and nutrition adherence, sleep, energy, hunger and stress. Your coach can see what changed in the weeks you felt bloated or off, reply with written feedback and adjust your nutrition targets, while anything that needs a doctor goes to a doctor.

If you want a plan that suits your body and your stomach, apply for coaching.