Probiotics and the Gut Microbiome: What They Are, Which Foods Have Them and What the Studies Show

Microbiome science is young and the industry moves faster than the trials. That is why it pays to start with the words, because each one promises something different, and then look at what has actually been measured.

Probiotics, prebiotics and fermented foods are not the same thing

The practical consequence is that not every fermented food is a probiotic. If the food is heated after fermentation, like sourdough bread or jarred pasteurised sauerkraut, the microorganisms do not arrive alive. And even if they do, calling it a probiotic requires an identified strain, in a specific amount, with a proven benefit. That is why two products with the word “probiotic” on the label may have nothing in common.

The trial that compared fermented foods and fibre

In 2021, a Stanford team randomly assigned 36 healthy adults to one of two diets for 10 weeks (Wastyk, 2021). One group raised fermented foods from 0.4 to about 6 servings a day (yogurt, kefir, kombucha, kimchi and other fermented vegetables; a serving was 6 ounces, about 177 ml, of drink or yogurt, or a quarter cup of vegetables). The other raised fibre from 21.5 to 45 g a day.

• Fermented foods: microbiome diversity rose steadily and several blood markers of inflammation went down.
• Fibre: diversity did not change over those 10 weeks, although the bacterial enzymes that break down fibre increased; the immune response varied according to each person’s starting microbiome.
• The limit: it is a small, short study in healthy people. It measures the microbiome and blood markers, not diseases.

Fibre: what your bacteria eat

EFSA considers 25 g of fibre a day adequate for an adult. The microbiome responds quickly to what you eat: in 10 volunteers, five days on an all-animal or all-plant diet were enough to change its composition (David, 2014). The famous “30 plants challenge” comes from the American Gut Project (McDonald, 2018): people who said they ate more than 30 types of plants a week had a more diverse microbiome than those who ate 10 or fewer. It is an association, in about 85 people from a citizen-science project, not a proven threshold; the underlying idea, variety, is reasonable. This is the fibre in common servings according to the USDA:

A plate of lentils, a pear, an apple and 40 g of oats add up to about 28 g of fibre, above EFSA’s 25 g. If you want to check the calories of other foods, see the food calorie table, and for more ideas the article on high-fibre foods.

What the meta-analyses say, problem by problem

• Antibiotic-associated diarrhoea in children: the 2019 Cochrane review (33 trials, 6,352 children) found it in 8% of those who took probiotics and 19% of those who did not; 9 need to be treated to prevent one case. With high doses (5 billion CFU a day or more) the effect was larger, with moderate certainty, although the authors call for a large trial to confirm the benefit of those doses (for example, of Lactobacillus rhamnosus or Saccharomyces boulardii).
• Irritable bowel syndrome: across 53 trials with 5,545 patients, some combinations and strains seemed to improve symptoms and pain, but it was not possible to conclude which ones (Ford, 2018). Evidence with caveats: it may help some people, but no specific product can be recommended.
• Weight: across 15 trials with 957 people with overweight or obesity, 0.6 kg and 0.27 BMI points less than placebo over 3-12 weeks, with no significant change in fat mass (Borgeraas, 2018). A small effect.
• Cleanses, “detoxes” and microbiome tests: none of the conditions with favourable trial results has anything to do with them.

Notice that every result depends on the strain, the dose and the problem: what worked in one trial cannot be transferred to another product. That is why there are no regimens here. If you are thinking of taking them, for example alongside an antibiotic, ask your doctor or pharmacist which one and how.

You do not need expensive supplements to look after your gut: you need enough fibre, a variety of plants and some fermented foods. Log what you eat with Renzy for a few days and you will see whether your plate is heading in that direction.