Probiotics and microbiome tests: what your money is wasted on

The short answer

Live-bacteria products are not neutral when your immunity is low, and microbiome tests sold online cannot tell you whether your gut is healthy. Talk to your team before taking any product of this kind. The money spent on them does more good in a grocery basket.

Probiotics contain live bacteria and yeasts. That is the whole point of them.

Macmillan Cancer Support says it plainly: there is a small risk that these microorganisms could cause an infection. That risk matters more in oncology, because some treatments lower your white blood cell count — exactly the cells that stop infections.

That is why their recommendation includes live-culture yogurts too, not just capsules: discuss it with your oncologist first.

Cancer Research UK goes further. For people undergoing chemotherapy, it states plainly to avoid supplements and probiotic products, such as bio yogurts. The reason is the same: they contain live bacteria.

This is not a lifelong ban, and it does not mean that everyone who takes probiotics will have a problem. It means the timing matters, and during treatment the timing is wrong.

In 2021, a team followed patients with melanoma undergoing immunotherapy and recorded their dietary habits.

Among the 128 patients followed, those who ate more fiber had longer periods without the disease progressing. The best outcome appeared in people who ate enough fiber and did not take commercial probiotics.

Those taking such supplements showed no advantage. Pay attention, though, to what the study does not say: in humans, the difference between those on probiotics and those without was not clear enough to draw a conclusion. The clear effect against probiotics appeared in mice, not in patients.

The study is observational — it followed what people were already doing, without changing anything. It does not prove cause and effect. But the direction is hard to ignore: food came out ahead of the box.

They send back a result that looks scientific: lists of bacteria, percentages, sometimes a score.

Some researchers put this to the test. They took a single stool sample, split it into identical parts, and sent it to seven companies selling this kind of test.

The number of bacteria found ranged from 34 to 906, depending on the company. Only three types appeared in all of them. And one company, which received the same sample three times, reported twice that the microbiome was healthy and once that it was not.

The explanation starts before the laboratory. There is no accepted definition of a healthy microbiome, so there is no reference result to compare against.

An international group of researchers wrote plainly that such tests have no proven value in clinical practice. What they can lead to, instead, is wasted money — both the patient's and the healthcare system's.

Fiber from food remains the one thing in this area with solid evidence, and it does not come with a subscription.

During periods of low immunity, food safety rules matter more than any supplement: well-cooked food, fruit and vegetables washed carefully, caution with unpasteurized products.

And one more thing that is cheap and useful: put everything you take on your own onto a list, then show the list.

• On which days after a treatment session is my immunity lowest?
• Am I allowed live-culture yogurt or kefir during those periods?
• Does this live-bacteria product conflict with my treatment?
• Am I on immunotherapy? Does that change what I eat?
• Is it worth paying for a microbiome test in my situation?

Gather the boxes and jars of supplements you currently take and photograph the labels. Bring the photos to your next check-up, before you buy anything else.