WHY WE STARTED THINKING ABOUT GUT HEALTH

Gut health became another part of the enormous puzzle we were trying to understand after Stu was diagnosed. We became interested in it when we started exploring immunotherapy. Stu was having treatment through CeGaT and I wanted to understand anything that might influence how well his immune system was functioning. That led us into the gut microbiome, which was a considerably less mainstream conversation in cancer treatment when we were doing this than it is now.

We knew that things like antibiotics, illness, treatment and diet could affect the microorganisms living in the gut, and we started looking at whether supporting Stu's gut health could form part of the wider approach we were taking. As with so much of what we did, we weren't looking at this instead of treating his cancer. We were looking at the whole picture and asking whether there were things we could do alongside his treatment that might support his body as well as possible.

WHAT IS THE GUT MICROBIOME?

The gut microbiome is the enormous community of microorganisms living in our digestive system, including bacteria, viruses and fungi. These microorganisms interact with the body in incredibly complicated ways. They are involved in digestion and metabolism, produce substances that can affect other parts of the body and interact extensively with the immune system.

The science around this has moved on enormously since Stu was ill. The gut isn't an isolated tube dealing with whatever you had for dinner. There is constant communication between the gut, immune system, metabolism and other organs. What is much harder is moving from that broad understanding to saying that a particular bacterial result is "good" or "bad", or that changing someone's microbiome in a particular way will alter the course of their cancer. That is where the science becomes much less certain.

TESTING STU'S MICROBIOME

We had Stu's microbiome tested as part of a series of tests we did with Patrice Surley and Dr Callebout. The GI-MAP test we used through Nordic Labs looked at organisms and markers in Stu's stool and reported bacteria, pathogens, parasites, fungi and viruses, with various things shown as high, low or within range.

At the time we interpreted some of those results as possible additional demands on Stu's immune system when we desperately wanted his immune system concentrating on his cancer. I would be more careful about drawing that conclusion now.

Stool and microbiome testing can provide information about what is present in a sample, but translating that into what is clinically important for an individual person is much more complicated. The microbiome changes over time and is influenced by diet, medication, illness and numerous other factors. Different tests also analyse different organisms and markers.

There are also limitations with some commercial microbiome tests. One published evaluation of the GI-MAP assay found problems with specificity when it was tested for gastrointestinal pathogens, including false-positive results. That doesn't mean our test told us nothing useful, but it does mean I wouldn't now look at a commercial stool report and assume that every flagged organism represented a problem that needed treating. PubMed Central (PMC). I still think investigating Stu's gut health was reasonable. I just understand the limitations of what those results could tell us much better now.

WHAT WE CHANGED

The test wasn't the only reason we changed things. Gut health became part of the way we thought about Stu's food and general health. We tried to give him a more varied diet with lots of different plant foods. We increased vegetables and other sources of fibre, included fermented foods and reduced highly processed foods. We didn't stop eating meat altogether, although we tended towards chicken and fish. We ate wholegrains and very little ordinary bread. When we did buy bread it was generally sourdough.

There was a lot of overlap between what we were doing for Stu's gut and the wider changes we made to his diet, which I've written about separately on the Food & Diet page. Looking back, I like that this part didn't depend on finding one magical food or supplement. It was mostly about variety and eating a broad range of decent food.

PROBIOTICS

Stu took probiotics. Early on he was taking probiotics recommended by his kinesiologist Hilary. After speaking to Dr Castro we changed to VSL#3, a high-concentration probiotic, which Stu drank every morning. I also came across Symprove through other cancer patients and later took it myself. The old version of this page even had my Symprove discount link on it, which feels like a slightly strange relic of that period of our lives now. Would I tell someone with glioblastoma in 2026 to take a probiotic because Stu did? No.

That isn't because probiotics are nonsense. Different probiotic strains are being studied for different conditions and there is research into their use for some treatment-related problems. But "probiotic" describes a huge range of different microorganisms and products. You can't assume that because one strain has evidence for one particular purpose, another product will do the same thing.

There is also an important cancer-specific safety issue that I don't think we appreciated enough at the time. Probiotic supplements contain live microorganisms. Some cancer treatments can reduce white blood cells and suppress the immune system, and there is a small risk of infection from the bacteria or yeasts in probiotic products. Macmillan now advises anyone having cancer treatment to speak to their cancer doctor, specialist nurse, pharmacist or dietitian before taking probiotics. So this is firmly in the category of this is what Stu took, not an instruction for what you should take.

FOOD & THE MICROBIOME

One of the least complicated ways of thinking about gut health is food. Different gut microorganisms use different components of what we eat. Dietary fibre is particularly important because some gut bacteria ferment fibre and produce compounds including short-chain fatty acids.

Current UK advice recommends around 30g of fibre a day for adults, and sources include wholegrains, vegetables, fruit, beans and other pulses, nuts and seeds. Eating a variety of plant foods also gives the gut bacteria a variety of things to work with. World Cancer Research Fund
That overlaps considerably with what we were already doing through The Rainbow Diet and the other changes we made to Stu's food. But cancer complicates everything. Someone who is losing weight, struggling to eat, recovering from surgery or dealing with treatment side effects may have completely different nutritional priorities. More fibre isn't automatically better for every person at every point in treatment. As with the Food & Diet page, I'd be much less interested now in creating a theoretically perfect diet and much more interested in what the individual person needs.

THE GUT-BRAIN CONNECTION

The gut and brain communicate in both directions through what is usually called the gut-brain axis. That communication involves the nervous system, immune system, hormones, metabolism and substances produced by gut microorganisms. When I first started reading about this, the idea that bacteria in your bowel could have anything whatsoever to do with your brain sounded extraordinary. It doesn't sound particularly extraordinary anymore.

But "the gut and brain communicate" and "changing the gut microbiome will treat a brain tumour" are two very different statements. The first is well established biology. The second is something researchers are still trying to understand. And that distinction matters when you're the person Googling this stuff at 2am because someone you love has glioblastoma.

THE MICROBIOME & GLIOBLASTOMA

This is where things have become particularly interesting since Stu was ill. Research is increasingly looking specifically at the relationship between the gut microbiome and glioblastoma. A 2026 review brought together clinical and laboratory research examining differences in the gut microbiome in GBM, possible effects on inflammation and immune signalling, microbial metabolites and potential relationships with treatment response. PubMed

There is human research too. A prospective phase I/II study published in 2025 investigated newly diagnosed glioblastoma patients receiving an anti-PD-L1 immunotherapy alongside standard treatment. Researchers found that particular gut microbiome features were among a number of biological characteristics associated with outcomes. Nature. That is fascinating to me because the immune system and immunotherapy were exactly why we became interested in Stu's microbiome in the first place.

But associated with is doing a lot of work in that sentence. It doesn't establish that those bacteria caused the better outcomes. It doesn't tell us that taking a probiotic containing particular bacteria would reproduce them. And it doesn't establish a microbiome treatment for glioblastoma. So I look at this research with enormous interest rather than as vindication that everything we did was right. There is a genuine scientific question here. Researchers are trying to answer it. That's very different from having an intervention we already know improves survival.

POO!
I couldn't rewrite this page and remove the poo. I wonder whether it's a British thing or just a human thing that so many people don't like talking about anything to do with our guts and poo. There isn't a creature on the planet that doesn't poo in some way or another.

I come from a family that doesn't get particularly uncomfortable about these things, so I never found it strange talking to Stu about poo. And I've seen some things wives aren't usually privy to. But in my heart, all of those things brought us closer. There was nothing we couldn't talk about, including poo. Cancer strips away an extraordinary number of boundaries. You end up discussing bowels, bodily functions, medication, sex, sickness and things neither of you ever imagined would become normal conversation. Poo was just another one.

HINDSIGHT

WHAT WE KNOW NOW THAT WE WISH WE'D KNOWN THEN

I don't regret becoming interested in Stu's gut health. If anything, it's fascinating to see how much research there now is into the microbiome, immunity, cancer and even glioblastoma specifically. What I would change is the amount of certainty I attached to some of the information we found.
When you're trying to keep someone alive, a test result that says something is high or low feels like an instruction. Fix this. Increase that. Remove this. Add that. I wanted every piece of information to give me something I could do. Now I understand better that the microbiome is an extraordinarily complicated ecosystem and that measuring something isn't the same as knowing how to manipulate it or whether changing it will alter someone's cancer.

I also wouldn't assume that because something is described as supporting the immune system it must therefore help someone with cancer. Cancer biology and the immune system are considerably more complicated than that. But I don't think we were wrong to look. The most interesting thing about revisiting this page in 2026 is that something we were exploring because it seemed biologically plausible has become an increasingly serious area of cancer research. We were just much further ahead of the evidence than I understood at the time.

If I could go back, I'd still feed Stu well. I'd still care about his gut health. I'd still ask questions about antibiotics, diet and the microbiome. And if we were considering probiotics or microbiome testing, I'd discuss what we hoped to achieve with someone who understood both oncology and nutrition. I'd just carry considerably less responsibility for trying to engineer the perfect collection of bacteria inside my husband's bowel. There were quite enough things I was already trying to engineer.

CURRENT INFORMATION · 2026

GUT MICROBIOTA & GLIOBLASTOMA · 2026 REVIEW
A recent review specifically examining current research into the gut microbiota-brain axis in glioblastoma, including immune signalling, metabolism, treatment response and potential future therapeutic approaches. Much of this field is still experimental.

NATURE COMMUNICATIONS · GBM IMMUNOTHERAPY & GUT MICROBIOME
A 2025 prospective phase I/II study in newly diagnosed glioblastoma which found that gut microbiome characteristics were among several biological features associated with outcomes in patients receiving anti-PD-L1 treatment alongside standard therapy. Interesting emerging human evidence, but not evidence that manipulating the microbiome improves survival.

MACMILLAN · DIET, FOOD SUPPLEMENTS & PROBIOTICS
Current cancer-specific guidance including important information about probiotics during treatment and the potential infection risk when someone's immune system is suppressed.

BRITISH DIETETIC ASSOCIATION · PROBIOTICS & GUT HEALTH
An accessible overview of probiotics, prebiotics and gut health, including the importance of seeking professional advice if the immune system is not functioning normally.

WORLD CANCER RESEARCH FUND · FIBRE, WHOLEGRAINS & CANCER
Useful information about fibre, wholegrains, gut bacteria and current dietary recommendations. The strongest cancer evidence here relates to cancer prevention, particularly bowel cancer, rather than treatment of glioblastoma.

gut health