Leukaemia

Why Gut Bacteria Matter During Leukaemia Treatment

The evidence here is strong and comes from one of the largest studies of its kind, spanning four hospitals on two continents. Leukaemia is cancer of the blood-forming cells in bone marrow. For many people the treatment offering the best chance of cure is a stem cell transplant from a donor. That treatment is also where the gut turns out to matter most, because of what happens to the bacterial community along the way.

A treatment that flattens the gut community

A stem cell transplant involves destroying the existing bone marrow with chemotherapy or radiotherapy, then replacing it with donor cells.

That process is brutal on the gut. The chemotherapy damages the intestinal lining directly. Patients receive broad-spectrum antibiotics because a wiped-out immune system makes infection lethal. Eating is often difficult, so fibre intake collapses.

Researchers profiled 8,767 stool samples from 1,362 patients undergoing transplants at four different centres.

The pattern was consistent. Bacterial variety fell sharply, and communities became dominated by single organisms. Domination was defined as one bacterial group occupying at least 30% of the entire community.

That's the opposite of a healthy gut, where no single organism holds anything like that share.

Source: Microbiota as Predictor of Mortality in Allogeneic Hematopoietic-Cell Transplantation

What variety predicted

The reason this study mattered is what happened next to those patients.

Higher bacterial variety around the time the donor cells took hold was associated with significantly lower mortality. Lower variety, both before the transplant and during that critical window, predicted worse two-year survival.

The clearest signal concerned graft-versus-host disease, where donor immune cells attack the recipient's own tissues. It's one of the main causes of death after transplant, and the gut is among the organs it attacks.

Among patients with higher bacterial variety, 17 of 244 died from that complication. Among those with lower variety, 26 of 184 did. That's a hazard ratio of 0.49, meaning roughly half the risk.

The study didn't find more cases of the complication in the low-diversity group. It found worse outcomes among those who developed it.

Why the immune system is the link

The connection runs through what the donor immune cells encounter when they arrive.

A transplant essentially installs a new immune system. That system has to learn what to tolerate, and the gut is where the largest share of that education happens.

A varied bacterial community produces the compounds that support regulatory immune cells, the ones restraining attacks on the body's own tissue. A community reduced to one dominant organism produces far less of that restraint.

The gut lining matters too. Damaged and short of the fuel its cells run on, it becomes leaky, and bacterial fragments crossing into the blood drive the inflammation that makes the attack worse.

That fits the finding precisely. Variety didn't prevent the complication. It changed how severe it became.

What can and can't be done about it

Some caution is needed on what follows from this.

Antibiotics during transplant are not optional. They prevent infections that would otherwise kill people, and nobody is suggesting withholding them.

A trial did test whether nutritional support during transplant protected the bacterial community. It found no significant difference between the intervention and control groups, with major depletion of bacteria and their products happening regardless.

That's an honest null and worth knowing. Feeding people better during the acute phase didn't rescue the community.

What did track with survival in that same trial was informative. One-year mortality was higher in patients with lower variety three weeks after transplant, and higher abundance of Blautia, a fibre-fermenting group, was found in survivors.

Trials of stool transplants to restore the community after treatment are now under way.

Source: Mortality and microbial diversity after allogeneic hematopoietic stem cell transplantation: secondary analysis of a randomized nutritional intervention trial

What this means for you

Leukaemia needs urgent specialist care, and treatment decisions including whether to have a transplant belong with a haematology team. In addition, the research above found that people whose gut bacteria stayed varied through a stem cell transplant were significantly more likely to survive, with roughly half the risk of dying from graft-versus-host disease. A varied, plant-rich diet in the periods when eating normally is possible supports the same gut bacteria this research ties to better survival. That's the same idea behind GutLab, everyday gut health starts with regularly feeding the microbiome a broad range of plant-rich ingredients.