- A growing body of research suggests that gut bacteria can change how the body uses nutrients in ways that matter for cancer.
- In one study, a single dietary amino acid acted like a switch, feeding tumours in some circumstances but strengthening cancer-killing immune cells in others.
- The result is a more complicated picture of “diet and cancer” where the gut microbiome can tilt the balance towards tumour growth or better treatment response.
Cancer is often described as a disease of mutated cells, but tumours do not grow in isolation.
They sit inside a “microenvironment” made up of blood vessels, structural tissue, signalling molecules and immune cells that can either attack the tumour or be pushed aside by it.
What is easy to miss is that this battleground is partly supplied by the gut.
Nutrients absorbed from food, along with microbial by-products created in the intestines, flow into the bloodstream and can influence both tumour cells and the immune cells trying to control them.
In this work, researchers focused on asparagine, an amino acid the body uses to build proteins and support cell survival.
Asparagine matters because both sides of the cancer fight want it. Tumour cells, especially in the cramped, nutrient-poor conditions inside a growing tumour, can rely on amino acids to keep building and repairing themselves.
At the same time, CD8+ T cells, the immune system’s front-line “killer” cells that can recognise and destroy cancer cells, also need the right fuel and building blocks to stay active and effective over time.
The new twist is that gut microbes can set the baseline for how much asparagine leaves the gut and enters the circulation. The team studied a common gut bacterium, Bacteroides ovatus, which can carry a gene called bo-ansB. That gene produces an enzyme that breaks down asparagine.
In mouse models carrying human-like gut microbiota, bacteria with an intact bo-ansB gene consumed more asparagine in the intestine.
With more of the amino acid being used up in the gut, less was absorbed into the bloodstream and delivered to tissues elsewhere in the body, including tumours.
To test whether this microbial “sink” was truly responsible, the researchers removed the bo-ansB gene from the bacterium.
Without it, the bacteria could no longer deplete asparagine in the intestine in the same way. More asparagine then passed into the bloodstream and reached the tumour.
That simple genetic change in a gut microbe altered the nutrient landscape that both tumours and immune cells had to compete within.
The most striking finding was that the consequences depended on who benefited from the asparagine in the tumour.
In colorectal cancer models given extra dietary asparagine, mice colonised with the bacteria that could deplete asparagine were more likely to show tumour growth.
When the bacteria lacked bo-ansB, the same asparagine-rich diet produced the opposite pattern: more asparagine reached the tumour and was taken up by CD8+ T cells.
Those immune cells shifted into a “stem-like” state, a long-lived mode that helps maintain a renewable supply of cancer-fighting T cells over time. In practical terms, this stem-like pool can keep generating fresh waves of killer cells rather than burning out quickly.
Mechanistically, higher asparagine levels in the tumour environment were linked to increased expression of a transporter protein, SLC1A5, on CD8+ T cells.
Think of this as the immune cell putting more “doors” on its surface to pull in the amino acid it needs.
When researchers blocked SLC1A5, the immune advantage disappeared, which suggests the transporter is a key part of how asparagine availability is translated into stronger anti-tumour immunity.
Taken together, the study argues against a one-size-fits-all view that a nutrient either “feeds cancer” or “starves cancer”.
The same nutrient can support tumour cells and immune cells, and the gut microbiome can decide which side gets more access.
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That has practical implications.
Instead of targeting the tumour alone, future strategies might try to reshape gut bacteria or adjust diet in ways that reduce tumour advantage while supporting durable immune responses.
It also hints at a monitoring angle: microbial genes and enzymes involved in nutrient metabolism could potentially serve as biomarkers that help predict disease progression or response to immunotherapy.
There are still important caveats.
These are controlled experimental models, not a ready-to-use dietary rule for people with cancer. Human microbiomes vary massively, diets are complex and tumours differ in how they use nutrients.
The real value here is the concept: diet, microbiota and immune function form a linked system, and cancer care may eventually use that system deliberately rather than treating it as background noise.
