If you are receiving chemotherapy treatment for melanoma and wonder why the chemo isn’t working as well as doctors had told you, one of the reasons could be your gut. Or you might be receiving chemotherapy for lung cancer, or leukemia and wonder why it’s not working the way doctors said it should. This can and often is quite discouraging, especially when your life is ultimately on the line.

But how can your gut affect treatment for skin cancer, lung cancer or leukemia?

With over 200 known cancers affecting more people these days, researchers are constantly working to find new and more effective treatments for many of them. In laboratory tests, a chemotherapy may work very effectively in destroying cancer cells, but when it’s administered to real patients, the results are different. In some people, the chemo is very effective, but in others, it’s not nearly as effective. Why?

Two different research teams believe they now know why and it could be nothing more than a person’s gut.

Inside your gut lives a host of different microbes that helps digest food, absorb different nutrients and generally keeps your body functioning. Ever hear the old adage of starve a fever? The reason is that a fever kills off a significant number of those microbes, referred collectively as the microbiome. Without the microbes, your gut can’t process the food properly and you end up vomiting. That’s why when a person runs a fever, they should eat a bland diet of things easily digested. Harder and greasier foods like hamburgers and steaks don’t do well and you’ll often taste them going down and coming back up.

The content of your gut microbiome also affects your body’s immune system and that’s why it can hinder the effectiveness of some chemotherapies. According to one report:

“The scientists found that those who had the most diverse gut microbes were most likely to respond to the immunotherapy. And tumour growth was reduced in mice that received faecal transplants from people who responded to immunotherapy.”

“The type of microbe was also linked to differences in responses to treatment, the researchers discovered. For example, people whose guts contained a lot of bacteria from a group called Clostridiales were more likely to respond to treatment, whereas those who had more Bacteroidales bacteria were less likely to respond.”

Researchers also found that some antibiotics can also make a difference on the effectiveness of chemotherapy. Many cancer patients develop secondary infections that are treated with anti-biotics and this is found to have a similar impact on chemo as the gut microbiome. According to the report:

“A second study showed that people who received antibiotics to treat infections shortly before or after starting immunotherapy did not respond as well to PD-1-blocking therapies.”

PD-1 and PD-L1 are proteins which are often targeted by certain chemotherapies. The chemo blocks either or both of these proteins, which in turns allows the drugs to more effectively kill cancer cells, but some antibiotics prevent the chemo from blocking the proteins, rendering the chemo less effective.

If you or someone you know isn’t responding to chemo the way doctors expect, it may be wise to examine what medications are being taken and have a stool sample tested for microbes. If lacking one or more important microbes, a simple repopulation of those microbes could help the chemo work more effectively.