But only recently did it become apparent that these microbes might affect cancer outcomes. Scientists at the University of Chicago discovered that mice with a strain of gut bacteria known as Bifidobacterium had a stronger immune response against melanoma tumors than mice who lacked the bacteria. They found that giving Bifidobacterium to the deficient mice slowed tumor growth. What’s more, combining the bacteria with an immunotherapy drug known as a checkpoint inhibitor nearly abolished the tumors.
Human studies showed that these checkpoint inhibitors were also more effective in cancer patients whose guts had more microbial diversity, as well as a greater abundance of several microbes, including Akkermansia muciniphila and Bifidobacterium longum. Patients with low levels of these and other microbes were less likely to respond to the treatment.
Dr. Jennifer McQuade of MD Anderson, a melanoma specialist, pointed out that some of the gut microbes that appear to improve how patients respond to immunotherapy are known to thrive on fiber. “These are bacteria that help us break down and utilize starch and fibers,” she said.
Their team examined the diets of 128 melanoma patients and found that those who regularly ate large amounts of fiber from fruits, vegetables, and other plant foods had better outcomes on immunotherapy than patients who ate the least amount of fiber. Their findings, published in Science in December, showed that every five-gram increase in daily fiber intake was associated with a 30 percent lower risk of death or cancer progression.
In the new study, patients are given daily meals that include as much as 50 grams of daily fiber from foods like beans, lentils, farro, brown rice, fruits, and vegetables — about twice the recommended amount of 25 to 30 grams of fiber a day. By comparison, the average American eats roughly 15 grams a day. (A control group will eat a healthy diet that follows guidelines from the American Cancer Society.)