Obesity elevates the risk for at least 13 major cancers, including those of the breast, colon and liver.

It also impairs immune responses that target tumours and are stimulated by cancer immunotherapies.

But it has long been unclear whether these effects stem from the sheer adiposity—or mass of fat—in people living with obesity or from the specific dietary fats they consume.

Now, a decade-long study led by Ludwig Princeton’s Lydia Lynch and reported in Nature Metabolism has provided a compelling answer to that question.

“Our study reveals that the source of dietary fat, not adiposity itself, is the primary factor that influences tumour growth in obese mice,” said Lynch.

“We found that high-fat diets derived from lard, beef tallow or butter compromise anti-tumour immunity and accelerate tumour growth in several tumour models of obese mice. Diets based on coconut oil, palm oil or olive oil, meanwhile, do not have this effect in equally obese mice. Our findings have implications for cancer prevention and care for people struggling with obesity.”

Lynch and her colleagues—including Marcia Haigis, a senior author of the study and a member of the Ludwig Centre at Harvard University—note that swapping out animal fats for plant fats might be a helpful dietary intervention for obese patients undergoing treatment for cancer.

Such dietary changes could also potentially lower cancer risk for people living with obesity.

Lynch, Haigis and others have previously shown that obesity induces changes in the immune system and in the microenvironment of tumours that contribute to tumour progression.

It does so by impairing the body’s cancer surveillance system, undermining the ability of immune cells—namely cytotoxic T cells (CTLs) and natural killer (NK) cells—to infiltrate tumours and, once there, kill their cancerous quarry.

In the current study, Lynch and her colleagues took those findings a step further.

They elucidate the mechanisms by which the metabolic derivatives of dietary animal fats impair NK cells and CTLs.

They also show that it is this immune dysfunction that helps hasten tumour progression in obese mice—an effect not seen in obese mice fed plant-based fats.

In fact, a palm oil-based diet even boosted anti-tumour immunity and slowed tumour growth in obese mice.

The researchers identified several metabolic intermediates of dietary fats, especially long-chain acylcarnitine species, that potently suppress NK cells and CTLs.

These metabolites were especially elevated in obese mice reared on butter, lard and beef tallow diets, but not in obese mice on plant-fat diets.

They report that, in CTLs, the molecules cause deep metabolic dysfunction in organelles known as mitochondria—the powerhouses of cells—compromising their cytotoxic function.

This saps anti-tumour CTLs of their vitality, undermines their production of a factor (IFN-γ) critical to their function and disables their cell-killing machinery.

The palm oil-based diet, on the other hand, prevented metabolic paralysis in the NK cells of obese mice, apparently by amplifying the activity of a master regulator of cellular metabolism known as c-Myc.

The researchers found that Myc expression was reduced in mice fed animal fat—and also in NK cells from people living with obesity.

“These findings highlight the significance of diet in maintaining a healthy immune system,” said Lynch.

“More important, they indicate that modifications to fat in the diet may improve outcomes in obese people undergoing treatment for cancer and suggest such measures should be clinically evaluated as a potential dietary intervention for such patients.”

Source: Ludwig Institute for Cancer Research