Swapping out animal fats for plant fats might be a helpful dietary intervention for obese patients undergoing treatment for cancer
A new study published in Nature Metabolism suggests that the type of fat we consume—not just how much we weigh—can influence cancer risk. In the study, researchers from Ludwig Cancer Research, Harvard Medical School, and Trinity College Dublin found that obese mice fed diets rich in animal fats like butter, lard, or beef tallow developed faster-growing melanomas than equally obese mice fed plant-based fats like olive oil, palm oil, or coconut oil.
The difference wasn’t weight, but biology. “Our study provides an important proof of principle that dietary fat can regulate immune function in obesity-related cancers,” write the researchers.
“We found that high-fat diets derived from lard, beef tallow or butter compromise anti-tumor immunity and accelerate tumor growth in several tumor 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,” said Lydia Lynch, a leader in the research.
Lynch and her colleagues from Harvard—including Marcia Haigis, a senior author of the study and a member of the Ludwig Center 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.
The Science: It’s About Acylcarnitines, Not Calories
The researchers discovered that animal-based fats caused a buildup of long-chain acylcarnitines—molecules that impaired CD8+ T cells and natural killer (NK) cells, both critical for detecting and killing cancer cells. These metabolites interfered with mitochondrial function, reducing the immune system’s ability to fight tumors.
By contrast, mice consuming olive oil, palm oil, or coconut oil did not experience this immune suppression—even though they gained the same amount of weight.
According to Lydia Lynch, senior author of the study and an immunologist at Harvard and Trinity,
“It’s not just obesity—it’s what kind of fat you eat. We show that some fats can paralyze the immune system’s anti-cancer response.”
These findings were also observed in human NK cells, where acylcarnitine buildup similarly impaired mitochondrial function, indicating the mechanism may be relevant to humans, though more research is needed.
What About the Butter Backlash?
This new study may challenge recent nutrition trends—particularly those backed by figures like Robert F. Kennedy Jr., who advocate for a return to traditional, animal-fat-heavy diets and question the safety of seed oils. While Kennedy and others have popularized the idea that saturated fats like butter are unfairly vilified, this research suggests animal fats could pose specific risks to cancer immunity, especially in the context of obesity.
It’s important to note, however, that the study did not examine seed oils like soybean, canola, or sunflower, which are high in polyunsaturated fatty acids (PUFAs) and the subject of their own scientific debates.
In short, this study is not a vindication of seed oils, but it does raise red flags about animal fats—at least in obese contexts and in mice.
Not All Saturated Fats Are the Same
Interestingly, the study showed that palm oil (also saturated) did not have the same cancer-promoting effects as butter or lard. This challenges simplistic categorizations of “saturated = bad” and points to a more nuanced relationship between fat structure, metabolism, and immune function.
It also underscores that obesity-related cancer risk may depend as much on what type of fat is stored and metabolized as on the amount of body fat itself.
