The researchers focused on one particular fat found in the blood of the mice fed a ketogenic diet: palmitic acid, which is commonly found in animal fats and dairy products. Remarkably, mice fed a normal diet who were injected with palmitic acid also became more susceptible to sepsis.

In this study, Napier and colleagues learned that palmitic acid can trigger trained immunity. The fat acts as a "brief pulse of inflammation" that alters the function of stem cells in the mouse's bone marrow so that they produce more inflammatory innate immune cells in the future. This means that when the innate immune system encounters a second inflammation stimulus later on, it responds much more strongly. Sometimes, as in the case of sepsis, this response is too strong.

It's this double-edged sword where if you have exposure to high fat and then exposure to a disease where more inflammation exacerbates the disease, then it's a bad thing," says Napier. "But if you're in the context where you eat high fat and then you get an infection and more inflammation helps you clear infection quicker, it's a good thing.