When we think of clogged arteries, we tend to vilify hamburgers, french fries, and other high-fat indulgence foods.

But these fatty foods have a quiet accomplice: sugar. Or rather, the immune cells that over-indulge on it.

Researchers from Stanford University have found that immune cells lurking in plaque buildups in arteries tend to over-consume sugar. When they do, they produce an over-zealous inflammatory response, which likely contributes to coronary artery disease (CAD)—a condition in which constricted arteries restrict or block blood-flow around the heart, leading to heart attacks, heart failure and death. The disease affects about 16 million (pdf) Americans annually. Their research was published (paywall) Monday (Feb. 29) in the Journal of Experimental Medicine.

“CAD is not just a disease of french fries and hamburgers, but a disease of the immune system,” Cornelia Weyand, a rheumatologist and immunologist at Stanford and lead author of the study, told Quartz. Weyand and her colleagues found evidence that as certain types of white blood cells, which help fight infections, consume sugar in the blood, they produce a hormone called interleukin-6, which is associated with inflammation. Our bodies also produce IL-6 when we’re experiencing high levels of stress.

Inflammation is a crucial part of our immune systems; without it, we wouldn’t be able to recover from anything from superficial cuts to live-saving surgeries, Weyand said. But equally critical to summoning an immune response is our body’s ability to turn it off. For years, scientists have understood that chronic inflammation plays a role in coronary artery disease and other conditions like rheumatoid arthritis, but they haven’t understood the specific mechanisms.

For this work, Weyand and her team examined the blood of 140 patients with CAD who had suffered at least one heart attack, and over 100 samples of blood from healthy patients. The immune cells from patients with CAD tended to consume more sugar than the immune cells from healthy patients.

“They love to eat sugar. They pull in sugar, and they burn it. As they burn it, they make a few mistakes,” Weyand says.

First, these cells produce extra IL-6, which causes inflammation. And in doing so, they also produce free-radicals, which are highly reactive chemicals that can cause damage to other parts of cells, like DNA or proteins needed for healthy cell function.

One of the proteins Weyand found to be damaged by these free radicals was a protein that normally helps derive energy from sugar. The alteration causes this protein to make its way into the cell’s nucleus, and hijack the cells normal activity to tell it to produce more inflammatory hormones. It’s something of a snowball effect.

Weyand says that these changes are likely a result of many sets of genes in a person’s body, and is currently working with her team to study the epigenetics—or family history of genes—that may lead to these other inflammatory side effects.

Medications that can target immune cells that produce too much IL-6 don’t currently exist. Weyand and her team proposed several possible pharmaceutical routes that could be pursued. Simply altering your diet to keep fats and sugars low isn’t enough keep your heart and arteries healthy, if you’re at risk for CAD, Weyand said. If there is sugar in your blood—and you should always have some—over-active immune cells will just keep gobbling it up.

The best thing you can do to keep your arteries clear and healthy, though, is exercise. “Exercise is an important way of regulating glucose levels,” Weyand says. When we exercise, our muscles pull in glucose to use for energy. Without that extra sugar in our blood streams, it’s unavailable for immune cells to over-indulge.