Among the many questions of what constitutes unhealthy food, one that seems to emerge time and again is about what kinds of fat are good and bad. Artificial trans fats are an easy one, the most common ones being partially hydrogenated oils. They’ve been shown to raise cholesterol and risk of heart disease — moving the Food and Drug Administration to ban them. But what about saturated and unsaturated fats? How do they affect the heart? A new study from the University of Illinois, Chicago, has that answer.

The researchers’ study compared the effects of oleate (also called oleic acid), a common dietary fat found in olive and peanut oil, to palmitate, another common dietary fat found in palm oil and thus, some foods like cookies, crackers, and microwave popcorn. Specifically, they were looking at how the two affected rats with heart failure. Heart failure affects about 5.1 million people in the U.S., and occurs when the heart can’t pump enough blood through the body. This causes a chain reaction in which the heart muscles try to compensate by enlarging and thickening to gain more strength, blood vessels narrow to keep blood pressure high, and blood is diverted from less essential organs. Eventually, the heart can’t keep pace and it fails.

Part of the heart’s ability to pump blood through the body comes from energy stores inside cells called lipid droplets. When the heart needs energy, it can tap into these droplets and release the fat for fuel. But, as the new study shows, the type of fat used makes a difference. When the rats were given oleate, the researchers saw “an immediate improvement in how the hearts contracted and pumped blood,” senior author of the study E. Douglas Lewandowski said in a press release. Palmitate, however, caused an imbalance in fat metabolism that forced cells to struggle for energy, while also producing toxic byproducts that can worsen the disease.

The effects of oleate were even seen down at the molecular level, with the researchers finding that it was able to activate genes for enzymes that metabolize fat — these genes are normally suppressed in enlarged organs like failing hearts. “This gives more proof to the idea that consuming healthy fats like oleate can have a significantly positive effect on cardiac health,” Lewandowski said.

The difference between oleate and palmitate is that the former is a monounsaturated fat while the latter is a saturated fat. The American Heart Association (AHA) says that monounsaturated fats can help reduce bad cholesterol levels while also providing nutrients like vitamin E that help develop and maintain the body’s cells — Mediterranean diets, high in monounsaturated fats have been shown to improve heart health. Saturated fats do the opposite, raising cholesterol and risk of heart disease or stroke. The AHA says that saturated fats should be replaced with monounsaturated fats, although all of them should be eaten in moderation.

Source: Lewandowski ED, Lahey R, Carley A, Wang X. Circulation. 2014.