Men seem to have the short end of the stick when it comes to heart disease. Even men who eat right, exercise,don’t smoke, and maintain a healthy body weight are at higher risk of it. No one’s known for sure why this is the case, but a new study suggests it may simply be that they carry the Y chromosome in their cells.

The researchers who headed the new study looked at the presence of coronary artery disease (CAD) in men, in which the arteries become narrowed over time. Narrowed arteries means that not enough blood (and therefore oxygen) can reach the heart, which puts it at significant risk of damage. The team analyzed the men’s genomes, and found that 90% of the men had one of two “haplogroup” gene variants: haplogroup I and haplogroup R1b1b2.

It is possible that having an extra X chromosome (that is, being a woman) might confer some sort of protective effect on the heart. But more research will of course be needed to sort these issues out.

The men with the haplogroup 1 variant had a 50% increased risk of developing CAD over the study’s five-year period. What was particularly telling was that this increased risk was present even when other classic risk factors like smoking, blood pressure, and cholesterol levels were controlled for.

"The major novelty of these findings is that the human Y chromosome appears to play a role in the cardiovascular system beyond its traditionally perceived determination of male sex,” said author Maciej Tomaszewski in a new release. Of course, the task now will be to determine exactly why these variants on the Y chromosome could influence heart risk, in the absence of other risk factors.

One possibility is that the immune system is the missing link. The team found that men with the halpogroup 1 variant also had “downregulations” in certain pathways in their immune systems. The immune system is also thought to play a role in heart disease risk, and increased levels of inflammation in the body are well known to be a predictor of cardiovascular disease.

It is possible that having an extra X chromosome (that is, being a woman) might confer some sort of protective effect on the heart. But more research will of course be needed to sort these issues out. Though it seems like the study offers more evidence that our health is largely a matter of our genes, just as much research shows that the opposite is true too: one’s lifestyle can significantly affect one’s risk level for many health problems, so making the best lifestyle choices is still the best route to take.

Tomaszewski is a researcher at the University of Leicester; the study was published in the The Lancet.