Researchers at VA hospitals across the country are finding that vets with post-traumatic stress disorder are more likely to suffer from heart disease as well.

However, there’s no scientific explanation for why it’s happening.

“There’s now a large body of evidence that unequivocally links trauma exposure to poor physical health,” Dr. Paula Schnurr of the VA’s National Center for Posttraumatic Stress Disorder told VA Research Currents. And some doctors are seeking a causal link between PTSD and coronary heart disease.

Last year, researchers at the San Francisco VA Medical Center published a study of 663 vets at two sites in California. About a third of the vets met the criteria for PTSD, and 17 percent of them also had reduced blood flows to their hearts, a condition known as ischemia. Only 10 percent of the non-PTSD vets experienced ischemia.

After researchers corrected for clinical and demographic factors influencing heart disease (age, past heart disease, inflammation, obesity, alcohol use, sleep quality, depression and social support), PTSD was associated with twice the risk for ischemia, according to VA Research Currents.

Furthermore, researchers found that the more severe the PTSD, the greater the risk of heart disease.

That’s in line with another study published last year analyzing data from the VA’s Vietnam Era twin (VET) registry, which includes about 7,000 twin pairs, all of whom served in the U.S. military during the Vietnam era. It also found heart disease more than twice as likely among vets with PTSD.

Findings like those leave researchers grappling with a host of questions.

Inflammation has been associated with PTSD, and it’s also a major factor in heart disease. Could this prove to be a causal link? Or does the problem lie with the bad health habits frequently associated with PTSD, things like smoking and drinking and not exercising? Does the inability to sleep well diminish the immune system’s ability to protect the heart?

Or do stress hormones damage blood vessels over time? Many combat vets are locked into a state of hyperarousal in which they’re reliving past traumas and preparing for future ones. This is a very physical process with norepinephrine and other stress hormones making the heart beat faster, constricting the arteries for higher blood pressure, and pumping more glucose into the blood for instant energy. This is great for an emergency, but if it happens over and over again, it can wear out arteries and heart muscle.

All these factors are plausible, and it’s likely that they’re all interrelated in ways we don’t fully understand yet.

Dr. Schnurr told VA Research Currents that she leans toward the allostatic load theory as an explanation. That holds that each of these physiological stress factors contributes to the overall wear and tear on the body cumulatively.

“For the most part, the biological changes we see are not clinically remarkable,” she was quoted as saying. “So it’s hard to argue that any by themselves could lead to the types of physical health changes that we see in PTSD. So conceptually, this theory makes sense.”

And it also suggests strongly that treating PTSD can reduce the risk of heart disease.