Smoking cigarettes has long been established as a major cause of heart disease deaths; now, there's growing evidence that electronic cigarettes may hurt the heart, too.

In two separate studies that will presented at the upcoming meeting of the American Heart Association, e-cigarettes use was shown to impact cholesterol, as well as the body's ability to pump blood.

In one study, researchers at the Boston University School of Medicine compared the cholesterol levels among four groups of adults: people who used e-cigarettes, people who smoked regular cigarettes, those who used both products, and nonsmokers. All 476 participants were basically healthy and had not been diagnosed with heart disease.

But those who vaped, the study found, had higher levels of unhealthy LDL cholesterol, on average, compared with nonsmokers. And levels of healthy HDL cholesterol were lower among people who used both traditional and e-cigarettes.

"This is a snapshot of what is happening right now" in the bodies of people who vape, said Dr. Sana Majid, a study author and postdoctoral fellow in vascular biology at Boston University School of Medicine.

"There is a lot we still don’t know about electronic cigarettes. It’s going to take time for us to understand how e-cigarettes affect your heart health" in the long-term, she told NBC News.

A second, smaller study looked at how smoking either traditional cigarettes or electronic cigarettes with nicotine impacted the heart's ability to pump blood through the body at rest and during exercise. That research included 19 smokers in their 20s and 30s, as well as an additional small group of people who didn't use either tobacco product.

Normally, blood flow increases during exercise. This was predictably observed in the nonsmoker group, using a type of ultrasound imaging called myocardial contrast echocardiogram.

Traditional cigarette smoking is known to reduce blood flow during exercise, which was also observed in the study. After exercise, the study found, blood flow returned to normal in smoking participants.

But the researchers found something surprising when they measured the blood flow of e-cigarette users: Their hearts' ability to pump blood was diminished both during exercise and rest.

"It’s evidence that there’s something wrong with the blood flow regulation in smokers and maybe even more so in e-cigarette smokers," said study author Dr. Florian Rader, medical director of the Human Physiology Laboratory and assistant director of the Noninvasive Laboratory at the Smidt Heart Institute at Cedars-Sinai Medical Center in Los Angeles.

"These products are marketed as healthy alternatives, and yet we see more and more evidence that they’re definitely not healthy," he added.

This is not the first time vaping has been found to impact blood vessel function. Research published in August by the University of Pennsylvania also showed e-cigarette use reduced blood flow, and the effect lasted for about an hour after vaping.

In that study, the e-cigarettes contained no nicotine, only flavorings and sweeteners.

Vaping's effect on lungs

Meanwhile, vaping's effects on the lungs is well documented. The new findings come amid a growing epidemic of what health officials are calling EVALI, short for "e-cigarette or vaping product use associated lung injury." For several months, the Centers for Disease Control and Prevention has been steadily increasing the number of confirmed cases of the vaping-related illness, which now stands at 2,051.

Patients arrive at the hospital with severe symptoms: trouble breathing, cough, fever, extreme fatigue, vomiting and diarrhea. Many have needed mechanical assistance breathing in intensive care units.

For some, the illness has been fatal. At least 39 patients have died of EVALI. And more deaths remain under investigation.

No one brand or ingredient has been singled out as the cause. However, the CDC reported a "breakthrough" last week, pointing to vitamin E oil as a cause.

The Trump administration is expected to take some kind of action on vaping this week, though it's unclear whether that will include a ban on mint, candy and fruity e-cigarette flavors, which have been shown to be popular among kids and young adults who vape.

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