Vaping may increase the risk of a heart attack or stroke, warns a new study.
Researchers found that the flavouring liquid used in popular electronic cigarettes may increase the risk of heart disease when inhaled - with cinnamon and menthol dubbed "particularly harmful".
Cinnamon and menthol flavours most harmful
They investigated the effect of e-liquids - also known as vape juice - on cells that line the interior of blood vessels, called endothelial cells.
The researchers discovered that, when grown in a lab, endothelial cells exposed to the e-liquids - or to blood collected from e-cigarette users shortly after vaping - are less viable and exhibit "significantly increased levels" of molecules implicated in DNA damage and cell death.
They said the cells were also less able to form new vascular tubes and to take part in wound healing.
The severity of the damage - aspects of which occur even in the absence of nicotine -varied among popular flavours, according to the findings, with cinnamon and menthol the most harmful.
'Not a safe alternative to cigarettes'
Study senior author Professor Joseph Wu, of Stanford University School of Medicine in the US, said, "Until now, we had no data about how these e-liquids affect human endothelial cells. This study clearly shows that e-cigarettes are not a safe alternative to traditional cigarettes.
"When we exposed the cells to six different flavors of e-liquid with varying levels of nicotine, we saw significant damage.
"The cells were less viable in culture, and they began to exhibit multiple symptoms of dysfunction."
Endothelial cells line the interior surface of blood vessels and play a critical role in heart and cardiovascular health.
Although some studies have suggested that e-cigarettes deliver lower levels of carcinogens to users than do conventional cigarettes - perhaps decreasing the risk of cancer - the effect of e-cigarette use on vascular health had not been clear.
Different flavours do more damage
The use of e-cigarettes has rocketed since their introduction about a decade ago, particularly among young people.
Today, as many as 19 per cent of people in Britain vape, up from 17 per cent in 2016 (Photo: Shutterstock)
Study co-lead author Dr Won Hee Lee, now an Assistant Professor at the University of Arizona, said, "One in five high school students have tried e-cigarettes, perhaps because they feel they are relatively safe.
"But we found the e-liquids caused changes in the endothelial cells that are closely related to those seen during the development of cardiovascular disease."
The researchers investigated the effect of six different popular e-liquid flavors - fruit, tobacco, sweet tobacco with caramel and vanilla, sweet butterscotch, cinnamon, and menthol - with nicotine levels of zero, six, and 18 milligrams per millilitre on endothelial cells.
They found that while several of the liquids were "moderately toxic" to the endothelial cells, the cinnamon and menthol-flavored e-liquids "significantly decreased" the viability of the cells - even in the absence of nicotine.
The findings, published online by the Journal of the American College of Cardiology, showed that exposure to the e-liquids also increased the relative levels of reactive oxygen species - molecules that can cause DNA damage - and the levels of molecules associated with cell death.
The researchers also found that exposure to the cinnamon and menthol flavoured e-liquids "significantly disrupted" the ability of the cultured cells to form capillary-like tubular structures associated with the growth of new blood vessels.
The e-liquid flavoured with caramel and vanilla also disrupted growth, but not as severely.
Same levels of nicotine as cigarettes
The team also compared the levels of nicotine in the blood serum of people after they had vaped e-cigarettes with the levels in people who smoked traditional cigarettes.
They found that the amounts of nicotine in the blood were similar between the two groups after 10 minutes of smoking at a constant rate.
Prof Wu said, "When you're smoking a traditional cigarette, you have a sense of how many cigarettes you're smoking.
"But e-cigarettes can be deceptive. It's much easier to expose yourself to a much higher level of nicotine over a shorter time period.
"And now we know that e-cigarettes are likely to have other significantly toxic effects on vascular function as well."
He added, "It's important for e-cigarette users to realise that these chemicals are circulating within their bodies and affecting their vascular health."