Adding flavouring liquid to electronic cigarettes may increase heart disease risk, recent findings suggest.

According to the study, the flavouring liquid added to e-cigarettes may increase the risk of cardiovascular diseases when inhaled.

As part of the study, the scientists investigated the effect of the e-liquids on cells called endothelial cells that line the interior of blood vessels. They found that, when grown in a laboratory, 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.

The cells are also less able to form new vascular tubes and to migrate and participate in wound healing.

The severity of the damage, aspects of which occur even in the absence of nicotine, varies among popular flavours, the researchers said. Cinnamon and menthol were found to be particularly harmful.

“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 flavours 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,” said Joseph Wu, lead author of the study.

The researchers studied human endothelial cells generated in the laboratory from what are called induced pluripotent stem cells, or iPS cells. Human iPS cells can become many different cell types, and they provide an ideal way for researchers to closely study cells that would be difficult to isolate directly from a patient.

The researchers investigated the effect of six different popular e-liquid flavours – fruit, tobacco, sweet tobacco with caramel and vanilla, sweet butterscotch, cinnamon, and menthol – with nicotine levels of 0, 6, and 18 milligrams per millilitre on endothelial cells derived from human iPS cells.

They found that while several of the liquids were moderately toxic to the endothelial cells, the cinnamon and menthol flavoured e-liquids significantly decreased the viability of the cells in culture even in the absence of nicotine.

The findings of the study were published in the Journal of the American College of Cardiology.

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 programmed 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. The cells exposed to cinnamon, caramel and vanilla flavours exhibited an increased uptake of low-density lipoproteins and lipids – processes commonly associated with inflammation and endothelial dysfunction – and a reduction in their ability to migrate to heal wounds or scratches.

Some of the effects of exposure to the various e-liquids were dependent on the nicotine concentration, but others, like cellular migration and decrease in cell viability, were independent of nicotine, suggesting a combined effect of nicotine concentrations and flavouring components.

Finally, the researchers 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.