I have what I believe to be a lipoma developing in my bicep, has been for over a year now.. I haven't been to the doc specifically for the tumor, so I'm not sure exactly what it is, but I have 95% certainty that it's a lipoma. I've been trying to think of the causes of this condition, and the only three things I can think of are vaping, heavy weight lifting, and random occurrence. Of course, it's natural to blame the "least healthy" habit on the reason behind lipoma development, or any unhealthy condition for that matter.. I'm sure if we developed skin cancer we would be wondering if vaping was to blame, but the unfortunate reality is that there is no scientific literature regarding the correlation between lipoma and vaping. It will be a long time before any reputable studies can be done regarding the long term effects of vaping.
NOW I will tell you this. Look up the chemical structure of triacylglycerol. Now look up the chemical structure of propylene glycol. Notice that the chemical structures are almost exactly the same. The only difference is that propylene glycol is missing a hydroxy group on the third carbon, with a hydrogen atom taking it's space. Note that fatty acids are stored in the body as triglycerides. Triglycerides are formed by the dehydrogenation reaction between triacylglycerol and free fatty acids. I would not be surprised if propylene glycol could undergo a similar reaction, where the two hydroxy groups of the propylene glycol of the electronic cigarette undergo dehydrogenation and fatty acids bind the propylene glycol. I'm 100% positive this reaction could happen in vitro, the only question is whether or not the body has biochemical pathways to facilitate this reaction. IF the body has mechanisms to bind fatty acids to propylene glycol, AND if the body does not have ways to metabolize fatty acids bound to propylene glycol, THEN, I would not be surprised to see cases of lipoma containing not only triglycerides with trace amounts of propylene glycol bound to fatty acids. I would love to do a biopsy on my lipoma and see if that was the case, because if it was the case, then it would mean the downfall of the vaping industry.
EDIT: I wouldn't love it because I dislike vaping... but I'd sure make a few dollars for the discovery!