While the term "high cholesterol" is usually associated with health problems, there is at least one way in which most people would prefer to have high cholesterol: In their inner ear, so that they will continue to have strong hearing as they get older. At least, that is the premise a recent study in the scientific journal PLOS Biology, whose authors found that plant-based substances called phytosterols — which look and act like cholesterol — improved the hearing in mice who had grown deaf while losing their inner ear cholesterol.
"Aging triggers cholesterol loss from sensory cells of the inner ear."
The Argentinian scientists speculated that because hearing loss occurs as people get older and the outer hair cells (OHCs) in their inner ears lose their elasticity, there could be a connection between this tendency and the fact that the brain loses cholesterol with age. Similarly, they wanted to test whether a common over-the-counter drug could help treat this condition.
To test their hypothesis, the scientists measured an enzyme called CYP46A1 in mice inner ear OHCs because its presence is inversely correlated to the presence of cholesterol. After determining that older mice had more CYP46A1 in their inner ears than younger mice, and therefore had less cholesterol, the researchers used an HIV drug called efavirenz to induce hearing loss in mice. This allowed them to test whether the presence of cholesterol was linked to hearing.
They discovered that when CYP46A1 was over-activated in younger mice, they suffered hearing loss. Yet when the researchers treated the mice with a plant-based cholesterol-like compound called phytosterols, the artificially deafened mice experienced improvement in their hearing.
"Our findings are very promising because they provide the first proof-of-principle supporting phytosterols supplementation as a possible approach for prevention or treatment of hearing loss," the authors wrote, concluding, "our findings point towards the importance of cholesterol homeostasis in the inner ear as an innovative therapeutic strategy in preventing and/or delaying hearing loss.".
"Our findings point towards the importance of cholesterol homeostasis in the inner ear as an innovative therapeutic strategy in preventing and/or delaying hearing loss."
Phytosterols, which can be easily purchased in many pharmacies and nutritional stores, are currently used to help treat cholesterol problems. They work by having a cell structure that looks and acts like cholesterol, and therefore competes with cholesterol in the digestive system. As a consequence of this competition, the body digests the phytosterols instead of the cholesterol and removes some of that cholesterol as waste. Phytosterols can thereby lower the unhealthy form of cholesterol known as low-density lipoprotein cholesterol (LDL), which can clog the heart and arteries and thereby cause serious cardiovascular diseases.
That is not to say that phytosterols are without risk. If people have a genetic condition called sitosterolemia, cholesterol will build up in the body when consumed and so will phytosterols. There is also the possibility that taking too many phytosterols can lead to atherosclerosis, a disease in which the walls of arteries develop abnormalities or lesions. Pregnant people or children should also avoid them.
The recent study is also significant because, as the authors explain, "the role of cholesterol homeostasis in the physiopathology of the inner ear has not been studied."
Of course, just because this treatment was promising in mice, that doesn't always mean it will translate to humans. But it does give scientists a place to start and in general, phytosterols are well-tolerated supplements. It could one day lead to a treatment for age-related hearing loss.
That said, further research on the subject is necessary. As the authors point out in their paper, other scientists will need to replicate their studies on other strains of mice before doctors can feel comfortable testing this on human beings. In addition,"although our results show that in aged cochlear tissue there is a reduction in cholesterol content in sensory cells of the inner ear — assessed by filipin labeling — and that this reduction is correlated with an increase in the immunofluorescence of CYP46A1, it will be important to test alternative methods for CYP46A1 quantification in specific cell populations."
