For years doctors have urged people to watch what they eat. A big part of this has involved maintaining low levels of cholesterol, a complex lipid that is essential for animal cell membranes. Cholesterol is naturally synthesized in the liver, but it can also be absorbed from what we eat. While some level of cholesterol is necessary to maintain healthy cell function, elevated cholesterol can have severe consequences. Increased cholesterol has been shown to play a role in cardiovascular disease, with cholesterol “plaques” blocking blood flow and leading to heart attacks and strokes. Because of these health concerns, many people believe in low cholesterol diets, avoiding cheeses, eggs, and meats.
While these health-conscious diets might make sense for humans, they would be baffling to polar bears (Ursus maritimus). Due to their extremely fatty diets, consisting largely of blubbery seals, polar bears can develop huge fat deposits. In some cases, fat can make up ~50% of a polar bear’s weight. Cholesterol levels in these bears can be through the roof: one study found that fasting bears (having not eaten for 1-2 months) had an average total cholesterol level of 381mg/dl, almost double the recommended level for a healthy human. So how do polar bears stay healthy with such unhealthy cholesterol levels?
A recent paper published in the journal Cell might help shed some light. The authors started by examining polar bears and their close cousin the brown bear (Ursus arctos). By sequencing the polar bear and brown bear genomes, the authors found that the two species have only diverged around 343-479 thousand years ago (a tiny amount of time, evolutionarily speaking). Despite the short time span, the authors found that a number of genes had been mutated in the polar bear. Specifically, genes related to cholesterol regulation and vascular disease.
One of the genes showing significant changes was APOB, a protein component of low-density lipoprotein (LDL), the so-called “bad” cholesterol. The authors found 9 missense mutations in the polar bear APOB gene, compared with other species of bears. It is hypothesized that these changes are one of the factors that helps polar bears survive with such extremely high cholesterol levels! Future studies will have to be conducted, but it is possible that these findings can be translated into humans (we also have the APOB gene), helping those with elevated cholesterol.
For hands on experiments into cholesterol detection and familial hypercholesterolemia (FH), a genetic condition that results in elevated cholesterol, check out Edvotek’s cholesterol experiments!