On Monday, August 9th, the Archives of Internal Medicine published a ten-year study of over ten-thousand individuals that showed that waist circumference, even independent of body mass index (BMI), is directly correlated with higher rates of death. Now, this is hardly new news, but the study, covered as it was by the Washington Post, the Associated Press, and many others, reinvigorated a question that is becoming more and more prevalent in America: what’s the deal with fat?
Why would waist circumference matter independent of BMI? Why isn’t fat in one place as good or bad as fat anywhere else?
Well, the current theory goes like this: there are many different kinds of adipose tissue (fat). The one you’re most likely to picture in your head is subcutaneous fat (I would call it “subcute” fat if I were a hipster in the metabolic world)– the fat that sits below the skin, and the fat you feel when you pinch your belly. As far as researchers can tell, subcutaneous fat isn’t the “bad” fat– the fat that causes major health problems like type 2 diabetes and cardiovascular complications. The human body, it seems, can mostly handle subcutaneous fat.
The problem occurs when subcutaneous fat is just the tip of the iceberg– and underneath the skin and the first layer of fat and the organs is visceral fat. (Now that just sounds villainous, doesn’t it?) Visceral fat, also known as abdominal fat, sits deep inside the body, packed in between organs in the body. And visceral fat screws the balance of the internal ecosystem up. How, why, and in what manner isn’t exactly clear yet– but the current picture involves the fact that adipose cells recruit macrophages– important players in the innate immune system– to the adipose tissue. These macrophages cause an inflammatory response within the body, and the delicate balance of hormones, proteins, and other cellular mechanisms within the body is thrown off-kilter. Levels of important proteins like adiponectin, leptin, and of course insulin get thrown off; the secretion patterns of cytokines, proteins that help control immune processes, changes; and when these molecular changes are followed to their ends, serious health conditions like type 2 diabetes and, implied by the recent study, death can result.
But, like I said, that’s old news. The role of visceral fat and abdominal adipose tissue has been a popular topic for researchers for years now. The reemergence of this news, though, reminded me, though, that one form of visceral fat that doesn’t make the headlines: epididymal white adipose tissue.
Now, visceral fat is the parent category of several different specific kinds of fat, including perirenal fat, which acts as a little blanket for your kidneys.
But my favorite visceral fat by far is epididymal fat. I first heard about it in a study published in Nature looking at phosphorylation in adipose tissue cells caused by agents like rosiglitazone (Avandia, for example); looking up the term, I found that epididymal adipose tissue is a very popular object of study, since the mouse and rat versions are particularly sensitive to changes in insulin and glucose levels, and are relatively easy to extract from the animals.
But, as soon as I found out what epididymal fat actually was, I couldn’t help but giggle. You see, epididymal fat is the fat that lives around the epididymis. And the epididymis? The epididymis is the tangled-up tube that lives at the end of the vas deferens and helps to transport sperm out of the testes.
In other words, many researchers are very closely analyzing mouse balls. Or, better still, mouse ball fat.
The existentialist in me laughs at the absurdity of the mental image: a whole nation, up in arms over obesity, staring intently at rat testicles to find an answer.
And the twelve-year-old girl in me? Giggles. Giggles and giggles because I read about balls in Nature, and because I just got you to listen to me talk about it!