Friday, July 03, 2009
Believe it or not fat cells are some of the coolest cells in the human body. When I first came to do my Master's at Queen's, I just assumed that fat cells were a bad thing, and the fewer you had the better. Sure, you needed a minimum amount of fat to protect your organs and avoid reproductive problems, but otherwise I didn't think that fat cells were all that important. Fat cells are just a passive depot that stores excess calories, right? Wrong.
One of the very coolest things about fat tissue is that it is an active endocrine gland, secreting hormones which have effects throughout the body. These include inflammatory hormones like IL-6 and TNF-alpha, which can directly increase the risk of heart disease and diabetes, but also include the anti-inflammatory hormone adiponectin, which increases insulin sensitivity and reduces liver fat. In a perfect world you would have lots of adiponectin, and not very much IL-6 or TNF-alpha.
When fat cells are small and healthy, they tend to secrete mostly adiponectin, which helps reduce inflammation, and keeps the body sensitive to insulin. Not surprisingly, high adiponectin levels are associated with reduced risk of contracting diabetes or heart disease. However, when fat cells grow too large they begin to secrete large amounts of the inflammatory hormones, and decrease their secretion of adiponectin, and these changes are thought to be a key mechanism linking excess body fat with metabolic risk (visceral fat, the "bad" fat stored around the mid-section, is thought to secrete especially high amounts of inflammatory hormones). This is one of the reasons why obesity is such a heterogenous condition - if you have lots of body fat, but it is stored in a large number of small fat cells, you are likely to be quite healthy. In contrast, if you have even a small amount of body fat, but it is stored in a small number of ballooning fat cells, you are likely to have very poor metabolic health (this is what is seen in individuals with lipodystrophy resulting from HIV treatment).
All of this brings me to a new paper published ahead of print this week in the Journals of Gerontology (Series A) which was authored by myself (my first as a lead author!), Peter, Dr Lance Davidson, Dr Jean-Pierre Despres, Dr Bob Hudson, and my Master's supervisor Dr Bob Ross. We examined the links between body fat distribution and adiponectin levels in abdominally obese, elderly men and women. What we found surprised us - although adiponectin levels were strongly and negatively associated with visceral fat levels in elderly women (which was expected), there was no association between adiponectin levels and visceral fat in elderly men (very unexpected). Further, while adiponectin was strongly associated with other markers of metabolic health like insulin sensitivity and triglyceride levels in women, there were no such associations in the men.
For some reason, all of the normal associations between adiponectin and other factors were present in the elderly women in our sample, but not in the elderly men.There are a few reasons why this could be happening. Adiponectin is thought to be cleared by the kidneys, and some papers report that kidney function drops off more in elderly men than it does in elderly women. In line with this theory, adiponectin levels were associated with age in men, but not women in our sample. Thus adiponectin might just build up in older men, obscuring the true relationship between adiponectin and markers of metabolic health or body fat distribution. As of right now though, that is mainly speculation. This is important though, because there is some evidence that in elderly men, adiponectin may actually increase, rather than decrease the risk of mortality. No one knows why this is the case, but it's clear that something very strange happens to adiponectin levels in elderly men, and I hope that this study helps us figure out why that is.
Now the title of the paper ("Associations of the limb fat to trunk fat ratio with markers of cardiometabolic risk in elderly men and women") may not seem related to adiponectin, and that is because the main impetus for the paper was to clarify some statistical issues which were raised by a study from another group. I find the adiponectin data very interesting though, so I thought I'd focus on that here instead (if anyone out there has an unbridled desire to discuss the marginality principle or other statistical issues, I would be more than happy to oblige). To read the paper in full, cite it, or nominate it for some sort of cool award for first-time lead authors, please visit the Journals of Gerontology website.
Have a great weekend,
Saunders, T., Davidson, L., Janiszewski, P., Despres, J., Hudson, R., & Ross, R. (2009). Associations of the Limb Fat to Trunk Fat Ratio With Markers of Cardiometabolic Risk in Elderly Men and Women The Journals of Gerontology Series A: Biological Sciences and Medical Sciences DOI: 10.1093/gerona/glp079
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