A recent study from the Journal of Bone and Mineral Research found that women with anorexia had much higher levels of fat in their bone marrow than women without AN (Ecklund et al, 2010). The study was generally publicized as "OMG! Anorexics have FAT on their bony bodies!" Which, as an interesting irony and news hook, I'll give you. But the story goes much deeper than that, which some of the news coverage touched on but really didn't delve into (they appeared to get stuck on the "WTF- could anorexics be fat?!?" part).
Eating disorders are associated with an increased risk for osteoporosis--and it ain't no joke. I've learned that the hard way, with three broken bones and several stress fractures. There are many hypotheses for this increased risk, including deficits in estrogen, high levels of cortisol, and high levels of leptin. I'm guessing each of these plays a role in the decrease in bone mass and density through either the metabolism of bone cells and/or a dramatic decrease in the formation of new bone cells during malnutrition.
This study points to a new mechanism for the dramatic bone density decrease seen in eating disorders in general and anorexia in particular. At the center of larger bones is the bone marrow, one type of which is the red bone marrow and produces new blood cells. The other type is the yellow bone marrow and contains fat cells that can be used as an energy source in cases of extreme starvation. Furthermore, the two types of bone marrow can be interchangeable--in cases of extreme blood loss, the yellow marrow can be converted to red marrow. What Ecklund et al found in this most recent study is that red marrow can be converted to yellow marrow if the body is profoundly starved, which can result in premature osteoporosis.
The study subjects with anorexia had much higher levels of yellow marrow than red marrow, and the researchers hypothesized that the body had prioritized the formation of extra fat for future energy needs at the expense of red blood cell formation (I'm wondering whether this also helps to explain the high levels of anemia seen in people with eating disorders). The innate intelligence of the body never ceases to astound me. In a starving person, fat (which is essentially energy) is much more useful than red blood cells. Without energy, the body shuts off. With fewer red blood cells, you may be more easily fatigued, but mild levels of anemia are rarely out-and-out life threatening.
It will be interesting to see if there is follow-up research done to see how weight restoration and recovery change the ratio of red and yellow marrow, and whether these changes persist for a long period of time after recovery.