Three new studies published in the April 9, 2009 issue of The New England Journal of Medicine show conclusive proof that adult humans do indeed have appreciable amounts of brown adipose tissue. Why is this important? For at least two reasons: 1) it puts to rest the issue that adult humans have this cell type (more on this below), and 2) if the numbers or the activity of these cells could be increased it could help in the fight against obesity.
So what exactly is brown adipose tissue? Well, when most people think of adipose tissue they think of white fat - the cell type that stores fat for future energy needs of the body (though experts think it is also useful for keeping fats away from other critical organs, like the liver and muscle, and preventing the excess fat from inhibiting their function). But brown fat has another purpose entirely - it burns fats and carbs to release heat, which in turn keeps the body warm. For animals that can shiver, like adult humans, it was believed that brown fat wasn't needed or if it was present it was a vestigial organ that wasn't important for normal physiology. These three studies show that cold temperatures induce the occurrence of this tissue and that it is indeed likely important for normal physiology. More importantly, though, the findings also suggest that because these cells are present they could be targeted to fight obesity, as mentioned above. Indeed, as one of the papers points out, if as little as 0.1% of a person's body weight is converted to brown adipose tissue it could account for ~20% of the adult body's daily energy expenditure.
But there is also an interesting background to these three studies, which all three papers cite and two explain a bit, but it might be interesting to spell out a little further here. The technique the three papers used to identify the brown fat is to give volunteers radiolabeled glucose ([18]F-fluorodeoxyglucose) followed by PET-CT scans. But this technique has been around for awhile. It was originally devised because it was noticed that tumors are quite energy intensive and thus more likely to take up this radiolabel more quickly than normal, healthy cells. Thus it was hoped the technique would allow advanced tumors and their metastases to be visualized. But around 2002-2004 a number of reports started to appear in the radiological literature pointing out that patients tested in this way showed several 'blobs' of staining in the supraclavicular area. Given what we know about the energy expenditure of brown fat, the authors of those earlier studies suggested that adult humans do indeed have appreciable levels of brown fat. But it wasn't until the new studies published this month that this staining was shown conclusively to be cold-inducible and, more importantly, upon biopsy that the cells are indeed brown adipose tissue, as characterized by histology and molecular marker analysis.
Time will only tell now if this cell type in adult humans can indeed be manipulated to keep us trim.
