I’ve done a fair amount of work against drug targets for metabolic disorders, so a recent letter in Nature caught my eye. The authors have used an ingenious technique to determine the number and age of the adipocytes (fat cells) that an individual has, and have tracked that cell population year by year.
One thing that comes out is confirmation of the fact that people basically set their number of fat cells during childhood and/or adolescence, and that number is then constant through their adult life. Several subjects in this study put on or took off weight during it, but that made no real difference to their number of adipocytes. And though liposuction does reduce the number of fat cells (by brute force!), they’re back to their original count after three years or so. So weight changes, as other studies have also indicated, are almost entirely due to individual fat cells becoming larger and smaller.
But that doesn’t mean that you’ve got the same fat cells all the way through. Most interestingly, this study found that about 8% of the adipocyte population turns over every year, which is a higher fraction than anyone realized. Half the fat cells in the body, then, have been replaced after about eight years have gone by. That also means that the stable total number results from a balance between adipocyte death and new cell formation, and it would certainly be interesting to know how these are tied together so well. The authors suggest that this relatively high turnover could be a potential target for weight loss drugs. If we could figure out how, say, to keep the fat cell population from being renewed so exactly, their numbers might naturally decrease. (On the other hand, perhaps the rate at which they die would drop to keep the balance – no one knows yet).
So, how do you tell how old a fat cell is, anyway? That’s the ingenious part I mentioned above, and it involves the same sort of techniques used in radiocarbon dating. The amount of carbon-14 in the atmosphere is relatively constant, with a few minor variations over the last fifty thousand years or so. Well, relatively constant except for the 1950s and 1960s, when we as a species reset the counter but good by atmospheric testing of atomic and nuclear weapons. Those tests released a much larger than usual amount of 14C into the world - in 1963 the count had doubled over normal background - and that's since cycled into the biosphere through uptake by plants and other living creatures.
That process has sent the atmospheric levels of radioactive carbon down steeply over the years, but there’s plenty of signal to detect, and we know just how much it’s gone down every year. In effect, every year of the last 50 or 60 has an anomalous carbon-14 reading, and each one is unique and vintage-dated. We take up the carbon through our food, and as a cell is formed, the particular carbon isotope signature of your body at the time is in all its parts. Many of these are recycled constantly – but the DNA isn’t. Extracting the DNA from cells and looking at the carbon-14 levels through mass spectrometry gives you a “production date” stamp for when that cell was born. (See here for a longer discussion of carbon isotope mass spectrometry as it relates to detection of banned steroid hormone use, specifically in the Floyd Landis case. That post, by the way, led to the longest comment thread ever seen on this blog). The same technique is being used for other cell populations as well.
The confirmation that the number of fat cells seems to be set before adulthood also ties in with the obesity trends seen in the general population. The great majority of obese adults were also obese as children, and the great majority of non-obese children do not become obese as adults. What factors set this adipocyte count in a person’s early life, and how many of them are environmental and could be modified, will be very useful to know. . .