Question of the Week goes to my friend Dylan, who works at Kidapt. Paraphrased:
A lot of people say that fat cells never die, and that they stay with you for life. Is this all true, partially true, or completely bunk?
GREAT question. That is actually a myth. Your fat cells die just like every other cell.
However, what does stay relatively constant is the overall number of fat cells you have. Once you are in adulthood, this number does not seem to change that much; this is why scientists used to think that the fat cells never died. But now with carbon dating, scientists can tell how old a fat cell actually is, which is what led them to discover that fat cells do indeed turnover. It seems that when fat cells die, they get replaced at relatively the same rate, whether you are obese or lean. It’s estimated that every 8 years, half of the fat cells in our subcutaneous fat mass are replaced! (So those little cellulite guys on my cheeks are not the same as the ones I fell on when I was 9 trying to skateboard down that steep hill…but the ones I am sitting on right now are the same as the ones I fell on yesterday when trying to bike-run my dog.)
So what happens when you lose weight?
The volume of the cells decreases, but the overall birth and death rate of the cells stays the same. Many people try to get rid of their fat cells by exercising more or eating less, but the cells just lose some of the stored energy rather than going away completely. (1)
What about gaining weight?
If you consume more than your body needs, you start to store some of the extra energy in the fat cells. As you gain more weight, you keep filling up the cells more, so they increase in size, or volume.
But what happens if these cells are overstuffed? Don’t we run out of space and make more cells?
It’s unclear… Many obese, and almost all severely obese, people have more than the average number of fat cells, but it’s unclear if they had that number going into adulthood or if they developed them as they gained weight. There haven’t been long-term (past 2 month) studies measuring the number of fat cells people have at baseline, and then measuring the number after the people have undergone massive weight gain.
There have been some shorter-term studies implying that different areas of the body may respond to overfeeding differently. For example, one recent-ish study found that overfeeding people for 2 months can result in an increase in number of fat cells in the leg area, but an increase in size, or volume, of fat cells in the abdominal region. The authors think that the ability of people to make more fat cells in leg fat can actually be a positive buffer. The increase in number of fat cells in the legs sequesters fat away from the abdominal area, and it is the overstuffing of the abdominal fat cells that is linked to insulin resistance.
Another study following the same weight gain protocol found that after the weight gained was then lost, the number of increased fat cells in the leg area stayed the same, but the size of all the cells decreased. Therefore, they never lost the number of cells they grew during the weight gain period. (Bummer for the participants!)
Thanks for the question, Dylan! If there’s more interest, I’ll do more detailed blog of this.
MS, RD, PhD
(1) Unfortunately the body also has some irritating mechanisms in place that fight against an artificially smaller fat cell size. Among other things, smaller fat cells result in decreased amounts of leptin, a hormone that regulates your appetite, so it makes it easier to take in more than your body needs. This isn’t to say you can’t maintain small fat cells, or ones who are filled less than they are used to; it just is difficult, as many of us know.
PS- Yes, that IS the phantom shadow from Scooby Doo!
For a clearly written update on fat cell turnover, check out:
Arner, P., & Spalding, K.L. (2010). Fat cell turnover in humans. Biochemical and Biophysical Research Communications, 396, 101-104.
New evidence as to our ability to develop new fat cells in response to overfeeding can be found here:
Tchoukalova, Y.D., Votruba, S.B., Tchkonia, T., Giorgadze, N., Kirkland, J.L., & Jensen, M.D. (2010). Regional differences in cellular mechanisms of adipose tissue gain with overfeeding. PNAS, 107(42), 18226-18231.
A study on imposed weight loss after an imposed weight gain:
Singh, P., Somers, V.K., Romero-Corral, A., Sert-Kuniyoshi, F.H., Pusalavidyasagar, S., Davison, D.E., & Jenson, M.D. (2012). Effects of weight gain and weight loss on regional fat distribution. American Journal of Clinical Nutrition, 96, 229-33.