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Obesity is epidemic in many countries, presenting a public health threat by increasing the risks for multiple conditions, including heart disease and diabetes, and causing great cosmetic distress for many dermatology patients. Despite widespread concern, little is known about the dynamic nature of fat-cell turnover.
Investigators in Sweden evaluated changes in the number of adipocytes during adulthood in 687 participants. Interestingly, although the total number of fat cells increased during childhood and adolescence, it stabilized in adulthood in both lean and obese individuals. Therefore, the difference in the number of fat cells between lean and obese people is established during childhood. The investigators also found that fat volume decreased after bariatric surgery, but the number of fat cells did not.
To study whether fat cells undergo apoptosis and replacement throughout life (adipocyte dynamics), the investigators made use of measures of carbon 14 (14C) in human DNA. Above-ground nuclear bomb tests conducted from 1955 to 1963 notably increased the atmospheric concentration of 14C, which is incorporated into human DNA through the consumption of plants and of animals that eat plants. Because the rate of dispersion of atmospheric C is predictable, investigators can derive the birth date of a cell by its C level. Measures of 14C levels in adipose tissue collected during liposuction or abdominal wall reconstruction in 35 adult patients made it possible to estimate that the median turnover rate of these cells is 8.4% per year, with half of all adipocytes replaced every 8.3 years. Obese patients added significantly more new fat cells per year than lean patients did, but the proportion (i.e., the turnover rate) was the same in lean and obese individuals.
Spalding KL et al. Dynamics of fat cell turnover in humans. Nature 2008 Jun 5; 453:783.
Comment
These results show that fat-cell number is set early and remains relatively stable during adulthood, but the cells themselves constantly turn over. Weight loss in adulthood reduces fat-cell volume but not number, a tight homeostasis that clearly works against weight-loss efforts. Eliminating fat “stem” cells through liposuction may not have long-term benefits, because new adipocytes will replace those that were lost, to defend the established number of fat cells.