LETTERS Dynamics of fat cell turnover in humans Kirsty L. Spalding 1 , Erik Arner 1 , Pa ˚l O. Westermark 2 , Samuel Bernard 3 , Bruce A. Buchholz 4 , Olaf Bergmann 1 , Lennart Blomqvist 5 , Johan Hoffstedt 5 , Erik Na ¨slund 6 , Tom Britton 7 , Hernan Concha 5 , Moustapha Hassan 5 , Mikael Ryde ´n 5 , Jonas Frise ´n 1 & Peter Arner 5 Obesity is increasing in an epidemic manner in most countries and constitutes a public health problem by enhancing the risk for cardiovascular disease and metabolic disorders such as type 2 dia- betes 1,2 . Owing to the increase in obesity, life expectancy may start to decrease in developed countries for the first time in recent history 3 . The factors determining fat mass in adult humans are not fully understood, but increased lipid storage in already developed fat cells (adipocytes) is thought to be most important 4,5 . Here we show that adipocyte number is a major determinant for the fat mass in adults. However, the number of fat cells stays constant in adulthood in lean and obese individuals, even after marked weight loss, indicating that the number of adipocytes is set during childhood and adolescence. To establish the dynamics within the stable population of adipocytes in adults, we have measured adipocyte turnover by analysing the integration of 14 C derived from nuclear bomb tests in genomic DNA 6 . Approximately 10% of fat cells are renewed annually at all adult ages and levels of body mass index. Neither adipocyte death nor generation rate is altered in early onset obesity, suggesting a tight regulation of fat cell number in this condition during adulthood. The high turnover of adipocytes establishes a new therapeutic target for pharmacological intervention in obesity. The fat mass can expand by increasing the average fat cell volume and/or the number of adipocytes. Increased fat storage in fully dif- ferentiated adipocytes, resulting in enlarged fat cells, is well docu- mented and is thought to be the most important mechanism whereby fat depots increase in adults 4,5 . To analyse the contribution of the fat cell volume in adipocytes to the size of the fat mass, we first analysed the relationship between fat cell volume and total body fat mass (directly measured with bioimpedance or estimated from body mass index (BMI), sex and age in a large cohort of adults). As expected, there was a positive correlation between the measures of fat mass and fat cell volume both in subcutaneous fat (Fig. 1a–c), which represents about 80% of all fat, and in visceral fat (Fig. 1d), which has a strong link to metabolic complications of obesity. However, the relationship between fat cell volume and fat mass markedly differed from a linear relationship (likelihood ratio test P , 0.001, and Akaike information criterion, described in Supplementary Information 1) in both sub- cutaneous and visceral adipose regions and both sexes, indicating that fat mass is determined by both adipocyte number and size. In the nonlinear case, both fat cell number and fat cell size determine fat mass. If the relationship had been linear, fat cell volume would be the only important determinant of fat mass. The generation of adipocytes is a major factor behind the growth of adipose tissue during childhood 7 , but it is unknown whether the number of adipocytes changes during adulthood. We assessed the total adipocyte number in 687 adult individuals and combined this data with previously reported results for children and adolescents 8 . Although the total adipocyte number increased in childhood and adolescence, this number levelled off and remained constant in adult- hood in both lean and obese individuals (adults over 20 yr, grouped in 5-yr bins; ANOVA, lean P 5 0.68, obese P 5 0.21; Fig. 2a and Supplementary Information 3). Thus, the difference in adipocyte number between lean and obese individuals is established during childhood 7,8 and the total number of adipocytes for each weight category stays constant during adulthood (Fig. 2b). The small vari- ation in adipocyte number for each BMI category demonstrates that this is a stable cell population during adulthood. To analyse whether alterations in adipocyte number may contri- bute to changed fat mass under extreme conditions, we next asked whether fat cell number is reduced during major weight loss (mean body weight loss, 18 6 11%, mean 6 s.d.) by radical reduction in calorie intake by bariatric surgery (reduction of the stomach with the purpose of facilitating weight loss). The surgical treatment resulted in a significant decrease in BMI and fat cell volume; however, this failed to reduce adipocyte cell number two years post surgery (Fig. 2b, c and Supplementary Information 4), in line with previous studies using different methodology 9–12 . Similar results were found in a complementary longitudinal study 13 . Ref. 13 found that significant weight gain (15–25%) over several months in non-obese adult men resulted in a significant increase in body fat, which was accompanied by an increase in adipocyte volume, but no change in adipocyte number. Similar to our findings, subsequent weight loss back to baseline resulted in a decrease of adipocyte volume, but, again, no change in adipocyte number. Although we cannot rule out that a more prolonged period of weight gain in adulthood could result in an increase in adipocyte number, these results and ours indicate that fat cell number is largely set by early adulthood and that changes in fat mass in adulthood can mainly be attributed to changes in fat cell volume. This may indicate that the number of adipocytes is set by early adulthood with no subsequent cell turnover. Alternatively, the generation of adipocytes may be balanced by adipocyte death, with the total number being tightly regulated and constant. We next set out to establish whether adipocytes are replaced dur- ing adulthood, and, if so, at what rate. Adipocytes can be generated from adult human mesenchymal stem cells and pre-adipocytes in vitro 14 and may undergo apoptosis or necrosis 15–17 , but it is unclear whether adipocytes are generated in vivo 14 . Cell turnover has been difficult to study in humans. Methods used in experimental animals, such as the incorporation of labelled nucleotides, cannot readily be adapted for use in humans owing to potential toxicity. The detection of cells expressing molecular markers of proliferation can give 1 Department of Cell and Molecular Biology, Karolinska Institute, SE-171 77 Stockholm, Sweden. 2 Institute for Theoretical Biology (ITB), Humboldt University Berlin and Charite´, Invalidenstrasse 43, 10115 Berlin, Germany. 3 Institute of Applied and Computational Mathematics, Foundation of Research and Technology, 71110 Heraklion Crete, Greece. 4 Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, 7000 East Avenue, L-397, Livermore, California 94551, USA. 5 Department of Medicine, Karolinska University Hospital, SE-141 86 Stockholm, Sweden. 6 Division of Surgery, Department of Clinical Science, Danderyds Hospital, Karolinska Institutet, SE-182 88 Stockholm, Sweden. 7 Department of Mathematics, Stockholm University, 106 91 Stockholm, Sweden. Vol 453 | 5 June 2008 | doi:10.1038/nature06902 783 Nature Publishing Group ©2008