Owing to the increase in obesity, life expectancy may start to decrease in developed countries for the first time in recent history. In humans the generation of fat cells (adipocytes) is a major factor behind the growth of adipose tissue during childhood. The factors determining the fat mass in adults, however, are not fully understood. Increased fat storage in fully differentiated adipocytes, resulting in enlarged fat cells, is well documented and thought to be the most important mechanism whereby fat depots increase in adults. Very little is known about the maintenance of fat cells (adipocytes) in humans, how different fat depots are maintained and how (or if) this is altered in obesity. Recently I developed a method that is based on the incorporation of 14C from nuclear bomb tests into genomic DNA, which allows for the analysis of cell and tissue turnover in humans. Using this novel methodology we now have a strategy for studying cell turnover in humans. One tissue of great interest and significant clinical relevance is adipose tissue. Excess adipose tissue, resulting in obesity, is currently one of the most serious threats to human health on a global level. The current proposal aims to determine the dynamics of human adipose tissue maintenance and investigate any differences in regulation of the fat mass in lean and obese individuals. Understanding the dynamics of adipocyte turnover may shed new light on potential treatments for obesity.