Adipocyte numbers are set during childhood and tightly regulated in adults

Spalding KL et al. (2008) Dynamics of fat cell turnover in humans. Nature 453: 783–787

Enlargement of adipocytes is the main mechanism of weight gain in adults, but total fat is also influenced by adipocyte numbers—obese people, on average, have more adipocytes than nonobese people. Spalding and colleagues investigated adipocyte turnover by measuring levels of ¹⁴C (derived from nuclear bomb tests) in adipocyte genomic DNA, which reflect atmospheric ¹⁴C concentrations when the cell underwent its last division.

Adipocyte numbers were calculated for 687 adults (age >20 years), and compared with previously reported values for children and adolescents. Adipocyte numbers increased during childhood and adolescence but remained constant during adulthood in both lean and obese individuals; however, adipocyte expansion started at age ∼2.1 years in individuals with early-onset obesity, compared with age ∼5.7 years in those who remained lean. Differences in adipocyte number between lean and obese individuals were, therefore, established during childhood.

Adipocyte generation occurred during adulthood: individuals who became adults before the first bomb tests in 1955 had raised ¹⁴C levels in adipocyte genomic DNA, suggestive of post-1955 generation, whereas individuals born after bomb tests ceased in 1963 had decreased ¹⁴C levels, suggestive of recent generation. Consequently, adipocyte turnover is tightly regulated to maintain constant cell numbers. The researchers calculated that the adipocyte turnover rate was ∼10% per year across all BMI categories, and remained constant in adulthood regardless of age and energy balance—bariatric surgery decreased adipocyte size significantly, but their numbers remained unchanged.

Feedback mechanisms that control adipocyte turnover might offer novel targets for prevention as well as treatment of obesity.