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Objective:To evaluate the ontogeny of adipose tissue dynamics in obese and lean Zucker rat pups, from suckling to puberty.Methods:The trial had a two-group parallel design. Sixty-two male Zucker rat pups shared within 15 litters received deuterated water for 5 days, prior killing at different age. Adipose tissues were collected for 2H-enrichment analyses using mass spectrometry to determine fat cell proliferation and lipid synthesis rates. Rats were assigned to obese and lean rat groups by genotyping.Results:The time course (from days 13 to 55) of all adipose tissue growth showed that the highest fractional rates of fat cell proliferation, triacylglycerol (TG) synthesis and de novo lipogenesis (DNL) took place during early suckling in all rat pups. The appearance of excessive fat mass growth in the obese rats, as compared with lean rats, was first shown through a significant increase in DNL at the end of suckling (P<0.05). The TG synthesis rate was enhanced (P<0.05) from the end of suckling and early postweaning until day 55 (from 122±10 to 498±78 in obese pups and from 25±6 to 75±26 mg new TG per day in lean pups (median±s.e.m., P<0.01)). In contrast, only by day 55 did the fractional proliferation rate of fat cells in retroperitoneal and epididymal depots in the obese rats supersede that of the lean rats (P<0.05).Conclusion:The early suckling period constitutes the most active period for adipose tissue development in normal rats. In the obese Zucker rat model, adipose hypertrophy primarily contributes to the early onset of obesity, while hyperplasia increases after puberty.Early onset of adipose tissue growth may play a determinant role in the development of obesity later in life.International Journal of Obesity (2008) 32, 648–657; doi:10.1038/sj.ijo.0803787; published online 18 December 2007 [ABSTRACT FROM AUTHOR]