We assessed the contributions of human liver and adipose tissue de novo lipogenesis (DNL) to triacylglycerol (TAG) synthesis. Volunteers were fed a high-energy, high-carbohydrate diet (HC, n = 5) or a normocaloric diet (NC, n = 10). NC subjects remained in the fasting state (Study 1, n = 5) or received oral glucose (Study 2, n = 5) throughout the test (12 h). HC subjects remained in the fasting state (Study 3). They ingested deuterated water and [U-13C]acetate to trace lipogenesis. Adipose tissue fatty-acid (FA) synthase (FAS), acetyl-CoA carboxylase 1 (ACC1), and SREBP-1c mRNA were measured. Plasma TAG-FA was labeled by 13C and deuterium showing active liver lipogenesis, which was stimulated (P < 0.05) by oral glucose and HC diet. Adipose tissue TAG had no detectable 13C enrichment in any test, showing no significant incorporation of TAG-FA provided by liver lipogenesis, but were labeled by deuterium in all tests, showing active DNL in situ; however, rough quantitative estimates showed that adipose DNL was minimal (