Specific role of glucose in rapid lipogenic activation in vivo.

Lipogenesis from glucose C was previously found to be rapidly activated as soon as mice nibbled a fat-free, glucose-rich diet. We have studied here whether such rapid activation is a specific effect of dietary glucose. The flux of endogenous glucose C to total lipid fatty acids (TLFA) in mice fasted for 1 day was compared with the minimal average flux of exogenous dietary glucose to TLFA during a 40-min period after the ingestion of various glucose-rich test meals by previously fasted mice. The fasted mice were injected intravenously with [U-(14)C]glucose, and the flux of glucose C to TLFA and to all "end products" was estimated from serial plasma glucose specific activity measurements and (14)C incorporation into TLFA 30 min after (14)C injection. Only 0.6 to 0.8 micro g of glucose C/min/20 g body wt was converted to TLFA, whereas 208 +/- 16 micro g of glucose C/min/20 g body wt was converted to all "end products" in the fasted animals. Previously fasted mice were fed [(14)C]glucose in small test meals as a neat solid, as a 30% aqueous solution, or as a fat-free, 58% glucose diet. During the next 40 min, the average flux of glucose C into TLFA increased at least 50- to 60-fold, regardless of the form in which glucose was fed; however, when glucose was fed as part of a complete fat-free diet, glucose was utilized at a much lower plasma glucose level than in mice fed either pure solid glucose or an aqueous glucose solution. Rapid activation of lipogenesis from glucose requires only glucose as a dietary constituent.