During short-term fasting, substrate utilization in skeletal muscle shifts from predominantly carbohydrate to fat as a means of conserving glucose. In rats, this shift in intermediary metabolism is accompanied by the induction of specific genes (Hildebrandt & Neufer, 2000), a response that is quickly reversed upon refeeding. To determine the impact of short-term fasting and refeeding on the transcriptional regulation of metabolic genes in skeletal muscle of humans, nine male subjects completed two trials in which a 20 h fast was followed by either a high-carbohydrate (CHO trial) or high-fat meal (FAT trial) in isocaloric amounts (4 MJ). Muscle biopsies were obtained from the vastus lateralis muscle prior to fasting (3 h after a light, standarized meal), after 20 h of fasting and 1 h after finishing the refeeding meal. Real-time PCR-based techniques were used to determine transcription activity and mRNA content from isolated nuclei and total muscle RNA, respectively (Pilegaard et al. 2000). One-way ANOVA for repeated measures was used to evaluate the effect of fasting and refeeding using Student-Newman-Keuls post-hoc tests to locate differences. Fasting (20 h) induced a 10-fold increase in transcription of the pyruvate dehydrogenase kinase 4 (PDK4) gene, an enzyme that is responsible for phosphorylating and inactivating pyruvate dehydrogenase. Surprisingly, refeeding (1 h) further increased transcription of PDK4 to more than 50-fold over prefasting levels in both the CHO and FAT trials. Fasting also increased transcription of the uncoupling protein 3 (~5-fold; UCP3), the lipoprotein lipase (~2-fold; LPL) and the carnitine palmitoyl transferase 1 (~2.5-fold; CPT1) genes. In response to refeeding, LPL transcription increased further (6- to 10-fold over prefasting) while UCP3 and CPT1 transcription decreased relative to the fasted state. Although not as great in terms of fold-changes, similar responses to both fasting and refeeding were obtained at the mRNA level. These data demonstrate that short-term fasting in humans induces transcriptional activation of several genes related to fat metabolism in skeletal muscle. These findings also suggest that acute regulation of PDK4 expression in skeletal muscle may reflect the high metabolic priority given to limiting glucose oxidation during both fasting and the early stages of refeeding.

The study was approved by the Copenhagen and Frederiksberg Ethics Committee, Denmark and the Human Investigations Committee, Yale University, USA.