*Corresponding autor e-mail: mdiaz@inb.unam.mx Introduction: A continuous glucose contribution is essential for life, being the brains main fuel and the unique source of energy that cells that lack mitochondria can use. If the glycemia falls, like in fasting or between meals circulating glucose recovers by two processes: (1) glucogenolysis, contributing short term glucose from the reserves of glycogen (first12-24h of fasting); and (2) the gluconeogenesis, forming glucose from some aminoacids and from other molecules such as glycerol and lactate. Nevertheless, if the fasting period is last too long the organism also obtains energy from acetyl-CoA produced by the degradation of the free fatty acids (FFA) and ketonic bodies (KB). In previous studies, animals under restricted food schedule (RFS) and expressing of the food entrainment oscillator (FEO). Showed, high levels of circulating FFA and KB, partial reduction of hepatic glycogen, increased ATP and an oxidized redox state, suggesting a rheostatic adaptation of the liver metabolism. Hence, the aim of this project was to explore the process responsible of maintaining the levels of glycemia and the regulation of the hepatic gluconeogenesis, during the expression of FEO. Methods: We analyzed the key enzymes of the gluconeogenesis in cellular hepatic fractions: glucose-6-phosphatase (G6Pase; microsomes) and the phosphoenolpyruvate carboxykinase (PEPCK; cytosol), by measuring enzymatic activity and by Western blot. Results: We found significant differences in the glycemia, the enzymatic activities and the expression of G6Pase and PEPCK during circadian characterization, showing a phase shift around the food access and presentation of bimodal patterns. In addition, we found higher levels of glucose and lower levels of genetic expression (amount of protein) and of activity (enzymatic activity) of the G6Pase and the PEPCK (before accessing to the food), in comparison with a simple fasting (24 and 48h). Conclusion: These results reinforce the idea of a rheostatic state by the animals that express FEO, during the control of the hepatic gluconeogenic process. Our results suggest that hepatic metabolism depends more in lipid oxidation than in biochemical formation of glucose during the fasting period of FEO expression. Publication History Article published online: 16 June 2026 © 2009. Brazilian Sleep Academy. This is an open access article published by Thieme under the terms of the Creative Commons Attribution-NonDerivative-NonCommercial License, permitting copying and reproduction so long as the original work is given appropriate credit. Contents may not be used for commercial purposes, or adapted, remixed, transformed or built upon. (https://creativecommons.org/licenses/by-nc-nd/4.0/) Thieme Revinter Publicações Ltda. Rua Rego Freitas, 175, loja 1, República, São Paulo, SP, CEP 01220-010, Brazil
Pérez-Mendoza et al. (Thu,) studied this question.