Abstract: When food intake is insufficient, the energy expenditure in animals is typically reduced to prolong survival. However, the molecular mechanisms regulating such response are not fully understood. To address this gap, we evaluated the basal metabolic indicators and conducted transcriptomic analyses to investigate the effects of prolonged fasting on energy metabolism in female Japanese quails. There were a control group (no fasting) and three fasted groups, namely fasting group Ⅰ, fasting group Ⅱ, and fasting group Ⅲ, which were fasted for 1, 4, and 6-11 days, respectively. The results showed that fasting significantly reduced body mass of the birds. The rate of daily body mass loss exhibited a U-shaped pattern-Symbol~Binitially rapid, then slowing, followed by an increase. Both the basal metabolic rate (BMR) at the whole-animal level and the mass-specific BMR were significantly lower in fasting group Ⅱ compared with the control group. Fasting significantly reduced serum glucose levels, did not affect serum triglyceride levels, and significantly increased serum uric acid levels. KEGG enrichment analysis provided insights into the pathways involved in the response to insufficient food intake. The analysis revealed significant upregulation of 13 pathways in fasting group I compared with the control group. These 13 pathways were primarily involved in the catabolism of the three macronutrients: carbohydrate (7 pathways), lipid (2 pathways), and protein (3 pathways). Similarly, in fasting group Ⅱ, eight pathways were upregulated, five of which were related to protein/amino acid metabolism, including one protein degradation pathway and three glucogenic amino acid metabolism pathways. As for fasting group Ⅲ, 13 pathways were significantly upregulated, including the ‘autophagy-animal’ pathway. These findings suggest that prolonged fasting induces a marked reduction in energy metabolism in female Japanese quails. Fasting alters the metabolism of all three major nutrient classes. Carbohydrate and lipid catabolism were significantly enhanced, while protein metabolism shows increased catabolic and anabolic activity during Phases I and Ⅱ, with catabolism predominating in Phase Ⅲ. In addition, compared with the control, Phase Ⅰ, and Phase Ⅱ, the ‘autophagy-animal’ pathway was significantly upregulated in Phase Ⅲ, suggesting that birds under sustained energy stress enhanced autophagy to recycle protein-derived substrates and compensated for the reduced nutrient intake. In conclusion, this study provides molecular-level evidence that long-term fasting downregulates energy metabolism in birds and highlights the critical role of autophagy in maintaining energy balance and physiological homeostasis.

Key words: female Japanese quail, fasting, transcriptome, autophagy, basal metabolic rate