Short-term fasting increases skeletal muscle lipid content in association with enhanced mRNA levels of lipoprotein lipase 1 in lean juvenile red seabream (Pagrus major)

Cultured fish are often subjected to short-term fasting for a few days prior to transport or slaughter. This intervention reduces waste and oxygen consumption during transport and improves flesh quality, but little is known about how it affects the muscle lipid content. Even less is known for wild-caught fish that are also sometimes fasted before slaughter. To investigate the effects of short-term fasting on muscle lipid content, we performed three fasting experiments using adult, lean juvenile, and fatty juvenile red seabream ( Pagrus major ). In the first experiment using adult specimens (performed at 20 °C), fasting for 10 days caused up to 2.3-, 7.8-, and 3.7-fold increases in muscle mRNA levels of lipoprotein lipase 1 ( LPL1 ), LPL2 , and peroxisome-proliferator-activated receptor gamma ( PPAR γ), respectively (n = 6 each). These genes all participate in lipid accumulation. The second experiment was performed on 5 groups of lean juvenile red seabream [considered as wild-caught fish; 19 °C, n = 5 each except for one group (n = 4)]. Fasting for 5 days significantly increased muscle lipid content (percentage of wet weight) and LPL1 mRNA levels up to 2.2- and 11-fold, respectively. The mRNA levels of LPL2 and PPAR γ could not be determined possibly due to their low expression levels. The lipid amount (mg) in the liver decreased by 88% during fasting, whereas adipose tissue apparently disappeared after fasting for 10 days. These results suggest that lipid mobilization from liver and adipose tissue anticipated the muscular utilization of lipid, resulting in the counterintuitive increase in muscle lipid content during fasting. The third experiment was conducted on 7 groups of fatty juvenile red seabream [considered as cultured fish; 20 °C, n = 5 each except for one group (n = 4)]. Fasting up to 10 days did not increase muscle lipid content or LPL1 mRNA levels. The lipid amount in the liver, but not in the adipose tissue, decreased during the experiment. Fasting for 10 days was not enough to induce active lipid mobilization when fish were well fed prior to fasting. Contrary to our expectations, regression analysis using data from juvenile specimens found a significant inverse relationship between LPL1 mRNA levels and lipid amount in the liver, which might indicate that LPL1 mRNA levels are a reflection of temporal tissue demand for lipids. Overall, these results suggest that short-term fasting for ~ 5 days increases muscle lipid content of wild-caught lean fish possibly by an LPL1-dependent lipid transport mechanism. Statement of relevance We showed that short-term fasting improves flesh quality.