Changes in the plantaris muscle as an indicator of alterations in lean body mass of obese Zucker rats following prolonged energy restriction and subsequent partial recovery.

To study the effects of a prolonged (80 day), severe (64% body mass loss) energy restriction and subsequent refeeding on skeletal muscle tissue, specifically the plantaris muscle, 21 genetically obese Zucker rats were selected for this study. Six rats were initially killed and served as baseline (BASE), then 15 rats underwent severe energy restriction for 80 days. Seven of these restricted rats (RESTRICT) were then killed and assessed while the other eight rats (REHAB) were fed a dry rehabilitation diet that provided 100% of the recommended energy and 1.5 times the recommended protein for growth. Once the REHAB rats had recovered approximately 45% of their original mass loss, these animals were then evaluated. Within 20 min after being killed, the plantaris muscles from each animal in each group had been removed, weighed, and frozen. Analyses included total plantaris mass, as well as differences in fiber diameters, esterase activity, and fiber type distributions between three groups (BASE, RESTRICT, and REHAB). The extreme body mass loss of 64% in genetically obese Zucker rats resulted in significant tissue weight loss and reduced fiber diameters in the plantaris muscle. Refeeding resulted in larger muscle fiber diameters that approached baseline values but an 11% difference in muscle weight remained and may be due to a decreased fiber number. Esterase activity seemed to indicate an initial fat utilization for the RESTRICT group, followed by suppressed esterase activity in the REHAB group, suggesting increased fat storage. No significant differences were found in fiber type distribution between BASE, RESTRICT, or REHAB animals.