Caloric restriction in lean and obese strains of laboratory rat: effects on body composition, metabolism, growth and overall health

New Findings What is the central question of this study? How do lean and obese rats respond physiologically to caloric restriction? What is the main finding and its importance? Obese rats show marked benefits compared with lean animals. Reduced body fat is associated with improved longevity with caloric restriction (CR) in rodents. Little is known regarding effects of CR in genetically lean versus obese strains. Long–Evans (LE) and Brown Norway (BN) rats make an ideal comparison for a CR study because the percentage body fat of young adult LE rats is double that of BN rats. Male LE and BN rats were either fed ad libitum (AL) or were calorically restricted to 80 or 90% of their AL weight. The percentages of fat, lean and fluid mass were measured non‐invasively at 2‐ to 4‐week intervals. Metabolic rate and respiratory quotient were measured after 3, 6, 9 and 12 months of CR. Overall health was scored monthly. The percentage of fat of the LE strain decreased with CR, whereas the percentage of fat of the BN strain remained above the AL group for several months. The percentage of lean mass increased above the AL for both strains subjected to CR. The percentage of fluid was unaffected by CR. The average metabolic rate over 22 h of the BN rats subjected to CR was reduced, whereas that of LE rats was increased slightly above the AL group. The respiratory quotient of BN rats was decreased with CR. Overall health of the CR LE group was significantly improved compared with that of the AL group, whereas health of the CR BN rats was impaired compared with the AL group. Overall, the lean BN and obese LE strains differ markedly in fat utilization and metabolic response to prolonged CR. There appears to be little benefit of CR in the lean strain.