Effect of diet and age on energy metabolism in rats.

Abstract

Effects of age and diet on energy metabolism, body composition and lipogenic enzyme activities were determined. Sprague-Dawley rats (2, 9, 18 and 24 mo), housed in individual metabolic cages within an open-circuit indirect calorimeter, were fed three low-fat (starch, high-protein and sucrose) and three high-fat (corn oil, olive oil and animal fat) diets for 3 wk. Fatty acid synthase (FAS), malate dehydrogenase (MDH), glucose-6-phosphate dehydrogenase, and ATP-citrate lyase (CTL) were assayed in adipose tissue and liver. Body compositions were determined by total body electrical conductivity (TOBEC). Energy retention was higher for 2-month-old animals in relation to other age groups; while the 18-and 24-month-old rats had negative energy retentions. High-fat diets induced higher fat utilization and high-carbohydrate diets induced higher carbohydrate utilization as reflected in RQ values. The 18-and 24-month-old animals utilized 80% more fat, while the 2- and 9-month-old animals utilized 30% more carbohydrates. Thus, older animals utilized higher amounts of fat for energy and younger animals utilized a higher proportion of carbohydrate. Lipogenic enzyme activities were highest in the 2-month-old rats for all enzymes measured and in both tissues, and were reflected in higher fat mass depositions. Rats fed either olive oil or animal fat had lower adipose tissue CTL activities, and feeding high-fat diets induced lower activities of adipose tissue MDH as compared with the other diets. Diet had no significant effect on either body weight or lean mass changes; except rats fed animal fat had the lowest total weight gain. These results indicate that aging reduces basal metabolic rate, as measured by gas-exchange indirect calorimetry, and shifts substrate utilization from carbohydrate to fat. Activities of adipose tissue CTL and MDH were lower in animals fed high-fat diets than in those fed low-fat diets. Fat accumulation, associated with development of obesity, was not observed in these short-term studies, either as a result of aging or of feeding high-fat diets.