It is now well established that genetically obese rodents have an increased ability to convert dietary energy to body energy relative to their lean counterparts (1). Alterations in energy metabolism, as indicated by decreased rates of oxygen consumption, are observable in obese (ob/ob) mice by 3 weeks of age (24); however, few studies have focused on younger obese mice. Marked changes in oxygen consumption of obese and lean mice have been reported at the time of weaning. Oxygen consumption per g body weight decreased by 25-50% within the first several days after weaning (3, 4). These studies and others (5-9) indicate that alterations in energy metabolism may appear very early in the life of obese mice.The present study was designed to monitor oxygen consumption, an indirect indicator of energy expenditure, of lean and obese mice from birth to 112 days of age; major emphasis was focused on developmental changes prior to 21 days of age. Body fat content of lean and obese mice at 7 and 14 days of age was also determined.Materials and methods. Heterozygote breeding mice (C57BL/6J ob/+)2 were housed in solid-bottom cages with wood shavings for bedding. They were fed a stock diet3 ad libitum. Ambient temperature was 25 f 2". Pregnant dams were removed from the breeding cages and placed in separate similar cages. Litter size was routinely standardized to six pups per litter; however, in a few instances litters with five or seven pups were utilized. Pups were weaned at 21 days of age unless indicated otherwise.In the first experiment pups were individually identified at 1 day of age. From 1 to 19 days of age daily oxygen consumption measurements and body weights were obtained on individual pups. Litters were also killed at 7 and 14 days of age. Food residue was removed from the stomach and the carcasses were homogenized in an equal weight of water. Carcass fat was extracted with chloroform/methanol (2: 1) and quantitated gravimetrically .Oxygen consumption measurements and body weights of pups on days 18-20 and again on days 22-24 of age were obtained in the second experiment. Only lean mice were used. In one treatment pups remained with the dam; in the other treatments pups were weaned to a high-carbohydrate or a high-fat diet at 21 days of age. The high-fat diet was formulated to approximately simulate mouse milk on a protein-energy basis (10) and contained per 100 g diet: 43.5 g casein, 32.0 g tallow, 5.0 g corn oil, 9.6 g glucose, 5.0 g cellulose, 4.0 g mineral mix (1 I), 0.4 g vitamin mix (ll), 0.2 g choline chloride and 0.3 g methionine. The high-carbohydrate diet was formulated by replacing the tallow with glucose on an equal-energy basis.In the last experiment oxygen consumption measurements and body weights were recorded weekly from 3 weeks of age to 16 weeks of age. Lean and obese male and female mice were utilized.The apparatus described by Watts and Gourley (12) was utilized to obtain estimates of oxygen consumption in the mice. Food was available to the mice until the oxygen consumption measurements commence...

Certain strains of rodents become fatter when fed high-fat rather than high-carbohydrate diets while other strains are able to consume a high-fat diet without an appreciable increase in body fat content (1-4). The obesity prone rodents are less able to regulate their food intake when fed a high-fat diet; additionally, consumption of a highfat diet has also been shown to increase the efficiency of energy utilization in pair-fed rodents ( 5 , 6). Less attention has focused on the interrelationship of the dietary fat to carbohydrate ratio and the development of obesity in other species.The influence of the dietary fat to carbohydrate ratio on body weight and body fat has been studied in young growing dogs (7). The fat-free body weight gain was not influenced, but accumulation of body fat tended to be elevated in dogs fed high-fat diets. Body fat averaged 29% of body weight in the young dogs fed diets with more than 50% of energy from fat while the body fat content averaged 23% in dogs fed the diets which contained less than 50% of energy from fat. The dogs were only 10months-old at the end of the 8-month study; had the diets been fed to adult dogs, body fat content might have been altered to a greater extent.The purpose of the present study was to examine the influence of a high-fat diet ( 5 1 % of energy from fat) and a high-carbohydrate diet (59% of energy from carbohydrate) on food intake and on body weight and body fat changes in adult dogs. Dogs were fed these diets for 25 weeks. Materials and methods.Three-year old female beagle dogs2 were housed in individ-' ual cages with raised floors. Lights in the temperature-controlled (22-23") room were on from 7 A M to 7 PM and off from 7 PM to 7 AM. Prior to the present study the dogs were fed approximately 750 kcal daily for 110 days. The composition of this nutritionally adequate (8) canned diet has been described previously (Table I, Diet 6 in Ref. 7). The dogs approximately maintained their weight during this 11 0-day period. Food and water were offered ad lib. during the present study. Dogs were fed once daily. Body weights were recorded weekly.Two semipurified diets were prepared; one diet was high-fat and the other highcarbohydrate. Each diet was formulated to contain 20% of energy from protein. A p proximately 50% of the protein in the diets were derived from isolated soybean protein and 50% from beef kidneys. Corn oil (7% of energy) was added to provide a source of essential fatty acids. Corn starch and tallow were added to provide the desired carbohydrate to fat ratio. A mineral and vitamin supplement3 and cellulose (6% of dry matter) were added to each diet. The diets were canned.At monthly intervals samples of each diet were obtained and analyzed for moisture by drying to a constant weight at 50°, for gross energy with a bomb calorimeter and for nitrogen by the Kjeldahl procedure. Protein was determined by multiplying the Vitamin premix was added at the level of 550 mg/ kg diet. The vitamin premix contained per g: vitamin A-5344 IU; vitamin D3 = 132 IU; vi...