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Differences in Rat Strain Response to Three Diets of Different Composition

Human Nutrition Research Division, Agricultural Research Service, United States Department of Agriculture, Washington, D. C.

Male weanling rats were housed individually and fed ad libitum one of three diets until they were killed, without fasting, when 330 to 335 days old. A mixed strain (BHE), Holtzman and a strain of Wistar rats were studied. The three diets fed to all strains included a pelleted stock diet, a semipurified diet (SP) and a semipurified diet containing 75% of the SP diet and 25% reconstituted, cooked and re-dried whole egg (SPE).

Weight gain for rats fed the stock diet was similar for rats of all strains. But when fed the SP and SPE diets BHE and Holtzman rats ate more and were heavier than Wistar rats. Gross calorie intakes were similar for the SP and SPE diets for any one strain. Nitrogen balances at 155 days of age indicated that BHE rats stored significantly less nitrogen than Holtzman or Wistar rats, but by 325 days all three strains were storing nitrogen and there were no significant differences related to strain. The BHE and Holtzman rats fed stock or SPE diets excreted larger amounts of protein in the urine than did Wistar rats. When fed the SPE diet, BHE and Holtzman rats had larger livers, larger livers per 100 gm of body weight, a smaller percentage of liver protein and more liver fat and cholesterol, more kidney damage, larger kidneys, larger adrenals, a smaller percentage of carcass protein, more carcass fat and higher serum cholesterol than did Wistar rats.

It was concluded that certain measurements such as body growth, food intake, urinary protein, liver, kidney and adrenal weights, carcass protein, carcass fat and serum cholesterol are influenced by strain of rat as well as by diet. The similarity for all three strains in digestibility of the dietary nitrogen and energy supply, and of gains per 100 gm of food and gains per 100 Cal. consumed in contrast with the differences in liver and body composition indicates that differences in response among strains must be related to differences in inherent metabolic characteristics of the rats.