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Oral Amino Acid Load Test of Protein Nutrition: Effect of Protein Quantity and Quality in the Rat^1,2,

Department of Food Science, Cook College, Rutgers-The State University, New Brunswick, New Jersey 08903

Glutamate-pyruvate transaminase (GPT) is known to increase with the level of protein nutrition. An oral load test was designed to make GPT rate-limiting in the catabolism of alanine and provide a measure of protein nutrition in the rat. When the alanine dose was administered on a body weight basis, GPT activity was reflected in the subsequent excretion of urinary nitrogen, or of the urea-plus-ammonia fraction. This load test was administered to 4 groups of young rats of equal age, which had been fed casein-lactalbumin (CL) purified diets at four precise levels of protein restriction, and thus had different body weights. The load test was also administered to 4 groups of young rats of equal age and body weight, but with differing dietary histories of protein (CL) quantity. In addition, the load test was administered to rats that were adjusted to two levels of each of four proteins (CL, soybean, peanut flour, wheat gluten) of different quality, as measured by weight gain and nitrogen balance. Urinary nitrogen excretion for 12 hours following the load increased as the level of protein nutrition increased in every case except when an extremely imbalanced protein (wheat gluten) was tested. A decrease in either protein quantity or protein quality decreased the percentage of ingested nitrogen excreted in the urine. Doubling the amount of wheat gluten had no effect on nitrogen retention. These results suggest the potential of an oral alanine load test for the assessment of protein nutrition, including the marginal protein malnutrition found in human populations.

KEY WORDS: • protein malnutrition • GPT • load test • alanine • urinary nitrogen

¹ Supported in part by Northeast Regional Research Project NE-73.

² Presented in part at the 58th Annual Meeting of the Federation of American Societies for Experimental Biology. Atlantic City, N.J., April 12, 1974. Federation Proc. 33, 712 (1974).

³ Current address: Grocery Manufacturers of America, Inc., 1425 K St. NW, Washington, D.C. 20005.

Manuscript received 7 June 1976.