The fate of amino acids deaminated following protein ingestion is uncertain. Presumably, the majority of the carbon skeletons of the amino acids are converted into glucose in the liver. In the present study, tritiated glucose dilution tracer studies have been used to determine the effect of a protein meal on the glucose appearance rate in plasma. Five normal male subjects ingested 50 g of protein in the form of cottage cheese. The glucose appearance rate was determined using a constant infusion of 3H-glucose, and compared to the glucose appearance rate following the ingestion of just water in the same subjects over an 8-hour period. The total amount of protein deaminated and converted to urea also was quantitated. Urea production could account for the metabolism of 29.3 g of protein ingested, or 58.5%. Glucose appearing in the circulation as a result of amino acid metabolism determined by tracer methodology was 9.68 ± 5.7 g. Based on the gluconeogenic potential of cottage cheese (42.3 g of glucose from 50 g of cottage cheese protein), this could only account for at most 43% of protein metabolized, or 23% of the total amount of protein ingested. The fate of the remaining amino acids metabolized remains to be determined.
Bibliographical noteFunding Information:
Supported by funds from the National Dairy Council, administered in cooperation with the National Dairy Promotion and Research Board, merit review funds from the Department of Veterans Affairs, and a grant from the National Institutes of Health #DK43018-01A1. We thank the subjects for their participation in this study, and the staffs of the Metabolic Research Laboratory, the Clinical Chemistry Laboratory, the Special Diagnostic and Treatment Unit, and also Mary Adams, MT, for technical assistance, and Claudia Durand for secretarial services.