Lysine from Cooked White Rice Consumed by Healthy Young Men Is Highly Metabolically Available When Assessed Using the Indicator Amino Acid Oxidation Technique

Abstract

Cooked white rice (CWR) provides up to 71% of the dietary protein for many people worldwide. The protein digestibility-corrected amino acid (AA) score is the method adopted by FAO/WHO to evaluate protein quality. Our group has proposed the metabolic availability (MA) of AAs as another determinant of protein quality. It measures the percentage of an indispensable AA that is incorporated during protein synthesis. This study is the first to our knowledge to assess the MA of l-lysine (l-Lys) from CWR in humans using the indicator AA oxidation (IAAO) technique. Three amounts of l-Lys, 10, 15, and 19 mg · kg−1 · d−1 (= 28.5, 42.8, and 54.3% of the mean l-Lys requirement of 35 mg · kg−1 · d−1), were studied in 5 healthy young men in a repeated-measures design. To test the principle that the Maillard reaction has an effect on the MA of l-Lys, we also assessed the MA of l-Lys in oven-browned, cooked rice (n = 3) in the amount of 19 mg · kg−1 · d−1l-Lys. The MA of l-Lys was estimated by comparing the IAAO response with varying l-Lys intakes in rice compared with the IAAO response to varying l-Lys intakes in the reference protein (crystalline AA mixture patterned after egg protein) using the slope ratio method. The MA of l-Lys from CWR was high (97%), but the effect of the Maillard reaction reduced it to 70%. The results show that despite its relatively low content in rice, l-Lys has a high MA when the rice is cooked without being browned.