Bioavailable Lysine Assessed Using the Indicator Amino Acid Oxidation Method in Healthy Young Males is High when Sorghum is Cooked by a Moist Cooking Method

Abstract

Background: Sorghum is the fifth most consumed cereal grain but limiting in the indispensable amino acid lysine. Complementing sorghum with lentils can improve the quality of sorghum-based diets. However, knowledge of lysine bioavailability in sorghum is lacking.

Objectives: The study objectives were to determine the bioavailability of lysine in sorghum and to assess the effect of complementation of sorghum and lentils in a mixed-meal format.

Methods: We studied 5 healthy young men (≤30 years; BMI <25 kg/m2) in a repeated-measure design using the indicator amino acid oxidation (IAAO) method, with L-[1-13C] phenylalanine as the indicator. Each subject participated in 8 determinations in random order. On the reference diet, subjects received 4 amounts of L-lysine (5, 8, 12, and 15 mg. kg-1 . d-1) from a crystalline amino acid mixture patterned after egg protein. On the test diet, they received 3 levels of lysine (8.2, 12.5, and 15.7 mg. kg-1 . d-1) from sorghum, and on the complementation diet they received 1 level of lysine from a mixed meal of sorghum and lentils. The bioavailability of lysine in sorghum was estimated by comparing the IAAO response to the test diet with the IAAO response to the reference diet using the slope-ratio method. Effectiveness of complementation was assessed by comparing the IAAO response to the mixed meal to the IAAO response to the test protein.

Results: The bioavailability of lysine from sorghum was 94%. Upon complementation with lentils, there was a decline in the oxidation of L-[1-13C] phenylalanine by 19% (P < 0.0495), reflecting an improvement in available lysine in the mixed meal due to increased lysine intake.

Conclusions: Although the bioavailability of lysine in sorghum is high, its lysine content is limiting. Complementation with lentils in a 1:1 ratio is recommended to achieve the lysine requirement for adult men consuming a sorghum-based diet. This trial was registered at clinicaltrials.gov as NCT03411005.

Keywords: amino acids; bioavailability; complementation; humans; lentils; lysine; protein quality; sorghum; stable isotope.

© The Author(s) 2021. Published by Oxford University Press on behalf of the American Society for Nutrition.

Figures

FIGURE 1

The effect of lysine intake on the mean rate of phenylalanine oxidation in 5 healthy adult males who participated in determining the BA of lysine in sorghum. There was a linear decrease in F13CO2, with graded intakes of lysine resulting in negative significant slopes (means ± SDs) for the sorghum slope (–0.0330; P = 0.0008; SEM = 0.009; R2 = 0.55) and the reference slope (–0.0352; P = 0.0006; SEM = 0.009; R2 = 0.58). Both slopes on the same graph have a common intercept. The BA of lysine in the reference protein (crystalline amino acids) was assumed to be 100% (65). The BA of lysine in sorghum was calculated as the sorghum slope divided by the reference slope: [(−0.0330/−0.0352).100] = 94%. Abbreviations: BA, bioavailability; F13CO2, rate of phenylalanine oxidation.