Protein digestibility of methionine supplemented common beans (Phaseolus vulgaris) in adult human subjects.

The study reports on the protein digestibility of five cultivars of common beans (Phaseolus vulgaris) of different color, tested in young adult human subjects, as the sole dietary protein source. Cheese protein was used as reference protein. The cooked beans provided 0.65 g protein/kg/day, and energy intake was adjusted to 45 kcal/kg/day. Results indicated apparent protein digestibility to vary from 49.6 to 62.1%. White colored beans showed the highest value, while black beans gave the lowest. Cheese protein showed an apparent protein digestibility of 76.2%. The true protein digestibility was calculated using the endogenous nitrogen excretion values obtained in the study, as well as literature values. Digestibility increased as expected, but it is still low as compared to other protein sources. A high correlation was found between dry matter digestibility and protein digestibility. Fecal nitrogen was fractionated between soluble and insoluble nitrogen in a 0.02 N NaOH solution. This assay was also carried out in the cooked beans, and findings revealed that the soluble nitrogen fraction was highly correlated with protein digestibility, with a correlation coefficient of -0.94. This fraction, still to be identified, could very well be responsible for the low digestibility values found for common beans in human subjects.