Comparisons between True Digestibility of Total Nitrogen and Limiting Amino Acids in Vegetable Proteins Fed to Rats

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Abstract
Values (%) for true digestibility (TD) of protein and individual amino acids in some vegetable proteins were determined by the rat balance (fecal) method. Diets containing 8% crude protein (N × 6.25) from soaked and autoclaved samples of Trapper and Century field peas, lentil, pinto bean, seafarer bean, black bean or fababean and autoclaved samples of soybean, peanut, sunflower, rolled oat, rice + soybean and corn + pea were tested in two rat balance studies. In the case of blends, each protein source provided 50% of total protein. The beans, peas and lentil proteins were limiting in sulphur amino acids, tryptophan and threonine, whereas sunflower and rolled oat were most limiting in lysine. In beans, peas and lentil, the TD values of methionine (51–82), cystine (46–85), tryptophan (47–90) and threonine (62–84) were considerably lower than the TD values of total nitrogen (72–90). Similarly, in sunflower and rolled oat, the TD values of lysine (81–83) were lower than the TD values of total nitrogen (90–91). These data suggested that crude protein digestibility may not be a good predictor of bioavailability of limiting amino acids in vegetable proteins. Amino acid scores of the vegetable proteins were 62–96%. The corrections for true digestibility of protein and individual amino acids lowered the scores by 6–15 and 11–47 percentage units, respectively.