The problem of curvature in slope assays for protein quality

The dose-response curves in a valid slope--ratio assay must be linear and meet at the zero protein dose. However in rat bioassays at least 3 types of curvature are found when the method is applied to a wide range of samples; the types of curvature result from: 1) excessive protein 2) lysine deficiency and 3) threonine deficiency. Whole wheat flour which was limiting in lysine and threonine showed marked downward curvature after supplementation with threonine. It can be argued from these results that the slope ratio and relative protein value method underestimated the protein quality of lysine--deficient foods.