Available (ileal digestible reactive) lysine in selected pet foods

A recently developed assay for determining available lysine (true ileal digestible reactive lysine) in foods and feedstuffs was applied to 20 commercially available cat foods. Semisynthetic diets, containing cat food as the sole protein source, were prepared. Titanium dioxide was included as an indigestible marker. The diets were fed to growing rats and digesta from the terminal ileum collected and analyzed, along with the diets, for reactive lysine. True reactive lysine digestibility was determined after correction for endogenous lysine loss at the terminal ileum of rats fed an enzyme-hydrolyzed casein-based diet. The amounts of digestible total lysine (conventional method) were also determined. Ileal total lysine digestibility significantly (P<0.05) underestimated (3.6-10.2%) lysine availability (ileal reactive lysine digestibility) for most of the cat foods tested. Ileal digestible total lysine significantly (P<0.05) overestimated the amount of dietary available lysine for all of the cat foods tested by between 38 and 143%. Total lysine digestibility determined using the conventional method of lysine analysis was inaccurate when applied to commercially available cat foods.