The aim of the present study was to determine whether the type of protein ingested influences the efficiency of catch-up (CU) growth and bone quality in fast-growing male rats. Young male Sprague-Dawley rats were either fed ad libitum (controls) or subjected to 36 d of 40 % food restriction followed by 24 or 40 d of re-feeding with either standard rat chow or iso-energetic, iso-protein diets containing milk proteins - casein or whey. In terms of body weight, CU growth was incomplete in all study groups. Despite their similar food consumption, casein-re-fed rats had a significantly higher body weight and longer humerus than whey-re-fed rats in the long term. The height of the epiphyseal growth plate (EGP) in both casein and whey groups was greater than that of rats re-fed normal chow. Microcomputed tomography yielded significant differences in bone microstructure between the casein and whey groups, with the casein-re-fed animals having greater cortical thickness in both the short and long term in addition to a higher trabecular bone fraction in the short term, although this difference disappeared in the long term. Mechanical testing confirmed the greater bone strength in rats re-fed casein. Bone quality during CU growth significantly depends on the type of protein ingested. The higher EGP in the casein- and whey-re-fed rats suggests a better growth potential with milk-based diets. These results suggest that whey may lead to slower bone growth with reduced weight gain and, as such, may serve to circumvent long-term complications of CU growth.
Keywords: AL (ad libitum) controls fed regular chow; AVD apparent volume density; BV bone volume; Biomechanics; CU (catch-up) re-fed regular chow; Cas (casein) re-fed casein-formulated chow; Casein; Catch-up growth; Ct.Ar cortical area; EGP epiphyseal growth plate; IGF-I insulin-like growth factor-I; LT long-term; Microcomputed tomography; OC osteocalcin; RES (restriction) restricted to 40 % of daily intake of regular chow; TV total volume; Tb.Th trabecular thickness; Tt.Ar total area; Whey; Whey (whey) re-fed whey-formulated chow; µCT microcomputed tomography.