Vitamin A nutrition of the equine: growth, serum biochemistry and hematology.

Four diets were fed to pony fillies for 40 weeks. One group received a basal diet low in carotene, designated mildly deficient. Other groups were fed basal diet plus vitamin A propionate equivalent to 12 (control), 1,200 (mildly intoxicated) or 12,000 (severely intoxicated) microgram retinol/kg body weight/day. The mildly deficient group exhibited impairment of growth and hematopoiesis with decreased serum concentrations of iron, albumin and cholesterol prior to depletion of liver vitamin A. Growth was depressed in mildly intoxicated and severely intoxicated fillies. The latter became debilitated and died in less than 40 weeks with reduced serum concentrations of iron, albumin and cholesterol. Significant associations were calculated between dietary vitamin A and vitamin A concentrations in plasma, liver and kidney, and between plasma vitamin A and liver and kidney vitamin A. Plasma retinyl ester increased logarithmically with plasma total vitamin A and linearly with kidney vitamin A. The data suggest that increases in retinyl ester due to excessive dietary vitamin A occur prior to appearance of clinical signs and perhaps reflect kidney vitamin A levels. Calculated estimates of optimal vitamin A intake fo maximal growth averaged 1.4 times the current National Research Council recommendation, for liver secreted serum constituents 5.4 times and for red blood cell criteria 10 times. Thus the recommended intake of 12 microgram/kg for weanling horses appears less than optimal. The data suggest hematologic criteria and serum biochemistries may be more sensitive indicators of vitamin A nutriture than growth.