Vitamin A antagonizes the action of vitamin D in rats

Interactions between vitamin A and vitamin D have been suggested for severa l decades but have not been established. In particular, vitamin A has been proposed to intensify the severity of the bone mineralization disease, rick ets and inhibit the ability of vitamin D to cure this disease. To investiga te this hypothesis, weanling Holtzman rats were fed a 1.2% calcium, 0.1% ph osphorus diet and 15.5 ng ergocalciferol (vitamin D-2) every 3 d for 21 d i n the presence of increasing amounts of retinyl acetate (0 mu g to 8621 mu g/d). The increasing amounts of retinyl acetate produced a progressive and significant decrease in total bone ash (P < 0.001) and an increase in epiph yseal plate width (P < 0.001). The same experiment conducted with increasin g amounts of Vitamin D-2 (0 to 645 ng/d) indicated that the antagonism by r etinyl acetate could be demonstrated at all vitamin D-2 dosages. To further investigate this antagonistic relationship, weanling Holtzman rats were fe d a 0.47% calcium, 0.3% phosphorus diet and 15.5 ng vitamin D-2 every 3 d f or 33 d in the presence of increasing retinyl acetate (0 to 3448 mu g/d). I n the absence of retinyl acetate, these rats maintained a normal serum-calc ium level (2.34 mmol/L). increasing retinyl acetate, however, eliminated th e ability of vitamin D-2 to elevate the level of serum calcium (1.35 mmol/L ). These results illustrated in vivo antagonism of vitamin D-2 action on in testine and bone by retinyl acetate.