Vitamin A antagonizes calcium response to vitamin D in man

For unknown reasons, the highest incidence of osteoporosis is found in nort hern Europe. In these populations, the sunlight exposure is limited and the vitamin A intake is high. The interaction between vitamin A and D has been the subject of several in vitro and animal studies. We have studied the ac ute effects of vitamin A and D on calcium homeostasis in 9 healthy human su bjects. We compared the effect of (i) 15 mg of retinyl palmitate, (ii) 2 mu g of 1,25-dihydroxyvitamin D-3 [1,25(OH)(2)D-3], (iii) 15 mg of retinyl pal mitate plus 2 mug of 1,25(OH)(2)D-3, and (iv) placebo in a double-blind cro ssover study. The subjects took vitamin preparations at 10:00 p.m. and the following day blood samples were collected five times from 8:00 a.m. to 4:0 0 p.m. Serum levels of 1,25(OH)(2)D-3 and retinyl esters increased (1.7-fol d and 8.3-fold, respectively; p < 0.01). As expected, serum calcium (S-calc ium) increased (2.3%; p < 0.01) and S-parathyroid hormone (PTH) decreased ( -32%,; p < 0.05) after 1,25(OH)(2)D-3 intake. In contrast, retinyl palmitat e intake resulted in a significant decrease in S-calcium. when taken alone (-1.0%; p < 0.05) and diminished the calcium response to 1,25(OH)(2)D-3 aft er the combined intake (1.4%; p < 0.01). S-PTH was unaffected by retinyl pa lmitate. No significant changes in serum levels of the degradation product of C-telopeptide of type I collagen (CrossLaps), or U-calcium/creatinine le vels were found. In conclusion, an intake of vitamin A corresponding to abo ut one serving of liver antagonizes the rapid intestinal calcium response t o physiological levels of vitamin D in man.