Vitamin A antagonizes calcium response to vitamin D in man.

ABSTRACT

For unknown reasons, the highest incidence of osteoporosis is found in northern 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 acute effects of vitamin A and D on calcium homeostasis in 9 healthy human subjects. We compared the effect of (i) 15 mg of retinyl palmitate, (ii) 2 microg of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], (iii) 15 mg of retinyl palmitate plus 2 microg of 1,25(OH)2D3, and (iv) placebo in a double-blind crossover study. The subjects took vitamin preparations at 1000 p.m. and the following day blood samples were collected five times from 800 a.m. to 400 p.m. Serum levels of 1,25(OH)2D3 and retinyl esters increased (1.7-fold and 8.3-fold, respectively; p < 0.01). As expected, serum calcium (S-calcium) increased (2.3%; p < 0.01) and S-parathyroid hormone (PTH) decreased (-32%; p < 0.05) after 1,25(OH)2D3 intake. In contrast, retinyl palmitate 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)2D3 after the combined intake (1.4%; p < 0.01). S-PTH was unaffected by retinyl palmitate. No significant changes in serum levels of the degradation product of C-telopeptide of type I collagen (CrossLaps), or U-calcium/creatinine levels were found. In conclusion, an intake of vitamin A corresponding to about one serving of liver antagonizes the rapid intestinal calcium response to physiological levels of vitamin D in man.