LAUSR

We thank Landrier and colleagues for their thoughtful comments regarding our recent publication regarding the role of decreased expression of CYP2R1 as a basis for reduced circulating levels of 25(OH)D in obese mice. First, Landrier and colleagues point out one of the weaknesses that we ourselves had noted in our article, specifically that the cholecalciferol content of the two diets was not identical. They further assert that when diet composition is adjusted for vitamin D intake, obese mice show no differences from lean mice in serum 25(OH)D3 concentrations. We agree that the effect of obesity on circulating levels of 25(OH)D is less clear in mice than it is in humans, which is why we extended our observations to conditions in which we altered intake of cholecalciferol. Here we think the situation is at least more consistent if not entirely clear. Similar to Seldeen and colleagues and Park and colleagues, we also found no differences in baseline levels of 25(OH)D of lean and obese mice. And, in agreement with Seldeen, we found that obese mice receiving high but not low intakes of cholecalciferol had lower serum levels of 25(OH)D3 at multiple time points between 4 and 24 weeks of supplementation. These results suggested that under conditions of high cholecalciferol intake, obese mice were less capable of 25‐hydroxylating cholecalciferol than lean mice. And in agreement with Park and colleagues, we found that mRNA levels of the principal hepatic 25‐hydroxylase, Cyp2r1, were significantly lower in the obese group than in the lean group. Second, we have no empirical explanation for the observation that serum levels of cholecalciferol were similar in obese and lean mice despite differences in the vitamin D3 contents of the two diets. It is possible that intestinal absorption of vitamin D3 in mice is facilitated by a high‐fat diet, as it is in humans (4) and rats, and that obese mice receiving the high‐fat diet actually had a vitamin D3 intake that was equivalent to that in lean mice receiving the low‐fat diet. Indeed, Dawson‐Hughes and colleagues have shown that mean peak (12‐hour) plasma levels of cholecalciferol are 32% (95% confidence interval 11% to 52%) greater after a single 50,000 IU dose of vitamin D3 in subjects consuming fat‐containing compared with fat‐free meals (P= 0.003). Further, we believe the cholecalciferol contents of both diets we used (standard CHOW and the high‐fat diet) were more than sufficient to provide adequate amounts of substrate for CYP2R1‐dependent conversion to 25(OH)D, making CYP2R1 activity limiting. Supporting this notion, it has been reported that raising the amount of cholecalciferol from a range below both of our diets (from 1000 IU/kg in lean mice and 1282 IU/kg in obese mice) to a range above both of our our diets (4000 IU/kg in lean mice and 5169 IU/kg in obese mice) has no effect on serum 25(OH)D3 concentrations. Thus, in sum, we agree that variation in vitamin D 25‐hydroxylase activity including CYP2R1 is not the only cause for decreased serum 25(OH)D3 in obesity, but we believe that our work continues to provide strong support for the novel possibility that it is the primary reason for this difference.