Excessive fructose intake causes 1,25-(OH)2D3-dependent inhibition of intestinal and renal calcium transport in growing rats

Abstract

We recently discovered that chronic high fructose intake by lactating rats prevented adaptive increases in rates of active intestinal Ca(2+) transport and in levels of 1,25-(OH)2D3, the active form of vitamin D. Since sufficient Ca(2+) absorption is essential for skeletal growth, our discovery may explain findings that excessive consumption of sweeteners compromises bone integrity in children. We tested the hypothesis that 1,25-(OH)2D3 mediates the inhibitory effect of excessive fructose intake on active Ca(2+) transport. First, compared with those fed glucose or starch, growing rats fed fructose for 4 wk had a marked reduction in intestinal Ca(2+) transport rate as well as in expression of intestinal and renal Ca(2+) transporters that was tightly associated with decreases in circulating levels of 1,25-(OH)2D3, bone length, and total bone ash weight but not with serum parathyroid hormone (PTH). Dietary fructose increased the expression of 24-hydroxylase (CYP24A1) and decreased that of 1α-hydroxylase (CYP27B1), suggesting that fructose might enhance the renal catabolism and impair the synthesis, respectively, of 1,25-(OH)2D3. Serum FGF23, which is secreted by osteocytes and inhibits CYP27B1 expression, was upregulated, suggesting a potential role of bone in mediating the fructose effects on 1,25-(OH)2D3 synthesis. Second, 1,25-(OH)2D3 treatment rescued the fructose effect and normalized intestinal and renal Ca(2+) transporter expression. The mechanism underlying the deleterious effect of excessive fructose intake on intestinal and renal Ca(2+) transporters is a reduction in serum levels of 1,25-(OH)2D3. This finding is significant because of the large amounts of fructose now consumed by Americans increasingly vulnerable to Ca(2+) and vitamin D deficiency.