We recently showed that excessive fructose consumption, already associated with numerous metabolic abnormalities, reduces rates of intestinal Ca(2+) transport. Using a rat lactation model with increased Ca(2+) requirements, we tested the hypothesis that mechanisms underlying these inhibitory effects of fructose involve reductions in renal synthesis of 1,25-(OH)(2)D(3). Pregnant and virgin (control) rats were fed isocaloric fructose or, as controls, glucose, and starch diets from d 2 of gestation to the end of lactation. Compared to virgins, lactating dams fed glucose or starch had higher rates of intestinal transcellular Ca(2+) transport, elevated intestinal and renal expression of Ca(2+) channels, Ca(2+)-binding proteins, and CaATPases, as well as increased levels of 25-(OH)D(3) and 1,25-(OH)(2)D(3). Fructose consumption prevented almost all of these lactation-induced increases, and reduced vitamin D receptor binding to promoter regions of Ca(2+) channels and binding proteins. Changes in 1,25-(OH)(2)D(3) level were tightly correlated with alterations in expression of 1α-hydroxylase but not with levels of parathyroid hormone and of 24-hydroxylase. Bone mineral density, content, and mechanical strength each decreased with lactation, but then fructose exacerbated these effects. When Ca(2+) requirements increase during lactation or similar physiologically challenging conditions, excessive fructose consumption may perturb Ca(2+) homeostasis because of fructose-induced reductions in synthesis of 1,25-(OH)(2)D(3).

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.

Excessive fructose consumption inhibits adaptive increases in intestinal Ca2+ transport in lactating and weanling rats with increased Ca2+ requirements by preventing the increase in serum levels of 1,25(OH)2D3. Here we tested the hypothesis that chronic fructose intake decreases 1,25(OH)2D3 levels independent of increases in Ca2+ requirements. Adult mice fed for five wk a high glucose-low Ca2+ diet displayed expected compensatory increases in intestinal and renal Ca2+ transporter expression and activity, in renal CYP27B1 (coding for 1α-hydroxylase) expression as well as in serum 1,25(OH)2D3 levels, compared with mice fed isocaloric glucose- or fructose-normal Ca2+ diets. Replacing glucose with fructose prevented these increases in Ca2+ transporter, CYP27B1, and 1,25(OH)2D3 levels induced by a low Ca2+ diet. In adult mice fed for three mo a normal Ca2+ diet, renal expression of CYP27B1 and of CYP24A1 (24-hydroxylase) decreased and increased, respectively, when the carbohydrate source was fructose instead of glucose or starch. Intestinal and renal Ca2+ transporter activity and expression did not vary with dietary carbohydrate. To determine the time course of fructose effects, a high fructose or glucose diet with normal Ca2+ levels was fed to adult rats for three mo. Serum levels of 1,25(OH)2D3 decreased and of FGF23 increased significantly over time. Renal expression of CYP27B1 and serum levels of 1,25(OH)2D3 still decreased in fructose- compared to those in glucose-fed rats after three mo. Serum parathyroid hormone, Ca2+ and phosphate levels were normal and independent of dietary sugar as well as time of feeding. Thus, chronically high fructose intakes can decrease serum levels of 1,25(OH)2D3 in adult rodents experiencing no Ca2+ stress and fed sufficient levels of dietary Ca2+. This finding is highly significant because fructose constitutes a substantial portion of the average diet of Americans already deficient in vitamin D.

We recently showed that excessive fructose (F) consumption inhibits adaptive increases in intestinal Ca transport in lactating and weanling rats with enhanced Ca needs, by preventing the increase in blood levels of the biologically active vitamin D, calcitriol. Here we tested the hypothesis that chronic F intake decreases calcitriol levels independent of Ca status. In the control study, adult mice fed for 5 wk a 40% glucose (G)‐low Ca diet displayed expected adaptive increases in intestinal and renal Ca transporter expression and activity as well as in serum calcitriol levels, compared with mice fed G‐ or F‐normal Ca diets. However, dietary F clearly prevented increases in calcitriol and Ca transporter expression to Ca deficiency. When the 40% F‐normal Ca diet was fed to adult rats for 3 mo, calcitriol levels and expression of CYP27B1 that synthesizes calcitriol decreased markedly compared to those in G‐fed mice. Ca status and Ca transporter expression were normal and independent of dietary sugar. Expression of CYP27B1 also decreased and of CYP24A1 increased in adult mice fed for 3 mo a 60% F‐normal Ca diet. Thus, chronic F intakes stimulate renal catabolism and impair synthesis of calcitriol, regardless of Ca status. This finding is highly significant because F constitutes a substantial portion of the average diet of Americans now highly vulnerable to F‐induced deficiencies in calcitriol