Christine Simpson

POSTTRANSPLANT BONE DISEASE: EVIDENCE FOR A HIGH BONE RESORPTION STATE

Loss of bone is a significant problem after renal transplant. Although bone loss in the first post transplant year has been well documented, conflicting data exist concerning bone loss after this time.It is equally unclear whether bone loss in long-term renal transplant recipients correlates with bone turnover as it does in postmenapausal osteoporosis. To examine these issues, we conducted a cross-sectional study to define the prevalence of osteoporosis in long-term (>1 year) renal transplant recipients with preserved renal function (mean creatinine clearance 73±23 ml/min). Bone mineral density (BMD) was measured at the hip, spine and wrist by DEXA in 69 patients. Markers for bone formation (serum osteocalcin) and bone resorption [urinary levels of pyridinoline (PYD) and deoxypyridinoline (DPD)] were also measured as well as parameters of calcium metabolism. Correlations were made between these parameters and BMD at the various sites. The mean age of the patients was 45±11 years. Eighty eight percent of patients were on cyclosporine (12% on tacrolimus) and all but 2 were on prednisone [mean dose 9±2 mg/day)]. Osteoporosis (BMD more than 2.5 SD below peak adult BMD) at the spine or hip was diagnosed in 44% of patients and osteopenia was present in an additional 44%. Elevated levels of intact parathyroid hormone (i PTH) were observed in 81% of patients. Elevated urinary levels of PYD or DPD were present in 73% of patients and 38% had elevated serum levels of osteocalcin. Levels of calcium, and of 25(OH) and 1,25(OH)2 vitamin D were normal. In a stepwise multiple regression model that included osteocalcin, PYD, DPD, intact PTH, age, years posttransplant, duration of dialysis, cumulative prednisone dose, smoking, and diabetes: urinary PYD was the strongest predictor of bone mass. These results demonstrate that osteoporosis is common in long-term renal transplant recipients. The data also suggest that elevated rates of bone resorption contribute importantly to this process.

Circulating Levels of Soluble Klotho and FGF23 in X-Linked Hypophosphatemia: Circadian Variance, Effects of Treatment, and Relationship to Parathyroid Status

CONTEXT Circulating fibroblast growth factor (FGF)-23 is variably elevated in individuals with X-linked hypophosphatemia (XLH), and klotho has recently been shown to effect renal phosphate handling, yet limited data are available on circulating FGF23 and klotho in XLH. OBJECTIVE The objective of the study was to characterize circulating FGF23 and klotho in XLH. DESIGN Children and adults with XLH withheld medication for 14 d. Fasting serum FGF23, PTH, klotho, phosphate, and 1,25 dihydroxyvitamin D were obtained. Treated adults were also sampled, and circadian sampling was performed in selected individuals. SETTING The study was conducted at a hospital research unit at an academic medical center. PATIENTS AND OTHER PARTICIPANTS Participants included 23 individuals with XLH and eight controls. INTERVENTIONS There were no interventions. MAIN OUTCOME MEASURES Serum klotho and FGF23 were measured. RESULTS FGF23 was greater in XLH than in controls and greater in treated XLH subjects compared with XLH subjects not receiving medical therapy. Children had higher klotho levels than adults, but values in XLH were similar to controls. A strong positive correlation between FGF23 and PTH was found in XLH subjects, whereas there was no relationship between these variables in controls. Circulating klotho, but not FGF23, has a diurnal pattern. CONCLUSIONS Serum klotho declines with age and demonstrates circadian variation but is normal in XLH. Serum FGF23 is similar in children and adults, is elevated in XLH, further increases with therapy, and demonstrates no diurnal variation. The direct relationship between FGF23 and PTH in subjects with XLH suggests that FGF23 regulation of PTH secretion is aberrant in this disorder.

Inhibiting gastric acid production does not affect intestinal calcium absorption in young, healthy individuals: a randomized, crossover, controlled clinical trial.

Proton pump inhibitors (PPIs) are the most potent gastric acid suppressing drugs available, and their use is widespread. An emerging concern about chronic PPI therapy is whether these drugs impair intestinal calcium absorption, resulting in a negative calcium balance and thereby potentially causing bone loss. The objective of this study was to evaluate the acute effect of the PPI esomeprazole or placebo on intestinal calcium absorption in healthy adults. Twelve young adults participated in a placebo‐controlled, double‐blind, crossover study. There were two 3‐week interventions that included a 14‐day adjustment period (designed to stabilize calcium homeostasis) followed by 6 days of a diet containing 800 mg of calcium and 2.1 g/kg of protein (intervention). During the last 3 days of the adjustment period and throughout the intervention period, subjects consumed esomeprazole or placebo. Half the subjects underwent 24‐hour continuous gastric acid pH monitoring. Intestinal calcium absorption was measured using dual‐stable calcium isotopes at the end of each intervention. Treatment with esomprazole significantly increased gastric pH (mean pH on PPI 5.38 ± 0.13, mean pH on placebo 2.70 ± 0.44, p = .005). Neither calcium absorption (PPI 34.2% ± 2.4%, placebo 31.5% ± 2.1%, p = .24) nor urinary calcium (PPI 321 ± 38 mg/34 hours, placebo 355 ± 37 mg/34 hours, p = .07) differed between the PPI and placebo groups. It is concluded that short‐term gastric acid suppression by PPIs does not attenuate intestinal calcium absorption in healthy young adults.

Vitamin D binding protein is a key determinant of 25-hydroxyvitamin D levels in infants and toddlers.

Circulating 25‐hydroxyvitamin D (25‐OHD) levels vary among human populations. Only limited information regarding determinants of these measures is available for infants and children, particularly in minority groups at greatest risk for vitamin D deficiency. We identified demographic determinants of circulating 25‐OHD in a large cohort of minority children, and now extend our studies to examine potential roles of vitamin D binding protein (DBP) as a determinant of 25‐OHD levels. Serum DBP level and common single nucleotide polymorphisms (SNPs) at positions 432 and 436 in the GC gene, encoding DBP, were examined. We confirmed self‐reported ancestry using ancestry informative markers (AIMs), and included quantitative AIMs scores in the analysis. The multivariate model incorporated previously identified demographic and nutritional determinants of 25‐OHD in this cohort, as well as GC SNPs and circulating DBP. Genetic variants in GC differed by self‐reported ancestry. The 1f allele (D432/T436) was enriched in African Americans, occurring in 71%. Homozygosity for the 1f allele (DDTT) occurred in 53% of African Americans but only 6% of Caucasians and 13% of Hispanics. Circulating DBP was significantly correlated with 25‐OHD. GC SNPs were associated with both circulating DBP and 25‐OHD. It appears that progressive substitution of lysine for threonine at the 436 position results in lower circulating 25‐OHD. Multivariate analysis revealed that genetic variance in GC significantly contributes to circulating DBP as well as 25‐OHD. Moreover, the effect of GC SNPs on 25‐OHD are evident after adjusting for their effects on circulating DBP. Thus in young children genetic variance of the common GC T436K SNP affects circulating levels of the DBP protein, which in turn affects circulating 25‐OHD. However, the GC genotype also affects circulating 25‐OHD independently of its effect on circulating DBP. These findings provide data that may be important in the interpretation of vitamin D status in children of varying ancestral backgrounds.

Parameters of high bone-turnover predict bone loss in renal transplant patients: a longitudinal study.

BACKGROUND Osteoporosis is a serious complication of kidney transplantation. Various factors have been postulated to contribute to posttransplant bone loss, among them treatment with corticosteroids, the use of cyclosporine and cyclosporine-like agents, and persistent hyperparathyroidism. In a previous cross-sectional study of long-term renal transplant recipients, we observed that osteoporosis or osteopenia was present in 88% of patients. Because biochemical markers of bone formation (serum osteocalcin) and bone resorption (urine pyridinoline, PYD, and deoxypyridinoline, DPD) were elevated in the majority of study subjects, we hypothesized that elevated rates of bone-turnover contribute to posttransplant bone loss in long-term renal transplant patients. This study was performed to examine this hypothesis. METHODS The study population was composed of 62 patients who were more than 1-year postrenal transplantation and who had preserved renal function. They were followed prospectively for 1 year. Biochemical markers of bone-turnover were measured at study entry, and patients were classified as having high bone-turnover based on elevated urinary levels of at least one marker of bone resorption (i.e., PYD or DPD) and/or serum osteocalcin (group 1). If none of these were present, they were classified as having normal bone-turnover (group 2). Bone mineral density (BMD) was measured by dual energy x-ray absorptiometry (DEXA) at time of entry into the study and again after 1 year of follow-up. The changes in BMD at the lumbar spine, hip, and wrist over the period of the study were compared between the high and normal bone-turnover groups. RESULTS Forty-three patients (69%) were classified as having high bone-turnover (Group 1), and 19 patients (31%) were classified as having normal bone-turnover (Group 2). There was a statistically significant difference in change in BMD between the two groups at the lumbar spine (-1.11+/-0.42%, high bone-turnover, vs. 0.64+/-0.54%, normal bone-turnover; P=0.02) and the hip (-0.69+/-0.38%, high bone-turnover, vs. 1.36+/-0.66%, normal bone-turnover; P=0.006). Whereas group 2 had stable bone mass, group 1 exhibited bone loss at these skeletal sites. CONCLUSIONS Our results indicate that bone loss is greater in renal transplant recipients with elevated biochemical markers of bone-turnover, suggesting that these markers may be useful in identifying patients at risk for continued bone loss. These data support the hypothesis that continued bone loss in long-term renal transplant recipients is associated with high bone-turnover. If accelerated bone resorption does play a role in posttransplant bone loss, this would provide a strong rationale for use of antiresorptive therapy for the prevention and treatment of this complication.

Treatment of Osteoporosis and Osteopenia in Long-term Renal Transplant Patients with Alendronate

Bone mineral density (BMD) and biochemical markers of bone‐turnover were evaluated in a 2‐year study in 58 long‐term renal transplant recipients with good renal function. In the first year of study, data were collected and patients with osteoporosis and parameters of high bone turnover were classified as being at high risk for on‐going bone loss (Group A; n = 29). Patients with lesser degrees of bone loss or without biochemical parameters of high bone turnover were followed longitudinally (Group B; n = 29). Group A patients were then placed on alendronate 10 mg/day and both groups were followed for an additional year. Changes in regional BMD and bone‐turnover markers between the first and second year within each group were analyzed using paired tests. BMD in Group A, which had declined at the lumbar spine (− 1.6 ± 0.5%) and total femur (− 1.5 ± 0.4%) during the first year of the study, increased on alendronate therapy at both the lumbar spine (+ 3.4 ± 0.6%, p = 0.001) and total femur (+ 1.6 ± 0.6%, p < 0.001). These patients also experienced a significant decline in levels of serum alkaline phosphatase, osteocalcin, urinary levels of deoxypyridinoline and pyridinoline. In contrast, neither BMD nor biochemical markers changed significantly over 2 years in Group B. The current results demonstrate that renal transplant patients with osteoporosis and biochemical parameters of high bone turnover are at continued risk for bone loss. Therapy with a bisphosphonate can reverse this bone loss and even increase bone mass in these patients. Whether patients with lesser degrees of bone loss and/or patients without parameters of high bone turnover can also benefit from bisphosphonate therapy deserves further study.

Relationship of Serum S100B Levels and Intracranial Injury in Children With Closed Head Trauma

OBJECTIVE: To determine if serum levels of S100B are higher in children with CHT and ICI as detected by cranial CT and if long bone fractures affect the level of S100B in children with CHT and skeletal injury. METHODS: Children <18 years of age who presented to an urban pediatric emergency department or were transferred from a referral hospital within 6 hours after accidental closed head trauma and who underwent cranial computed tomography were enrolled prospectively. Mean serum S100B levels for children with or without intracranial injury (ICI) and long-bone fractures were evaluated through analysis of covariance. RESULTS: One hundred fifty-two children, 24 with ICI and 128 without ICI, were enrolled prospectively. Twenty-five children had long-bone fractures. Children with ICI were significantly younger than those without ICI (6.9 vs 9.8 years; P = .01). The time of venipuncture after injury was significantly later in children with ICI (P = .03). Mean S100B levels were significantly greater for children with ICI (212.9 vs 84.4 ng/L; P = .001), children with long-bone fractures (P = .008), and nonwhite children (P = .03). After controlling for time of venipuncture, long-bone fractures, and race, mean S100B levels were still greater for children with ICI (409 vs 118 ng/L; P = .001). The ability of serum S100B measurements to detect ICI, determined as the area under the curve, was 0.67. CONCLUSIONS: After controlling for time of venipuncture, long-bone fractures, and race, S100B levels were still higher in children with ICI than in those without ICI. However, the ability of serum S100B measurements to detect ICI was poor.

Demographic, dietary, and biochemical determinants of vitamin D status in inner-city children

BACKGROUND Reports of clinical rickets are particularly evident in minority infants and children, but only limited analyses of vitamin D are available in this demographic group. OBJECTIVE We sought to characterize circulating 25-hydroxyvitamin D [25(OH)D], 1,25-dihydroxyvitamin D [1,25(OH)(2)D], and their determinants, including circulating parathyroid hormone (PTH), total alkaline phosphatase activity (ALP), calcium, and phosphorus, in minority infants and children. DESIGN We obtained demographic information and blood samples for measurement of PTH, ALP, 25(OH)D, and 1,25(OH)(2)D in >750 6-mo- to 3-y-old children. Dietary intake data were obtained and analyzed. RESULTS The mean (±SD) 25(OH)D concentration was 66 ± 22 nmol/L (26.3 ± 8.7 ng/dL). A total of 15% of children had 25(OH)D concentrations less than the recommended target threshold of 50 nmol/L. Combined elevations of PTH and ALP occurred in only 2.5% of children. Determinants of 25(OH)D included vitamin D intake, age (decreasing with age), skin type (greater concentrations in lighter-skinned children than in darker-skinned children), formula use (higher intakes), season (greater concentrations in the summer and fall than in the winter and spring), and, inversely, PTH. The mean 1,25(OH)(2)D concentration was 158 ± 58 pmol/L (60.6 ± 22.5 pg/mL), which was consistent with a reference range of 41-274 pmol/L or 15.7-105.5 pg/mL. Determinants for 1,25(OH)(2)D were age (decreasing with age), sex (greater concentrations in girls than in boys), skin type (greater concentrations in lighter-skinned children than in darker-skinned children), and, inversely, serum calcium and phosphorus. CONCLUSIONS Although 15% of subjects were vitamin D insufficient, only 2.5% of subjects had elevations of both PTH and ALP. The greater 25(OH)D concentrations observed with formula use confirm that dietary vitamin D fortification is effective in this demographic group. Circulating 1,25(OH)(2)D is higher in infants than in older children and adults and, in contrast to 25(OH)D, is not directly correlated with nutrient intakes.

The effect of a whey protein supplement on bone mass in older Caucasian adults

CONTEXT It has been assumed that the increase in urine calcium (Ca) that accompanies an increase in dietary protein was due to increased bone resorption. However, studies using stable Ca isotopes have found that dietary protein increases Ca absorption without increasing bone resorption. OBJECTIVE The objective of the study was to investigate the impact of a moderately high protein diet on bone mineral density (BMD). DESIGN This was a randomized, double-blind, placebo-controlled trial of protein supplementation daily for 18 months. SETTING The study was conducted at two institutional research centers. PARTICIPANTS Two hundred eight older women and men with a body mass index between 19 and 32 kg/m(2) and a self-reported protein intake between 0.6 and 1.0 g/kg participated in the study. INTERVENTION Subjects were asked to incorporate either a 45-g whey protein or isocaloric maltodextrin supplement into their usual diet for 18 months. MAIN OUTCOME MEASURE BMD by dual-energy x-ray absorptiometry, body composition, and markers of skeletal and mineral metabolism were measured at baseline and at 9 and 18 months. RESULTS There were no significant differences between groups for changes in L-spine BMD (primary outcome) or the other skeletal sites of interest. Truncal lean mass was significantly higher in the protein group at 18 months (P = .048). C-terminal telopeptide (P = .0414), IGF-1 (P = .0054), and urinary urea (P < .001) were also higher in the protein group at the end of the study period. There was no difference in estimated glomerular filtration rate at 18 months. CONCLUSION Our data suggest that protein supplementation above the recommended dietary allowance (0.8 g/kg) may preserve fat-free mass without adversely affecting skeletal health or renal function in healthy older adults.

The Effect of Dietary Protein on Intestinal Calcium Absorption in Rats

Increasing dietary protein intake in humans acutely increases urinary calcium. Isotopic absorption studies have indicated that, at least in the short term, this is primarily due to increased intestinal Ca absorption. To explore the mechanisms underlying dietary proteins effect on intestinal Ca absorption, female Sprague Dawley rats were fed a control (20%), low (5%), or high (40%) protein diet for 7 d, and Ca balance was measured during d 4-7. On d 7, duodenal mucosa was harvested and brush border membrane vesicles (BBMVs) were prepared to evaluate Ca uptake. By d 7, urinary calcium was more than 2-fold higher in the 40% protein group compared with control (4.2 mg/d vs. 1.7 mg/d; P < 0.05). Rats consuming the 40% protein diet both absorbed and retained more Ca compared with the 5% protein group (absorption: 48.5% vs. 34.1% and retention: 45.8% vs. 33.7%, respectively; P < 0.01). Ca uptake was increased in BBMVs prepared from rats consuming the high-protein diet. Maximum velocity (V(max)) was higher in the BBMVs prepared from the high-protein group compared with those from the low-protein group (90 vs. 36 nmol Ca/mg protein x min, P < 0.001; 95% CI: 46-2486 and 14-55, respectively). The Michaelis Menten constant (K(m)) was unchanged (2.2 mm vs. 1.8 mm, respectively; P = 0.19). We conclude that in rats, as in humans, acute increases in protein intake result in hypercalciuria due to augmented intestinal Ca absorption. BBMV Ca uptake studies suggest that higher protein intake improves Ca absorption, at least in part, by increasing transcellular Ca uptake.