Demonstration of a difference in urinary calcium, not calcium absorption, in black and white adolescents

Resistance to bone resorbing effects of PTH in black women

Black women have a lower incidence of vertebral and hip fractures than white women, possibly due to differences in skeletal and mineral metabolism. One suggested mechanism is that blacks have decreased skeletal sensitivity to parathyroid hormone (PTH). To test this hypothesis, we infused h(1-34)PTH in healthy premenopausal black (n = 15) and white (n = 18) women over 24 h and measured serum and urine indices of bone turnover and calcium metabolism throughout the infusion. At baseline, the mean 25-hydroxyvitamin D (25(OH)D) concentration was significantly lower in black women (46%). There were also nearly significant trends toward higher PTH and lower urinary calcium and pyridinoline levels in black women. During infusion, there were no racial differences in the mean (1-34)PTH levels achieved or in resultant elevations of serum calcium or 1,25-dihydroxyvitamin D (1,25(OH)2D) levels. Endogenous parathyroid suppression (measured by (1-84)PTH levels) was also similar between blacks and whites. There was an initial decline in urinary calcium/creatinine in both groups with a greater reduction in black women early in the infusion period (p < 0.05 at 8 h). Furthermore, blacks had lower levels of urinary calcium/creatinine throughout the infusion (p < 0.05 group difference). Bone formation markers (carboxy-terminal propeptide of type I procollagen and osteocalcin) decreased within 8 h and continued to decline throughout the infusion with no distinguishable racial differences (p < 0.05 time trend for both). The most dramatic difference between black and white women in response to PTH infusion was represented by the bone resorption markers. Three separate metabolites of bone resorption (cross-linked N-telopeptide of type I collagen, cross-linked C-telopeptide of type I collagen, and free pyridinoline) all showed substantially greater elevations in white (mean peak increments 399, 725, and 43%) compared with black women (mean peak increments 317, 369, and 17%) during the infusion (p < 0.05 group differences for all three variables). These data strongly suggest that blacks have decreased skeletal sensitivity to the acute resorptive effects of increased PTH. This finding indicates that calcium homeostasis may be accomplished in blacks (during times of relative calcium deficiency) by greater conservation of calcium from nonskeletal sources (most likely renal) with relative preservation of skeletal tissue. These differences in calcium economy could account, at least in part, for the increased bone mass and lower incidence of osteoporotic fractures in black women.

Reduced rates of skeletal remodeling are associated with increased bone mineral density during the development of peak skeletal mass

Two related studies were conducted to assess the associations between markers of skeletal modeling and remodeling in healthy children. Members of monozygotic twin pairs, aged 6-14, enrolled in a clinical trial of calcium supplementation, were studied at the end of the period of supplementation and for 3 years thereafter. Supplemented children had significantly higher rates of gain in bone mineral density (BMD) (+3% on average) during the period of supplementation accompanied by significantly lower concentrations of serum osteocalcin (OC, -15%). During postsupplement follow-up, both differences in BMD and OC disappeared. Black females, age matched to the baseline ages of the white children, had significantly lower serum concentrations of both OC and tartrate-resistant acid phosphatase (TRAP) at all ages and higher BMDs. When stratified on serum TRAP concentrations, regardless of race, children with lower concentrations had significantly higher BMDs, and no racial differences were apparent. In regression models accounting for 70-80% of the variability in BMD in children, body size and TRAP, but not race, remained significantly associated with BMD. The skeletal advantages seen with calcium supplementation and black race appear to be associated with reduced rates of skeletal turnover. Given that markers of turnover during growth reflect both skeletal modeling and remodeling, and there is no apparent advantage to reduced skeletal modeling, it seems probable that reduced remodeling is the factor that accounts for the increases in bone mass.

Comparison of body composition in black and white premenopausal women

We examined 23 pairs of black and white premenopausal women to determine whether there were ethnic differences in body composition. The pairs were matched on weight and height. Each woman had measurements of total body water taken by a tritiated water dilution technique, total body nitrogen, and total body carbon by neutron-activation, mineral ash by dual x-ray absorptiometry, and body potassium by whole body counting. Differences between blacks and whites were compared with the use of both the two-compartment and four-compartment models. The two-compartment model showed that in premenopausal black and white women of similar age, heights, and weights, blacks had significantly more lean mass. The four-compartment model resulted in similar conclusions. The black women had larger protein, mineral, and water compartments and less fat than whites. It may be that body weight measurements as an indicator of obesity should be adjusted for black versus white women.

Risk for osteoporosis in black women

Models of involutional bone loss and strategies for the prevention of osteoporosis have been developed for white women. Black women have higher bone densities than white women, but as the black population ages there will be an increasingly higher population of black women with osteoporosis. Strategies should be developed to reduce the risk of black women for fragility fractures. Dual energy X-ray absorptiometry measurements of the total body, femur, spine, and radius were performed on 503 healthy black and white women aged 20-80 years. Indices of bone turnover, the calcitrophic hormones, and radioisotope calcium absorption efficiency were also measured to compare the mechanisms of bone loss.The black women had higher BMD values at every site tested than the white women throughout the adult life cycle. Black women have a higher peak bone mass and a slightly slower rate of adult bone loss from the femur and spine, which are skeletal sites comprised predominantly of trabecular bone. Indices of bone turnover are lower in black women as are serum calcidiol levels and urinary calcium excretion. Serum calcitriol and parathyroid hormone levels are higher in black women and calcium absorption efficiency is the same in black and white women, but dietary calcium intake is lower in black women. Black and white women have a similar pattern of bone loss, with substantial bone loss from the femur and spine prior to menopause and an accelerated bone loss from the total skeleton and radius after menopause. The higher values for bone density in black women as compared with white women are caused by a higher peak bone mass and a slower rate of loss from skeletal sites comprised predominantly of trabecular bone. Low-risk strategies to enhance peak bone mass and to lower bone loss, such as calcium and vitamin D augmentation of the diet, should be examined for black women. The risk vs. benefits of hormonal replacement therapy should be determined, especially in older women.

Variables related to urinary calcium excretion in young girls

The relations among dietary and calcium kinetic factors and 24-h urinary mineral excretion were evaluated in a group of 89 healthy girls (51 white and 38 black) aged 4.9-16.7 years. Nutrient intakes were calculated for each participant using a weighed intake of all food and beverage on the day of the 24-h urine collection study and two subsequent 24-h food records. A significant relation was noted between urinary calcium and sodium excretion (r = 0.55; p < 0.0001). No significant relations were found between urinary calcium and (a) calcium intake (r = 0.08), (b) protein intake (r = 0.14), or (c) phosphorus intake (r = 0.11). Urinary calcium was not significantly related to fractional calcium absorption (r = 0.03) or net calcium absorption (r = 0.11), but was significantly associated with the bone calcium deposition rate (r = 0.24; p < 0.03). Using a multiple regression model, both urinary sodium and the bone calcium deposition rate were independent predictors of urinary calcium excretion in this population (r = 0.57; p = 0.0001). A substantial number of the children in this population had urinary calcium excretion > 4 mg/kg/day (12%). The incidence of hypercalciuria differed between the racial groups and was markedly higher in the white than in the black children (17.6 vs. 5.3%). Over a range of usual calcium intakes, during the rapid-bone-growth period in childhood and early adolescence, urinary calcium appears relatively unaffected by calcium intake and is most strongly associated with urinary sodium levels.

Vitamin D deficiency in veterans with chronic spinal cord injury

Chronic spinal cord injury (SCI) is associated with osteopenia, increasing the prevalence of long-bone fractures. Although disuse may be the primary cause of osteopenia, identification of any additional mechanisms of bone loss may lead to potential therapeutic interventions. We investigated the relationships of serum calcium (Ca), phosphorus (PO4), albumin, alkaline phosphatase (Alk P), and parathyroid hormone (PTH) with serum 25-hydroxyvitamin D [25(OH)D] in 100 subjects with chronic SCI and 50 control subjects. in a subgroup of 50 subjects with SCI and 50 control subjects, we correlated these parameters with serum 1,25-dihydroxyvitamin D [1,25(OH)2D]. Mean ages for the group with SCI and the controls were the same. In subjects with SCI, the duration of injury was 20 +/- 1 years (mean +/- SD). Thirty-two of 100 subjects with SCI, as compared with eight of 50 controls, had serum 25(OH)D levels less than the normal range (chi2 = 4.36, P < .05). In subjects with SCI, a negative correlation was demonstrated between serum 25(OH)D and PTH (r = .29, P < .005). Mean serum 1.25(OH)2D levels were significantly elevated in subjects with SCI as compared with controls (61 +/- 21 v 46 +/- 18 pg/mL, P < .0005). Twenty of 50 subjects with SCI had serum 1.25(OH)2D levels greater than 62 pg/mL, as compared with 10 of 50 controls (chi2 = 4.76 P < .05). A positive correlation was found between serum PTH and 1,25(OH)2D in subjects with SCI and controls (r = .41, P < .005 and r = .30, P < .05, respectively). Twelve subjects with SCI had serum PTH levels greater than the normal range. In this high-serum PTH subgroup, serum 15(OH)D concentration was significantly lower (P < .05) and serum 1,25(OH)2D and Alk P concentrations were significantly higher (P < .005 and P < .05, respectively) as compared with the subgroup with serum PTH values within the normal range. In subjects with SCI, 17 had a serum Ca concentration less than 8.5 mg/dL. In persons with SCI, depressed levels of serum 25(OH)D, as well as other factors, may result in forces inclined to reduce the serum calcium concentration. A state of mild secondary hyperparathyroidism may result, thus increasing the conversion of serum 25(OH)D to 1.25(OH)2D. These data suggest that in chronic SCI subjects, as in the general population, secretion of PTH and the increase of circulating 1.25(OH)2D are subject to control by negative-feedback mechanisms. Higher levels of serum PTH would be expected to accelerate bone resorption of a skeleton already regionally osteoporotic as a consequence of the bone mineral loss due to acute immobilization.

Differences in calcium absorption and kinetics between black and white girls aged 5-16 years

To assess racial differences in calcium (Ca) metabolism, we measured Ca absorption and kinetics in 89 girls (38 black and 51 white) aged 4.9-16.7 years using a dual-tracer stable isotope technique. We found significantly greater rates of fractional (0.44 +/- 0.13 vs. 0.25 +/- 0.08, p < 0.0001) and total calcium absorption (406 +/- 142 vs. 234 +/- 82 mg/day, p < 0.0003) in black than in white postmenarcheal girls. Fractional absorption of Ca was also greater in black than in white premenarcheal girls (0.39 +/- 0.14 vs. 0.30 +/- 0.10, p = 0.01). Fractional absorption of Ca was more closely correlated to Ca intake in white (r = -0.42, p = 0.002) than in black girls (r = -0.25, p = 0.14). Urinary Ca excretion was significantly lower in pre- but not postmenarcheal black girls than in white girls. Calcium kinetic values associated with bone calcium deposition were greater in black girls, indicating a greater rate of bone Ca deposition in both pre- and postmenarcheal black girls. These results suggest that the greater bone mass accumulated during childhood and adolescence in black than in white females is due, in part, to greater rates of Ca absorption in black girls.

Demonstration that bone mineral density of the lumbar spine, trochanter, and femoral neck is higher in black than in white young men

The incidence of osteoporosis and fractures of the hip and spine is lower in black than in white subjects. To determine whether bone mass is increased in black men and to assess the influence of body weight and age, bone mineral density (BMD) of the lumbar spine, trochanter, and femoral neck was measured by dual-photon absorptiometry in 59 normal white men and 40 black men between the ages of 20 and 50 years. Body weight and age were not different from each other in the two groups. BMD of the midradius was measured by single-photon absorptiometry. Multivariate regression was used for independent analysis of each group and for analysis of the two groups together. After adjusting for body weight, age was inversely related to BMD of the femoral neck in both blacks and whites and of the trochanter in blacks. When body weight was analyzed independently of age, it was a positive predictor for BMD of the midradius of black men and of the femoral neck in white men. Despite the racial differences in age and weight on BMD, there were no significant interactions between race and age or race and weight when the data from black and white men were combined. Race had a highly significant effect on BMD of the lumbar spine, trochanter, and femoral neck midradius, and BMD was higher in blacks than in whites at these sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Risk factors for hip fracture in black women. The Northeast Hip Fracture Study Group

BACKGROUND

Although more than 1 percent of black women 80 years of age or older have hip fractures each year, little is known about risk factors for hip fracture in these women.

METHODS

We carried out a case-control study involving 144 black women admitted with a first hip fracture to 1 of 30 hospitals in New York and Philadelphia. The control were 218 black women living in the community who were matched to the case patients according to age and ZIP Code or telephone exchange and 181 hospitalized black women matched according to age and hospital. Information was obtained through personal interviews and was studied by multivariable logistic-regression analysis.

RESULTS

When the case patients were compared with the control subjects from the community, the women in the lowest quintile for body-mass index had a markedly increased risk of hip fracture as compared with the women in the highest quintile (odds ratio, 13.5; 95 percent confidence interval, 4.2 to 43.3). Postmenopausal estrogen therapy for one year or more was protective for women under 75 years of age (odds ratio, 0.1; 95 percent confidence interval, < 0.1 to 0.5). Factors associated with an increased risk of hip fracture included a history of stroke (odds ratio, 3.1; 95 percent confidence interval, 1.2 to 8.1), use of aids in walking (odds ratio, 5.6; 95 percent confidence interval, 2.7 to 11.5), and consumption of seven or more alcoholic drinks per week (odds ratio, 4.6; 95 percent confidence interval, 1.5 to 14.1). The results were similar when the case patients were compared with the hospitalized control subjects.

CONCLUSIONS

Among black women thinness, previous stroke, use of aids in walking, and alcohol consumption are associated with an increased risk of hip fracture. Postmenopausal estrogen therapy protects against hip fracture in women under 75 years of age.