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Post by djoser-xyyman on Jul 21, 2010 7:36:03 GMT -5
Racial Differences in Bone Density between Young Adult
Black and White Subjects Persist after Adjustment for
Anthropometric, Lifestyle, and Biochemical
Differences*†
BRUCE ETTINGER, STEPHEN SIDNEY, STEVEN R. CUMMINGS,
CESAR LIBANATI, DANIEL D. BIKLE, IRENE S. TEKAWA, KIMBERLY TOLAN,
AND PETER STEIGER
Division of Research (B.E., S.S., I.S.T., K.T.), Kaiser Permanente Medical Care Program, Oakland
94611; Department of Medicine (S.R.C.), University of California, San Francisco 94143; Division of
Endocrinology (C.L.), Department of Medicine, Veterans Administration Medical Center, Loma Linda
92357; Mineral Metabolism Unit (D.D.B.), Veterans Administration Medical Center, San Francisco,
California 94121; Hologic, Inc. (P.S.), Waltham, Massachusetts 02154
ABSTRACT
This study tested whether racial differences in bone density can be
explained by differences in bone metabolism and lifestyle. A cohort of
402 black and white men and women, ages 25–36 yr, was studied at
the Kaiser Permanente Medical Care Program in Northern California,
a prepaid health plan. Body composition (fat, lean, and bone
mineral density) was measured using a Hologic-2000 dual-energy
x-ray densitometer. Muscle strength, blood and urine chemistry values
related to calcium metabolism, bone turnover, growth factors, and
level of sex and adrenal hormones were also measured. Medical history,
physical activity, and lifestyle were assessed. Statistical analyses
using t- and chi-square tests and multiple regression were done
to determine whether racial difference in bone density remained after
adjustment for covariates. Bone density at all skeletal sites was statistically
significantly greater in black than in white subjects; on
average, adjustment for covariates reduced the percentage density
differences by 42% for men and 34% for women. Adjusted bone density
at various skeletal sites was 4.5–16.1% higher for black than for white
men and was 1.2–7.3% higher for black than for white women. We
concluded that racial differences in bone mineral density are not
accounted for by clinical or biochemical variables measured in early
adulthood. (J Clin Endocrinol Metab 82: 429–434, 1997)
Discussion
By using DXA area measurements, we confirmed that
young, adult, black men and women have substantially
greater BMD than white persons (2– 6); this finding is true at
all skeletal sites, and we have extended these observations to volumetric estimates of spinal BMD. Despite an extensive
search for clinical and biochemical explanations, we were
unable to account for most of the racial differences in BMD.
The racial differences we observed in BMD do not appear
to be artifacts of the DXA method. Although the greater area
bone density observed after measurement with DXA could
be caused by larger bone size and therefore may not represent
a true increase in volumetric density, our measurement
of area and volume of both spine and hip show that black
persons do not have larger vertebrae or upper femurs than
white persons. Further, the magnitude of the BMD differences
in spinal volumetric density are similar to those for
spinal area density.
Our large, population-based study had sufficient power to
detect racial differences in clinical and biochemical variables
on the order of 0.5 standard deviations. Clinical and biochemical
assessment was extensive and enabled us to study
a comprehensive list of variables that others have considered
relevant to bone mass. After multiple adjustments for these
variables, about half the racial differences in BMD still remained.
This finding suggests that other factors than those
controlling muscle mass and body size must act specifically
on the skeleton.
Weconclude that racial differences inBMDare established
early in childhood (33) and are not explained by clinical and
biochemical variables measured in young adulthood. Studies
of adolescents might find differences in metabolic or lifestyle
factors that account for a larger share of the racial differences
in bone mass than those that we observed. By midadolescence,
black boys and girls have 10–15% greater bone density
than their white counterparts (33). However, most of the
difference would be expected to remain evident 10–20 yr
later. Thus, the appearance of such a large racial difference
in young adults cannot be attributed to persistent differences
in metabolic or lifestyle factors and supports the view that
bone density differences result from influences operating
during childhood and adolescence.
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Post by djoser-xyyman on Jul 21, 2010 7:37:32 GMT -5
Racial and Ethnic Differences in Bone Turnover Markers in Men
Benjamin Z. Leder, Andre B. Araujo, Thomas G. Travison and John B. McKinlay
Endocrine Unit (B.Z.L.), Department of Medicine, Massachusetts General Hospital, Boston, Massachusetts 02114; and New England Research Institutes (A.B.A., T.G.T., J.B.M.), Watertown, Massachusetts 02472
Context: Whereas racial and ethnic differences in fracture risk and bone mineral density (BMD) in men have been well described, the influence of race and ethnicity on biochemical markers of bone turnover is less clear.
Methods: To examine the relationship between bone turnover, BMD, and race and ethnicity in men, we measured BMD, serum intact osteocalcin (OC), and serum C-terminal telopeptides of type 1 collagen (CTx) in 1029 men (aged 30–79 yr) enrolled in the Boston Area Community Health/Bone Survey, a population-based random sample of Black, Hispanic, and White. Men with diseases or on medications known to affect bone metabolism were excluded from the analysis. Mean serum levels of OC and CTx were adjusted for age, month and time of blood sample, and 25-hydroxyvitamin D.
Results: Compared with Black men, adjusted mean OC levels were 17.6 and 20.5% higher in Hispanic (P = 0.02) and White men (P < 0.01), respectively. There was no significant difference between White and Hispanic men. Adjusted mean CTx levels were 14.3% higher in White men, compared with Black men (P = 0.04), but no other differences were significant. OC declined by 0.4%/yr from age 30 to 65 yr and increased thereafter by 2.1%/yr. The age trend in CTx appeared to follow a pattern consistent with a quadratic function of age. Model-estimated annual percent changes within age decade were as follows: 30–39 yr, –2.5%; 40–49 yr, –1.4%; 50–59 yr, –0.3%; 60–69 yr, +0.9%; 70–79 yr, +1.7%. There was no variation in the shape of the age trend in OC or CTx by race or ethnic group. Correlations between bone turnover markers and BMD (adjusted for age, height, weight, serum 25-hydroxyvitamin D, and PTH and month and time of blood sample) were generally weak.
Conclusions: Bone turnover markers are lower in Black men, compared with White and Hispanic men. Age trends in bone turnover markers are not influenced by race or ethnicity. Future studies in this cohort and others are needed to explore further these reported differences in bone metabolism among Black, Hispanic, and White men.
In this study we report that adjusted serum OC is lower in Black men, compared with White and Hispanic men. CTx is also lower in Black men compared with White men, but not different from Hispanic men. In previous studies performed in women, most, but not all, have similarly reported that bone formation markers are lower in Black compared with White subjects (9, 33, 34, 35, 36) whereas differences in markers of bone resorption were inconsistently found (9, 33, 34, 35, 36, 37, 38). In men, the Third National Health and Nutrition Examination Survey study reported lower serum OC among Black subjects but reported no between-group differences in another biochemical marker of bone formation, bone-specific alkaline phosphatase (31). Data on racial differences in bone resorption are less robust, although the present study generally confirms the results of previous small studies (9, 39).
The mechanisms underlying the lower serum levels of OC and CTx in Black men are unclear. It has been suggested that these differences could be due to racial differences in the skeletal sensitivity to PTH. That hypothesis is supported by a study in women that demonstrated a greater increase in bone resorption markers in White vs. Black women undergoing PTH infusion (40). The PTH-induced response in bone formation markers in this study, however, showed no racial differences, making this mechanistic explanation less pertinent to our finding of decreased bone formation in Black men. Alternatively, because 1,25-dihydroxyvitamin D directly stimulates OC gene transcription and synthesis (41), the lower serum 25(OH)D in Black men could be contributing to the observed lower serum OC. Similarly, because 1,25-dihydroxyvitamin D directly stimulates osteoblast/stromal cell expression of receptor activator of nuclear factor-B ligand, down-regulates osteoblast/stromal cell expression of osteoprotegerin, and stimulates osteoclastic receptor activator of nuclear factor-B expression (42), the lower 25(OH)D in Black men may also help explain differences in CTx. These latter hypotheses are consistent with our findings that removing serum 25(OH)D from the linear regression narrowed between group differences in OC and CTx (42). Clearly, however, the influence of serum 25(OH)D is modest, pointing to more complex mechanisms including potential racial/ethnic differences in vitamin D receptor polymorphisms or other genetic factors (43, 44, 45, 46).
In this study we also describe the age-related changes in OC and CTx in men. The increase in OC we observed in the later decades was also reported in the National Health and Nutrition Examination Survey cohort and in some, but not all, smaller studies (31, 47, 48). The observed age-related changes in CTx we report also generally agree with previous reports of smaller numbers of patients (48, 49). We then further attempted to explore the relationship between bone turnover and BMD in Black, Hispanic, and White men. Whereas associations between bone turnover and BMD were found, they were inconsistent, relatively weak, and not influenced by race and ethnicity. Taken together, these results suggest that neither differences in overall bone turnover nor differences in the effect of age on bone turnover are likely to explain the observed differences in BMD between racial/ethnic groups.
The interpretation of our study is limited by the fact that a single measurement of a single bone resorption and formation marker may not consistently represent that subject’s steady-state level of bone turnover. Additionally, the fact that the markers were not consistently measured in the fasting state likely added to their variability (particularly CTx). Nonetheless, the strengths of this study, including the large racially diverse population-based cohort, provide important information regarding the influence of race and ethnicity on bone turnover in men. The conclusion we have drawn, namely that bone turnover markers are generally lower in Black men compared with other ethnic groups, and that the relationship between bone turnover and bone density is generally weak and not influenced by race and ethnicity should help inform ongoing research in this area.
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Post by djoser-xyyman on Jul 21, 2010 7:42:27 GMT -5
Racial differences in skeletal calcium retention in adolescent girls with varied controlled calcium intakes1,2,3
Michelle Braun, Cristina Palacios, Karin Wigertz, Lisa A Jackman, Rebecca J Bryant, Linda D McCabe, Berdine R Martin, George P McCabe, Munro Peacock and Connie M Weaver
Background: Higher bone mass in blacks than in whites has been related to greater calcium utilization efficiency. Dietary calcium requirements for maximal skeletal calcium accretion during puberty may differ between the races.
Objective: This study compared the relation between calcium intake and calcium retention in black and white adolescent girls.
Design: A range of controlled calcium intakes (760–1981 mg Ca/d) were used in 3-wk controlled balance studies. Some subjects were studied more than once; a total of 182 observations from 55 black girls and 66 white girls were analyzed.
Results: Blacks had 185 ± 32 mg/d greater mean skeletal calcium retention than did whites (P < 0.0001) at all calcium intakes as a result of significantly greater net calcium absorption (P < 0.001) and lower calcium excretion (P < 0.0001).
Conclusions: Dietary calcium requirements did not differ with race. Higher calcium retention at all calcium intakes during adolescence may underlie the higher bone mineral content of adult blacks than of adult whites.
The high daily skeletal calcium retention rates we found in these balance studies in adolescents reflect the well-established fact that 40% of the total calcium phosphate mineral of the adult peak bone mass is deposited in the years around puberty (15, 16). The higher rates of calcium retention we found in blacks than in whites are in accord with the consistently documented finding that African Americans have higher peak bone mass than do white Americans (1, 17, 18). Indeed, in these samples of adolescent girls, we found that black girls already had significantly higher total BMD than did white girls (Table 1), which is in agreement with the findings from some other studies (2, 3). The differences between rates of skeletal calcium retention in black and white adolescents are highly likely to be largely due to genetic differences. This possibility is perhaps not unexpected, because a large proportion of the variation in adult peak bone mass in both whites and blacks is highly heritable (19-21).
We previously showed that a major factor determining skeletal calcium retention in white adolescents is the amount of calcium in the diet (6) and that there were significant differences in calcium intake between blacks and whites (7). In this current study of pooled data from our balance studies over a wide range of dietary calcium intakes, we found that the relation between dietary calcium intake and skeletal calcium retention was linear. The slopes between races were parallel; black girls had consistently higher average calcium retention—185 mg/d. Dietary calcium intake and race accounted for 26% of the variation in skeletal calcium retention.
The calcium intake associated with maximal calcium retention was used by the Dietary Reference Intake panel to set calcium requirements for adolescents in the United States and Canada based on data in white girls (5, 6). The relation between calcium intake and calcium retention was evaluated by using a nonlinear model (6), which showed a plateau in calcium retention in white girls at intakes >1300 mg Ca/d. A plateau in calcium retention at very high dietary calcium intakes is likely due to fecal calcium soap formation and reduced calcium absorption (22). No plateau in calcium retention was apparent for black girls. This may be because intakes >2 g/d were not tested in the black girls.
Racial differences in calcium retention and BMD from our studies are apparent by puberty. The magnitude of the increase in BMD as a person matures from Tanner stage 1 to Tanner stage 5 is greater in blacks than in whites (23). Earlier onset of sexual maturity has an earlier onset in black girls than in white girls (24), and postmenarcheal age predicted an additional 3.9% of the variation in calcium retention. The effect of calcium intake on calcium absorption, retention, and excretion and racial differences in these variables were not affected by the concentrations of 25(OH)D, 1,25(OH)2D, or PTH, which suggested that these adolescents were vitamin D sufficient.
Calcium absorption, calcium deposition in bone, and calcium retention all peak in girls just before the onset of menarche (6, 25, 26). At that time, the bone calcium deposition rate is 5 times that of adulthood. We observed that indexes of sexual maturity were key predictors of calcium retention. As sexual maturity progressed, calcium retention decreased, and the time after menarche was negatively related to calcium retention
Across all intakes, lower urinary calcium excretion was observed in black girls than in white girls, which is similar to results in earlier studies (7, 8, 10, 27). In the study by Abrams et al (8), the racial difference was observed only in premenarcheal girls, in contrast to the present study, in which racial differences occurred even at later stages of sexual maturity. Net calcium absorption was significantly higher in black than in white girls across all intakes, which is consistent with other studies using single calcium intakes (7, 8, 11). Thus, for any given calcium intake within the range studied (760–2195 mg Ca/d), black girls absorb and retain more calcium and excrete less calcium than white girls. However, for each unit of increase in calcium intake, black and white girls absorb and retain the same amount of calcium. For each additional 1-mg calcium intake, an average of 25% will be retained, 2% will be excreted in the urine, and 73% will be excreted in the stools. Thus, daily dietary calcium requirements are the same for both races.
The present study has several strengths. There was a large number of subjects, who were tested under highly supervised, controlled conditions. Furthermore, all girls were from the same geographic region and were tested in the same season of the year, which probably reduced variation. However, the variations in calcium absorption and retention among subjects at each calcium intake are wide. It may be argued that the method of calcium balance has a large variation. However, the daily collection of 24-h urinary over a 14-d period has a small error, and yet the variation among these subjects in urinary calcium was just as large as the variations in calcium absorption and retention. The mechanisms underlying the variation are unknown. Perhaps, like the differences between blacks and whites in calcium absorption, differences in calcium retention and urinary excretion may also be due to genetic variation. A limitation of our study is the absence of values at calcium intakes >2 g/d in black girls, which is necessary to establish a threshold intake. Thus, it cannot be ruled out that a plateau intake occurs at a higher calcium intake in black than white girls. Higher peak bone mass in black women than in white women, despite lower calcium intakes, suggests that, if our calcium intake recommendations slightly underestimate intake for maximal retention in black girls, bone health is not likely to be compromised.
We concluded that, at all calcium intakes, black girls retain more skeletal calcium than do whites, which may explain the higher peak bone mass in black adults than in white adults. However, the response to changes in calcium intake is similar in blacks and whites, and the calcium requirements are the same in both races.
REFERENCES
Pollitzer W, Anderson JB. Ethnic and genetic differences in bone mass: a review with a hereditary vs environmental perspective. Am J Clin Nutr 1989;50:1244–59. (Published erratum appears in Am J Clin Nutr 1990;52:181.)[Abstract/Free Full Text]
Hui SL, Dimeglio LA, Longcope C, et al. Difference in bone mass between black and white American children: attributable to body build, sex hormone levels, or bone turnover? J Clin Endocrinol Metab 2003;88:642–9.[Abstract/Free Full Text]
Bryant RJ, Wastney ME, Martin BR, et al. Racial differences in bone turnover and calcium metabolism in adolescent females. J Clin Endocrinol Metab 2003;88:1043–7.[Abstract/Free Full Text]
Bryant RJ, Wastney ME, Martin BR, et al. Racial differences in bone turnover and calcium metabolism in adolescent females. J Clin Endocrinol Metab 2003;88:1043–7.[Abstract/Free Full Text]
Abrams SA, O'Brien KO, Liang LK, Stuff JE. Differences in calcium absorption and kinetics between black and white girls aged 5–16 years. J Bone Miner Res 1995;10:829–33.[Medline]
Palacios C, Wigertz K, Martin BR, et al. Sodium retention in black and white female adolescents in response to salt intake. J Clin Endocrinol Metab 2004;89:1858–63.[Abstract/Free Full Text]
Wigertz K, Palacios C, Jackman LA, et al. Racial differences in calcium retention in response to dietary salt in adolescent girls. Am J Clin Nutr 2005;81:845–50.[Abstract/Free Full Text]
Peacock M. Calcium absorption efficiency and calcium requirement in children and adolescents Am J Clin Nutr 1991;54(suppl):261S–5S.[Abstract/Free Full Text]
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