AA’s Protective Body Against Osteoporosis


Image-10-21-13-at-4.30-PM-560x440By: Nikki Nies

Between 2012 and 2013, the growth of the Asian American (AA) community increased by 2.9%, making them the fasting growing ethnic group in the U.S.19 Representing 19.4 million of the American population, the AA bone biology and calcium and vitamin D intake deserve better understanding, as intake is often times lower than the recommended dietary guidelines.2  Many AA families have immigrated to the U.S., bringing not only physical possessions, but dietary and cultural practices as well.  AA tend to consume less dairy products due to perceived and/or actual lactose maldigestion and lack of culturally inundated use.7  Of the calcium rich foods consumed, AA tend to consume calcium more from orange  juice, soy and dark green leafy vegetables.5,18,19 Yet, there is a 47% prevalence of  vitamin D deficiency for Asian emigrants, with mean 25(OH)D 23.7-60.1 nmol/L for ethnic minorities in comparison to white at 65.4-79.6 nmol/L.13  vitamin D deficiency is  associated with female gender, dark skin, being covered while indoors and longer residency in host country (>2 years).13

AA women possess some of the same risk factors are associated with osteoporosis in Caucasians, even though rates of fragility fracture differs among groups.7,8 Additionally, AA hip fracture incidence is lower than Caucasians, which is hypothesized as due to shorter height, lower incidence or severity of falls, lower bone mass, better bone quality-including shorter femoral neck axis length (FNAL) and hip axis length (HAL) and/or differences in soft tissue thickness.3,6,12,15,16  The biological differences in Caucasians and AA body composition provide AA a protective layer, literally, against osteoporosis even with suboptimal calcium and vitamin D intakes. It has been surmised AA reach peak growth velocity sooner than other ethnicity groups, with earlier epiphyseal closure, which leads to shorter leg length and FNAL.3 Potential explanations of lower bone mineral density (BMD) may include AA skin pigmentation, reduced sun exposure, lower vitamin D intake, smaller bone size in hip geometry, with suggestions that AA have greater resistance to buckling, compressive and/or impact forces.3,6,8,16 Furthermore, use of imaging technology indicates the Chinese skeleton compensates for smaller bone size in trabecular and cortical bone compartments, which provides greater bone strength.3  Cortical bone has been found to be a protective barrier to fractures due to its thickness, cross sectional area, cortical porosity, mineralization, crystallinity and presence of microcracks.7calcium.figure2

During the menopause transition, a decline in ovarian function beginning about 2 years before the final menstrual period (FMP) is followed by an increase in bone resorption and then bone loss. The magnitude of the increase in bone resorption is inversely associated with body mass index (BMI).15  With a higher BMI, this may influence osteoporosis through increased weight bearing and increased adiposity, which is a source of leptin and estrone; they are known to positively affect the maintenance of BMD.15 AA tend to have lower BMIs than Caucasians, which may correspond in bone resorption, appearing to account for the ethnic variation in perimenopausal bone loss.15

With extensive research on postmenopausal women’s bone health, it is equally important to understand adolescent calcium needs as the growth period is crucial for optimal bone health due to the bone accumulation and as half of adult peak bone mass occurs at this time.2,19   A randomized crossover design studied 29 AA adolescents’ calcium intakes, ranging from 600-1600 mg/day, measuring bone turnover rate and calcium absorption using double stable calcium kinetic analysis.19 Both genders had low habitual mean intake, low mean serum 25-hydroxyvitamin D concentrations, with true fractional calcium absorption inversely related with calcium load and calcium retention increased with increased calcium consumption.19  Calcium retention is dictated by calcium intake, with absorption efficiency increased within the  first year after spermatorrhea in boys, but decreased with menstrual cycle in girls.20  The most important factors of calcium absorption include sex–higher absorption rate for boys, age, pubertal development  and retention.10,20

With the minimal calcium intake at 1110 mg/day for Chinese American boys and 970 mg/day for girls, it leads to maximal retention, which is considerably lower than that for Caucasian girls.  The study suggests lower calcium recommendations for AA adolescents than for Caucasian adolescents. Although, AA adolescents habitually have lower calcium and vitamin D intake, bone gain is similar in comparison to Caucasians, which due to increased calcium absorption efficiency and renal calcium conservation and due to the inverse relationship of calcium load and true fractional calcium absorption.9,10,19  Boys retained significantly more calcium than girls as calcium intake increased through lower fecal excretion and higher net absorption.  As a controlled feeding crossover study, both genders were studied, providing insight into adolescent populations’ calcium intake, which has a limited amount of data.  However, by using Chinese American participants, it is hard to generalize results to the entire AA adolescent population.  Future research can and should compare Wu et al’s results using more ethnically diverse AA participants.19

Talk about bone health is not complete without looking into AA women’s lifestyle predictors of peak BMD.4 In a cross sectional study of 48 Mexican American (MA) and 58 AA women, participants were tested for BMD, body composition, aerobic fitness and muscle function.  Pearson’s correlations and multiple linear regressions found AA hip BMD was lower in MA when adjusted for age, BMI, income and physical activity.4  Lean body mass was the strongest predictor of BMD for both ethnicities, but stronger for AA.4  A potential explanation of this mechanism includes earlier plateau in AA BMD compared to other ethnic groups, lower rates of skeletal remodeling, differences in hormone concentrations and effects.4 Also, AA had significantly lower total, spine and hip BMD (P<0.05).4

N (MA/AA) MA AA
Total BMD (g/cm2) 47/57 1.11 ± 0.008 1.05 ± 0.06**
Spine BMD  (g/cm2) 47/57 1.05 ± 0.13 1.00 ± 0.10*
Hip BMD (g/cm2) 47/57 0.98 ± 0.11 0.86 ±0.11**
Spine t score 47/57 -0.002 ± 1.14 -0.364 ± 0.90
Hip t score 47/57 0.130 ± 0.83 -0.741 ± 0.84**

*Significant at P <0.005, **significant at P <0.01

While these results are the first of its kind, dietary recall bias has to be noted and causal relationships can not be drawn from any variables and BMD.4 However, all questionnaires were given in the participants’ preferred language and questionnaire and lab assessments were performed by the same research team. Again, by using specific minority groups-MA and CA,  it is hard to generalize results to the entire AA population.4

Moreover, by observing CA cortical thickness and cortical volumetric BMD (Ct.BMD), it can help shed light on the microarchitectural differences between AA and Caucasian risk for fractures.1 In a cross sectional study, pre and postmenopausal women were observed, with CA having smaller bone area at radius and tibia, greater cortical volumetric bone density and thickness and greater trabecular thickness than white women, which provides additional resistance to fractures.1 In CA, lower cortical porosity and higher tissue mineral density contribute to higher mineral BMD, more dense, thicker cortices.1,9

Premenopausal white women (n=46) Premenopausal Chinese women (n=46) Postmenopausal white women (n=68) Postmenopausal Chinese women (n=29)
Height (cm) 165±7 162 ± 6** 162± 6 157± 5***
Weight (kg) 63± 17 56 ± 10* 66±12 58±8***
BMI (kg/m2) 23.1 ± 5.5 21.6±3.5 25.3± 4.9 23.6± 2.6±*
Calcium intake (mg/d) 1394 ± 1570 885 ± 557* 1557± 730 901± 544***
OCP duration (years) 7.4 ± 6.4± 3.7 ± 3.3* N/A N/A
Baecke sport index 1.6 ± 0.7 1.1 ± 0.6*** 1.2± 0.5 1.2± 0.7
PTH (pg/mL)b 31 ± 13 37 ± 13* 38± 12 37±11
25-hydroxyvitamin D (ng/mL)c 36 ± 14 25 ± 9*** 38± 14 31±10*

OCP=oral contraceptive use; PTH=serum intact parathyroid hormone  bData for 174/192 women  cData for 152/192 women

*p < 0.05 between race by menopausal status;**p <0.001 between race by menopausal status ***p < 0.001 between race by menopausal status

The above listed results show CA have thicker, denser cortices due to lower cortical porosity and higher tissue mineral density.1 Studies have shown a strong relationship between cortical porosity and bone strength, with each SD increase in cortical porosity increasing risk of fracture 22-55% depending on skeletal site.  However, with noninvasive assessment, this may have led to confounding errors in density assessment as thicker cortices may result in more beam hardening artifact of measurement.1image_(2)

Repeatedly, Asian women are found to have lower areal BMD, which is the amount of bone mineral divided by the bone scanned area, and lower wrist, arm and hip fracture rates than Caucasians.1,9,17 Using high resolution technology, a study led by Walker et al., 2010 aimed to better understand this paradox, assessing cortical and trabecular bone noninvasively.17 32 white and 31 Chinese American women’s 25-hydroxyvitamin D, serum calcium, creatinine, alkaline phosphatase activity, intact PTH and thyroid stimulating hormone were measured.  Biochemical data was analysed using two sided t test and Pearson’s correlation for BMD and microarchitectural variables.17

Results showed CA were shorter, at lower weights, higher parathyroid hormone and a trend of less sun exposure than Caucasians.17 At the radius and tibia, CA women’s trabecular BMD was 22% and 15% higher respectively.17  Although, CA have smaller bone size, cortical thickness was 22.5% greater in CA than white.  When weight and physical activity factors were adjusted, differences in bone size decreased in radius and tibia, which suggests protective effects.2,17 In this study, the physical difference between CA and Caucasian women provide AA lower rates of hip fractures: greater trabecular and cortical thickness at radius and tibia, at tibia, trabecular number is greater, trabecular separation and inhomogeneity are lower.11,17 Participants were obtained as a convenience sample, which may instill selection bias–with healthy premenopausal, not postmenopausal women used and it can not be discerned if the results are applicable to postmenopausal.  This variation in menarche may skew the results and considerations must be made in regards in FMP.

The AA population’s body composition has a lowered risk for osteoporosis and fractures, with  body composition changes that accelerate bone loss within the FMP. In a eight year cohort study, 3302 Japanese and African American women spanning premenopausal age and beyond, estradiol, FSH and urine N-telopeptide (NTX) was measured.14 Results showed that in postmenopausal women, urinary and serum type I collagen NTX, a marker of bone resorption, levels were higher in perimenopausal women due to decreased estrogen production in perimenopausal and postmenopausal ovaries.14 The mean increase in urinary NTX was most markedly see in those with BMI >25 kg/m2, with increases greatest in Japanese Americans.  The study concluded ethnic variation in BMI helped explain the variation in perimenopausal bone loss as there was a decline in ovarian functioning starting 2 years before FMP with an increase in bone resorption and bone loss.14  Limitations of the study included the sole use of Japanese Americans as the AA experimental population and the obtainment of hormone levels on basis of single annual sample, which may not provide the best portrayal of hormone levels as menstrual cycles are irregular and the timing of blood sampling could have impacted the hormone levels.14 Causal relationships can not be determined from the results, with BMI as a measure of obesity, but not an indicator of contribution to increased fat mass, lean mass or both.

While AA are classified at increased risk of osteoporosis due to their lower calcium intake, their bone biology and metabolism show protective effects against osteoporosis in comparison to Caucasians.  Future research should expand on past studies’ findings, including using additional sample sizes when feasible and more consistent use of the type of ethnic participants.  While CA are the largest sub-minority group of AA, if future research states study’s are looking at AA, ethnic groups beyond CA should be used to elevate studies’ findings.  In addition, longitudinal studies may provide greater insight in the long term effects of AA calcium intake, bone health and risk of fractures.

From this extensive research, it is understandable and reasonable to lower calcium and/or vitamin D recommendations for AA in comparison to Caucasians due to AA’s increased calcium absorption efficiency at lower intakse, while continuing to promote the importance of bone health education.   By explaining the AA physical differences– lower bone mass, bone quality-including shorter femoral neck axis length (FNAL) and hip axis length (HAL) and/or differences in soft tissue thickness, this may curb fears about suboptimal calcium intake.3,6, 12,15Additionally, it would be helpful to recommend additional dairy rich calcium products as many AA obtain calcium from orange juice, soy products and dark leafy green vegetables..19 These recommendations would be appropriate to enhance the AA diet.  While the AA bone biology and cultural differences physically provide a protective layer against osteoporosis even at lower calcium intakes than other ethnic groups, adequate bone health education should be provided to this particular population.

 

Photo Credit:Vegan American Princess, Asian American Business Expo and Pointe Med

References

  1. Boutroy S, Walker MD, Liu XS, et al. Lower cortical porosity and higher tissue mineral density in chinese american versus white women. Journal of Bone and Mineral Research. 2014;29(3):551-561.
  2. Burrows M, Jones A, Mirwald R, Macdonald H, McKay H. Bone mineral accrual across growth in a mixed-ethnic group of children: Are asian children disadvantaged from an early age? Calcified Tissue International. 2009;84(5):366-378.
  3. Cong E, Walker M. The chinese skeleton: Insights into microstructure that help to explain the epidemiology of fracture. Bone Research. 2014;2.
  4. Crespo N, Yoo E, Hawkins S. Anthropometric and lifestyle associations of bone mass in healthy pre-menopausal Mexican and Asian American women. Journal of Immigrant and Minority Health. 2011;13(1):74-80.
  5. CROSS NA, KIM KK, YU ESH, CHEN EH, KIM J. Assessment of the diet quality of middle-aged and older adult korean americans living in chicago. J Am Diet Assoc. 2002;102(4):552-554.
  6. Finkelstein J, Lee M, Sowers M, et al. Ethnic variation in bone density in premenopausal and early perimenopausal women: Effects of anthropometric and lifestyle factors. J Clin Endocrinol Metab,. 2002;87(7):3057-3067.
  7. Jackson K, Savaiano D. Lactose maldigestion, calcium intake and osteoporosis in african-, asian-, and hispanic-americans. Journal of the American College of Nutrition. 2001;20(2):198S-207S.
  8. Khandewal S, Chandra M, Lo JC. Clinical characteristics, bone mineral density and non-vertebral osteoporotic fracture outcomes among post-menopausal U.S. south asian women. Bone. 2012;51(6):1025-1028.
  9. Nam H, Shin H, Zmuda J, et al. Race/ethnic differences in bone mineral densities in older men. Osteoporos Int. 2010;21:2115-2123.
  10. Opotowsky S. Dietary calcium intake, fractional calcium absorption, urinary calcium excretion, and levels of calcitropic hormones and bone markers in young, healthy chinese-american and caucasian women .DORIS DUKE MEDICAL STUDENTS’ JOURNAL. 2001-2002;I:44-50
  11. Renzaho AMN, Halliday JA, Nowson C. Vitamin D, obesity, and obesity-related chronic disease among ethnic minorities: A systematic review. Nutrition. 2011;27(9):868-879.
  12. Sowers M, Zheng H, Greendale G, et al. Changes in bone resorption across the menopause transition: Effects of reproductive hormones, body size, and Ethnicity. J Clin Endocrinol Metab. ;98(7):2854-2863.
  13. Thomas P. Racial and Ethnic differences in osteoporosis J Am Acad Orthop Surg. 2007;15(1):S26-S30.
  14. Tung W. Osteoporosis among asian american women. Home Health Care Management & Practice. 2012;24(4):205-207.
  15. Walker MD, Liu XS, Zhou B, et al. Premenopausal and postmenopausal differences in bone microstructure and mechanical competence in Chinese-American and white women. Journal of Bone and Mineral Research. 2013;28(6):1308-1318.
  16. WIECHA JM, FINK AK, WIECHA J, HEBERT J. Differences in dietary patterns of vietnamese, white, african-american, and hispanic adolescents in worcester, mass. J Am Diet Assoc. 2001;101(2):248-251.
  17. Wu L, Martin BR, Braun MM, et al. Calcium requirements and metabolism in chinese-american boys and girls. Journal of Bone and Mineral Research. 2010;25(8):1842-1849.
  18. Yin J, Zhang Q, Liu A, et al. Factors affecting calcium balance in Chinese adolescents. Bone. 2010;46:162-166.

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