BRIEF (MINI) ABSTRACT
Among 1866 Asian women (901 Filipina women, 654 Chinese women, and 311 Japanese women) who had vitamin D assessment prior to initiation of osteoporosis therapy, Filipina women had a lower prevalence of vitamin D deficiency compared to Chinese women, despite higher body mass index. In multivariable analyses that adjusted for age, body mass index, and smoking status, the relative risk of low vitamin D was significantly higher for Chinese women (relative risk 1.4, 95% confidence interval 1.1–1.7) but not Japanese women (relative risk 1.2, 95% confidence interval 0.9–1.6). The 40% higher risk of low Vitamin D in Chinese compared to Filipina women emphasizes the importance of disaggregating Asian race when examining nutritional health attributes.
Keywords: Vitamin D, osteoporosis, women, Chinese, Japanese, Filipina, ethnicity
INTRODUCTION
Optimizing vitamin D status is an integral component of osteoporosis and fracture prevention care.1 In a previous study, we observed that one in five US women who initiated osteoporosis therapy with an oral bisphosphonate drug had recent history of low vitamin D level and that the prevalence of low vitamin D varied by race and ethnicity.2 Compared to non-Hispanic White women where the prevalence of low vitamin D was 19%, the relative risk of low vitamin D was 1.2 (95% confidence interval, CI 1.1–1.3) for Asian women, 1.7 (CI 1.6–1.8) for Hispanic/Latina women and 2.2 (CI 2.0–2.4) for Black women.2 However, fewer studies have examined variation in vitamin D status among Asian subgroups, where there is now recognition that ethnic differences among Asian adults may impact care delivery and outcomes.3
Recognizing that Asian Americans comprise a heterogeneous group for whom fracture prevention care should be individualized, we further identified Asian ethnicity among those classified by Asian race and examined whether vitamin D status varied across the primary Asian subgroups identified among women who subsequently initiated osteoporosis treatment.
METHODS
This retrospective observational study examined the prevalence of low vitamin D among 2679 Asian women identified from Kaiser Permanente Northern California (KPNC) members who were aged 50–89 years, initiated oral bisphosphonate therapy during 2010–2013, had body mass index (BMI) measured and were not underweight, and had a 25OH-Vitamin D (25OHD) level measured within the prior two years.2 The study was approved by the KPNC Institutional Review Board with a waiver of informed consent.
Health plan databases were used to identify the subset of Asian women who were of Chinese, Filipina, or Japanese ethnicity, comprising 70% of the overall Asian cohort (1866 of 2679 women). Asian ethnicity was determined from self-reported data in electronic health record or administrative databases. The primary outcome was low 25OHD level defined as <20 ng/mL.4 Body mass index was classified as healthy (18.5–24.9 kg/m2), overweight (25–29.9 kg/m2), or obese (≥30 kg/m2) using standard cut-points and as healthy (18.5–22.9 kg/m2), overweight (23–27.4 kg/m2), or obese (≥27.5 kg/m2) using lower BMI intervention thresholds for adults of Asian race.5 Smoking status was classified based on self-reported data from the prior five years (current smoker, prior smoker, never smoked).
Differences between groups were examined using the Chi-squared or Fisher exact test. Log-binomial regression was used to examine the association of ethnicity and low 25OHD as previously conducted for the source cohort.2
RESULTS
There were 1866 Asian women of Filipina (901 women, 48%), Chinese (654 women, 35%), and Japanese (311 women, 17%) ethnicity included in these analyses. Compared to Filipina women, Chinese women were similar in age, but Japanese women were slightly older (Table 1). Chinese and Japanese women also had slightly lower mean BMI, with about three-fourths classified as healthy weight and one-fourth as either overweight or obese using standard BMI thresholds and about half as healthy weight and half as either overweight or obese using Asian-specific BMI thresholds. In contrast, 59% of Filipina women had healthy weight, 32% overweight, and 9% obesity using standard BMI cut points and one-third had healthy weight and two-thirds had overweight or obesity using Asian-specific cut points. Chinese women (1%) were less likely to report current smoking when compared to Filipina women (3%), whereas Japanese women were similar (3%). Differences in smoking prevalence between Chinese and Japanese women did not reach statistical significance (p=0.06). A significantly higher proportion of Chinese women (21%, p= 0.02) had 25OHD <20 ng/mL compared to Filipina women (16%), but the proportions did not differ significantly for Japanese women (19%).
Table 1.
Age, weight and Vitamin D status of Filipina, Chinese, and Japanese women
| Filipina women N = 901 |
Chinese women N = 654 |
Japanese women N = 311 |
|
|---|---|---|---|
| Age, mean ± SD | 69.5 ± 8.0 | 69.1 ± 9.5 | 74.3 ± 8.7* |
| BMI, mean ± SD (kg/m2) | 24.9 ± 3.9 | 23.4 ± 3.4* | 23.3 ± 3.1* |
| Obese (≥ 30.0) | 81 (9%) | 28 (4%)* | 7 (2%)* |
| Obese (≥ 27.5) | 182 (20%) | 79 (12%)* | 31 (10%)* |
| Current smokerΔ | 28 (3%) | 7 (1%)* | 9 (3%) |
| 25-OHD <20 ng/mL | 147 (16%) | 138 (21%)* | 58 (19%) |
| Relative risk (low vitamin D) † | Referent | 1.4 (1.1 – 1.7) | 1.2 (0.9 – 1.6) |
BMI = body mass index
p < 0.05 vs Filipina women. For BMI comparisons, significant differences by ethnicity were seen for overweight and obese versus healthy BMI and for obese vs not obese BMI.
Smoking status examined for the study cohort was classified as current smoker (2%), former smoker (10%), never smoker (83%), or unknown smoking status (5%).
Relative risk (outcome low vitamin D) adjusted for age, smoking status, and BMI as a continuous variable.
Adjusting for age, BMI (as a continuous measure), and smoking status, the relative risk (RR) of low 25OHD was significantly higher for Chinese (RR 1.4, 95% CI 1.1–1.7) but not for Japanese (RR 1.2, 95% CI 0.9–1.6) women, when compared to Filipina women (Table 1). These results were unchanged when adjusting for BMI as a categorical variable using standard or lower Asian-specific thresholds.
DISCUSSION
This contemporary report compares vitamin D status among a large population of Filipina, Chinese, and Japanese American women who subsequently initiated osteoporosis therapy. Despite a higher proportion with overweight and obesity, Filipina women had a lower prevalence of vitamin D deficiency (16%) compared to Chinese women (21%). In analyses that adjusted for age, BMI, and smoking status, the risk of low vitamin D was 40% higher in Chinese women.
Data comparing Asian ethnic groups that include Filipina women in the same US clinical population are limited. In a Hawaii study of 223 older adults of White, East Asian, Native Hawaiian/Pacific Islander, or Filipino ethnicity, Filipino adults were least likely to be vitamin D insufficient and East Asian adults were most likely,6 supporting observations in our current study. In the Philippines, up to half of all working urban Filipinos (52% of 1446 adults in Metropolitan Manila) were reported to have 25OHD <20ng/mL,7 whereas in a separate study of 70 women with osteoporosis examined in Manila, 36% had 25OHD between 10 and <32 ng/mL and 64% had levels ≥32 ng/mL.8 In China, the majority of postmenopausal women identified across seven geographic regions had 25OHD <20 ng/mL (61%), with higher prevalence of low Vitamin D noted in urban areas.9 However, multiple factors may contribute to country-specific differences, including geography, sun exposure, supplement use, dairy and fish consumption, lifestyle, health, genetic factors, and health-related perspectives and attitudes.7–10 Data from the United States indicate a lower prevalence of Vitamin D deficiency among older compared to younger adults and among those taking vitamin D supplements.11
While our data are limited by lack of information on dietary and supplemental vitamin D intake as well as other health behaviors, these findings underscore the importance of disaggregating Asians when examining health attributes and the potential role of culturally-specific nutritional assessment and counseling in osteoporosis care. Future studies should include examination of other Asian subgroups including South Asians, other East and Southeast Asians, and Native Hawaiian and Pacific Islanders. A greater understanding of Asian health, ethnic differences, and contributing factors may inform approaches to optimizing skeletal outcomes for these women.
ACKNOWLEDGMENTS
FUNDING SOURCE:
This work was supported by a grant from the National Institute on Aging at the National Institutes of Health, 1R01AG047230. The opinions expressed in this publication are solely the responsibility of the authors and do not represent the official views of Kaiser Permanente or the National Institutes of Health.
Footnotes
CONFLICT OF INTEREST: Samantha Ho, Christina Li, Malini Chandra, and Joan Lo have no conflicts of interest to disclose.
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