Life-Course Predictors of Ultrasonic Heel Measurement in a Cross-sectional Study of Immigrant Women from Southeast Asia

Diane S. Lauderdale1, Talya Salant2, Katherine L. Han3 and Phuong L. Tran3,4

1 Department of Health Studies, University of Chicago, Chicago, IL.
2 Pritzker School of Medicine, University of Chicago, Chicago, IL.
3 Weiss Health Center, Chicago, IL.
4 Department of Medicine, University of Chicago, Chicago, IL.


    ABSTRACT
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Few studies address chronic disease risk for Southeast Asians in the United States. In 1999, the authors conducted a cross-sectional study of bone mineral density (BMD) estimated from ultrasonic calcaneal measurements in women born in Southeast Asia who then lived in Chicago, Illinois. The study addressed three questions: Do Southeast-Asian women have relatively low BMD? What factors before and after immigration are associated with BMD? Are factors that reflect the childhood/adolescent environment equally associated with BMD for postmenopausal and premenopausal women? An interviewer-administered bilingual questionnaire collected immigration, reproductive, and lifestyle data from 213 women (aged 20–80 years) born in Vietnam, Cambodia, or Laos. The authors found that the estimated mean BMD of postmenopausal Southeast-Asian women was lower than the reference values for White women. Four summary indicators of childhood/adolescent environment were predictive of higher BMD: more years of education, earlier age at menarche, lower height, and coastal birth; these indicators were more strongly associated with BMD for premenopausal (multiple-partial R2 = 0.21) than postmenopausal (R2 = 0.06) women. Young-adult exposures (e.g., early first pregnancy and age at immigration) and proximal lifestyle factors (e.g., smoking, physical inactivity, vegetarian diet, and betel nut use) were also assessed as potential predictors of BMD.

Asian Americans; bone density; calcaneus; emigration and immigration; women's health

Abbreviations: BMD, bone mineral density


    INTRODUCTION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
While few persons born in Vietnam, Cambodia, or Laos lived in the United States before 1970, the population has grown rapidly as a result of the nonnumerical provisions of refugee resettlement, which do not include quotas. Many endured traumatic events before and during immigration and arrived with limited material resources. Compared with other Asian Americans, a high proportion live in poverty (1Go). Prior epidemiologic studies of Southeast Asians have focused on distinctive problems: post-traumatic stress, depression, hepatitis B, tuberculosis, and cervical cancer (2GoGoGoGoGoGoGo–9Go). This population is relatively young, and there is little information upon which to base expectations of their health in the United States as they age. Adverse circumstances could lead to chronic disease morbidity that diverges sharply from that of other Asian-American ethnicities.

Among elderly White women, one important contributor to morbidity and disability is osteoporosis, with an estimated 40 percent lifetime risk of osteoporotic fracture (10Go). Osteoporosis is defined as low bone mineral density (BMD) relative to young healthy women (11Go). Based on a clinical perception of high fracture risk among Vietnamese women, our aims were threefold: to determine whether Southeast-Asian women have relatively low BMD compared with White women, to identify determinants of BMD, and to explore the extent to which childhood/adolescent factors are associated with BMD in postmenopausal versus premenopausal women. Because the current and former living environments are so different for these women, this may be an informative population in which to assess the association between early life circumstances and osteoporosis. BMD changes rapidly for women during two periods of life, increasing from the fetal period through the second year following menarche (12Go) and decreasing following menopause. Thus, we expected indicators of environmental circumstances before age 18 years—in Southeast Asia—to be associated more strongly with BMD for premenopausal than postmenopausal women.


    MATERIALS AND METHODS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Subjects
This cross-sectional study was conducted in 1999 in Chicago, Illinois. Adult women born in Southeast Asia were recruited by using several strategies. Advertisements in Vietnamese, Cambodian, and Laotian were distributed in the community. Participants were recruited at ethnic religious and social events, ethnic health fairs, and a bilingual English/Vietnamese clinic. Clinic participants and subjects recruited at community events reported similar rates of chronic conditions. The protocol was approved by the University of Chicago Institutional Review Board, and written informed consent was obtained.

Data collection
An interviewer-administered questionnaire collected data on demographics, immigration history, occupation at age 18 years, reproductive and medical history, medications, tobacco use, betel nut use, current physical activity, and diet (targeted at identifying vegetarians and consumers of dairy foods). Interviews were conducted in English, Vietnamese, Cambodian, or Laotian. Height and weight were measured.

BMD at the calcaneus was estimated by using the Sahara Clinical Bone Sonometer (Hologic, Waltham, Massa-chusetts), which measures the broadband ultrasound attenuation and speed of sound of an ultrasound beam as it passes through the heel. A linear combination of these measures estimates BMD. Since ultrasound passes through the heel, this measurement may reflect aspects of bone microarchitecture not captured by x-ray densitometry (13Go).

Analytical approach
Mean BMD by age was compared with reference values collected by Hologic for White and Asian-American women (defined by Hologic as US-born) (14Go). Potential predictors of BMD were grouped into three categories by life stage. Indicators of childhood/adolescent environment were place of birth, remembered age at menarche, height, and education. Birthplaces were categorized as coastal versus noncoastal, reflecting an hypothesized association with a coastal diet rich in seafood and less affected by drought-related famine. Age at menarche is influenced by energy intake, weight, and physical activity (15GoGoGo–18Go). Education is a summary indicator of socioeconomic status. While height is determined overwhelmingly by genetic factors in well-nourished populations, it reflects environmental factors in worse environments (19Go). For girls, peak BMD is achieved after maximum height velocity (20GoGo–22Go).

Early-adult indicators were occupation at age 18 years (farmer vs. nonfarmer), age at first pregnancy, parity, and age at immigration. We defined early pregnancy as within 5 years of menarche.

Proximal lifestyle factors were body mass index, tobacco use, physical activity, betel nut use, vegetarian diet, and consumption of dairy foods. Physical activity was assessed by using 7-day recall (23Go) but was simplified to sedentary hours per day since the heel is weight-bearing when a person stands. Betel, an addictive stimulant common in southern Asia (24Go, 25Go), is chewed as a quid with lime paste (calcium hydroxide). While betel is associated with deleterious health effects (26Go), we hypothesized that because of the calcium, it may be associated with increased BMD.

Ordinary least-squares regression was used to evaluate potential predictors. All models were adjusted for age, years since menopause (allowing for different age effects after menopause), and country of origin. Effects of early-life indicators were estimated and were compared by menopausal status. Early-adult and proximal factors were singly assessed in models adjusted for the four early-life factors, including interaction terms to allow for differential effects by menopausal status.


    RESULTS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Data were collected from 213 women (table 1). Figure 1 compares BMD by age for the Southeast-Asian women for whom Hologic reference values were available. We found that while the BMD of premenopausal Southeast-Asian women was similar to that of Whites, they seemed to experience a steeper age-related decline. BMD means of Asian Americans were much higher than those of Southeast Asians, although the age effects were similar.


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TABLE 1. Characteristics of women from Southeast Asia participating in a cross-sectional study of bone mineral density, Chicago, Illinois, 1999

 


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FIGURE 1. Mean calcaneal bone mineral density (BMD) (g/cm2, estimated by quantitative ultrasound), by decade of age, of women from Southeast Asia and reference values for Whites and Asian Americans determined by the manufacturer of the ultrasound densitometer used (Hologic, (Waltham, Massachusetts) (21Go).

 
Earlier age at menarche, more years of education, and coastal birth were all significantly associated with increased BMD (table 2). There was an inverse association between height and BMD. Table 3 compares these associations by menopausal status; all were stronger for premenopausal women. The multiple-partial R2 value for the four factors was 0.21 for premenopausal women and 0.06 for postmenopausal women.


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TABLE 2. Association of four indicators of childhood/adolescent environment with estimated calcaneal bone mineral density among women from Southeast Asia, Chicago, Illinois, 1999

 

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TABLE 3. Comparison of point estimates for effects of four indicators of childhood/adolescent environment* on estimated calcaneal bone mineral density in women from Southeast Asia, Chicago, Illinois, 1999

 
Of the early-adult factors, being a farmer at age 18 years was positively associated with BMD (table 4). Early first pregnancy and older age at immigration were associated with lower BMD. Of the proximal risk factors, smoking and sedentary hours showed the strongest evidence of associations. Smoking was associated with lower BMD. The number of sedentary hours per day was inversely associated with BMD. Body mass index was not related to BMD, modeled either continuously or in categories. The direction of the effect for betel nut use (n = 8) was positive. While few women were vegetarians, the negative association was of similar magnitude to smoking.


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TABLE 4. Association of each early-adult and proximal factor with estimated calcaneal bone mineral density after adjustment for the four indicators of childhood/adolescent environment,* among women from Southeast Asia, Chicago, Illinois, 1989

 

    DISCUSSION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
We found that after age 50 years, immigrant Southeast-Asian women had a tendency toward low estimated calcaneal BMD compared with White women. At all ages, Southeast-Asian women had lower mean BMD than the reference values for US-born Asian Americans. That difference may be due to either nativity or ethnicity, since a sample of US-born Asian Americans would be unlikely to include Southeast Asians. This study provides some evidence of an association between age at immigration and BMD, with younger age associated with higher BMD. Previous work has found nativity effects for BMD among Japanese Americans (27Go), and Chinese, Japanese, and Korean elderly who have immigrated more recently experience higher hip fracture rates than those who have been in the United States longer (28Go).

Indicators of the childhood/adolescent environment, including age at menarche, education, height, and coastal birth, were associated with BMD, and the associations were stronger for premenopausal than postmenopausal women. Previous studies have found that delayed menarche is associated with low BMD (29GoGo–31Go). Explanations generally stress late menarche as a reflection of irregular ovulation or a determinant of total years of endogenous estrogen exposure. However, since we found no effect for years from menarche to menopause after adjustment for age at menarche, age at menarche here likely represents the adolescent environment, such as adolescent body weight. Few studies have assessed the relation between education and BMD (32Go). While the implications of coastal birth are specific to this population, there is strong evidence of geographic variation in fracture rates in the United States and elsewhere as well as evidence that where persons lived earlier in their lives is a stronger predictor than residence at time of fracture (33Go). The weak inverse association with height may reflect the difficulty in attaining bone mineralization for a larger skeleton associated with greater height.

Several early-adult factors were associated with BMD. Farming at age 18 years—an indicator of physical labor and indeterminant factors—increased BMD. Consistent with findings from previous studies, early first pregnancy decreased BMD (34Go). Associations with proximal lifestyle factors were generally weaker, with evidence that smoking and sedentary hours per day were associated with lower BMD. As hypothesized, the direction of the association for betel nut use was positive, but few women reported either tobacco or betel nut use. A previous study found that Southeast-Asian women greatly underreported their tobacco use (35Go); underreporting may have occurred in the present study as well. The lack of association between body mass index and BMD is not consistent with previous studies (36GoGoGoGo–40Go). It may reflect the low probability that Southeast-Asian women with a higher body mass index now also had a higher body mass index during years of bone formation.

This study has several limitations. It was a cross-sectional convenience sample. Elderly women attending ethnic events may be atypically healthy and mobile. Ultrasound bone assessment is relatively new but has been shown to predict hip and vertebral fracture independently of x-ray densitometry (41GoGoGo–44Go). These data suggest that osteoporosis may be a significant problem for aging Southeast Asians.


    ACKNOWLEDGMENTS
 
Supported in part by the Washington Square Health Foundation, Inc.

The authors thank the Vietnamese Association of Illinois, the Chinese Mutual Aid Association, the Cambodian Association of Illinois, and the Laotian Association of Illinois for facilitating subject recruitment; Manivong Pe, Sareoun Soeun, Choulie Hok, and Tam Nguyen for translation assistance; Dr. Murray Favus and Dr. Kate Pickett for comments on the manuscript; and Dr. Paul Rathouz for statistical and editorial contributions.


    NOTES
 
Reprint requests to Dr. Diane S. Lauderdale, Department of Health Studies, University of Chicago, 5841 S. Maryland Avenue, MC 2007, Chicago, IL 60637-1470 (e-mail: lauderdale{at}health.bsd.uchicago.edu).


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Received for publication January 20, 2000. Accepted for publication July 18, 2000.