What Do We Know About Osteoporosis in Asian American and Native American
Women?

Zhao Chen, MPH, Ph.D.

University of Arizona

-   **[Learning Objectives]{.underline}**

1.  To study ethnic differences in bone mineral density.

2.  To review existing data on risk and prevalence of fractures among
    Asian American and Native American women in comparison with other
    race/ethnic groups.

3.  To examine distribution of osteoporosis risk factors in Asian
    American and Native American populations.

4.  To explore causes of ethnic variations and discordance in bone
    mineral density and rate of fracture.

5.  To understand the importance of considering ethnicity in
    osteoporosis prevention and treatment.

-   **[Background]{.underline}**

Osteoporosis affects all race/ethnic groups of people in the United
States. However, most of our knowledge on osteoporosis risk factors, and
prevalence of osteoporosis and fractures are derived from studies among
white postmenopausal women. Information regarding osteoporosis among
minority populations are limited, which is especially true for Native
American and Asian American populations.

To reduce the overall number of cases of osteoporosis and to reduce the
proportion of adults who are hospitalized for vertebral fractures
associated with osteoporosis are two objectives of Healthy People 2010
in the United States. Nonetheless, for Native American and Asian
American populations, data on osteoporosis and rate of vertebral
fractures either are statistically unreliable or have not been collected
in the nation-wide surveys that have been used as the baseline data
sources for evaluating outcomes of Healthy People 2010.

According to the Bureau of the Census, in 1990 there were 1,959,234
Native American living in about 500 tribes in the United States. Asian
Americans were about 7 million in 1990, and comprised approximately 3%
of the total US population. The Asian American population is diverse in
ethnicity, language, and country of origin. About one-half of Asians in
the United States are of Chinese, Japanese, or Korean ethnicity.
Overall, 66% of Asians were born in foreign countries. Because of the
diverse genetic and cultural backgrounds in Asian and Native American
populations, preventing and managing osteoporosis presents many
challenges.

-   **[Bone Mineral Density]{.underline}**

Studies have consistently shown that Asian populations have lower bone
mineral density (BMD) as compared with the white population (Bhudhikanok
et al. 1996, Russell-Aulet et al. 1991, Russell-Aulet et al. 1993,
Larcos and Baillon 1998, Marcus et al. 1994). The discrepancies in BMD
between Asians and whites increase with aging. Since almost all the BMD
studies on ethnic comparisons are cross-sectional in nature, the
observed ethnic differences in BMD with aging is possibly due to secular
variations in peak bone mass among different ethnic groups rather than
various rates of bone loss. Longitudinal studies are needed to give
conclusive results on ethnic variation in rate of bone loss. Ethnic
difference in BMD appears starting as early as puberty. Being born in
the U.S or a foreign country does not seem to have a substantial impact
on BMD (Kin et al. 1993).

Comparison studies on BMD among Native American populations to white
populations are very few and the results are inconsistent, which may
result from different measurement sites and different age populations in
different studies. In general, younger Native American women have higher
BMD and older Native American women lower BMD than white women of the
same age (Evers, Orchard, and Haddad 1985, Perry et al. 1998).
Interestingly, a study among participants in the Women's Health
Initiative (WHI) suggests that postmenopausal Native American women may
have higher BMD at the hip, while lower or similar total and spine BMD
in comparison to non-Hispanic white postmenopausal women (Chen et al.
1998).

-   **[Fracture]{.underline}**

On average, the rate of hip fracture among Asian populations is only
about one half of that of the white population (Ho et al. 1993, Ross et
al. 1991, Silverman and Madison 1988). Prevalence of other fractures in
Asian Americans is less known. Some evidence indicates that Asians may
have a high rate of fracture at the spine (Ito et al. 1997).

The prevalence of osteoporotic fractures in Native American is unknown.
One study among American Indians in Canada showed that the rate of hip
fracture in American Indian was 80% higher in comparison to a
non-American Indian population living in the same region (Salamon et al.
1997). However, the attribution of osteoporosis to those fractures is
unknown. Another study suggests that fracture rate may be low among
Native Americans living in the south (Martin et al. 1993).

-   **[Osteoporosis Risk Factors]{.underline}**

According to the data from Behavioral Risk Factor Surveillance System
1997, Asian Americans and Native Americans are less likely to engage in
leisure-time physical activity, and a higher percentage of Native
American smoke cigarettes comparing with the whites. Lactase deficiency
is common in both Asian and Native American populations (Bertron,
Barnard, and Mills 1999a), which results low calcium intake in adults.

As in other ethnic groups, aging is significantly associated with low
BMD and increased risk of fracture in both Asians and Native Americans.
Asian Americans on average have lower weight and lower BMI than whites.
Calcium intake, physical activity, and BMI may play important roles in
peak bone mass among Asian populations. We know little about
osteoporosis risk factors in Native Americans. In the WHI, weight and
BMI are significant predictors of BMD in Native American postmenopausal
women.

-   **[Factors related to Ethnic Variations in BMD and
    Fracture]{.underline}**

Results from many studies, but not all, suggest that ethnic differences
in BMD may be explained by bone size, body weight, height and BMI
(Bhudhikanok et al. 1996). After controlling for bone size or body
weight, ethnic differences in BMD either diminished or disappeared.
Differences in calcium metabolism (Kung et al. 1998) and lifestyles
(Davis et al. 1999b, Yang et al. 1999, Hirota et al. 1992) attribute to
ethnic variation in BMD.

A notable phenomenon is the discordance in ethnic differences of BMD and
risk of hip fracture. Asian populations have lower BMD. Meanwhile
prevalence of hip fracture is also lower in Asians comparing with that
in the white population. This discordance reminds us that the cause of
fracture is multi-factorial and low BMD is only one of the risk factors
that attributes to risk of hip fracture. Accumulated evidence suggests
that there may be ethnic variations in bone microstructure (Kung et al.
1999), bone geometry (Chin et al. 1997, Kung et al. 1999, Cummings et
al. 1994, Wang et al. 1997) and risk of fall (Davis et al. 1999a, Davis
et al. 1999c). These variations may explain the lower risk of hip
fracture in Asian populations.

-   **[Preventing and Managing Osteoporosis]{.underline}**

Preventing and managing osteoporosis in Asian and Native American
populations is a great challenge because of lack of research in many
areas. Cultural differences in attitude, values and beliefs about
osteoporosis has been found in the first and second generation of Asian
Americans (Matsumoto et al. 1995). Since different ethnic groups may
have different preferences on types of food, nutrition intervention for
preventing and managing osteoporosis needs to be ethnic specific
(Bertron, Barnard, and Mills 1999a, Bertron, Barnard, and Mills 1999b).
There is no question that BMD predicts risk of fracture in all ethnic
groups (Huang, Ross, and Wasnich 1998). However, whether population
specific references or absolute fracture rates should be used for
diagnosis of osteoporosis among different race/ethnic groups is still in
debate (Marshall, Johnell, and Wedel 1996). Race/ethnicity is a proxy
measure of a group of genetic and lifestyle factors. It would be ideal
if we can precisely measure the major covariance of fracture risk in
clinical sittings, and convert them into absolute fracture rates.
However, there are some limitations to achieve this goal soon. Even if
absolute fracture rates can be constructed, gender, age and ethnic
specific references may still be useful in clinical diagnosis of
secondary causes of osteoporosis. It is most likely that ethnic
differences in response to osteoporosis medications is minimal (Wasnich
et al. 1999, Davis et al. 1995).

In summary, ethnic differences in BMD exist, which may be related to
bone size and body weight. Rate of fractures other than hip fracture is
less known among Asian Americans. We do not know the prevalence of
osteoporotic fractures in Native Americans. The distribution of
osteoporosis risk factors may vary by ethnicity. Non BMD factors, such
as bone microstructure, body balance, and bone geometry may attribute to
the observed ethnic variations in rate of hip fracture. Race/ethnicity
needs to be considered for osteoporosis prevention, diagnosis and
treatment in Asian and Native American populations.

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