Acculturation stress and allostatic load among Mexican immigrant women *

Objectives this case-control study compared levels of stress and allostatic load (AL) among Mexican women in the US ( n =19) and Mexico ( n = 40). Method measures of stress included the Perceived Stress Scale (PSS) and the Hispanic Women’s Social Stressor Scale (HWSSS). A composite measure of 8 indicators of AL (systolic and diastolic blood pressure, body mass index (BMI), waist-to-hip ratio, total cholesterol, glycated hemoglobin (hemoglobin A1C), triglycerides and C-reactive protein) was calculated. Results there were no significant group differences in AL between Mexican and Mexican immigrant women ( t = 1.55, p = .126). A principal component factor analysis was conducted on the 8 AL indicators; a 2-factor solution explained 57% of the variance. Group differences in the two AL factors were analyzed using MANOVA. BMI and waist-to-hip ratios were lower, but blood pressure and triglycerides were higher in the US group and were mediated by time in the US. Greater acculturation stress was significantly related to increased waist-to-hip ratio ( r = .57, p = .02). Final remarks findings suggest some measures of AL increased with time in the US, and acculturation stress may be a significant factor.


Introduction
The US and Mexico are neighbors sharing the longest and most active migration corridor in the world. More than 11.7 million Mexican immigrants currently reside in the United States (1) comprising the largest immigrant group in the country. Although migration from Mexico has slowed greatly in the past few years (1) , Mexico still has a greater percentage of its citizens living abroad (mostly in the US) than any other country in the world (2) . Moreover, more than onethird of these mexicanos en el extranjero (Mexicans living abroad) have resided in the US for 15 years or more, many with precarious legal status (3) .
Consistent with the Hispanic Paradox (4)(5) data, the risk for cardiovascular disease among immigrant Latinos is relatively low at the time of arrival but increases greatly with length of residence in the US (6) . Mexican-American women in particular have one of the world's highest rates (44%) of metabolic syndrome (MS), a disorder characterized by central (abdominal) obesity, insulin resistance/hyperinsulinemia, hypertension, and dyslipidemia (7) , all of which increase the risk for atherosclerotic disease.
Using a framework of allostatic load (AL) (8) , one factor that may contribute to MS among Mexican immigrant women is perceived stress (9) . Acculturationrelated factors such as family separation, cultural conflicts, low socioeconomic status, language barriers, racism and discrimination and low perceived control over employment may contribute to chronic stress and may predispose Mexican immigrant women to the development of MS. Recently, the threat of deportation is a major stressor for many families where one or more members are among los sin papeles (the undocumented). To date, there is little known regarding patterns of AL accumulation and the impact of chronic stress on AL among Mexican immigrant women (10) .
In its simplest form, acculturation can be conceptualized as a normative process that occurs when a person from one culture is exposed to another culture (11) . In contrast, acculturation stress is defined as a negative reaction to intercultural contact or the cultural adaptation process. Mexican immigrants to the US may experience acculturation stress when seeking housing, work, or education or because of racial/ ethnic discrimination and loss of social support (12) .
These conflicts are frequently encountered by new immigrants; however, if migration does not result in a substantially higher quality of life and financial security, acculturation stress may become a chronic state (13) . This is particularly true among the undocumented.
Acculturation stress has long been associated with poor mental health in a number of studies among Mexican immigrants (14)(15) . Consistent with the model of (AL), recent studies suggest that cumulative exposures to high levels of chronic psychological stressors may lead to a variety of physiologic conditions as well.
The biobehavioral process of AL ( Figure 1) provides a compelling framework to explain the link between cumulative exposure to chronic psychological and physiological stressors and the prevalence of chronic illnesses among minority groups. Allostasis is the bodily mechanism by which humans and other organisms cope with short-term physiological and psychological stress. Similar to acculturation, it is a normative process. However, the allostatic process may become ineffective if the stress itself persists over an extended period of time, the body does not recognize the stressor as having been resolved, or the body's mechanisms for shutting off the stressor are not functioning (16) . AL is the collective term used to refer to damage incurred by the body as it adapts to such psychosocial or physical stressors. It has been posited that AL may negatively affect the body through a variety of biochemical mechanisms (8)   Mexican Americans, and acculturation measures did not account for the difference (19) . Chronic work-related, financial, and caregiving stressors were associated with elevated ALs in a separate study, but immigrant women were not part of the sample (9)  (case) sample; the use of relatives as controls helped to assure the groups had significant overlap and helped to limit confounding by genetic factors thought to be related to MS and elevated AL (20) . The 1:2 sampling ratio was chosen based on published recommendations for case-control studies (21) . Likewise, it has been noted that when the number of cases in a study is small, the ratio of controls to cases can be raised to improve the ability to find significant differences (22) . The sample size of 59 women is consistent with the number of subjects required for pilot studies using a comparative design to calculate effect sizes to estimate power and sample size needed for a larger study (23) . Sample sizes of 15-25 per group were recommended for small to medium standardized effect sizes (24) . bilingual research assistants (Mexico) and promotoras (US) assisted participants by reading items to them when necessary. All participants completed a 14-item Spanish language version of the Perceived Stress Scale (PSS) (26) . This version (27) measured the degree to which social situations were appraised as stressful. It is widely used and has utility in predicting biomarkers of stress. Individual scores on the PSS can range from 0 to 56 with higher scores indicating higher perceived stress. Prior to use, the literacy level of the PSS and the Hispanic Women's Social Stressor Scale (HWSSS) (28) were assessed using the Fernandez-Huerta method (29) , and both were found to be at a 7 th grade level. The PSS has been tested among adults in Mexico with a reported internal consistency among women of α = .78 (30) . gas-permeability plastic bags with a desiccant packet added to reduce humidity. When the specimens were sufficiently dry, the blood spots were a dark brownish color and no bright red areas were visible (32) . The packets were then mailed to the laboratory from collection sites in both the US and Mexico. Elution and analyses of the DBS eluates were carried out in the laboratory using a standardized process (33) . Participants in the US were compensated with a $10 gift card to a popular "big-box" store. Following consultation with local leaders, participants in Mexico were given a bag of groceries containing rice, beans and tuna fish.   Levene's test was used to test for homogeneity of variance. While the original AL model was based on a single factor (8) , more recent research has suggested that a twofactor model might explain more of the variance in AL (35) .
More recently, it has been suggested that model invariance across subpopulations does not preclude the possibility that the measurement of AL may differ importantly by the studied sample (35) . Therefore, the one-factor and two-factor models need to be examined in this study to determine which model is a better fit to the data in the sample.
Descriptive statistics were computed for all variables.
Next, the eight components of AL were examined by factor analysis to determine whether a single-factor or a two-factor model was a better fit to the data.
For hypothesis 1, the AL scores for the two groups (US and Mexico) were compared by an independent t-test.
Multivariate analysis of variance (MANOVA) was used to identify the specific indicators that led to group differences in AL and to build successive models to identify covariates that explained the differences. Wilk's lambda was used to assess whether the MANOVAs were significant. Hypothesis 2 was tested using Pearson's r to identify the specific AL  Table 2.   Table   3, the women in the US had significantly higher levels of DBP, SBP and TG than women living in Oaxaca.

Discussion
Our findings support previous studies (6,18) , which linked AL among Mexican immigrants with increased time spent in the US. This relationship was particularly true in this study with regard to systolic and diastolic blood pressure and triglycerides. Given that the participants in the study were young to middle-aged women, the prevalence of hypertension in the US group was somewhat unexpected, although the results of other studies indicate that hypertension is likely underdiagnosed and untreated among immigrant Latinos (37) .
Despite the higher scores for blood pressure and triglycerides, we did not find a significant difference in AL between the two groups of women. This was likely due to the unexpectedly higher BMI and waist-to-hip ratios among women in the Mexican group. Although acculturation stress has traditionally been thought to contribute to weight gain among immigrant women (38)(39)(40) , an additional factor that may parallel this process among Mexican women is the nutrition transition. The nutrition transition is defined as a broad shift in dietary habits and physical activity that coincides with economic, demographic, and epidemiological changes (38) .  (41) . The two groups of women were well The Mexican community we visited was no exception.
When immigrants return to rural Mexico, they often create a demand for lifestyles they adopted in the US (43) . These preferences include building fast food restaurants and a greater reliance on automobiles as a means of transportation. These changes affect the community at large and may add to the obesogenic environment that characterizes Stage 4 of the nutrition transition in Mexico. On a related note, we did not directly measure physical activity and nutritional intake in this study; these two variables would add greatly to our understanding of the role that stress plays in AL and should be included in future studies. In this study, we limited our choice of biomarkers to measures that could easily be collected in a community setting. This decision precluded us from assessing neuroendocrine markers such as 24-hour urinary norepinephrine or epinephrine.
In addition, the laboratory we used was not able to accommodate DBS analysis for inflammatory biomarkers such as interleukin6 (IL-6) and tumor necrosis factor alpha (TNF-α). Focus groups may be needed to explore the potential for collecting additional biomarkers. Finally, it can be argued that the sample size (N = 59) was too small to conduct the two principal component factor analyses. There is no consensus in the factor analysis literature concerning a minimum sample size. While older references suggest there should be at least 10 cases per variable, with a total of at least 100 cases (44) , other more recent sources recommend limiting the number of variables and factors to assure moderate to high levels of communality (45) . Following the more recent guidelines, we restricted the number of variables to 8 and the number of factors in our final model to 2. In our study, communalities ranged from .43 to .75, placing it just below the .6 to .8 average recommended as a high range (45) . It has been suggested that small sample sizes may not be a problem when the data are highly reliable and communality levels are high (46) . Accordingly, we had highly reliable measures of biomarkers (systolic BP, HgbA1C, total cholesterol, triglycerides, and waist-tohip ratio), and our communality levels were moderate to high. Lastly, we achieved convergence using Promax rotation as a solution. Often, samples that are too small will fail to converge (47) . For these reasons, we argue that principal component factor analysis was appropriate in our study.

Conclusion
In summary, this study supports evidence of a link between length of residence in the US and acculturation stress in some indicators of AL among Mexican immigrant women. To attenuate the declines in health status among Mexican immigrants postulated by the Hispanic Paradox, further research will be needed to clarify the roles played by acculturation and acculturation stress in the genesis of allostatic load.