1 Coordinating Centers for Biometric Research, Division of Biostatistics, School of Public Health, University of Minnesota, Minneapolis, MN
2 Department of Social Medicine, University of Bristol, Bristol BS8 2PR, United Kingdom
Correspondence to Avis Thomas, University of Minnesota, 2221 University Avenue, Suite 200, Minneapolis, MN 55414 (e-mail: avist{at}ccbr.umn.edu).
Received for publication January 26, 2005. Accepted for publication April 20, 2005.
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ABSTRACT |
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cardiovascular diseases; cerebrovascular accident; diabetes mellitus, type 2; emigration and immigration; ethnology; Hispanic Americans; liver neoplasms; social class
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INTRODUCTION |
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MATERIALS AND METHODS |
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All men signed a brief consent form giving investigators explicit permission to use all the data collected. Men had every opportunity to decline the screening. Undocumented aliens or men planning to move away from the area may have thus been implicitly discouraged.
Data collection at screening is described in detail elsewhere (12). At the first screening visit, blood pressure was taken three times with a standard mercury sphygmomanometer; this study used the average of the second and third readings. Serum total cholesterol was measured from a nonfasting blood sample. Men were asked how many cigarettes a day they currently smoked, if they were presently taking medicine for diabetes, and whether or not they had ever been hospitalized for a heart attack for 2 weeks or longer. Current home address, Social Security number, and date of birth were collected. Men were asked: "Which of the following best describes the group to which you belong: White, Black, Oriental, Spanish American, American Indian, or other?" Men in the top 1015 percent group for coronary heart disease, but without history of heart attack or present use of medicine for diabetes, were invited to return for additional screening and possible inclusion in the randomized trial.
Home address ZIP codes were matched to 1980 US Census data to obtain ZIP code-based median household income and percentage Latino. White versus Latino race/ethnicity-specific median incomes were compared. Latino-specific data were not publicly available for some ZIP codes; to include as many men as possible, we chose to use ZIP code-based median income for Whites to estimate income for both Whites and Latinos.
To investigate the merits of adjusting for ZIP code-based median household income versus census tract-based median family income, we obtained census tracts for as many screenees as possible. Home addresses were geocoded using ArcView 8.2 and ArcGIS StreetMap USA (both by ESRI, Inc., Redlands, California) software and matched with 1980 US Census tract boundary files from the National Historical Geographic Information System.
Study participants
Men were included in this study if they self-identified as "White" or "Spanish American," they had complete baseline health data, and the median income for Whites in their ZIP code was known. Of the 361,662 men screened, there were 6,647 Latinos and 325,384 Whites. Of these, the 5,846 (88 percent) Latinos and 300,647 (92 percent) Whites with complete baseline data are the focus of this report. Among the 801 Latinos and 24,737 Whites who were excluded, 25.2 percent and 29.8 percent died during follow-up, respectively.
Mortality data
Participants' dates of birth, Social Security numbers (if available), and names were used to obtain mortality data from the Social Security Death Index and the National Death Index through 1999. National Death Index matching was based on a probabilistic scoring system developed by the National Center for Health Statistics; a match with slightly incorrect or missing Social Security numbers was still possible if the name and date of birth matched exactly, where names could be matched on the basis of either exact spelling or the New York State Identification and Intelligence System phonetic codes (13). For deaths through 1990, the Social Security Death Index, National Death Index, and MRFIT clinic records were used to determine dates of death; two or three nosologists then read each death certificate and coded the underlying cause of death. For deaths in 19911999, the cause and date of death were obtained from the National Death Index Plus service. Both the Social Security Death Index and the National Death Index capture deaths that occurred in Puerto Rico (14
).
The National Death Index matching criteria have been shown to have 97 percent specificity and 97 percent sensitivity for the general population (15), and National Death Index searches have been shown to produce high-quality results, even with incomplete or incorrect data (13
). A National Death Index search correctly identified 98.4 percent of the 191 known 19791980 deaths among men randomized into the MRFIT; 93.2 percent were found in a Social Security Death Index search (16
). However, it has not been clear whether matching results for Latinos are of the same high quality as for Whites. National Death Index documentation reports lower matching rates among non-Whites due to lower reports of Social Security numbers and incorrect spelling/recording of ethnic names (17
). To address this question, we randomly selected 500 Whites and 500 Latinos with a Social Security number available and searched for death records in the Social Security Death Index through January 2004. The results were used to evaluate the relative reliability of Latino and White Social Security numbers, names, and dates of birth, as well as the overall number of Latino and White deaths found.
Statistical methods
Baseline data were tabulated using percentages or means and standard deviations. Age-adjusted p values for Latino/White differences were calculated using linear or logistic regression.
Proportional hazards regressions were used to examine Latino/White mortality differences. Regressions were stratified by clinical center unless they involved state-by-state analysis. Regressions were adjusted first by age; then by age and risk factors (smoker (yes/no), cigarettes per day, serum cholesterol, systolic blood pressure, current treatment for diabetes, and prior heart attack); and finally by age, risk factors, and income. The proportional hazards assumption was checked for all-cause mortality by examining the hazard ratios for sequential 10-year time segments.
Similar analyses were repeated for specific medical and demographic subgroups. Among the subgroups considered were Latinos living in a ZIP code with a high versus a low percentage of Latinos. Those men in a ZIP code with a high percentage of Latinos could have lower mortality rates due to being less acculturated to mainstream US culture, on average, or due to greater availability of Spanish-speaking and/or culturally appropriate medical facilities or better quality social networks.
A separate analysis was also carried out for the subset of men for whom both ZIP code-based and census tract-based median incomes were available. We compared Latino/White hazard ratios for risk and income-adjusted models using each of the two income measures.
The state in which they were screened was used as an approximate proxy for Latinos' country of origin. To test the robustness of Latino/White hazard ratios across state and across other variables, we performed fully adjusted proportional hazards regressions separately by state, demographic variables, and risk factors. We tested for linear or categorical interaction between race/ethnicity and these variables by adding interaction terms to the regressions.
All analyses were conducted using SAS, version 8.2 (SAS Institute, Inc., Cary, North Carolina), software. All p values were two sided.
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RESULTS |
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Latino/White mortality by cause for all men combined
During the 25-year follow-up period, 1,085 (18.6 percent) Latinos and 73,807 (24.6 percent) Whites died. Table 2 summarizes Latino/White hazard ratios for several causes of death, before and after risk and income adjustments. The age-adjusted Latino/White hazard ratio for all-cause mortality was 0.84 (95 percent confidence interval (CI): 0.79, 0.90); it was 0.86 (95 percent CI: 0.81, 0.92) after further adjustment for risk factors and 0.82 (95 percent CI: 0.77, 0.87) after adjustment for age, risk factors, and income. The age- and income-adjusted, but not risk factor-adjusted, Latino/White hazard ratio was 0.77 (95 percent CI: 0.73, 0.82). Separate regressions for Latinos and Whites showed that they had similar associations of major risk factors and income with all-cause mortality (not shown).
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Mortality by cause for subgroups defined by state, demographic characteristics, risk factor levels, and income
Latino/White hazard ratios for all-cause mortality were also estimated for selected subgroups (table 3). Differences in Latino/White hazard ratios by state, percent Latino in ZIP code, income level, age at screening, diabetic status, smoking status, cholesterol level, blood pressure level, overall coronary heart disease risk level, availability of Social Security number, and presence at the second screening visit were small and in most cases not significant at conventional levels (p = 0.01 for overall coronary heart disease risk level; p = 0.06 for smoking; p > 0.13 for each of the others).
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Robustness of results
Latino/White differences in all-cause mortality remained constant over the 25 years of follow-up time. The fully adjusted Latino/White hazard ratios for all-cause mortality were 0.86 (95 percent CI: 0.75, 0.99) during years 110, 0.78 (95 percent CI: 0.71, 0.86) during years 1120, and 0.84 (95 percent CI: 0.76, 0.93) in years 21 and after.
To investigate the use of different census-based income estimates, we considered census tract-based and White-only ZIP code-based median income, both of which were available for 5,367 (92 percent) Latinos and 261,539 (87 percent) Whites. The age-, risk-, and income-adjusted Latino/White all-cause mortality hazard ratio was 0.82 (95 percent CI: 0.77, 0.88) when using ZIP code-based income and 0.81 (95 percent CI: 0.76, 0.87) when using census tract-based income. To include as many men as possible, we selected White-only, ZIP code-based median income as the income estimate in all the other analyses.
Quality of reported birth date and vital status ascertainment
To evaluate the quality of the National Death Index search for Latinos compared with Whites, we randomly selected 500 Latinos and 500 Whites with Social Security numbers from the study population and searched the Social Security Death Index database through January 2004 for information about them. The average age at screening was 44.7 years for Latinos and 46.2 years for Whites. Matches were sought using only the Social Security number and alternately using only the last name with soundex code (for more information: http://www.archives.gov/research_room/genealogy/census/soundex.html), first name (exact spelling), and year of birth. The National Death Index search had identified 82 Latino and 111 White deaths for this group through 1999; the Social Security Death Index search found four additional deaths for this time period, all of them for Whites. Incorporating these four additional deaths reduced the unadjusted Latino/White all-cause mortality ratio from 0.78 to 0.75.
The Social Security Death Index database contained information on 123 Latino deaths and 155 White deaths through January 2004; among them, ten Whites (6.5 percent) and four Latinos (3.3 percent) had single-digit discrepancies in Social Security number when compared with the MRFIT database (table 4). None had larger discrepancies. By this criterion alone, the Social Security numbers for Latino MRFIT screenees were 3.2 percent more consistent than those for the Whites. Additionally, one Latino and six Whites had Social Security numbers in the MRFIT database that were assigned to someone else in the Social Security Death Index database. Soundex-adjusted surnames and dates of birth were 1.9 percent and 1015 percent less consistent for Latinos, respectively; investigations of first name yielded ambiguous results.
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DISCUSSION |
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In 19731975, most Latino Americans were foreign born; it is thus likely that a majority of MRFIT Latino screenees were also first generation immigrants. The low Latino/White all-cause mortality hazard ratio could thus reflect a healthy immigrant phenomenon. In this view, the immigrants' self-selection process favors individuals with above-average health, intelligence, and self-motivation. These characteristics would have favorably influenced immigrants' mortality across their life span (3). Studies have found more favorable self-reported health statuses for first generation immigrants (Hispanic, non-Hispanic White, and non-Hispanic Black) than for native-born Americans, particularly in the early years after immigration (18
, 19
). Other authors have suggested that first generation immigrants may be healthier because of lesser past exposure to pollutants, sedentary lifestyles, tobacco, and so on, in their country of origin (18
, 20
) or because they partially retain advantageous cultural characteristics and health behaviors (e.g., dietary differences, stronger social networks, and family cohesion) (21
, 22
). Possible genetic factors, including a propensity for obesity and diabetes, would work against this (23
).
Alternately, Latino/White mortality hazard ratios less than one have often been attributed to the "salmon hypothesis" (24), in which first generation Latino immigrants are disproportionately likely to return to their country of origin before death, thus skewing reported mortality rates. Three aspects of the current study mitigate against that possibility. First, several factors would have reduced the likelihood that undocumented aliens or those planning to leave the country would have chosen to participate. The men screened for the MRFIT were recruited from stable populations and gave researchers written permission to use their Social Security number and other confidential data. An overwhelming majority of both Latinos (96 percent) and Whites (98 percent) provided researchers with a plausible Social Security number, indicative of a long-term interest in US residency.
Second, had Latinos left the country at a significant rate, the Latino/White all-cause mortality hazard ratio would have decreased over time. However, the observed Latino/White differences in mortality rates remained constant over 25 years of follow-up.
Third, the salmon hypothesis applies only to immigrants from Mexico, Central America, and South America and not to those from Puerto Rico (because deaths in Puerto Rico are included in the National Death Index and Social Security Death Index databases) or Cuba (who have tended to remain in the United States for political reasons). It would thus predict lower observed Latino/White hazard ratios in California or Illinois, where most Latinos were Mexican American, versus New York, New Jersey, and Florida, where Cubans and Puerto Ricans predominate. However, no such pattern existed.
There has been considerable discussion in the literature regarding incomplete ascertainment of Latino deaths (8, 9
, 17
, 25
), which led us to more carefully examine the quality of the MRFIT data and the National Death Index search. Based on the analysis of a random sample of screenees (500 Latinos and 500 Whites), the Latinos in this study had Social Security numbers that were 3 percent more consistent than did Whites; soundex-adjusted surnames were 2 percent less consistent. Because of the preeminence of the Social Security number in the National Death Index's search algorithm, we conclude that the National Death Index results are approximately as complete for Latinos as for Whites, and that the low Latino/White hazard ratios are not an artifact of inconsistent or incomplete identifying information. In order for the underlying crude death rates among the White and Latino screenees to be equal, an additional 351 (6.0 percent) of the Latinos would have to have died but been missed by the National Death Index search, which is a much greater error rate than that found in the random sample.
We found significantly lower coronary heart disease and cardiovascular disease mortality among Latinos than Whites but significantly higher stroke mortality. Stroke has been shown to be particularly dependent on deprivation in early life (2628
), which may be more common for first generation Latino immigrants than for the general population. The lower risk-adjusted coronary heart disease mortality could reflect the fact that risk factors were measured only in middle age. It is becoming clear that lifetime exposure to cigarette smoking, elevated cholesterol, and elevated blood pressure is associated with coronary heart disease risk and that adulthood measures alone underestimate their importance (29
). Thus, while the Latinos have a higher prevalence of smoking (although fewer cigarettes per day) in the early 1970s, they may have started smoking at a later age than Whites and, thus, had a shorter and lower level of lifetime exposure to cigarette smoke; for diseases with long latency periods, this would be related to lower risk. Support for this comes from the lower lung cancer and chronic obstructive pulmonary disease mortality among Latinos, which is unchanged by adjustment for measures of smoking in middle age. Similarly, blood pressure-level and cholesterol-level profiles across the lifetime of Latinos may have been more favorable than those of the Whites in a way that is not captured by the relative similarity of measures taken in middle age.
Combining cardiovascular disease and diabetes deaths resulted in a fully adjusted combined hazard ratio of 0.80, nearly the same as for cardiovascular disease alone. The favorable cardiovascular disease result is, thus, not due to such misclassification of cause of death.
The fully adjusted Latino/White hazard ratio for all cancers combined was 0.69; similar hazard ratios were found for most site-specific cancers. In contrast, the Latino/White hazard ratios of 2.02 and 1.46 for liver cancer and stomach cancer, respectively, were markedly elevated. The hazard ratios for chronic obstructive pulmonary disease and infection were 0.62 and 1.69. Each of these hazard ratios was consistent with hazard ratios found in studies based on regional cancer registries and census population counts (6, 7
) and based on the National Longitudinal Mortality Study (4
).
Marmot et al. (30) showed that, for several causes of death in various English immigrant populations, mortality rates for first generation immigrants were between those of England and their country of origin; their work underscored the long-term influence of culture and environment on health. Additionally, studies of men of Japanese birth or descent living in Japan, Hawaii, and California have shown that coronary heart disease risk factors and mortality grade between those of Japanese living in Japan and White Americans, with Japanese Americans living in Hawaii having intermediate values. The risk factors and mortality of Japanese-American immigrant communities approached those of White Americans only after a few generations (31
37
). For first generation Latino immigrants, the lower overall cancer and chronic obstructive pulmonary disease mortality rates seen in this study may likewise reflect the long-term effects of a healthier environment and lifestyle in their country of origin. Likewise, a higher rate of liver cancer, consistent with the higher rates found in Latin America, may be partially explained by the greater incidence of hepatitis B and hepatitis C in Latin American populations. Latin American countries also have more humid climates and consequently greater contamination of foods with fungus-based aflatoxins and mycotoxins, known liver carcinogens (6
). A higher rate of stomach cancer may be attributed to greater rates of Helicobacter pylori infection (38
, 39
) or dietary salt. H. pylori and hepatitis B are often acquired in childhood, through horizontal transmission, and have been taken as indicators of inadequate resources for maintaining hygiene. This evidence of exposures consequent on greater early life deprivation among Latinos is also congruent with the elevated risk of stroke. Ecologic studies have demonstrated how early life deprivation and/or greater dietary salt intake may underlie the international correlation between stroke and stomach cancer risk (40
42
).
There were no significant state-by-state variations in Latino/White mortality ratios. This supports the thesis that Latino/White differences are due to either a healthy immigrant phenomenon or pan-Latino social, cultural, and behavioral factors. Latino/White hazard ratios also did not have any statistically significant material variations by age, risk factor levels, income, or percentage of Latinos in the ZIP code of residence.
Adjustment for ZIP code-based and census tract-based median incomes had a similar effect on Latino/White hazard ratios. Among the MRFIT randomized men, adjustment for personally reported family income rather than neighborhood income produced a similar effect.
Palloni and Arias (25) speculate that migrant Mexican populations living farther from California or Texas would be more heavily preselected for vigorous health, and they observe that Mexicans living outside those two states enjoy an 8 percent lower mortality rate. The all-cause mortality hazard ratios reported here are 0.84 (95 percent CI: 0.76, 0.93) in California and 0.74 (95 percent CI: 0.61, 0.90) in Illinois, with no clinical centers in Texas; these data weakly support their hypothesis.
Limitations of this MRFIT study include a selection process that would have tended to exclude men who were undocumented aliens. In addition, the MRFIT Latinos probably included few US-born men. The MRFIT Latinos, who were recruited in 19731975, may not be representative of the current Latino-American population. The study is also limited by its exclusion of women.
In conclusion, with 25-year follow-up on 5,846 Latinos and 300,647 Whites, Latinos had substantially lower risk- and income-adjusted all-cause mortality than did Whites, particularly for coronary heart disease and most cancers. Risk factor- and income-adjusted stroke, liver cancer, and infection mortality rates were significantly higher among Latinos, as was risk factor-adjusted diabetes, and point to the need for culturally appropriate targeted interventions. The mortality results, both by cause and overall, did not vary significantly over time or by state and did not appear to be an artifact of return migration or poor data. These pan-Latino results may reflect a healthy immigrant phenomenon, as well as possible genetic, cultural, and environmental differences. Understanding the mechanisms behind the differences may provide insights that can be used to improve the health of all.
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ACKNOWLEDGMENTS |
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The authors thank Gabriela Vazquez-Benitez for her assistance in compiling the US census data. They also thank Dr. Karl Eschbach for sharing his insights into Latino immigration patterns and National Death Index matching issues.
Conflict of interest: none declared.
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References |
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