1 Epidemiology and Surveillance Research, American Cancer Society, Atlanta, GA.
2 Nutrition and Health Sciences Program, Graduate Division of Biological and Biomedical Sciences, Emory University, Atlanta, GA.
3 Division of Nutrition and Physical Activity, Centers for Disease Control and Prevention, Atlanta, GA.
4 Division of General Medicine, Department of Medicine, School of Medicine, Emory University, Atlanta, GA.
5 Division of Cancer Prevention and Control, Centers for Disease Control and Prevention, Atlanta, GA.
Received for publication July 13, 2001; accepted for publication January 6, 2003.
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ABSTRACT |
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cardiovascular diseases; diet; diet surveys; mortality; neoplasms; nutrition; nutrition assessment; nutrition surveys
Abbreviations: Abbreviations: CI, confidence interval; CPS II, Cancer Prevention Study II; DQI, Diet Quality Index; ICD-9-CM, International Classification of Diseases, Ninth Revision, Clinical Modification.
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INTRODUCTION |
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Only a few indexes of diet quality have been examined for associations with measures of health status. In each of 10 such studies, the index of diet quality predicted all-cause mortality in at least one gender group (1, 68, 1824). The dietary diversity score created by Kant et al. (22, 23) was associated with cardiovascular disease mortality in men and women and cancer mortality in men only. Kants Recommended Food Score (24) was associated with reduced cardiovascular disease and cancer mortality in women. Two recent cohort studies examined incidence of cardiovascular disease and cancer by Healthy Eating Index score (25, 26). Diet quality had a moderate inverse association with cardiovascular disease in men and was weakly associated with cardiovascular disease in women; it was not associated with cancer in either study.
The Diet Quality Index (DQI) was developed by Patterson et al. (5) in 1994 to measure overall dietary intake patterns and to use these patterns to predict chronic disease risk. As the DQI was constructed, a higher DQI was indicative of a poorer-quality diet and was therefore expected to be positively associated with increased mortality. For the current study, we assumed that a low-quality diet is positively associated with chronic disease risk. This assumption was based on the weight of evidence from studies of associations between individual foods and nutrients and chronic disease risk (913). The ability of the DQI to predict chronic disease morbidity or mortality has not yet been determined. The objective of this study was to investigate whether the DQI is positively associated with short-term all-cause, all-circulatory-disease, and all-cancer mortality while controlling for other known risk factors for mortality.
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MATERIALS AND METHODS |
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The CPS II Nutrition Cohort included 184,193 women and men. For the current analyses, we limited the Nutrition Cohort to White and African-American women and men aged 5079 years, because there were very few deaths among persons in other race and age strata. We excluded participants who reported a history of cancer, heart attack, or stroke on the 19921993 survey or who left more than 15 percent of the items on the food frequency questionnaire blank (table 1).
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Mortality follow-up
Mortality follow-up for the cohort was complete through December 1996. Mortality and cause-of-death codes were determined through linkage with the National Death Index (32), and deaths were coded according to the International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) (33). Between the time of the cohorts baseline survey in 19921993 and December 1996, 869 women (1.4 percent) and 1,736 men (3.3 percent) died (table 4). The mortality analyses were not stratified by race, because there were very few deaths among African Americans.
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We adjusted for age in all Cox models in these analyses by including single year of age at the baseline survey in the strata statement of the PHREG model. In multivariate-adjusted models, data were controlled for the following factors: race (White, African American), occupation (white-collar, blue-collar, homemaker, unknown), education (less than high school graduate, high school graduate, some college, college graduate, graduate school, unknown), smoking status (never, former smoker of <10 pack-years, former smoker of 10 pack-years who had quit smoking
10 years previously, former smoker of
10 pack-years who had quit smoking <10 years previously, current smoker, unknown), physical activity (metabolic equivalent hours per week: 0, 13.5, >3.514, >1428, >28, unknown), alcohol intake (never, 13 drinks per week, >3 drinks per week to 1 drink per day, >12 drinks per day, >2 drinks per day, unknown), use of dietary supplements (never or <1 pill per week, 16 pills per week,
1 pill per day), use of aspirin (never or irregular use, regular use of <1 pill per day, regular use of 1 pill per day, regular use of >1 pill per day), and, for women only, mammography history (never or >3 years before, 13 years before, <1 year before, unknown) and hormone replacement therapy (ever or never). For women, level of physical activity and dietary supplement use were the most important correlates of diet quality. For men, level of physical activity and body mass index (weight (kg)/height (m)2) were the most important correlates of diet quality (Seymour, unpublished manuscript). We conducted additional analyses adjusting for the factors listed above and other factors (body mass index, high blood pressure, diabetes mellitus, elevated cholesterol, emphysema, and family history of cancer). These factors may have been on the causal pathway but they were also potential confounders of associations between diet quality and chronic disease risk, because they are only partly a result of a poor-quality diet and are influenced by other risk factors for chronic disease mortality. Inclusion of the additional covariates in the models had no effect on the findings except to reduce the weak positive association between DQI and all-cause mortality in men (data not shown). We conducted a trend test using the DQI as a continuous variable in the Cox models. We performed a diagnostic test to verify that the independent variables used in the multivariate survival models were not collinear (36).
Graphic techniques and time-extended Cox models were used to check the proportional hazards assumption for this study (37). No violations of this assumption were apparent. Multiplicative interaction terms between diet quality and each of the demographic and health-related variables were added to separate multivariate models for examination of possible interactions between diet quality and each variable. The likelihood ratio test (38) was used to determine whether interaction terms for each variable were statistically significant at the p 0.05 level. Few of the chronic disease risk factors significantly modified the relations between diet quality and mortality (data not shown). When statistically significant interactions did occur, they were fewer than would be expected by chance alone, and they lacked a discernible pattern of effect modification for any of the characteristics. Thus, there did not appear to be any strong modifiers of the relation between diet quality and all-cause, all-circulatory-disease, or all-cancer mortality.
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RESULTS |
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In models that adjusted for age only, DQI was positively associated with all-cause and all-circulatory-disease mortality in both women and men and with all-cancer mortality in men (tables 5 and 6). The all-cause mortality rate ratio for women with the lowest-quality diet as compared with the highest-quality diet was 1.86 (95 percent confidence interval (CI): 1.28, 2.70), and for men it was 1.78 (95 percent CI: 1.43, 2.22). For circulatory disease, positive associations were found for medium, medium-low, and low DQI (versus high DQI) in women and for medium-low and low DQI in men. For cancer, a rate ratio of 1.49 (95 percent CI: 1.04, 2.14) was found for men with the lowest-quality diet.
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In multivariate-adjusted models examining individual components of the DQI (data not shown), the cholesterol component was positively associated with all-cause, all-circulatory-disease, and all-cancer mortality for both women and men, indicating that a higher intake of cholesterol was positively associated with increased mortality. For women only, the fruit and vegetable component was positively associated with all-circulatory-disease mortality and the calcium component was positively associated with all-cause mortality, indicating that lower intakes of fruits and vegetables and calcium were positively associated with mortality.
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DISCUSSION |
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Several previous studies of dietary indexes found small but significant inverse associations between high-quality diets and all-cause mortality (1, 68, 24). However, in each of these studies, only a few potentially confounding factors were included in multivariate models, and it is possible that the associations would have been greatly attenuated if additional potentially confounding factors had been taken into account. The recent examinations of the Healthy Eating Index (25, 26) were more comparable to the present work, although they examined incident disease rather than mortality. Data for many potentially confounding factors were included in multivariate-adjusted models as part of the evaluation of the Healthy Eating Index. No association was found between a high-quality diet, as measured by the Healthy Eating Index, and cancer, and only a weak, inverse relation was seen with cardiovascular disease.
The findings from examinations of the Healthy Eating Index and results of the current study suggest that recent dietary guidelines, as measured by dietary indexes, are not associated with cancer occurrence or mortality. Present dietary guidelines may not adequately describe intake that is associated with reduced risk of chronic disease. In this study, only the cholesterol component of the DQI was positively related to mortality in men; the cholesterol, fruits-and-vegetables, and calcium components were positively related to mortality in women. The majority of cohort participants met the dietary recommendation for cholesterol intake (92 percent of women and 74 percent of men). The cholesterol component may be capturing and acting as a proxy for a truly low-quality diet. It may be useful to develop an index of diet quality by first evaluating the individual components of the index for their association with morbidity and mortality. This process would identify the appropriate components to include in the final index.
Several explanations are possible regarding the general lack of a positive association between the DQI and risk of death in this cohort. The DQI may not separate dietary intake patterns into truly high-quality or low-quality dietary intakes. Total fat intake comprises a mixture of saturated and unsaturated (both mono- and polyunsaturated) fats. Studies have observed protective effects of unsaturated fats against certain chronic diseases, such as breast cancer and coronary heart disease (18, 3941). In addition, some of the components of the DQI may need to be more narrowly defined. Intake of fruits and vegetables has been associated with a lower risk of cardiovascular disease as well as a lower risk of many diet-related cancers, but fruits and vegetables vary in terms of how protective they are (9, 10). Dark green and deep yellow fruits and vegetables, citrus fruits, tomatoes, and cruciferous vegetables may be more highly associated with reduced risk of many chronic diseases than other fruits and vegetables such as iceberg lettuce and potatoes (9, 10). This distinction is not recognized in the DQI.
Some components of the DQI, such as protein, may have inappropriate scoring. A high protein intake does not appear to be positively associated with risk of circulatory disease or cancer (42, 43), but a low protein intake has been related to increased risk of chronic disease (44). The DQIs current scoring favors a low protein intake, and thus it "rewards" people for activity that is not beneficial to diet quality. When we examined the protein component alone, higher consumption of protein among women was inversely related to all-cause and all-cancer mortality. This finding contradicts Patterson et al.s (5) assumption that a lower protein intake, within the range of US consumption, is part of a high-quality diet. For men, high protein consumptionmore than 150 percent of the Recommended Dietary Allowance (45)was positively associated with all-cause and all-circulatory-disease mortality, but moderate protein consumption was not. Moderate protein intake, 100150 percent of the Recommended Dietary Allowance, may be a more appropriate level for high-quality diets. Finally, some foods or nutrients that may be important in a high-quality diet were not included in the index. For example, fish intake and nut consumption have both been associated with lower risk of chronic disease (4649), but these dietary components were not measured by the DQI.
The current study had several limitations. Dietary data came from a 68-item food frequency questionnaire that was self-administered by Nutrition Cohort participants in 19921993. The original DQI, however, was constructed using a 2-day diet record and one 24-hour dietary recall. A food frequency questionnaire may not be able to capture enough detail on dietary intake to separate participants according to dietary quality. However, a recent study by Newby et al. (50) found that DQI assessed using two different food frequency questionnaires was reasonably valid (r = 0.65 and r = 0.61) when compared with DQI assessed using 2 weeks of diet records. Newby et al.s study supports the use of a food frequency questionnaire to rank-order a population according to quality of dietary intake. Furthermore, the food frequency questionnaire used for the CPS II Nutrition Survey was found to be reasonably valid when assessed for validity with four 24-hour dietary recalls in a subset of the Nutrition Cohort. Correlation coefficients for the eight components of the DQI ranged from 0.33 to 0.66 for women, with four components above 0.6. For men, they ranged from 0.29 to 0.75, with five components above 0.57 (51). However, misclassification of diet quality from food frequency questionnaire information cannot be ruled out as a contributor to the failure to observe strong associations between diet quality and mortality in this study.
The food frequency questionnaire used for the current study was completed in 19921993 and assessed the respondents diet in the preceding year. The dietary data collected during this time period may not have captured the dietary intake that played a role in the disease process. The relevant exposure may actually have occurred 1020 or more years prior to death. Because there were only 4 years of follow-up in this cohort, to be counted as deceased participants had to be diagnosed with a disease and then die within a 4-year period. Thus, most of the deaths recorded were due to very aggressive cancers and serious circulatory disease. The DQI may predict chronic disease mortality better with more years of follow-up, when there are sufficient numbers of deaths from more clearly diet-related chronic diseases.
All data from the CPS II and CPS II Nutrition Survey questionnaires were self-reported. This population is highly educated and therefore may be expected to accurately self-report health-related data. Conversely, this cohort is also fairly elderly, which may have affected memory or accuracy of recall (52).
The study had several strengths. The Nutrition Cohort is large, with 115,833 participants being included in the analysis cohort. The data for this study were collected in a population that was disease free at the time of survey completion, which eliminated the possibility of disease-related recall bias. Data were available for many potential confounders of diet qualitydisease associations, whereas previous studies controlled for few covariates other than age, smoking, and physical activity. The availability of data on a wide array of covariates enabled us to control for many demographic and health-related factors that are likely to confound associations between diet quality and disease.
There is great interest in indexes of diet quality and their ability to predict chronic disease morbidity and mortality. This study and other studies conducted by McCullough et al. (25, 26) suggested that these indexes may not predict chronic disease morbidity and mortality as well as previously thought. Before more dietary indexes are produced, more research is needed to determine whether current indexes are valuable and for what purposes they should be used.
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NOTES |
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REFERENCES |
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