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Original Research Communication |
1 From the the Division of Epidemiology, School of Public Health, University of Minnesota, Minneapolis.
See corresponding editorial on page 701.
2 Supported by National Cancer Institute grant CA3974.
3 Address reprint requests to L Harnack, Division of Epidemiology, 1300 South 2nd Street, Suite 300, Minneapolis, MN 55454-1015. E-mail: harnack{at}epi.umn.edu.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONSUBJECTS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Objective: We examined the association of compliance with the Dietary Guidelines and incident cancers.
Design: Data from a population-based cohort of postmenopausal women (n = 34 708) were examined. A dietary guidelines index was derived as a summary measure of compliance with the Dietary Guidelines, and the association of this index and cancer incidence was examined for all cancers combined and for sitespecific cancers with > 100 events.
Results: For all cancers combined, the relative risks associated with the upper 4 quintiles of the dietary guidelines index in reference to the bottom quintile were 0.95 (95% CI: 0.87, 1.05) for quintile 2, 0.88 (95% CI: 0.80, 0.97) for quintile 3, 0.88 (95% CI: 0.80, 0.96) for quintile 4, and 0.85 (95% CI: 0.77, 0.93) for quintile 5 (P for trend < 0.01). Similar patterns in relative risks were found for cancers of the colon, bronchus and lung, breast, and uterus. In contrast, ovarian cancer incidence was positively associated with the dietary guidelines index.
Conclusion: Our findings suggest that adherence to the cluster of nutrition-related behaviors included in the Dietary Guidelines for Americans may be associated with a lower risk of cancer.
Key Words: Dietary Guidelines for Americans • cancer • women • diet • nutrition • diet quality
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONSUBJECTS AND METHODSRESULTSDISCUSSIONREFERENCES |
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2 y about food choices that promote health and prevent disease, serve as the basis for federal nutrition policy and nutrition education activities. To ensure that the Dietary Guidelines remain current, every 5 y the 2 agencies appoint a Dietary Guidelines Advisory Committee of experts in nutrition and health to review scientific and medical knowledge and recommend changes to the Dietary Guidelines. The committee’s recommendations are then reviewed, edited, and published by the USDA and the Department of Health and Human Services, most recently in 2000. Although scientific knowledge regarding the influence of nutritional factors on health and disease serves as the basis for the specific recommendations included in the Dietary Guidelines for Americans, limited empirical epidemiologic data are available to verify that adherence to the cluster of nutrition-related behaviors included in the Dietary Guidelines will reduce the incidence of disease. Indeed, it is possible that the recommendations in total may not be beneficial for some diseases because the Dietary Guidelines were designed for the prevention of an array of chronic diseases, resulting in the inclusion of recommendations that may be protective for some diseases but not for others.
To our knowledge, just 3 studies have examined the relation of compliance with the Dietary Guidelines to health outcomes (2–4). Kant et al (2) examined the association of mortality with a recommended food score developed as a measure of compliance with the food-based recommendations included in the 1995 Dietary Guidelines for Americans. In that prospective cohort study of adult women, subjects in the fourth quartile of the recommended food score had a relative risk for all-cause mortality of 0.69 (95% CI: 0.61, 0.78) compared with those in the lowest quartile. In contrast, McCullough et al (3, 4) found that adherence to the food and nutrient recommendations included in the 1995 Dietary Guidelines (as measured the USDA Healthy Eating Index) was only weakly associated with the incidence of cardiovascular disease in men and women enrolled in the Health Professionals Follow-up Study and the Nurses’ Health Study cohorts. Furthermore, no association was found between cancer incidence and the Healthy Eating Index in either cohort.
The aim of our study was to examine the association of compliance with the Dietary Guidelines and incident cancers. We evaluated data from the Iowa Women’s Health Study (IWHS), a population-based cohort of postmenopausal women in whom diet was measured before the diagnosis of cancer.
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SUBJECTS AND METHODS |
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TOPABSTRACTINTRODUCTIONSUBJECTS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Identification of cases
Information on cancer diagnosis was ascertained through the State Health Registry of Iowa, which is part of the National Cancer Institute’s Surveillance, Epidemiology, and End Results Program. Vital status of cohort members was determined through computer linkage of participant identifiers with Iowa death certificates, through mailed follow-up questionnaires in 1987, 1989, 1992, and 1997, and via the National Death Index for nonrespondents to the mailed follow-ups.
Response rates for the follow-up questionnaires were 91%, 89%, 83%, and 79%, respectively. Through 31 December 1998, after 13 y of follow-up, 5038 cancers, excluding nonmelanotic skin cancer, were recorded.
Data collection
A self-administered questionnaire administered at baseline included, among other items, questions regarding diet, smoking, physical activity, weight, height, reproductive history, family and personal history of cancer, and demographic information. To assess physical activity, participants were asked to report how often they took part in moderate and vigorous physical activities. Reported body weight and height were used to calculate body mass index (in kg/m2). Diet was assessed with a semiquantitative food-frequency questionnaire that was almost identical to that used in the 1984 Nurses’ Health Study (8). Participants were asked to report their average consumption, over the past year, of 127 food items, including 29 vegetables; 15 fruits; 13 dairy foods; 14 meat, poultry, seafood, or egg items; 17 breads, cereals, or starches (including 8 whole-grain items); 14 beverages (including 4 alcoholic beverages); and 25 sweets, baked goods, and miscellaneous items. Daily intakes of nutrients were calculated from the food frequency by multiplying the frequency of consumption of the specified unit of each food by the nutrient content of that unit of food. The food-frequency questionnaire used in the present study was found to account for 93% of total energy intake in a validation study with 194 female nurses (9). In a validation study in the Iowa population, the correlations between total fat, saturated fat, and cholesterol intake estimates derived from the food-frequency questionnaire compared with the average of five 24-h dietary recalls were 0.62, 0.59, and 0.21, respectively (10).
Measurement of compliance with the Dietary Guidelines for Americans
We constructed a dietary guidelines index to measure compliance with the Dietary Guidelines. The components of the index, outlined in Table 1
, were combined to calculate a single score for each study participant, allowing for ranking of individuals according to compliance with the fifth edition of the Dietary Guidelines for Americans (1). With the use of baseline data, we were able to include all but one of the major guidelines, "keep foods safe to eat," because food handling practices were not assessed.
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30 were classified as obese. In cases where the Dietary Guidelines were not quantitative, criteria provided from another set of national recommendations were sought. As an example, dietary recommendations provided by the National Research Council’s Committee on Diet and Health (11) were used to establish quantitative criteria for assessing compliance with the guideline to "choose a diet that is low in saturated fat and cholesterol and moderate in total fat." Where quantitative criteria were not available through the Dietary Guidelines or other national recommendations, cut points that allowed for differentiation among individuals were selected. For example, cut points for the variety of fruits consumed were determined so that study participants were classified into thirds. The scoring system was based on the premise that each major guideline should contribute equally to the total dietary guidelines index score. The maximum score for each major guideline was 2, with the maximum across all 9 index items totaling 18. An index score of 18 represented full compliance with all guidelines included in the index, and a score of 0 represented total noncompliance.
Analyses
Women with a history of cancer at baseline other than skin cancer (based on self-report) were excluded from all analyses (n = 3830), as were those with more than 29 blank items on the food-frequency questionnaire or with calculated energy intake of < 500 or > 5000 kcal (n = 2790). Those with missing covariate information were also excluded from analyses, leaving 34 708 women.
The association of the dietary guidelines index and cancer incidence was examined for all cancers combined and for site-specific cancers with > 100 events. Cox proportional hazards regression was used to derive relative risks adjusted for age and other potentially confounding variables (12). Covariates included in the regression models for specific cancers were those variables found to be associated with both the dietary guidelines index and the cancer of interest. The regression model examining the incidence of all cancers combined included all covariates in the models for specific cancers. Trends in relative risks across levels of the dietary guidelines index (coded ordinally) were tested by a chi-square statistic. All analyses were performed with SAS (version 6.12; SAS Institute, Inc, Cary, NC).
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RESULTS |
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TOPABSTRACTINTRODUCTIONSUBJECTS AND METHODSRESULTSDISCUSSIONREFERENCES |
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). Subjects with a higher dietary guidelines index score were more likely never to have smoked and to report having used hormone replacement therapy compared with those with a lower index score. In addition, those with a higher index score were somewhat older and had a lower energy intake, compared with those with a lower score. As expected, subjects with a higher index score had lower intakes of sweets, total fat, saturated fat, cholesterol, and sodium than did subjects with a lower index score.
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, the relative risk of all cancers associated with the upper 4 quintiles of the dietary guidelines index in reference to the bottom quintile was 0.95 (95% CI: 0.87, 1.05) for quintile 2, 0.88 (95% CI: 0.80, 0.97) for quintile 3, 0.88 (95% CI: 0.80, 0.96) for quintile 4, and 0.85 (95% CI: 0.77, 0.93) for quintile 5 (P for trend < 0.01). Similar patterns in relative risks were found for several cancers at specific sites. Incidence of colon cancer decreased progressively with increasing scores on the dietary guidelines index (P for trend < 0.01). The incidence of colon cancer in those in the top quintile of the dietary guidelines index was 74% (95% CI: 0.60, 0.99) of the incidence of those in the lowest quintile. Progressive decreases in relative risks across quintiles of the dietary guidelines index were also seen for incident cancers of the bronchus and lung (P for trend = 0.01), breast (P for trend < 0.01), and uterus (P for trend < 0.01). Relative risks suggested an inverse (nonsignificant) association between the dietary guidelines index and incident upper digestive tract, rectal, hematopoietic, and lymphatic cancers. In contrast, the incidence of ovarian cancer was positively associated with the dietary guidelines index. Relative risks across the upper 4 quintiles in reference to the bottom quintile were 1.59, 1.94, 1.79, and 2.05 (P for trend = 0.02).
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, for all subgroups incidence of cancer was lower among those in the highest quintile of the dietary guidelines index than among those in the lowest quintile, although the relative risks were not statistically significant for 2 groups (those with more than a high school level of education and those who were current or former hormone replacement therapy users).
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, the relative risks of all cancers associated with the upper 4 quintiles of the modified dietary guidelines index in reference to the bottom quintile were attenuated to 0.87 (95% CI: 0.80, 0.97) for quintile 2, 0.88 (95% CI: 0.80, 0.97) for quintile 3, 0.87 (95% CI: 0.79, 0.96) for quintile 4, and 0.92 (95% CI: 0.83, 1.02) for quintile 5 (P for trend = 0.15). With the exception of cancer of the bronchus and lung, the significant inverse associations observed for other site-specific cancers were attenuated or no longer evident in the model with the modified dietary guidelines index.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONSUBJECTS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Because of the wide variety of methods that have been used to evaluate compliance with the Dietary Guidelines and the use of different outcomes (eg, all-cause mortality or cancer incidence), it is difficult to compare the findings of studies of compliance with the Dietary Guidelines and cancer. The recommended food score used by Kant et al (2) was based on reported consumption of foods recommended in the 1995 Dietary Guidelines for Americans. The food score included 23 food items, 15 of which were fruits and vegetables. Thus, fruit and vegetables contributed heavily to the food score, and nutrient-based recommendations in the Dietary Guidelines were not included. In the McCullough et al (3, 4) studies, the healthy eating index developed by the USDA as a measure of compliance with the diet-related recommendations in the 1995 Dietary Guidelines for Americans (13) was used. This index consists of 10 equally weighted components, each representing different dietary recommendations from the Food Guide Pyramid and the 1995 Dietary Guidelines for Americans. In contrast to previous studies, in an effort to comprehensively evaluate the Dietary Guidelines for Americans our index included both diet and nondiet recommendations in the Dietary Guidelines. Thus, level of physical activity and weight status (as measured by body mass index) were included in our measure of compliance with the Dietary Guidelines. In addition, our dietary guidelines index was based on the 2000 Dietary Guidelines. Although most of the recommendations in the 1995 and 2000 Dietary Guidelines are consistent, some differences are evident. For example, in comparison to the 1995 Dietary Guidelines the 2000 version includes a greater focus on the need to include whole-grain foods in the daily diet.
Consistent with findings for all cancers, incidence of cancers of the colon, bronchus and lung, breast, and uterus were significantly lower with greater compliance with the Dietary Guidelines, and relative risks were suggestive of an inverse association between compliance with the Dietary Guidelines and upper digestive tract, rectum, hematopoietic, and lymphatic cancers. In contrast, incidence of ovarian cancer increased across levels of the dietary guidelines index. This positive association appears to be attributable to the physical activity component of the index. In a previous analysis of data from the IWHS, Mink et al (14) found the incidence of ovarian cancer to be higher among women in the cohort with moderate and high than with low levels of physical activity. Consistent with this, in our analysis in which physical activity and body mass index were excluded from the dietary guidelines index and included as covariates, the relative risk of ovarian cancer for a high compared with a low level of physical activity was 1.54 (95% CI: 1.06, 2.23), and the relative risks of ovarian cancer associated with the upper 4 quintiles of the revised dietary guidelines index in reference to the bottom quintile were attenuated to 0.70 (95% CI: 0.40, 1.22) for quintile 2, 1.14 (95% CI: 0.69, 1.87) for quintile 3, 1.42 (95% CI: 0.89, 2.29) for quintile 4, and 1.01 (95% CI: 0.60, 1.71) for quintile 5 (P for trend = 0.22).
Findings from other studies that have examined the association of physical activity with ovarian cancer have been equivocal. In one study little evidence of an association between physical activity level and ovarian cancer was found (15). In other studies inverse (16–18) or positive (19) associations have been reported. More research is needed to clarify the relation of physical activity to ovarian cancer.
With the exception of cancers of the bronchus and lung, the significant associations observed between the dietary guidelines index and cancers (all cancers and site-specific cancers) were attenuated or no longer evident in analyses conducted to examine the association of the diet-based Dietary Guidelines with cancer occurrence. These findings suggest that associations observed between the dietary guidelines index and cancer incidence may be attributable in part or in total to the physical activity and weight-related recommendations included in the Dietary Guidelines.
Although our findings suggest that the Dietary Guidelines are associated with reduced cancer risk, it is possible that another set of recommendations might be more efficacious, with greater cancer prevention possible. Further research is warranted to evaluate the relative efficacy of various dietary recommendations and eating patterns. Specifically, eating patterns that have previously been found to be associated with better health outcomes (20–28) should perhaps be evaluated within a study population so that an optimal dietary recommendation for prevention of cancer and other chronic diseases may be formulated.
We were unable to conduct analyses to evaluate the magnitude of risk reduction that may be associated with nearly complete compliance with the Dietary Guidelines relative to very poor compliance, because few women in the study cohort were found to be highly compliant. Dietary guidelines index scores for those categorized in the highest quintile averaged 13.4 (range: 12.2–17.6), indicating that many of those in the highest quintile did not comply with several of the specific recommendations in the Dietary Guidelines. Conversely, index scores for those in the lowest quintile averaged 7.1 (range: 2.1–8.2), indicating some compliance.
It is important to recognize that the IWHS is not ideal for evaluating the Dietary Guidelines for Americans in relation to cancer occurrence because previous findings from this study have contributed to the scientific base used in formulating the Dietary Guidelines. It is possible that the associations observed in this study reflect this circularity. Ideally, the Dietary Guidelines would be evaluated with the use of data from a study in which diet-cancer hypotheses have not previously been explored.
Several methodologic issues must be considered in interpreting our findings. First, the source of dietary information in our study was a single food-frequency questionnaire administered at baseline. Although the food-frequency questionnaire used in the IWHS has been shown to provide reasonably valid intake estimates for many foods and nutrients (9, 10), measurement errors inherent in retrospective dietary assessment are of concern (29). Also, consideration should be given to the quality of our measure of compliance with the dietary guidelines index. Some of the Dietary Guidelines were difficult to measure using the data collected as part of the study. Furthermore, the scoring system we devised for the dietary guidelines index gave equal weight to each of the major recommendations included in the Dietary Guidelines (each major recommendation was worth a maximum of 2 points). An arbitrary weighting scheme such as this may not be ideal. This was not a randomized trial and is therefore subject to confounding by lifestyle choices that are related to cancer. We attempted to control for measured confounding factors, but we cannot rule out the possibility of residual confounding. Finally, because participants in this study were older, predominantly Caucasian women residing in Iowa, strictly interpreted, the study findings should be generalized only to similar women. Further evaluation of the Dietary Guidelines in different population groups is needed so that the findings reported here may be considered in the context of other population groups.
In conclusion, findings from the IWHS suggest that greater adherence to the cluster of nutrition-related behaviors recommended in the Dietary Guidelines for Americans may be associated with a lower risk of cancer. Specifically, risk in postmenopausal women may be reduced for cancers of the colon, bronchus and lung, breast, and uterus. Better compliance with the Dietary Guidelines was associated with an increased risk of ovarian cancer, but fortunately its incidence is a small proportion of total cancer.
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