1 Department of Public Health, Kanazawa Medical University, Ishikawa, Japan.
2 Department of Preventive Medicine, Feinberg School of Medicine, Northwestern University, Chicago, IL.
Received for publication June 14, 2003; accepted for publication October 24, 2003.
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ABSTRACT |
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blood pressure; diet; follow-up; fruit; nutrition; population studies; vegetables
Abbreviations: Abbreviations: DASH, Dietary Approaches to Stop Hypertension; DBP, diastolic blood pressure; SBP, systolic blood pressure.
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INTRODUCTION |
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Dietary factors play a critical role in the prevention and treatment of hypertension. For example, in the Dietary Approaches to Stop Hypertension (DASH) trial, a "combination" dietary pattern, including increased intake of fruits, vegetables, poultry, fish, and low-fat dairy products and reduced intake of red meats, fats, cholesterol, and sweets, substantially lowered blood pressure during an 8-week intervention (3). Sodium reduction further contributed to blood pressure lowering in another 4-week DASH trial (4). Dietary guidelines for prevention and treatment of hypertension, and of cardiovascular diseases in general, emphasize a healthy eating pattern according to food (not only nutrient) intake and avoidance of a high salt and alcohol intake (5, 6). Although the DASH combination diet is effective in lowering blood pressure, influences of specific foods and food groups (e.g., vegetables, fruits, fish, red meats) on blood pressure have not been well studied over the long term. Information is especially sparse on relations of food groups to blood pressure change in populations followed prospectively for years.
We report on associations between baseline food group intake and subsequent 7-year blood pressure change in the Chicago Western Electric Study cohort. These data offer an unusual opportunity to contribute to the issue of dietary factors influencing long-term blood pressure change. The Chicago Western Electric Study data encompass both high-quality assessment of baseline food intake and annual blood pressure follow-up in a middle-aged working population.
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MATERIALS AND METHODS |
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Blood pressure measurements
With the participant seated, research-trained clinicians measured blood pressure by using standard mercury sphygmomanometers at the beginning of each yearly medical examination. Left arm measurements were used consistently. Only limited information was obtained concerning use of blood pressurelowering medication; however, data were collected when few people with hypertension were being treated with medications. In 1958, for example, only 11 percent of examinees reported a diagnosis of hypertension; of these, 62 percent were treated and considered controlled (7 percent of the entire cohort). Because of the absence of detailed annual information on use of antihypertensive medication, and its relative infrequency, no effort was made to control for medication use in these analyses.
Dietary assessment
Dietary intake information was obtained at the first (19571958) and second (19581959) examinations, 1 year apart, by two nutritionists using standardized interviews and questionnaires based on Burkes comprehensive dietary history method (9). Baseline nutrient intake data are means from these two examinations (19571958 and 19581959) to reduce classification error, that is, regression dilution bias. The dietary interview, lasting about 1 hour, asked about usual eating pattern (what, when, and where) on a typical workday and weekend, special diets now and previously, and changes in eating habits during the preceding 20 years. These questions were followed by a detailed cross-check of 195 specific food items to determine number of times in the previous 28 days that each food item had been eaten and quantity per serving. As an aid in determining portion sizes, models of commonly used foods and dishes of varying sizes were used for reference. Information was obtained from the dietary department of the company regarding standard portions and types of foods served in the cafeteria. Supplementary information regarding food preparation was obtained from a questionnaire mailed to participants wives and returned by participants at interview. Information on food preparation was also obtained from neighborhood restaurants and bakeries. When a man reported habitual consumption of a dish not on the list of 195 foods, the recipe was obtained and analyzed regarding its component parts for nutrient assessment. No data were collected on dietary supplement use, given its presumed rarity in 19571959.
The nutritionists coded dietary information to indicate kinds and quantities of foods and beverages (alcoholic and nonalcoholic) consumed by each participant during the preceding 28 days. These data were analyzed on the basis of a food table derived from several sources (1012) to obtain each mans usual daily caloric intake and consumption of animal and vegetable protein; animal and vegetable fat; total carbohydrates; total saturated fatty acids; total unsaturated fatty acids; linoleic acid; linolenic acid; arachidonic acid; cholesterol; calcium; phosphorus; iron; vitamins A, C, and D; thiamine; riboflavin; and niacin. Quantities of linoleic, linolenic, and arachidonic acid were summed to estimate total polyunsaturated fatty acids. Subsequently, beta-carotene intake was also estimated by using data still available on food group intake and on total vitamin A (13, 14). Information on dietary sodium chloride, potassium, magnesium, or fiber intake was not obtained.
Food profile scores, based on information obtained at the second examination only, indicated level of consumption of 26 foods over the previous 28 days: soft drinks; coffee; decaffeinated coffee; whole milk; skim milk; cream; cheese; eggs; ice cream; puddings or custards; soups; fish; beef, veal, or lamb (beef-veal-lamb); pork, ham, or bacon; liver; poultry; mixed dishes; vegetables; breads or cereals; potatoes; fruits; pastries; sweets or sugars; butter; margarine; and fried foods. Each item was coded on a four-point scale (0 for none and 1, 2, or 3 for increasing levels of consumption). Six food groups (vegetables, fruits, fish, beef-veal-lamb, pork-ham-bacon, and poultry) were selected for this report to explore relations of blood pressure change to food groups markedly modified in the DASH combination diet. Consumption of low-fat dairy products, with the exception of skim milk, was not surveyed at baseline. For most items described in this report (except fish), the lower two categories were combined because there were no or few participants in the "none" stratum.
Follow-up
Study participants were invited for annual follow-up examinations through 1966, that is, for 7 years after the second dietary survey was conducted. These examinations included measurement of blood pressure, weight, and serum cholesterol; medical history and physical examination; electrocardiogram; and other itemsmethods have been described previously (7, 8).
Exclusions
Some of the original 2,107 participants were excluded for one or more of the following reasons (not mutually exclusive): missing baseline dietary assessments (n = 191), missing baseline blood pressure (n = 72), missing data on educational attainment (n = 223), previously diagnosed diabetes mellitus (n = 31), prior myocardial infarction (n = 44), and/or fewer than three follow-up examinations between 1960 and 1966 (n = 184). The cohort remaining for the analyses of the relations of baseline food profiles to change in blood pressure during 7 follow-up years totaled 1,710.
Statistical analyses
Analyses were conducted with the generalized estimating equation method for longitudinal data (15) to estimate the relation of baseline dietary factors to average yearly change in systolic blood pressure (SBP) or diastolic blood pressure (DBP), with adjustment for baseline blood pressure and likely confounders. Blood pressure measured at the second examination was set as baseline blood pressure because the food score survey was performed at the second examination. The relation of each food group level to blood pressure change was measured by using the coefficient of the cross-product (interaction) term between food group level and a time variable, t (t = 0, 1, ..., 7).
Analyses of the relation of a food group to blood pressure change were serially adjusted for confounding factors in five generalized estimating equation models, that is, a model adjusted for baseline age only, a model adjusted for baseline age and weight at each year, and three multivariate-adjusted models (models 1, 2, and 3). Model 1 was adjusted for seven potentially confounding factors (baseline age, weight at each year, height, education, smoking, alcohol consumption, intake of total energy). In addition to the factors in model 1, model 2 was also adjusted for intake of the five other food groups so we could look at all food groups simultaneously. Model 3 was additionally adjusted for daily intake of 12 nutrients (average of years 1 and 2)total carbohydrate (percentage of kilocalories), total protein (percentage of kilocalories), saturated fatty acid (percentage of kilocalories), polyunsaturated fatty acid (percentage of kilocalories), dietary cholesterol (mg/1,000 kcal), iron (mg/1,000 kcal), thiamine (mg/1,000 kcal), riboflavin (mg/1,000 kcal), niacin (mg/1,000 kcal), vitamin C (mg/1,000 kcal), beta-carotene (index units/1,000 kcal), and retinol (index units/1,000 kcal)to assess whether the relation between a food group and blood pressure change was possibly attributable to these nutrients present in the food group. Total energy intake was omitted as a covariate in model 3 because the nutrients were expressed as caloric densities. For each potential baseline variable in the model, a cross-product term with time was included to adjust for the relation of the confounder with blood pressure change. Adjustment for other food groups was conducted by including dummy variables for other food groups and the cross-product terms between these dummy variables and time. In this report, results are expressed as change in blood pressure/year coefficients, which represent the difference in annual blood pressure change between a higher intake group and the reference intake group; p values are given for coefficients.
Average values of baseline blood pressure by baseline food group intake, adjusted for age and other confounders, were also compared by using analysis of covariance.
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RESULTS |
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Of the 1,710 participants, 1,322 completed all seven annual follow-up examinations through 1966. The remaining participants underwent at least three follow-up examinations between 1960 and 1966. Mean baseline blood pressures of those with three to six follow-up examinations were not significantly different from those of participants completing all seven examinations.
Relation of baseline food group intake to baseline SBP and DBP
Average values of baseline SBP and DBP by each food intake category, adjusted for age, body mass index, education, total energy intake, cigarettes per day, and alcohol consumption, were calculated (data not shown). Adjusted averages of baseline blood pressure were not significantly different by each food intake group, except for DBP according to intake of beef-veal-lamb, where adjusted baseline mean DBP was significantly higher in men whose intake of beef-veal-lamb was lower (p < 0.05), and for SBP by intake of fruit, where adjusted mean SBP was significantly lower in men whose fruit intake was the lowest (i.e., <14 cups/month or <0.5 cups/day) compared with those who consumed 1442 cups of fruit per month (about 0.51.5 cups/day) (p < 0.05).
Relation of baseline food group intake to average annual SBP change during 7-year follow-up
Independent relations of baseline food group intake to change in SBP during 7-year follow-up, with control for confounding factors, were evaluated in five generalized estimating equation models (table 2). In multivariate-adjusted models, men who consumed at baseline 1442 cups of vegetables per month (about 0.51.5 cups/day) showed a significantly smaller SBP increase per year compared with men in the lowest intake category, except in model 3. For the former group, SBP increased an estimated 2.8 mmHg less 7 years later compared with men who consumed less than 14 cups per month (<0.5 cup/day) (model 2). After adjustment for 12 nutrients (expressed as caloric density; refer to the Materials and Methods section of the text) and the corresponding cross-product terms between nutrients and time, the relation between vegetable intake and SBP diminished (model 3). Further analyses with adjustment for each nutrient separately showed that the association between vegetable intake and SBP was weakened, especially by adjustment for beta-carotene intake (data not shown).
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Although not significant, higher intake of fish tended to relate to a lower rise in SBP over time. In all models, men whose beef-veal-lamb intake was higher had a significantly greater rise in SBP. Men who reported eight to 20 or more than 20 servings of 120-g portions of meat had a 5.4 mmHg or 6.0 mmHg, respectively, greater increase in SBP over 7 years compared with men who reported less than eight servings per month (model 2). A similar association prevailed for pork intake. In all models, men whose poultry intake was the highest had a significantly greater rise in SBP.
Nutrient adjustment did not greatly impact the fish, meat, and poultry associations (model 3). Results (model 3) were similar with control for vegetable protein intake instead of total protein intake or with control for absolute daily intake of nutrients and energy instead of intake based on caloric density.
Relation of baseline food intake to average annual DBP change during 7-year follow-up
Table 3 presents independent relations of baseline food intake to change in DBP during 7-year follow-up. Men with a higher intake of vegetables at baseline showed significantly less increase in DBP, or a borderline significant increase, compared with men in the lowest intake category in all models except nutrient-adjusted model 3. Men who consumed 1442 cups or more than 42 cups of vegetables per month were estimated to have 1.2 mmHg or 1.5 mmHg, respectively, less of an increase in DBP 7 years later compared with men who consumed less than 14 cups per month (model 2). With further adjustment for the 12 nutrients (expressed as caloric density), the association between vegetable intake and DBP decreased notably (model 3). Further analyses with adjustment for each nutrient separately showed that the association between vegetable intake and DBP was weakened, especially by adjustment for beta-carotene intake (data not shown).
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Men with the highest intake of beef-veal-lamb had a significantly greater increase in DBP. Higher poultry intake was also associated with a greater increase in DBP. Nutrient adjustment did not greatly impact the meat and poultry associations (model 3). Results (model 3) were similar with control for vegetable protein intake instead of total protein intake or with control for absolute daily intake of nutrients and energy.
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DISCUSSION |
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By the early 1990s, based on scientific evidence available at the time, nutritional guidelines for prevention and control of high blood pressure recommended weight control, reduced intake of sodium chloride (salt), avoidance of heavy alcohol consumption, and increased dietary potassium intake (1, 16). The DASH study recently added further dietary approaches to reduce blood pressure in both nonhypertensives and hypertensives by using a "combination" dietary pattern (3, 4, 17). This pattern emphasizes higher-than-usual intakes of fruits and vegetables (five to nine servings/day) and low-fat dairy products (two to four servings/day). It also includes selection of whole grains, poultry, fish, and nuts as well as reduced intake of total fats, saturated fats, cholesterol, red meats, sweets, and sugar-containing beverages. This diet is high in potassium, magnesium, phosphorus, calcium, fiber, and protein. The DASH-Sodium trial also showed that the combination diet plus reduced salt intake (at about 50 mmol/day) yielded substantial combined reductions in SBP and DBP in both nonhypertensive and hypertensive persons. The DASH dietary concept, emphasizing a healthy pattern based on food rather than nutrient intake, has been incorporated into recent dietary guidelines by the American Heart Association (5).
Previous observational studies reported that dietary patterns characterized by a high intake of fruits and vegetables were associated with lower risks of developing heart disease and stroke (1821), and vegetarians were reported to have lower blood pressure than general populations (22). A report from the Nurses Health Study showed that blood pressure level after 4 years (not blood pressure rise in 4 years) was inversely related to baseline intakes of fruits and vegetables (23). To our knowledge, however, relations of these food groups to long-term blood pressure rise with age have not been reported. Our results suggest that a higher vegetable intake substantially suppresses blood pressure increase over years independent of fruit intake, as does higher fruit intake independent of vegetable intake, although the lack of dose-response relations suggests a possibility that very low intake of fruits or vegetables is related to a higher blood pressure increase. The strength of the findings for vegetables diminished with adjustment for 12 nutrients, including total protein, lipids, and antioxidants and other vitamins, and the data indicated that the association of vegetable intake was explained in part by beta-carotene intake. The fact that the relation of vegetable intake did not change after adjustment for vegetable protein is noteworthy because data are available indicating that this nutrient may be inversely related to blood pressure change (3, 4, 24). Furthermore, since data on sodium, potassium, magnesium, or fiber were lacking in the Chicago Western Electric Study, it was not possible to control for these nutrients.
The DASH combination diet included increased intake of fish and reduced consumption of red meat. The DASH design precluded definitive assessment of the effect of individual food groups on blood pressure. Our results suggest modest inverse associations of lower fish intake and significant direct associations of higher beef-veal-lamb and pork intake with an increase in blood pressure over time. With regard to fish intake, several interventional studies showed that omega-3 polyunsaturated fatty acids, commonly found in fish oil, lowered blood pressure (2527), although the doses of these fatty acids in these trials were much higher (average, >3 g/day) than those consumed by US populations (e.g., <0.2 g/day in the Multiple Risk Factor Intervention Trial (MRFIT)) (28). In addition, it has been suggested that protein or amino acids in fish lower blood pressure (29, 30). Our results on fish intake support the inclusion of fish in the DASH combination diet. On the other hand, there have been few studies on red meat intake and long-term blood pressure rise (23). The influence of red meat intake on blood pressure has been unclear, possibly because red meat includes animal (saturated) fats and cholesterol as well as protein. Cross-sectional data from the INTERSALT Study suggest that higher dietary total protein intake has a favorable influence on blood pressure (31). Although direct relations to blood pressure of saturated fatty acids, dietary cholesterol, and Keys score have been reported (28, 32), and the DASH diet is low in these nutrients, our results showed that relations of red meat with blood pressure changed little after adjustment for intake of saturated fatty acids and dietary cholesterol. Our results on beef-veal-lamb and pork intake support the concept that lower intake of red meat has a favorable influence on blood pressure; this aspect of the DASH combination diet may have contributed to its blood pressurereducing effects. The relation of higher poultry intake to greater blood pressure increase over time, independent of other dietary factors, was unexpected. The DASH combination diet involves no reduction in poultry intake. This aspect of food intake in relation to blood pressure needs to be investigated further.
Our findings are complementary and additive to a previous report from this same cohort on the potential role of specific nutrients in long-term blood pressure change (24). In those analyses, total and animal protein; total, saturated, monounsaturated, and polyunsaturated fatty acids; cholesterol; Keys dietary lipid score; calcium; alcohol; and average annual change in weight were positively and significantly related to average annual change in SBP. Vegetable protein and total carbohydrate, beta-carotene, and antioxidant dietary vitamin score were inversely related to average yearly SBP change. The present findings, focusing on specific foods with adjustment for these nutrients, suggest that habitual intakes of specific foods and food groups have additive effects beyond those attributable to nutrients in contributing to blood pressure rise with age. Thus, our findings lend further support to the promotion of a DASH-style dietary pattern not only for treatment of high blood pressure but also for long-term prevention of blood pressure rise.
The study has both strengths and limitations. Among its strengths are the highly detailed habitual dietary intake data on participants, virtually unique in long-term nutritional epidemiology studies. The Chicago Western Electric Study cohort has been recognized as contributing important observations on the relation of dietary fats and cholesterol, fish intake, and other dietary factors to heart disease risk throughout many years of follow-up (8, 33). The availability of annual blood pressure measurements for 7 consecutive years also allowed for the long-term observations reported here. The infrequent diagnosis of hypertension and the relatively rare use of blood pressurelowering medications also permitted us to examine dietary influences largely separate from those related to drug treatment. In contrast, data on use of blood pressurelowering medications in the National Health and Nutrition Examination Survey indicate that 72 percent of hypertensives (judged at that time to be nearly 50 percent of the population) were using blood pressure medications in 19881991 (34). Therefore, a study such as the Chicago Western Electric Study investigation may be difficult to carry out at the present time because of the extensive use of blood pressure medication now. In addition, the proportion of people taking dietary supplements, including vitamins and minerals, was almost certainly much lower than it is today (although specific information on this matter was not collected); hence, our study results were apparently affected less by such supplements and better reflect the effects of food on blood pressure in this population, independent of influences of supplements.
Limitations of these findings should also be acknowledged. First, data collection for this study took place over 40 years ago; consequently, lifestyle aspects, including dietary habits and food products, were different from those prevalent now in the United States. For example, average total fat intake in this study (43 percent of total energy) was considerably higher than recent values, and low-fat dairy products were generally unavailable. Furthermore, the available study data contain no information on dietary sodium chloride, potassium, magnesium, or fiber intakes. Therefore, these nutrients could not be included as covariates in the statistical models. It is thus possible that some of our findings are related to these unmeasured nutrients. For instance, higher meat intake may have also been associated with higher salt intake, and our inability to adjust for this association is an acknowledged limitation.
Three other limitations merit discussion. Given the classification of food intake into only three or four food frequency categories, there was undoubtedly misclassification of intake for each food assessed. Such misclassification likely reduced our ability to detect food intakeblood pressure change relations. This fact may also account for the lack of dose-response relations in several of the foodblood pressure change relations. Second, residual confounding by unmeasured nondietary factors is likely. Although the models accounted for major covariates such as weight at each year and alcohol intake, we could not assess physical activity, for example. Thus, residual confounding might have led to false-positive relations. Finally, given possible differences in the pathophysiology of hypertension across population subgroups, these findings may (or may not) be generalizable beyond middle-aged, non-Hispanic White males.
In conclusion, results of this 7-year blood pressure follow-up study extend prior epidemiologic and short-term dietary trial data. They also lend support to the concept that blood pressure increase with age may be prevented by consuming a diet rich in fruits and vegetables and reduced in meat (except fish), in addition to other influences not studied here, such as reduced salt intake, avoidance of heavy alcohol consumption, and weight control.
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ACKNOWLEDGMENTS |
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The authors express appreciation to the officers, executive leadership, and labor force of the Western Electric Company, Chicago; their cooperation made this study possible. They also acknowledge the roles of the founders of the Chicago Western Electric Study, of the two nutritionists who collected the dietary data on the Western Electric men, and of the many physicians who participated in the annual examinations. For a list of their names, refer to Paul et al. (7) and Stamler et al. (8).
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NOTES |
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REFERENCES |
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