Relation between Intake of Flavonoids and Risk for Coronary Heart Disease in Male Health Professionals

Methods

The Health Professionals Follow-up Study is a prospective investigation of 51 529 U.S. men who were 40 to 75 years of age in 1986. Of these men, 29 683 were dentists, 10 098 were veterinarians, 4185 were pharmacists, 3745 were optometrists, 2218 were osteopathic physicians, and 1600 were podiatrists; all were recruited from their professional organizations. The study began in 1986, when each participant completed a detailed questionnaire on his diet and medical history. We mailed biennial follow-up questionnaires to obtain updated information on exposures and to learn about newly diagnosed events related to coronary heart disease [13, 14]. In 1990, participants completed a semi-quantitative food frequency questionnaire. More than 94% of eligible participants were followed in 1988, 1990, and 1992 [15]. The nonresponders were assumed to be alive if they were not listed in the National Death Index, a nationwide registry of deaths in the United States [16]. Because men who receive a diagnosis of cardiovascular disease or related conditions may alter their diets, we excluded men who reported having prevalent cardiovascular disease on the baseline questionnaire in 1986.

Dietary Assessment

The dietary questionnaires completed in 1986 and 1990 asked about the average frequency of intake of 131 foods during the previous year. Intake of specific nutrients was computed by multiplying the frequency of consumption of an item by its nutrient content, which was derived primarily from U.S. Department of Agriculture sources [17]. We excluded men (about 3% of the sample) whose reported daily energy intake was less than 800 kcal or more than 4200 kcal or those who left 70 or more items blank on their dietary questionnaires.

We determined each participant's intake of three flavonols and two flavones by using food tables (based on analyses done by Hertog and colleagues [3, 18]) that were supplemented with values for additional U.S. foods (apples, onions, teas, and red wines [three varieties of each]; avocado; cantaloupe; watermelon; blueberries; green beans; corn; sprouts; yellow squash; green pepper; tofu; and apple juice). Foods were analyzed [4, 18] by the same laboratory at the State Institute for Quality Control of Agricultural Products in Wageningen, the Netherlands. Levels of flavones (such as apigenin and luteolin) in foods are very low. Using information about 147 foods and recipes that contain flavonoids, we determined the average intake of quercetin, myricetin, kaempferol, luteolin, and apigenin and summed the totals for these five compounds. Quercetin, kaempferol, and myricetin composed more than 90% of the flavonoids ingested by our study sample.

In our baseline 1986 questionnaire, we did not assess intake of onions in sufficient detail. Because much of the flavonols present in the diets of persons in developed countries comes from onions [19, 20], we assumed that the intake of onions reported on the 1990 questionnaire (in response to two separate questions: one about onions as a garnish and one about onions as a vegetable, rings, or soup) had not changed during the previous 4 years and could be added to the dietary information reported in 1986. Because of this limitation, our main analyses were limited to the 34 789 men who responded to both the 1986 and the 1990 dietary questionnaires.

To assess the validity of the dietary questionnaire, we compared the intake of nutrients and food as reported on the food frequency questionnaire with that reported in two 1-week dietary records (spaced approximately 6 months apart) kept by a subsample of 127 men [21, 22]. Although we could not directly test the validity of values for intake of flavonoids as derived from the questionnaire, the Pearson correlation coefficients between the questionnaire and the dietary records were high for most nutrients (average, 0.65 [range, 0.30 to 0.92]). We were unable to measure the validity of values for self-reported intake of onions. However, correlations between the questionnaire and the dietary records were good for other main sources of flavonoids: tea (r = 0.77), apples (r = 0.70), and broccoli (r = 0.46) [22].

Case Ascertainment

We considered only nonfatal myocardial infarction end points that occurred between the date on which the 1986 questionnaire was returned and 31 January 1992. Because onion consumption as reported on the 1990 questionnaire was needed to calculate total intake of flavonoids, men who had had a fatal myocardial infarction between 1986 and 1990 could not report onion consumption and were not included in this analysis. Participants who reported an incident myocardial infarction on the 1988, 1990, or 1992 questionnaires were sent a letter confirming the report and requesting permission to review medical records. Myocardial infarctions were confirmed using criteria from the World Health Organization [23]: compatible symptoms plus either typical electrocardiographic changes or elevated cardiac enzyme levels.

For our secondary analyses of all incident coronary end points between 1990 and 1992, we included nonfatal and fatal myocardial infarction as well as cardiovascular revascularization procedures.

Deaths were reported by next-of-kin, coworkers, and postal authorities in addition to being ascertained using the National Death Index. We confirmed fatal infarctions by using medical records or autopsy reports. Further details have been published elsewhere [13, 24].

Statistical Analysis

Our primary analyses examined the association between energy-adjusted [25] intake of flavonoids as reported on the 1986 dietary questionnaire (supplemented with intake of onions as reported on the 1990 questionnaire) and nonfatal myocardial infarction. For the secondary analyses, we examined the association between self-reported intake of flavonoids from 1990 and total incident coronary events.

For the primary analysis, each participant contributed follow-up time from the date on which he returned his 1986 questionnaire until diagnosis of nonfatal myocardial infarction, death, or 31 January 1992. Relative risks were calculated as the incidence of coronary heart disease among men in a given quintile of intake of flavonoids divided by the incidence among men in the lowest quintile of intake, adjusted for age (in 5-year strata) using the Mantel-Haenszel method [26]. To adjust for other risk factors, we used multiple logistic regression to generate odds ratios as a valid estimate of relative risk. We did the secondary analyses in a similar way, except that intake of flavonoids, disease history, and other variables used in the multivariate model were assessed using the 1990 questionnaire.

Results

For men in our study, the main sources of flavonols and flavones were tea (25%), onions (25%), apples (10%), and broccoli (7%). The average total intake was 20.1 mg/d; the three primary flavonols ingested were quercetin (15.4 mg/d), kaempferol (3.6 mg/d), and myricetin (0.9 mg/d). Men with a higher intake of flavonoids were slightly older, drank less alcohol, smoked less, ate more dietary fiber, and were more likely to take vitamin E supplements (Table 1).

During 6 years of follow-up in 34 789 men who completed the 1986 and 1990 dietary questionnaires, we documented 486 nonfatal myocardial infarctions. Compared with men in the lowest quintile for intake of flavonoids, men in the highest quintile had an age-adjusted relative risk for coronary heart disease of 1.06 (95% CI, 0.81 to 1.40) (Table 2). After multivariate adjustment, the relative risk was essentially unchanged (relative risk, 1.08 [CI, 0.81 to 1.43]). Controlling for intake of dietary fiber, saturated fat, or cholesterol did not substantially alter these results. No individual flavonols or flavones were associated with an appreciable reduction in risk for coronary heart disease. For example, the multivariate relative risk of quercetin (for the highest compared with the lowest quintile of intake) was 1.14 (CI, 0.86 to 1.51).

The association between total intake of flavonoids and risk for nonfatal myocardial infarction was not appreciably modified by age or the use of vitamin E supplements. Among the 404 men who were not taking vitamin E supplements, the relative risk for nonfatal myocardial infarction (for the highest compared with the lowest quintile of intake of flavonoids) was 1.09 (CI, 0.80 to 1.48); the relative risk was 0.94 (CI, 0.65 to 1.36) after men who reported taking multivitamin supplements were excluded from the analysis.

For the secondary analyses, we calculated the relative risk for all incident fatal and nonfatal coronary events using only the dietary information collected on the 1990 questionnaire. Between 1990 and 1992, we documented 373 incident coronary events: 122 nonfatal myocardial infarctions, 44 deaths from coronary heart disease, and 207 cardiovascular revascularization procedures. As before, intake of flavonoids was not associated with risk for total coronary heart disease (relative risk, 0.94 [CI, 0.68 to 1.31] for the highest compared with the lowest quintile of intake).

We calculated the relative risk for death from coronary heart disease between 1990 and 1992 in the 38 036 men, 4838 of whom had prevalent coronary heart disease. Between 1990 and 1992, we documented 140 deaths from coronary heart disease. The age-adjusted relative risk for death from coronary heart disease was 0.64 (CI, 0.39 to 1.04). After further adjustment for cardiovascular risk factors, including indicator variables for prevalent coronary heart disease, the relative risk was greater (relative risk, 0.59 [CI, 0.35 to 0.98]). However, after controlling for intake of dietary fiber, carotene, and saturated fat, the relative risk was substantially attenuated (relative risk, 0.77 [CI, 0.45 to 1.35]). We repeated the 2-year analyses (1990 to 1992) using only the 4814 men (105 deaths from coronary disease) who had prevalent coronary heart disease. The multivariate relative risk for death from coronary heart disease for the highest compared with the lowest quintile of intake was 0.63 (CI, 0.33 to 1.20). Thus, the inverse association between total intake of flavonoids and risk for death from coronary heart disease was limited to the men who had previously had cardiovascular disease and was not statistically significant.

Although intake of onions was not assessed until 1990, we did examine the association between consumption of apples and tea and the 6-year risk for death from coronary heart disease among men who had no previous cardiovascular disease. In a multivariate model that controlled for nondietary risk factors for coronary heart disease in addition to intake of dietary fiber, saturated fat, and vitamin E, men who consumed two or more cups of tea daily had a relative risk for death from coronary heart disease of 1.59 (CI, 0.98 to 2.28) compared with men who did not drink tea. Compared with men who did not consume apples, men who ate one or more apples daily had a relative risk for death from coronary heart disease of 1.01 (CI, 0.57 to 1.80).

Discussion

These data from more than 34 000 men suggest that intake of flavonoids is not an important determinant of risk for incident coronary heart disease among male health professionals in the United States. In our primary analyses, we found no substantial association between intake of flavonoids and incidence of nonfatal myocardial infarction. Among men with prevalent coronary heart disease and a higher intake of flavonoids, we did find a possible reduction in the risk for death from coronary heart disease.

Hertog and coworkers [11] reported a strong inverse association between the highest and lowest tertiles of total intake of flavonoids (relative risk, 0.32 [CI, 0.15 to 0.71]) and risk for death from coronary heart disease in the Zutphen Elderly Study, which included 112 of 805 men (13.9%) who had previously had myocardial infarction. Among the subsample of 693 men who had not previously had coronary heart disease, the relative risk for incident fatal and nonfatal myocardial infarction (38 cases) was no longer significant (relative risk, 0.52 [CI, 0.22 to 1.23]), but the relative risk for death from total coronary heart disease (20 cases) remained significant (relative risk, 0.29 [CI, 0.09 to 0.93]). As do our results, this finding suggests that intake of flavonoids is inversely associated only with death from coronary heart disease. We found a suggestion of a risk reduction only among the men with previous cardiovascular disease (relative risk, 0.63 [CI, 0.33 to 1.20] for the highest compared with the lowest quintile of intake) and not among the men who were free of cardiovascular disease at baseline (relative risk, 1.31 [CI, 0.42 to 3.05]). Our partial agreement with the results of the Zutphen Elderly Study may point to a specific effect on thrombosis [9, 10] among men with established coronary heart disease. Knekt and colleagues [12] followed 2748 men and 2385 women for as long as 25 years and found that participants in the upper quartile of intake had a relative risk for death from coronary heart disease of 0.54 (CI, 0.33 to 0.87) compared with participants in the lower quartile. The relative risk was somewhat attenuated after adjustment for intake of antioxidant vitamins, dietary fat, and dietary fiber (relative risk, 0.73 [CI, 0.41 to 1.32] in men and 0.67 [CI, 0.44 to 1.00] in women). In the Seven Countries Study, Hertog and coworkers [20] reported a significant inverse association between estimated average intake of flavonoids and risk for death from coronary heart disease during a 25-year follow-up period. The results of this cross-cultural study are suggestive but should be interpreted cautiously, because many other factors could explain the differences seen in death from coronary heart disease in these countries.

The prospective nature of our study reduces the potential for bias caused by differential recall of intake by cases and noncases of coronary artery disease. Measurement error in the assessment of flavonoids from our dietary questionnaire may have attenuated our reported relative risks; however, we previously reported associations between diet (as assessed by our questionnaire) and relevant biomarkers in blood [27, 28] and risk for coronary heart disease [13, 29-31]. Because intake of flavonoids tends to be linked with factors that are both positively associated (such as age, lower alcohol intake, and hypertension) and inversely associated (such as intake of dietary fiber and nonsmoking status) with risk for coronary heart disease (Table 1), it is unlikely that our results would be substantially altered even if we could account for unmeasured confounding.

The overall range of intake of flavonoids in our study was similar to that in the Zutphen Elderly Study, except that the percentage of flavonoids obtained from tea was only 25% in our study and 61% in the Zutphen Elderly Study. We previously reported that no appreciable association existed between intake of tea and risk for coronary heart disease [24], and this finding persisted after further follow-up. Of the primary contributors to total flavonoid intake in the Zutphen Elderly Study, only tea had a significant inverse association with death from coronary heart disease (none of these foods were significantly associated with reduced incident myocardial infarction). Mean daily intake of tea in our study was less than 0.5 cups; in the Zutphen Elderly Study, it was 3 to 4 cups. However, we found no inverse association between tea consumption and death from coronary heart disease, even when 2 or more cups were consumed per day. Constituents of tea other than flavonoids may contribute to the reduction in risk for coronary heart disease [32]. Alternatively, tea consumption may serve as a marker for other health-conscious behaviors.

Certain flavonoids, including quercetin, can be strong antioxidants. Flavonoids may preserve vitamin E in low-density lipoprotein cholesterol [7] and thus may be cardioprotective only in populations with low intake of vitamin E or with intake of flavonoids that is much lower than the intake reported in our study [12].

In summary, our data do not support an important inverse association between the flavonoids studied and the incidence of coronary heart disease. We did find that flavonols may be inversely associated with death from coronary heart disease among men with prevalent coronary heart disease. Alternatively, such an association might be explained by bias or confounding attributable to the severity of prevalent disease and recent changes in lifestyle. Currently, intake of flavonoids at the levels typically consumed in the United States cannot be considered an established protective factor for coronary heart disease.

Dr. Katan: Department of Human Nutrition, Agricultural University, Bomenweg 2, 6703 HD Wageningen, the Netherlands.

Dr. Stampfer: Channing Laboratory, 180 Longwood Avenue, Boston, MA 02115.