Dietary fiber is prominent in recommendations for prevention of both coronary heart disease and cancer. The National Cancer Institute gives primary emphasis to dietary fiber in its recommended food choices. A range of intake of 20 to 30 g/d is suggested for US adults.1 The National Heart, Lung, and Blood Institute, through its National Cholesterol Education Program, emphasizes reduction in fat intake and compensatory energy replacement through the increased intake of other foods, including those with high fiber content. However, no quantitative target is provided for fiber intake.2 The report by Pietinen et al3 of the Alpha-Tocopherol Beta-Carotene (ATBC) Study in this issue of Circulation provides new epidemiological evidence that should have an effect on dietary recommendations for prevention of coronary heart disease.
What is dietary fiber, and how has its role been investigated? Much of the research on dietary fiber through the 1980s is reviewed in Diet and Health: Implications for Reducing Chronic Disease Risk, the encyclopedic 1989 report of the National Research Council.4 Dietary fiber is plant material consisting of nonstarch polysaccharides and lignins (polymers of phenylpropane residues), which are resistant to digestion by enzymes secreted in the human alimentary tract. Components of dietary fiber may be characterized by their specific chemical structures, their properties of solubility or insolubility, and the foods in which they are found. Food composition data for fiber are difficult to compile because of the complexity of their chemical properties and variations in available analytic methods. Quantification of fiber intake in the usual diet has not, therefore, been possible until very recently, and it remains somewhat tentative due to these limitations. Nevertheless, there is agreement that the principal food sources for fiber in general are cereals, fruits, and vegetables, with several foods providing multiple types of fiber.
How much fiber is consumed by US adults? The mean value of fiber intake estimated from the Second National Health and Nutrition Examination Survey (NHANES II, 1980 to 1984) was 11.1 g/d for both men and women.4 The more recent report from NHANES III (Phase 1, 1988 to 1991) gives mean values of 16.6 to 20.0 g/day for men and 12.5 to 14.7 g/day for women in age groups of 20 to 29 through 80 and older. The apparent increase in intake reported in the NHANES III data and the differences in fiber intake among ethnic groups (least among non-Hispanic blacks, most among Mexican Americans, and intermediate in non-Hispanic whites) are noteworthy.5
Why have population studies of dietary fiber and health been difficult? Several potential problems in such studies are emphasized in the National Research Council report, including (1) interactions among components of fiber, so that effects may be dependent on the particular fiber composition of a given diet; (2) correlation of fiber intake with that of total energy and fat, so that these aspects of diet must also be taken into account; (3) modification of the effects of fiber by other dietary components, such as fat intake, so that only certain subgroups with distinct dietary characteristics may exhibit a given effect of fiber intake; and (4) lack of a sufficient range of fiber intake in some populations for associations to be found.4 To these may be added general considerations about the practicalities of dietary assessment in population studies, such as their validity and reliability, and the relation in time between the reference period of the dietary history and the onset of the disease process. These factors led the National Research Council reviewers to state that “the evidence for a protective role of dietary fiber per se in CHD . . . [and other conditions] is inconclusive.”4 Their recommendations for further research included epidemiological studies with adequate sample size, sufficient interindividual variation in fiber intake, improved dietary assessment, and analysis of each of the major components of dietary fiber.
In view of this background, what does the ATBC Study contribute to the evidence concerning dietary fiber and the risk of coronary heart disease? The ATBC Study was a large clinical trial of α-tocopherol and β-carotene supplementation in the prevention of lung cancer among >29 000 male smokers age 50 to 69 years in southwestern Finland. The study design, population characteristics, dietary methods, and trial results were reported previously; a recently developed nutrient database for Finland was used to obtain an estimation of intake of specific components of dietary fiber.6 7 8
For the present analysis, the total ATBC Study population was considered to be a cohort, regardless of intervention groups. The risk of subsequent coronary events was evaluated in relation to the history of dietary fiber intake over the year before study entry, as obtained through previously validated methods.7 All men were smokers. After appropriate exclusions were made, 21 930 of the 29 133 men randomized into the trial were included for analysis. Determination of nonfatal myocardial infarction (specifically, ICD code 410) was based on the National Hospital Discharge Register and that for fatal events (ICD codes 410–414) was based on the Central Population Registry. Over the total follow-up period (median duration, ≈6 years), 1399 “major coronary events” (fatal and nonfatal combined) were identified, including 635 coronary deaths. Relative risks for successive quintile groups by intake of fiber, as well as trends for continuous risk estimates, were presented after adjustment for energy intake (ie, g/d per 1000 kcal of total energy). Multiple components of fiber were addressed: total fiber, soluble fiber, insoluble fiber and its components (insoluble noncellulosic polysaccharides, lignin, and cellulose), and food sources of fiber (cereal, vegetables, and fruits). For coronary deaths, further analysis was provided for rye products, other cereal products, potatoes, vegetables, and fruit and berries.
The main result of this analysis of the ATBC Study was lower risk estimates for coronary death with higher prestudy levels of dietary fiber intake after adjustment for age, treatment group, smoking, body mass index, blood pressure, education, physical activity (at leisure), and intakes of energy, alcohol, saturated fatty acids, beta-carotene, and vitamins C and E. For example, the highest quintile group for total fiber intake, relative to the lowest quintile group, had twice the intake (34.8 versus 16.1 g/d) and exhibited an adjusted relative risk of 0.73 (95% confidence limits, 0.56 to 0.95) and a significant linear trend over all levels of intake (P=.004). Total mortality was also inversely related to total fiber intake (P for trend, .006), and other nutrients were found to be less strongly associated with coronary events than was fiber (results for fatty acids are to be reported separately) (P. Pietinen, personal communication, 1996).
The extensive analysis in this study provides a great deal of information for readers of the report—much food for thought. Points deserving special comment include the following:
1. This study conforms closely to the features proposed by the National Research Council report for more definitive studies in its sample size, range of fiber intake, quality of dietary assessment and underlying nutrient data base, and detailed consideration of specific components of dietary fiber.4 In these respects, this study represents an important addition to the available evidence on fiber intake and the risk of coronary heart disease, as does a recent report by Rimm et al9 on the Health Professionals Follow-up Study.
2. Analyses contrasting the highest and lowest quintile groups of fiber intake revealed differences in other risk factors for coronary heart disease: The highest quintile group for total fiber intake smoked less heavily, consumed less fat (total and saturated fat) and cholesterol, and had higher levels of leisure physical activity and intakes of beta-carotene and vitamins C and E. Intake of rye products was fivefold higher and intake of fruit and berries was twofold higher in this group; notably, alcohol intake was lower in this group, being one third that of the lowest fiber quintile group. This underscores the caution expressed by Wynder et al10 in commenting on the Health Professionals Follow-up Study, which had similar findings although there were many differences in lifestyle, and dietary patterns of fiber intake differed widely among groups.
3. The gradient in coronary heart disease risk in relation to fiber intake is summarized from a linear model in terms of 10-g/d increments, corresponding to a 17% decrement in risk. However, inspection of the many quintile group analyses in the ATBC Study shows that lower risk is often limited to the highest or next-to-highest level of fiber intake. Thus, not “every” increment from the lowest intake is accompanied by lesser risk. A 10-g/d increment in fact represents the difference in median total fiber intake from that of the very lowest to that of the third-to-fourth quintile group in the ATBC Study, a relatively large part of the reported range of intake. This quantitative result is strikingly similar to that of the Health Professionals Follow-up Study, with an increment of 10 g/d corresponding to a 19% decrement in coronary risk. However, in that study, the 10-g gradient represented the entire difference in average intake between the first and the fifth quintile groups.9 Relatively large differences in fiber intake, then, are required to demonstrate substantial differences in the risk of coronary heart disease.
4. It is of interest that the relation of fiber intake to coronary heart disease in the ATBC Study is stronger for fatal events than for nonfatal ones, as inferred from the weaker associations for all major events relative to those for coronary deaths. No explanation of this observation is proposed. It is plausible that definite nonfatal events were selectively determined over only possible coronary events in the ATBC Study, which relied on ICD 410 for case identification. Humble et al11 noted in reporting on the Lipid Research Clinics/Coronary Primary Prevention Trial analysis of fiber intake in the placebo group of that study that “suspect events” were approximately twofold as strongly related to fiber intake as definite events (fatal and nonfatal events were not distinguished in the report). Thus, underdetermination of possible versus definite nonfatal coronary events might be expected to weaken the apparent association with fiber intake. In the Health Professionals Follow-up Study, the risk of nonfatal coronary events was significantly less in the highest versus the lowest quintile group of total fiber intake, although somewhat weaker than for fatal events, with relative risks of 0.65 (0.49 to 0.88), and 0.45 (0.28 to 0.72), respectively.9
5. The components of fiber and food sources of fiber that contribute most importantly to the difference in risk remain undetermined. Soluble fiber and rye products stand out in the ATBC Study analysis, but characteristics of the Finnish diet—with rye products, containing both soluble and insoluble fiber, being a major source—limit the ability to separate specific components, despite their detailed characterization in this study (P. Pietinen, personal communication, 1996).
6. Several mechanisms have been suggested by which higher fiber intake could reduce the risk of coronary heart disease.4 The ATBC Study reported the association of soluble fiber intake and coronary heart disease to be independent of serum cholesterol concentration. Rimm et al9 suggested that lower fiber intake accounted for the observed association between saturated fat intake and coronary heart disease in the Health Professionals Follow-up Study, a point with which Wynder et al10 disagreed. The complexity of relations among dietary fiber, other aspects of dietary patterns, and other risk factors poses difficulties in disentangling specific components, and the results may be different in different populations. In any case, much more explicit presentation of data analysis would be necessary to convincingly support such arguments.
7. Recommendations to increase dietary fiber intake for the prevention of coronary heart disease are based largely on evidence from epidemiological studies, which are supported by laboratory and clinical investigation but lack data from intervention trials. The present study is a major contribution to the population data. Concern that findings in a population of Finnish smokers might have limited generality is diminished by the similarity of findings in US health professionals, although the particularities of dietary behavior among different populations must be borne in mind. The present results reinforce the existing recommendations to increase fiber intake; they suggest that levels of intake similar to those in the higher-exposure groups in these latter studies may be optimum and support a target at least as high as that of the National Cancer Institute recommendations, in the 20 to 30 g/d range.1 More definitive recommendations, and possibly stronger support, will await intervention studies and perhaps further examples of large, well-designed, and well-conducted studies in populations with increasingly diverse dietary patterns.
The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
- Copyright © 1996 by American Heart Association
Office of Communications, National Cancer Institute. Diet, Nutrition & Cancer Prevention: The Good News. Bethesda, Md: US Department of Health and Human Services, Public Health Service, National Institutes of Health, National Cancer Institute; NIH publication No. 87-2878, 1986.
National Cholesterol Education Program Expert Panel on Population Strategies for Blood Cholesterol Reduction. Report of the Expert Panel on Population Strategies for Blood Cholesterol Reduction. Bethesda, Md: US Department of Health and Human Services, Public Health Service, National Institutes of Health, National Heart, Lung, and Blood Institute, National Cholesterol Education Program; NIH publication No. 93-3046, 1993.
Pietinen P, Rimm EB, Korhonen P, Hartman AM, Willett WC, Albanes D, Virtamo J. Intake of dietary fiber and risk of coronary heart disease in a cohort of Finnish men: the ATBC Study. Circulation.. 1996;94:2720-2727.
Committee on Diet and Health, Food and Nutrition Board, Commission on Life Sciences, National Research Council. Diet and Health: Implications for Reducing Chronic Disease Risk. Washington, DC: National Academy Press; 1989.
Alaimo K, McDowell MA, Briefel RR, Bischof AM, Caughman CR, Loria CM, Johnson CL. Dietary intake of vitamins, minerals, and fiber of persons ages 2 months and over in the United States: Third National Health and Nutrition Examination Survey, Phase 1, 1988-1991. Advance Data.. 1994;258:1-26.
Pietinen P, Hartman AM, Haapa E, Rasanen L, Haapakoski J, Palmgren J, Albanes D, Virtamo J, Huttunen JK. Reproducibility and validity of dietary assessment instruments. Am J Epidemiol.. 1988;128:655-666.