One of these beliefs is that high fiber diets will reduce the risk of heart attacks (myocardial infarcts, MI) and other heart diseases(see Mozaffarian et al for references). This belief comes from a paper by Rimm et al (1996) who claimed to demonstrate such an inverse relationship. This seemed to be a reasonable assumption and one I would like to believe, but after analyzing the data I am not convinced.
In the study by Rimm et al (1996) a total of 43,757 male health professionals 40 to 75 years of age were studied for 6 years. During this time 734 cases of myocardial infarction were documented and these were analyzed with respect to dietary intake of fiber. The results of this study are shown in Figure 1. You can see that no significant effect on MI is seen in any group but group 5 which was the highest intake category (28.9 grams/day).
So if you are willing to spend most of you time eating fiber containing foods all day you might be less likely to have a heart attack. However, anytime you have data points which suddenly go from insignificant, as in category 4, to significant, as in category 5 you should worry that the value in category 5 may be a fluke.
Figure 1. Relative risk of myocardial infarct (MI) and dietary intake of fiber.
Each intake category represents increasing intake of total dietary fiber. The data points are relative risks and 95% confidence intervals taken form the data in table 2 of Rimm et al 1996. The quantity of fiber in the diet increases from 1 to 5.
The next study from this same group involved 39,876 women health professionals who were followed for 6 years (Liu et al 2002). The incidence of cardiovascular disease (CVD) and MI were examined in this study and their results for total dietary fiber intake are shown in Fig 2. No significant associations were observed between total fiber intake and CVD (Fig 2 A), and the women in category 4 showed marginal significant decrease in MI (Fig 2 B). Since all other RR are insignificant these data do not support the contention that dietary fiber intake is associated with a decreased risk of CVD or MI.
Figure 2. A. Relative risk of cardiovascular disease (CVD) and B. myocardial infarct (MI) as a result of dietary intake of total fiber. Data taken from tables 3 and 4 in Liu et al 2002.
These authors also examined the association between the various sources of dietary fiber. If any type of fiber would show a protective effect one would think it would be vegetable fiber. Think again. The data in Figure 3 demonstrated that no decrease or increase in relative risks of CVD or MI result from the consumption of various amounts of vegetable fiber.
Figure 3. Relative risk of cardiovascular disease (CVD) and myocardial infarct (MI) as a result of dietary intake of vegetable fiber. Data taken from table 3 and 4 in Liu et al 2002.
If no associations could be shown with total fiber or vegetable fiber, then surely these would be found with soluble fiber. After all, soluble fiber was thought to be primarily responsible for the cholesterol lowering effect of dietary fiber, hence one would expect great things. Several studies claimed this to be true and were summarized in a meta-analysis by Brown et al 1999. However, if you read this paper carefully you would not expect much effect from soluble fiber. Brown et al 1999 found that the concentrations of total cholesterol and LDL cholesterol in the blood were decreased marginally and no effects on HDL cholesterol and triglycerides were found. The reduction of total cholesterol and LDL cholesterol were minus 0.045 mM ( 0.9%)and 0.057 mM ( 2.7%) respectively which are very small and probably physiologically insignificant effects. The authors point out that increasing soluble fiber intake can make only a small contribution to lowering plasma cholesterol. Little did they know, the effect was not just small but it was without consequence as a factor in decreasing the risk of CVD or MI (Figure 4).
Figure 4 . Relative risk of cardiovascular disease (CVD) and myocardial infarct (MI) as a result of dietary intake of soluble fiber. Data taken from tables 3 and 4 in Liu et al 2002.
Other types of fiber which were included in this study were cereal fiber, fruit fiber and insoluble fiber. These produced no significant reduction in the risk of CVD or MI.
Try as they might, all of the kings men could not put the story of fiber and heart problems together again. No evidence of a protective effect on CVD or MI from eating even very large quantities of any kind of fiber was found in these studies.
Brown L, Rosner B, Willett WW, Sacks FM 1999 Cholesterol lowering effects of dietary fiber: a meta-analysis. Am J Clin Nutr 69:30-42.
Liu S et al 2002 A prospective study of dietary fiber intake and risk of cardiovascular disease among women. J Am Coll Cardiol 39:49-56.
Mozaffarian et al 2006 Trans fatty acids and cardiovascular disease. NEJM 354:1601-1613.
Rimm EB et al 1996 Vegetable, fruit and cereal fiber intake and risk of coronary heart disease among men. JAMA 275: 447-51/