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FIBER and CARDIOVASCULAR DISEASE

Abstract:  A change in dietary Fiber of 1% of total calories reduces serum cholesterol by about 1.2 mg/dl.  This change in serum cholesterol explains how dietary fiber reduces risk of cardiovascular disease (CVD).  Soluble and insoluble fiber produce near equivalent benefits in reducing risk of CVD.

Dietary Fiber reduces risk of CVD.  The research results in Table F following that summarize results from the principal available studies of  Fiber on heart disease and stroke confirm this. Omitted here are results from a few much smaller studies that in aggregate found a similar result.  A problem noted was a usual relation between higher dietary Fiber and generally better health habits among individuals.  Including a modest adjustment for this, a mean adjusted risk ratio from the available research is about 0.73 for an average difference of about 15.5 grams per day in intake of Fiber. This result appears reasonable for both Heart Disease and Stroke. Thus each added gram per day of Fiber reduces CVD risk by a ratio of 0.980.  Note that added Fiber also reduces risk of Cancer.  

The relative merits of soluble and insoluble Fiber have been a subject of much discussion.  A broad review of the existing research on this by Kushi  (Am J Clin Nutr 1999,70:451S)  summarized the available results on coronary risk by type of Fiber.  The average Coronary disease risk ratios for total Fiber, soluble Fiber, and insoluble Fiber obtained in this review were 0.72, 0.82, and 0.76 respectively.  Soluble Fiber accounts for only about 20% of total Fiber.  Because both this and the fact that the available data do not identify a very significant difference in benefit from the two fiber types, an assumption in Life Ahead is that that all types of Fiber contribute similarly.

It long has been recognized that dietary Fiber can reduce serum cholesterol substantially.  Thus a key question becomes “Does Fiber produce its benefits by this mechanism?”   A comprehensive meta analysis of the results of 67 different controlled trials by Brown, (Am J Clin Nutr 69:30) showed via plots of this data that a change of 32 grams per day in overall Fiber reduced total and LDL cholesterol by 1 mmol/l or 39 mg/dl.  This is a reduction of 1.2 mg/dl of cholesterol per added gram per day of Fiber.

This change in serum cholesterol using Life Ahead formulas relating cholesterol to heart disease will account for a 0.984 risk ratio on coronary heart disease from each added gm of Fiber at age 50.  This equates to a risk ratio of 0.77 for the average 15.5 grams/day involved. (The Life Ahead formula of cholesterol and risk is in turn based results from the very large MRFIT study.) This risk ratio is quite close to the above actually measured effect of a risk ratio of 0.980 per gram of Fiber intake and a ratio of 0.73 for this same amount of Fiber. It thus seems likely that the benefit of dietary Fiber results nearly all or entirely from its effect in reducing cholesterol.

The Brown analysis found that Dietary Fiber also reduces HDL but to much smaller extent than that for total cholesterol at 0.077 (0.01-0.13) mg/dl per gm/day of Fiber. This is in agreement with the present assumption in Life Ahead that dietary factors do not change the concentration of or percentage of HDL in total cholesterol appreciably. 

The formula for CVD risk from Fiber now included in Life Ahead modifies the forecast level of total serum cholesterol with a correction to total cholesterol of  -1.2 mg/dl per gram of Fiber per day.  The correction is taken vs. the average population value of zero effect at a dietary intake of 18 grams of total fiber per day.  This value is well within the error margin of the Brown study and forecasts closely the average actual measured benefit of Fiber on Cardiovascular Disease from results in Table F.

The most acceptable approach today for increasing Fiber is via diet.  See the library values in Life Ahead for Fiber sorted in order of descending amounts of Fiber, and compare these values with those now in a usual diet. Cereals top the list for Fiber, but many other foods can contribute.  Several Fiber enhancing supplements are available and are widely advertised.  Psyllium can increase Fiber appreciably, and has been shown to reduce serum cholesterol.  Thus it probably will help in reducing risk of cardiovascular diseases.  But no actual direct research data were found confirming that a Fiber supplement actually reduces risk of either cardiovascular disease or cancer.

A problem here, however, is that confirming data on this are unlikely to become available.  Fiber probably protects via reducing the extent of atherosclerosis and thus will produces its benefit mostly after 10-20 years of duration of use.  Thus no practical clinical type study will be able to show an effect within a practically useful time period.