Fiber, Lipids, and Coronary Heart Disease

What Is Dietary Fiber?

The Life Sciences Research Office of the Federation of American Societies for Experimental Biology defines fiber as “the endogenous components of plant materials in the diet which are resistant to digestion by enzymes produced by humans.”⁴ Simply put, dietary fiber, including cellulose, hemicelluloses, pectin, and lignin, comes from the cell walls of plants plus other indigestible components of plants. Much of the difficulty in defining specific types of fiber is related to differences in analytical methodology used to quantify it.

Only as recently as the late 1970s was it recognized that fiber could be characterized by its solubility in the intestines.^{5 6} Nutrient tables developed before 1980 contain primarily crude fiber values. Crude fiber is measured after an exhaustive extraction of foods and does not provide an accurate estimate of the actual dietary fiber content of foods. In the United States the method developed by Prosky et al⁷ has been accepted by the Association of Analytical Chemists, is widely used, and provides a reliable estimate for total fiber content of foods. In the United Kingdom a method for determining nonstarch polysaccharides has been developed and is commonly used for assessing fiber content. These methods yield some slight differences in total fiber content, but they are relatively close in their estimation and far more accurate than the crude fiber assays of old.

One of the physical characteristics of polysaccharides is the ability to swell and hold water within a matrix. Fibers with a high water-holding capacity, such as pectin, gums, and psyllium, have been referred to as soluble fiber. Because it was presumed that defining the characteristics of fibers by their soluble and insoluble classification could facilitate distinction in biological responses, methods were developed to measure each of these fiber fractions. Improved methods for soluble fiber determination have resulted in increased accuracy regarding content, but it has since become evident that the physiological response to fiber sources based on this measurement is not necessarily predictable on the basis of solubility alone. Other factors, such as fermentability, viscosity, and bile acid binding ability, also contribute to the physiological response to fiber sources. Additionally, since all foods contain a mixture of polysaccharides, only isolated polysaccharides can be simply classified as soluble or insoluble fiber sources.

Despite the potential confounding of these classifications, both soluble and insoluble forms of fiber are important and appear to have different health properties in at least some settings.^{5 6 8} The AHA dietary guidelines for Americans³ emphasize the importance of consuming a variety of fiber sources to obtain the different types of fibers found in foods. Fiber is important for gastrointestinal health as well as cholesterol-lowering benefits. Foods containing fiber are good sources of several other essential nutrients, and, depending on the method of preparation, these foods are typically low in fat, saturated fatty acids, and cholesterol as well.

Sources of Dietary Fiber

Because of these limitations in chemical assessment methodology, quantifying dietary fiber composition of the food supply has historically been difficult and limited to crude fiber only. Within the past decade food composition databases have reflected technological advances by listing specific values for total, soluble, and insoluble dietary fiber. Wheat, rye, rice, and most other grains are primarily composed of insoluble fiber.^{9 10} Oats have a greater proportion of soluble fiber than any other grain. Barley offers both but is mainly insoluble fiber. Legumes, beans, and peas are also excellent sources of both soluble and insoluble fiber, with almost 7 g of total fiber and 2 g of soluble fiber in a one-half cup serving. Certain fruits and vegetables are better sources of both soluble and insoluble fiber than others. Usually more of these foods must be consumed to equal the fiber contributions of either beans or cereal grains. Table 1⇓ provides some examples across food groups. Choosing foods from all these groups ensures adequate intake of both types of fiber in a ratio of about 1 part soluble to 3 parts insoluble fiber. Numerous other nutrients are contributed by these foods as well, usually without adding fat or excessive calories.

Mechanisms of Serum Cholesterol Reduction

Specific mechanisms involved in serum cholesterol reductions observed with increased fiber intake remain somewhat inconclusive. Recent research provides evidence that viscous polysaccharides act in the gastrointestinal tract to reduce blood cholesterol by decreasing absorption of cholesterol or fatty acids and decreasing absorption of biliary cholesterol or bile acids.^{11 12} Fiber may also cause altered serum concentration of hormones or short-chain fatty acids that affect lipid metabolism. β-Glucan, the water-soluble fiber prevalent in oats and barley, has been shown in animal models to be the active agent causing the altered cholesterol metabolism.¹³

On the basis of results of studies of oats and barley, other proposed mechanisms include the influence of ∝-tocotrienols and compounds that have vitamin E activity and produce hepatic HMG-CoA (hydroxymethylglutaryl coenzyme A) reductase inhibition in animal models, but this has yet to be conclusively documented.¹⁴ It has been further suggested that the amino acid content of oats and the arginine-lysine ratio may also promote the hypocholesterolemic response.^{15 16} Further studies in humans are needed to delineate and quantify these mechanisms across different fiber sources and under different biological conditions.

Fiber Intake and Mortality

Several longitudinal observational studies have reported significant inverse associations between total fiber intake and both cardiovascular and all causes of mortality.^{17 18} Part of this phenomenon may reflect the accompanying inverse association that is often observed between fiber and fat intake when calories are controlled.¹⁹ For example, in the Zutphen Study,¹⁸ men in the lowest quintile of dietary fiber intake exhibited a four times higher rate of CHD mortality compared with men in the highest quintile, even though total caloric intake was about the same. Numerous studies comparing vegetarians and nonvegetarians have reported lower levels of serum cholesterol and lower mortality rates in the former, but whether fiber is the active agent involved or simply a reflection of greater intake of complex carbohydrates and less saturated fat is not easily deciphered.^{20 21 22}

Other epidemiological evidence is less consistent. The rate of CHD mortality was reported to be inversely associated with fiber intake across 20 industrialized nations, but adjustment for fat intake removed the association.²³ Similarly a 20-year cohort study of 1001 middle-aged men in Ireland and Boston reported significant inverse association between fiber intake and risk of CHD, but the association diminished when other risk factors were controlled.²⁴ In a 12-year follow-up study of 859 men and women aged 50 to 79 years, a 6-g increment in daily fiber intake was associated with a 25% reduction in ischemic heart disease mortality, independent of calories, fat, and other dietary variables.²⁵ A recent study among 850 men in the Yi province of China reported that lower serum cholesterol and blood pressure levels were associated with higher intakes of fiber from oats and buckwheat.²⁶ Total fat and dietary cholesterol intakes were also significantly lower in those with the highest fiber intakes, but caloric intakes were similar across all fiber groups. These studies and others illustrate the complexity of measuring the independent impact of fiber on lipids and/or mortality rates within the limitations of available diet assessment methodology, disparate food composition data, and the difficulty of controlling confounding factors.²⁷

Results of Clinical and Metabolic Studies

A growing number of metabolic research studies have reported total cholesterol reductions of 10% to 15% with diets enriched with fiber from oats,^{28 29 30} beans,^{31 32} or psyllium,³³ but these diets were also reduced in fat. Other studies have investigated adding supplements of pectin and guar gum with subsequent cholesterol reductions of 10% or more, but gastrointestinal side effects were more common.^{19 34 35}

In the past decade more than 30 clinical studies have evaluated the impact of oats and other fiber-rich foods as part of Step I or similar fat-modified diets in outpatient, free-living, and controlled settings. The majority of studies report the greatest lipid-lowering benefits occur among persons with elevated baseline cholesterol levels. A meta-analysis of pooled data from 13 randomized, controlled trials with baseline and follow-up dietary data further controlled for the impact of the fat-modified diet alone. Fiber from two servings of oats enhanced cholesterol reduction by an additional 2% to 3% beyond what was achieved by fat modification.³⁶

Some studies on dietary fiber and lipid response have reported equivocal results.^{37 38 39} In its review of evidence to determine whether to approve a food manufacturer’s petition for a health claim that links increased oat fiber intake with cholesterol lowering, the Food and Drug Administration ⁴⁰ applied rigorous evaluation criteria to all available data. Studies that did not support the soluble fiber lipid-lowering relation were often criticized for small sample sizes, inadequate dietary data to evaluate adherence to the recommended diet, and/or lack of standardized sources of fiber. Of all the studies reviewed, the majority favored the association with doses ranging from 34 g total dietary fiber (2.5 g soluble fiber) to 123 g (10.3 g soluble fiber). ⁴⁰ A dose-response study specifically designed to evaluate this question among hypercholesterolemic persons found maximal lipid lowering was achieved with 56 g versus 84 g of oat bran, suggesting a possible threshold effect.¹⁵ Fifty-six grams of oat bran is equivalent to approximately two servings (two-thirds cup dry).

Studies of fiber supplements containing psyllium have reported greater reductions of 15% in LDL-cholesterol levels as part of the usual American diet and 9% as part of a Step I diet.³³ More recently a fiber supplement containing a mixture of guar gum, pectin, soy fiber, pea fiber, and corn bran lowered LDL cholesterol by 7% to 8% in hypercholesterolemic participants after 15 weeks compared with those taking a placebo.⁴¹ These reductions persisted throughout the 51-week follow-up period with continued use of supplements. Potential risks of excessive use of fiber supplements include reduced mineral absorption and a myriad of gastrointestinal disturbances. Fiber from natural dietary sources is preferred to avoid these problems and supply numerous other nutritional benefits. A fiber supplement added to a diet otherwise high in saturated fat and cholesterol provides dubious cardiovascular advantages. Indeed, such an approach can be detrimental if it instills a false sense of security that precludes further attention to other aspects of the diet, such as high saturated fat intake.

Observational epidemiological evidence consistently demonstrates lower incidence of CHD and other long-term diseases among those with the highest intake of fruits, vegetables, and grains.^{25 42 43} Such a dietary pattern appears to offer protective effects that transcend lipid lowering and overall is typically lower in total fat, saturated fat, and cholesterol.

Effects of Fiber on Other Risk Factors

Inverse associations between fiber and blood pressure have also been reported.^{44 45 46} Some intervention studies among hypertensive and normotensive individuals have reported reductions in blood pressure in response to increased fiber intake, but these results are not conclusive.^{8 47 48} Confounding factors in these studies include obesity, use and amount of antihypertensive medication, and comorbidity. A vegetarian diet appears to induce blood pressure reduction in hypertensive individuals, but independent effects of dietary fiber have yet to be elucidated.

Similarly, high-fiber vegetarian diets have also been associated with reduced risk of obesity.^{8 20 49 50} It has been hypothesized that high-fiber foods may favorably impact satiety and slow gastric emptying, thereby sustaining a feeling of fullness that prohibits overeating.^{6 8} Intake of high-fiber foods may also improve glycemic control in diabetic individuals and reduce risk of insulin resistance.^{1 51} Clinical trials to address each of these factors and isolate independent effects of fiber on weight control versus glucose metabolism are needed.

Fiber and Children’s Diets

As a preventive strategy, it has been recommended that children older than 2 years should gradually adopt the Step I diet, reducing total and saturated fat intake to 30% and 10% of total calories, respectively. Children should also derive the majority of calories from complex carbohydrates.² Such a diet inherently offers the benefits of increased fiber and nutrient intake from a host of dietary sources as well, but lingering concerns about reduced energy intake and compromised growth have prompted suggested fiber guidelines from the pediatric community. It has been proposed that the “age plus 5” rule be applied when determining the appropriate amounts of dietary fiber for young children.⁵² This means, for example, a five-year-old should consume 5+5=10 g of fiber per day. This should be easy to accomplish within the boundaries of the Step I diet. Once a child’s caloric intake approaches that of an adult, ie, 1500 calories or more, 25 total grams should be well tolerated.

Conclusions

The greatest impact on lowering total and LDL cholesterol is derived from reduced intakes of saturated fat and cholesterol as well as weight reduction in obese persons. Diets high in complex carbohydrates and fiber are associated with reduced mortality rates from CHD and other chronic diseases. Fiber found in oats, barley, and pectin-rich fruits and vegetables provides adjunctive lipid-lowering benefits beyond those achieved by reductions in total and saturated fat alone. The AHA recommends a total dietary fiber intake of 25 to 30 g/d from foods, not supplements, to ensure nutrient adequacy and maximize the cholesterol-lowering impact of a fat-modified diet. Current dietary fiber intakes among adults in the United States average about 15 g, or half the recommended amount.⁵³