February 2011 Issue

Dietary Fiber — New Insights on Health Benefits
By Rita Carey Rubin, MS, RD, CDE
Today’s Dietitian
Vol. 13 No. 2 P. 42

Dietary fiber has long been associated with health benefits related to gastrointestinal function, weight maintenance, and satiety. Fiber’s benefits, however, are not likely limited to the alimentary movement and processing of food. In fact, significant research suggests that fiber may help prevent the development of seemingly unrelated conditions, including cardiovascular disease (CVD), inflammation, and osteoporosis.

CVD
Many studies have demonstrated an inverse relationship between the intake of dietary fiber and the risk of developing CVD. Although scientists cannot fully explain fiber’s cardioprotective effects, they have considered numerous mechanisms. Fiber likely has a direct physiologic effect on serum cholesterol, satiety, and postprandial glucose levels as well as more indirect effects on thrombosis, vascular inflammation, and body weight. Most researchers agree that fiber reduces the risk of CVD, but further studies are needed to understand all of the complex physiologic mechanisms involved.

Hyperlipidemia
The effect of dietary soluble fiber on serum cholesterol levels has been extensively documented and promoted (likely second only to fiber’s effect on laxation). Indeed, today’s aggressive marketing of products containing significant amounts of soluble fiber has made this relationship familiar to most people. A 2000 review of clinical research on dietary fiber as it relates to CVD cites numerous clinical trials investigating the effects of oat fiber, psyllium seed, guar gum, and pectin on serum lipid levels.1 In these studies, intakes of 9 to 16.5 g/day of a variety of soluble fibers (mostly psyllium and guar) produced net reductions in serum total and LDL cholesterol levels of 5.5% to 11% and 3.2% to 12.1%, respectively.

Soluble fibers are known to bind with bile acids in the small intestine, thereby removing them from the body and reducing the rate of bile acid recycling. The loss of bile acids in the stool stimulates the liver to increase cholesterol uptake from the circulation to replenish the bile acid supply. As a result, concentrations of serum total and LDL cholesterol are reduced, while HDL cholesterol and triglycerides are generally unaffected.1,2 Fiber may also indirectly inhibit the hepatic synthesis of cholesterol. Certain soluble fibers belonging to a class of compounds called oligosaccharides are fermented (in the lower gut) into short-chain fatty acids (SCFAs) and gases. When SCFAs enter the circulatory system, they may inhibit the liver from producing cholesterol and consequently have a direct hypocholesteremic effect.1,3

Insulin Resistance
A strong correlation exists between insulin resistance, hyperinsulinemia, and the development of risk factors for CVD.1,4 In this context, fiber may reduce CVD risk by modulating postprandial plasma glucose and insulin concentrations.1 Gerald M. Reaven, MD, an endocrinologist and professor emeritus in medicine at the Stanford University School of Medicine, has written multiple scientific papers on the connection between insulin resistance and CVD, including one published in 2006 in the American Journal of Clinical Nutrition. He argued that the metabolic syndrome, a cluster of conditions that predispose individuals to CVD, should more accurately be named “insulin resistance syndrome” because of the negative effects that elevated plasma concentrations of insulin have on cardiovascular health. According to Reaven, the combination of insulin resistance and compensatory hyperinsulinemia increases the likelihood that an individual will develop a plasma lipid profile typical of the metabolic syndrome: elevated plasma triglyceride and very–low-density lipoprotein levels along with depressed HDL levels. Elevated insulin levels may also stimulate the production of plasminogen activator inhibitor-1, a prothrombotic molecule that is positively correlated with the risk of CVD. Reaven and others also cite evidence that hyperinsulinemia may increase plasma concentrations of adhesion molecules (which bind leukocytes to the vascular wall in the beginning stages of atherogenesis) and promote the development of essential hypertension (via activity in the sympathetic nervous system and enhanced renal sodium reabsorption).1,4

Inflammation
Fiber may promote cardiovascular health by inhibiting vascular inflammatory processes. Inflammation has been implicated in nearly every phase of atherogenesis—from disease initiation and the movement of LDL cholesterol to macrophages in the vascular wall to the eventual rupture of unstable plaque, resulting in heart attack or stroke.5 C-reactive protein (CRP) is one of several inflammatory compounds formed during atherogenesis. Once considered too broad and nonspecific to predict future cardiovascular events, CRP is now an accepted biomarker of CVD risk.5,6 Data also suggest that CRP may play a direct role in the development of CVD. In this regard, CRP may aid the movement of leukocytes to the vascular wall and mediate the transfer of LDL into macrophages.5

Numerous studies have revealed positive correlations between dietary fiber intake and levels of circulating CRP. Data from the Seasonal Variation of Blood Cholesterol Levels Study, published by Ma et al in 2006 in the American Journal of Clinical Nutrition, found inverse associations between total dietary fiber, soluble fiber, insoluble fiber, and CRP in a cohort of 641 individuals followed for one year. These findings were consistent with an earlier study by Ajani et al, published in 2004 in the Journal of Nutrition, that used cross-sectional data from the 1999-2000 National Health and Nutrition Examination Survey (NHANES) to evaluate the relation between fiber and CRP. Publishing in the American Journal of Cardiology in 2003, King et al also examined 1999-2000 NHANES data and found that after controlling for important confounders (eg, age, sex, body weight, physical activity), fiber stood out as the sole nutritional component negatively associated with levels of CRP. Note that in this assessment the researchers found no relationship between CRP and total fat, saturated fat, cholesterol, fish, or total protein intake.

Osteoporosis
Significant quantities of cereal fiber have been associated with reduced absorption of several minerals, including iron, zinc, calcium, and magnesium. However, depressed nutrient absorption is primarily attributed to the phytate content of cereals rather than the fiber itself.7 In fact, soluble fibers classified as oligosaccharides or prebiotics may actually have a positive effect on mineral absorption and subsequent bone maintenance and development. Prebiotics reach the large bowel relatively undigested and intact. In the lower gut, these carbohydrates stimulate the growth of beneficial bacteria, particularly Bifidobacteria and Lactobacilli, and are thus fermented into SCFAs, hydrogen, methane, and carbon dioxide.8

Numerous animal and human studies have demonstrated improved mineral bioavailability and bone structure with the addition of dietary prebiotics. A review written by Scholz-Ahrens et al and published in the Journal of Nutrition in 2007 summarized the results of many of the studies performed over the previous 15 years. In animal studies, the prebiotics inulin and fructooligosaccharide (FOS) improved the absorption of bone-relevant minerals in rats and dogs. FOS was also effective in reducing bone loss in estrogen-deficient rats. In human studies, some outcomes showed no significant effect of prebiotics on mineral absorption, whereas others found a significant increase in the bioavailability of calcium and magnesium. The authors of this review note that the lack of significance in some studies may have been caused by differences in experimental design that allowed too little observation time and/or an insufficient dose of prebiotics. Positive studies with young men, adolescent boys and girls, and adolescent girls alone demonstrated improved calcium absorption in participants who consumed 40 g of inulin, 15 g of FOS, or 8 g of a combination of inulin and FOS per day, respectively.

The mechanisms underlying the potentially improved bioavailability of minerals with the consumption of prebiotics are unknown. Nonetheless, in their review article, Scholz-Ahrens et al outline a number of potential processes that have been proposed. SCFAs may improve the solubility of minerals, bacterial fermentation products may stimulate proliferation of enterocytes (cells lining the gut) and thus increase the absorptive surface area, and the fermentation of prebiotics may enhance expression of calcium-binding proteins. In addition, healthy gut flora may prevent gastrointestinal infections and subsequent damage to enterocytes.

2010 Dietary Guidelines for Americans
The 2010 Report of the Dietary Guidelines Advisory Committee (DGAC) on the Dietary Guidelines for Americans lists foods and nutrients that the majority of Americans underconsume. Fruits, vegetables, whole grains, and legumes are among the foods that 75% to 95% of almost all age-sex groups underconsume. Consequently, Americans’ diets are deficient in a number of nutrients that predominate in these foods. The DGAC gives special attention to four nutrients—vitamin D, calcium, potassium, and dietary fiber—because they are strongly linked to indicators of dietary inadequacy or disease prevalence.9

The science and research backing the DGAC report is impressive. The committee members conducted an extensive review of research to make their recommendations and found that fiber deserved special attention due to its connection with dietary adequacy and disease prevention. Based on 2003-2006 NHANES data, the committee found that less than 3% of adult men and less than 6% of adult women consume adequate fiber. The Adequate Intake (AI) for fiber is 14 g per 1,000 kcal per day. The AI for fiber is based on the median fiber intake associated with the lowest risk of CVD in prospective, cohort studies that were analyzed by the Institute of Medicine.9

The DGAC makes several conclusions regarding the health-protective effects of fiber based on available evidence. In its report, the committee states that a moderate body of evidence suggests dietary fiber protects against obesity, CVD, and type 2 diabetes. The cardioprotective effects of fiber are associated with lower blood pressure, improved serum lipid profiles, and reduced markers of inflammation. Whole grain intake is associated in studies with decreased risk of heart failure, lower blood pressure, and lower total and LDL cholesterol levels. Evidence connecting the glycemic index or glycemic load of foods with CVD is less conclusive, with more negative than positive data, according to the DGAC. However, some studies cited in the DGAC report found a positive correlation between the glycemic load and CVD in women with a body mass index (BMI) of greater than 23 kg/m2 but not in leaner individuals. Given the evidence linking insulin resistance with CVD, it is logical to wonder whether the individuals in this study with a higher BMI may have been more insulin resistant and hyperinsulinemic and thus at greater risk.

Room for More Research
The role that fiber plays in promoting health and preventing disease is clearly complex. High-fiber diets have been linked with multiple health benefits, but whether these positive effects are due to fiber alone or to the combined influence of nutrients, fiber, and phytochemicals found together in whole foods remains unclear. Future research will continue to elucidate the physiologic functions of fiber, which can only bolster what dietitians have known for a long time: A diet rich in fiber from whole grains, legumes, fruits, and vegetables is essential to good health.       

— Rita Carey Rubin, MS, RD, CDE, is a dietitian practicing in northern Arizona.

 

References
1. Pereira MA, Pins JJ. Dietary fiber and cardiovascular disease: Experimental and epidemiologic advances. Curr Athero Rep. 2000;2(6):494-502.

2. Glore SR, Van Treeck DV, Knehans AW, Guild M. Soluble fiber and serum lipids: A literature review. J Am Diet Assoc. 1994;94(6):425-436.

3. Davidson MH, Maki KC. Effects of dietary inulin on serum lipids. J Nutr. 1999;129(7 Suppl):1474S-1477S.

4. Reaven GM. Compensatory hyperinsulinemia and the development of an atherogenic lipoprotein profile: The price paid to maintain glucose homeostasis in insulin-resistant individuals. Endocrinol Metab Clin North Am. 2005;34(1):49-62.

5. Blake GJ, Ridker PM. Inflammatory bio-markers and cardiovascular risk prediction. J Int Med. 2002;252(4):283-294.

6. King DE. Dietary fiber, inflammation and cardiovascular disease. Mol Nutr Food Res. 2005(6);49:594-600.

7. Weaver CM, Heaney RP, Martin BR, Fitzsimmons ML. Human calcium absorption from whole-wheat products. J Nutr. 1991;121(11):1769-1775.

8. Cummings JH, Macfarlane GT, Englyst HN. Prebiotic digestion and fermentation. Am J Clin Nutr. 2001;73(2 Suppl):415S-420S.

9. USDA Center for Nutrition Policy and Promotion. Report of the Dietary Guidelines Advisory Committee on the Dietary Guidelines for Americans, 2010. Available at: http://www.cnpp.usda.gov/DGAs2010-DGACReport.htm. Accessed December 1, 2010.

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