Recommendations for the Intake of Fiber

 2005 Report of the Dietary Guidelines Advisory Committee 

 

Fibers are different from sugars and starches in that they are not digested and absorbed in the small intestine and converted to glucose. Humans do not have the necessary enzymes to break down fibers into their constituent parts so that they can be absorbed into the body. Therefore, fibers pass from the small intestine into the large intestine relatively intact. There they can be fermented by the colonic microflora to gases such as hydrogen (H2) and carbon dioxide (CO2) and to short chain fatty acids. Although fibers are not converted to glucose as are sugars and starches, some of these short chain fatty acids are absorbed and can be used for energy in the body. However, determining the amount of calories supplied by fiber is complex since it depends on such factors as the fermentability of the fiber, the individual’s colonic microflora, how long fiber stays in the colon, etc.

The IOM has set an Adequate Intake (AI) value for fiber of 14g of fiber per 1,000 kcal. This AI is based on the totality of the evidence for fiber decreasing the risk of chronic disease and other health-related conditions, but the actual numbers for the AI were derived from the data supporting a decreased risk for the development of coronary heart disease (CHD). The major food sources of fiber are fruits, vegetables (particularly legumes), and grains. Milk does not contain fiber although certain milk-containing products may.

What are the major health benefits of fiber-containing foods?

Conclusion

Diets rich in dietary fiber have a number of important health benefits including helping to promote healthy laxation, reducing the risk of type 2 diabetes, and decreasing the risk of coronary heart disease (CHD). Prospective cohort studies suggest that decreased risk of heart disease is associated with the intake of 14 g of dietary fiber per 1,000 calories. 

Rationale 

Overview

The conclusion regarding the recommended intake of dietary fiber is consistent with the IOM’s AI value of 14 g of fiber per 1000 kcals (IOM, 2002). This AI for fiber intake was based on the totality of the evidence for certain health benefits of dietary fiber, placing emphasis on fiber’s protective role against CHD but also including its effect on laxation (Burkitt 1972; Cummings, 1992; Kelsay et al., 1978) and diabetes  (Colditz et al., 1992; Salmeron et al., 1997a). The Committee evaluated the potential effects of fiber on laxation and diabetes and focused on the effects of fiber on CHD, since that was the basis of setting a value for fiber intake. Particular attention was paid to studies published since the IOM report. Studies on the association between fiber and diabetes are discussed under Question 2 in this section: How important is the glycemic response to carbohydrates to human health? Summaries of the studies on the relationship of fiber to healthy laxation and to risk of CHD are shown below.  

Review of the Evidence

Fiber and Laxation

Chronic constipation is one of the most common disorders in Western countries (Roma et al., 1999). Although there is no one accepted definition of what constitutes normal laxation, constipation has been defined as difficulty in passing stools or an incomplete or infrequent passage of hard stools (Anderson, 2003).  Epidemiological studies have reported a negative correlation between per capita fiber consumption and the incidence of chronic constipation (Graham et al., 1982). Dietary fibers from whole grains, fruits, and vegetables (including legumes) increase stool weight, which promotes normal laxation in children and adults. In general, the greater the weight of the stool, the more rapid the rate of passage through the colon (Birkett et al., 1997) the better the laxative effect. The water holding capacity and bulking ability that result in increased laxation are thought to reduce intracolonic pressure and lower the risk for diverticular disease as well (Bodribb et al., 1976). 

Several factors affect stool weight, including the fermentability of the fiber (the less fermentable, the greater the fecal bulk) the water holding capacity of the fiber, and the contribution of the fiber to microbial mass, which also contributes to fecal bulk (Bach Knudsen et al., 1997; Blackwood et al., 2000 Chen et al., 1998). In addition, certain fibers may contain unfermented gel, which acts as an emollient and a lubricant (Marlett et al., 2000). Cummings reviewed over 100 studies of the effect of fiber intake on stool weight and calculated the increase in weight of the stool as a function of fiber intake (Cummings et al., 1992). There was a wide range of the contribution of dietary fiber to fecal weight (e.g. an increase of 5.7 g fecal bulk per gram of wheat bran fed compared to an increase of 1.3 g per gram of pectin in the diet). A meta-analysis of 11 studies in which daily fecal weight was measured accurately in 26 groups of people (n = 206) on controlled diets of known non-starch polysaccharide content shows a significant correlation between fiber intake and mean daily stool weight (r = 0.84) (Cummings et al., 1992). Although stool weight continues to increase as fiber intake increases (Burkitt et al., 1972; Wrick et al., 1983), there is a plateau effect for both intestinal transit time and fecal frequency. In general, most studies show that once intestinal transit time was less than 1 day and fecal frequency reached two to three per day the only effect of extra fiber in the diet was increased stool weight (Haack et al., 1998). The fecal weight required to achieve normalcy is variable, but the effect on decreasing transit time appears to plateau at fecal outputs >160 to 180 g per day (Burkitt et al., 1972). Many fiber experts have interpreted this as fiber having a “normalizing” effect on laxation: once normal laxation has been achieved, additional fiber may contribute to other health benefits but not to laxation.

A number of recent feeding studies of healthy individuals provide further evidence to support the role of a fiber-rich diet in normal laxation and other purported benefits to colonic health ( Bach Knudsen et al., 1997; Blackwood et al., 2000; Chen et al., 1998 Haack et al., 1998). There are a large number of recent publications on the use of pre- and probiotics to alter the colonic microflora. Although a change in the microflora has been documented in several studies, functional endpoints are lacking at this time. (Cummings et al., 2002). Certain clinical studies have reported successful management of chronic constipation with fiber supplementation (Cummings, 1984; Hein et al., 1978; LoeningBaucke, 1994; Shafik et al., 1993).

Children

Consumption of adequate dietary fiber is associated with important health benefits throughout the lifecycle, but certain populations may require specific comment.  For example, since the new AI for fiber is based on a decreased risk for CHD, some may assume that meeting the AI for fiber is less important for children than for adults. However, chronic constipation is one of the most common causes of morbidity in childhood (Bakwin, 1971; Leung, 1996; Loening-Baucke, 1995). Some studies have shown that up to 10 percent of children have chronic constipation (Bakwin, 1971; Leung, 1996; Loening-Baucke, 1995), which accounts for 25 percent of visits to pediatric gastroenterology clinics (Loening-Baucke, 1994). Several cross-sectional surveys on U.S. children and adolescents found inadequate dietary fiber intakes (Champagne et al., 2004; Cavadini et al., 2000). A randomized study of Greek children (291 with constipation and 1,602 controls) age 2 to 14 found that constipated children had lower caloric and nutrient intakes (P < 0.001), lower body weight/height (P < 0.001), and reported a higher prevalence of anorexia (P < 0.001). Despite the age of onset of constipation, dietary fiber alone was inversely correlated with chronic constipation (P < 0.001) (Roma et al., 1999). Another study found that children with constipation consumed approximately half as much fiber as a control group that was not constipated (McClung, et al., 1995). Similarly, Morais et al. (1999) reported that children with chronic constipation ate less fiber than their age-matched counterparts. 

The elderly

There are a number of issues regarding healthy laxation and the elderly, some of which are summarized in Imershein et al. (2000). Some of these issues involve varied definitions of what constitutes normal laxation. However, there are also very real issues of drug interactions with laxation and lack of appropriate hydration due to concerns about urinary incontinence. Diuretics (often prescribed for hypertension) may result in hard stools, which are difficult to pass. Limitations on mobility also contribute to constipation. Not only is the prevalence of constipation higher in the elderly than in the general population (Grant, 1999), but the impact on quality of life appears to be greater (Pettigrew et al., 1997). Constipation may affect up to 20 percent of people over 65 years of age (Rouse et al., 1991).

Fiber and Chronic Heart Disease

Evidence from a large number of epidemiological studies supports a protective role for dietary fiber against CHD (Fraser et al., 1992; Humble et al., 1993; Kromhout et al., 1982; Kushi et al., 1985; Khaw and Barrett-Connor et al., 1987; Morris,1977; Liu et al., 1999; Pietinen et al., 1996; Rimm et al., 1996; Wolk et al., 1999).  The actual numbers used to set the AI were from three well-designed, adequately powered prospective epidemiological studies that measured the intake of fiber in healthy people and related the intake to later development of CHD (Pietinen et al., 1996; Rimm et al., 1996; Wolk et al., 1999) (see Table D5-1). Specifically, in the Health Professionals Follow-up Study, in which 43,757 men were followed, the relative risk for CHD for men in the highest quintile of fiber intake was 0.59 compared to 1.0 for the men in the lowest quintile of fiber intake (p<0.001). In the Nurses’ Health Study, involving 68,782 U.S. women, the relative risk for the highest quintile of fiber intake was 0.77 compared to 1.0 for the lowest (Wolk et al., 1999). In the Finnish Men’s Study, involving 21,930 men, the relative risk for CHD for men in the highest quintile of fiber intake was 0.45 compared to the lowest quintile (P <0.001) (Pietinen et al., 1996). As can be determined from data rows one through three of Table 5-1, the AI of 14 g of fiber per1,000 kcal was calculated using the average intake of fiber in the “protected group” in each of the studies (i.e., the highest quintile of fiber intake) and dividing that intake by the average energy intake for that quintile to obtain grams of fiber per calorie. Fiber intake then was expressed as grams per 1,000 calories to provide a more useable number. In brief, to be in the group with the lowest risk for CHD, an average intake of 14 g of fiber per 1,000 kcal would need to be consumed.  

Two more recent prospective cohort studies (Bazzano et al., 2003; Liu et al., 2002) provide further evidence that supports the AI of 14 g of fiber per 1000 kcal (Table D5-1).  Liu et al. used prospective data from the Women’s Health Study over a 6-year period to assess the relationship between total dietary fiber, soluble and insoluble fiber, and fiber sources on the risk of cardiovascular disease or myocardial infarction. A significantly smaller number of cardiovascular disease cases occurred in the highest quintile of intake than in the lowest quintile of intake (99 cases vs. 140 cases). The age and randomized treatment-adjusted relative risk (RR) of cardiovascular disease was 0.65 (P for the linear trend = 0.001) comparing the highest and lowest quintiles (Liu et al., 2002). Liu et al. also reported a pooled analysis of nine published dietary fiber and CHD epidemiological studies. This pooled analysis shows an RR of 0.83 associated with 10 g increases in dietary fiber intake (Liu et al., 2002). Bazzano et al. (2003) examined the relationship between total and soluble dietary fiber intake and the risk of CHD and cardiovascular disease in 9,776 adults who participated in the National Health and Nutrition Examination Survey 1 Epidemiologic Follow-up Study. They report that individuals in the highest quartile for dietary fiber intake (20.7 g per day) had an RR of 0.88 for CHD events compared to those in the lowest quartile (5.9 g per day).

A meta-analysis by Pereira et al. (2004) compared intakes of dietary fiber and its subtypes (cereal, fruit, and vegetable fibers) and risk of CHD. This pooled analysis of 10 large prospective studies reported that each 10 g per day increment of dietary fiber was associated with a 14 percent decreased risk of coronary death. Fiber from cereals and fruits also had a strong, inverse association with CHD risk (RR 0.75 and 0.70, respectively). This association was not found for vegetable fiber (RR 1.00).  

When setting the AI for fiber, the IOM also took into consideration small-scale clinical intervention trials and potential mechanisms for this observed protective effect against CHD. As reviewed by Fernandez (2001), a large number of relatively small-scale clinical intervention trials have shown that viscous fibers can lower serum cholesterol. It is generally accepted that a decrease in serum cholesterol is protective against CHD (see Part D, Section 4, for further information). Notably, in the studies in the Types and Sources of Dietary Fiber Summary Tables, Appendix G-3, total dietary fiber from foods was shown to be protective against CHD, not just those fibers that lower cholesterol. Whole grains, fruits, and vegetables are the food sources of fiber. Other possible mechanisms for the protective effect of high-fiber diets include the delayed absorption of macronutrients; a decrease in serum triglyceride levels; and a lowering of blood pressure.

Also, whole grains, fruits and vegetables contain substances, such as phytochemicals, that may contribute to their beneficial effect in protecting against CHD.

SUMMARY 

Carbohydrates—the sugars, starches, and fibers found in grains, fruits, vegetables, and  milk products—are an important part of a nutritious, healthy diet. …  To reduce the risk of CHD disease and promote healthful laxation, the Committee recommends the intake of 14 g of dietary fiber per 1,000 calories.

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