Health benefits of dietary fiber

Dietary fiber intake provides many health benefits. However, average fiber intakes for US children and adults are less than half of the recommended levels. Individuals with high intakes of dietary fiber appear to be at significantly lower risk for developing coronary heart disease, stroke, hypertension, diabetes, obesity, and certain gastrointestinal diseases. Increasing fiber intake lowers blood pressure and serum cholesterol levels. Increased intake of soluble fiber improves glycemia and insulin sensitivity in non-diabetic and diabetic individuals. Fiber supplementation in obese individuals significantly enhances weight loss. Increased fiber intake benefits a number of gastrointestinal disorders including the following: gastroesophageal reflux disease, duodenal ulcer, diverticulitis, constipation, and hemorrhoids. Prebiotic fibers appear to enhance immune function. Dietary fiber intake provides similar benefits for children as for adults. The recommended dietary fiber intakes for children and adults are 14 g/1000 kcal. More effective communication and consumer education is required to enhance fiber consumption from foods or supplements.     

Ten-Year Trends in Fiber and Whole Grain Intakes and Food Sources for the United States Population: National Health and Nutrition Examination Survey 2001–2010

Current U.S. dietary guidance includes recommendations to increase intakes of both dietary fiber and whole grain (WG). This study examines fiber and WG intakes, food sources and trends from 2001 to 2010 based on National Health and Nutrition Examination Survey (NHANES) data for children/adolescents (n = 14,973) and adults (n = 24,809). Mean fiber intake for children/adolescents was 13.2 (±0.1) g/day. Mean fiber intake for adults 19–50 years (y) was 16.1 (±0.2) g/day and for adults 51+ was 16.1 (±0.2) g/day. There were significant increases in fiber intake from 2001–2010 for children/adolescents and for adults 51+ y. Mean WG intake for children/adolescents was 0.52 (±0.01) oz eq/day. Mean WG intake for adults 19–50 y was 0.61 (±0.02) oz eq/day and for adults 51+ 0.86 (±0.02) oz eq/day. There were no significant changes in WG intake for any age group from 2001–2010. The main food groups contributing to dietary fiber intake for children/adolescents were vegetables (16.6%), grain mixtures (16.3%), other foods (15.8%) and fruits (11.3%). For adults 19+ y, the main sources of dietary fiber were vegetables (22.6%), other foods (14.3%), grain mixtures (12.0%) and fruits (11.1%). Major WG sources for children/adolescents included ready-to-eat cereals (RTEC) (31%), yeast breads/rolls (21%) and crackers and salty grain snacks (21%). The main sources of WG for adults 19+ were yeast breads/rolls (27%), RTEC (23%) and pastas/cooked cereals/rice (21%). Recommending cereals, breads and grain mixtures with higher contents of both dietary fiber and WG, along with consumer education, could increase intakes among the United States (U.S.) population.     

Position of the American Dietetic Association: health implications of dietary fiber

Dietary fiber consists of the structural and storage polysaccharides and lignin in plants that are not digested in the human stomach and small intestine. A wealth of information supports the American Dietetic Association position that the public should consume adequate amounts of dietary fiber from a variety of plant foods. Recommended intakes, 20-35 g/day for healthy adults and age plus 5 g/day for children, are not being met, because intakes of good sources of dietary fiber, fruits, vegetables, whole and high-fiber grain products, and legumes are low. Consumption of dietary fibers that are viscous lowers blood cholesterol levels and helps to normalize blood glucose and insulin levels, making these kinds of fibers part of the dietary plans to treat cardiovascular disease and type 2 diabetes. Fibers that are incompletely or slowly fermented by microflora in the large intestine promote normal laxation and are integral components of diet plans to treat constipation and prevent the development of diverticulosis and diverticulitis. A diet adequate in fiber-containing foods is also usually rich in micronutrients and nonnutritive ingredients that have additional health benefits. It is unclear why several recently published clinical trials with dietary fiber intervention failed to show a reduction in colon polyps. Nonetheless, a fiber-rich diet is associated with a lower risk of colon cancer. A fiber-rich meal is processed more slowly, which promotes earlier satiety, and is frequently less calorically dense and lower in fat and added sugars. All of these characteristics are features of a dietary pattern to treat and prevent obesity. Appropriate kinds and amounts of dietary fiber for the critically ill and the very old have not been clearly delineated; both may need nonfood sources of fiber. Many factors confound observations of gastrointestinal function in the critically ill, and the kinds of fiber that would promote normal small and large intestinal function are usually not in a form suitable for the critically ill. Maintenance of body weight in the inactive older adult is accomplished in part by decreasing food intake. Even with a fiber-rich diet, a supplement may be needed to bring fiber intakes into a range adequate to prevent constipation. By increasing variety in the daily food pattern, the dietetics professional can help most healthy children and adults achieve adequate dietary fiber intakes.     

The ‘Age+5’ Rule: Comparisons of Dietary Fiber Intake Among 4- to 10-Year-Old Children

Objective To determine children's sources of dietary fiber and to identify the food group choices made by those who met the “age+5” rule, which recommends that children daily consume an amount of fiber that is equal to their age plus an additional 5 g fiber.

Design This study used 24-hour dietary recalls and 1-day food records to assess the nutrient intake and food group choices of children who did and did not meet the recommendations of the age+5 rule.

Subjects The 1989-1991 US Department of Agriculture Continuing Survey of Food Intakes by Individuals provided the study sample of 603 children between the ages of 4 and 6 years and 782 children between the ages of 7 and 10 years.

Statistical analyses Differences in nutrient and food group intakes between age categories were determined by t tests.

Results Only 45% of 4- to 6-year-olds and 32% of 7- to 10-year-olds consumed adequate fiber to meet the age+5 rule. Those who met the age+5 rule did so by consuming significantly more high- and low-fiber breads and cereals, fruits, vegetables, legumes, nuts, and seeds. Children with low fiber intakes had significantly higher energy-adjusted intakes of fat and cholesterol, whereas those who met the age+5 rule had significantly higher energy-adjusted intakes of dietary fiber, vitamins A and E, folate, magnesium, and iron.

Applications The majority of the children had low intakes of dietary fiber, suggesting that they are at risk for future chronic disease. Parents and school foodservice personnel should strive to offer fiber-rich foods to children so their acceptance and consumption of them will be increased. J Am Diet Assoc. 1998;98:1418-1423.     

5～6岁儿童锌需要量的探讨

将膳食因素与锌摄入量作多元线性回归分析,得出多元回归方程锌摄入(mg)=-0.426+0.053×蛋白质摄入(g)+0.003×磷摄入(mg)+0.234×锌贮留(mg)+0.078×粗纤维摄入(g)(n=27,P＜0.001,R2=0.927)。初步认为5～6岁儿童每日摄入5～6mg锌就能保证发育,这个锌摄入水平可能是目前膳食条件下锌的需要量。从多元回归方程可以看出,锌的需要量受膳食因素影响。各国各地区居民膳食因素不同,所以锌需要量也不完全一致。     

Evaluation of dietary fibre intake from infancy to adolescence against various references--results of the DONALD Study

OBJECTIVE: To evaluate dietary fibre intake from infancy to adolescence against various reference standards expressed on a body weight, age or energy intake (EI) basis and to describe age and 15-year time trends. DESIGN AND SUBJECTS: The ongoing longitudinal (open cohort) Dortmund Nutritional and Anthropometric Longitudinally Designed Study with a total of 7810 3-day dietary records from 980 infants, children and adolescents aged 6 months to 18 years collected between 1990 and 2004. RESULTS: Absolute dietary fibre intakes (g/day) increased with age from infancy to adolescence; dietary fibre density (g/1000 kcal) reached a maximum at 1 year and remained constant after the transition to the family diet. Expressed as g/kg body weight, dietary fibre intake increased during infancy with a maximum at 1 year, but decreased thereafter during childhood and adolescence. Dietary fibre intake was low irrespective of the reference used and showed no time trends, with the exception of a small negative trend in 4-8-year olds and 9-13-year-old girls. CONCLUSION: As intake of dietary fibre in our sample was significantly associated with EI and fibre density remained constant after infancy, our data support an energy-based approach for establishing a reference value for children and adolescents.     

Position of the American Dietetic Association: health implications of dietary fiber

It is the position of the American Dietetic Association that the public should consume adequate amounts of dietary fiber from a variety of plant foods. Populations that consume more dietary fiber have less chronic disease. In addition, intake of dietary fiber has beneficial effects on risk factors for developing several chronic diseases. Dietary Reference Intakes recommend consumption of 14 g dietary fiber per 1,000 kcal, or 25 g for adult women and 38 g for adult men, based on epidemiologic studies showing protection against cardiovascular disease. Appropriate kinds and amounts of dietary fiber for children, the critically ill, and the very old are unknown. The Dietary Reference Intakes for fiber are based on recommended energy intake, not clinical fiber studies. Usual intake of dietary fiber in the United States is only 15 g/day. Although solubility of fiber was thought to determine physiological effect, more recent studies suggest other properties of fiber, perhaps fermentability or viscosity are important parameters. High-fiber diets provide bulk, are more satiating, and have been linked to lower body weights. Evidence that fiber decreases cancer is mixed and further research is needed. Healthy children and adults can achieve adequate dietary fiber intakes by increasing variety in daily food patterns. Dietary messages to increase consumption of high-fiber foods such as whole grains, legumes, fruits, and vegetables should be broadly supported by food and nutrition professionals. Consumers are also turning to fiber supplements and bulk laxatives as additional fiber sources. Few fiber supplements have been studied for physiological effectiveness, so the best advice is to consume fiber in foods. Look for physiological studies of effectiveness before selecting functional fibers in dietetics practice.     

Relation of dietary glycemic index, glycemic load, added sugar intake, or fiber intake to the development of body composition between ages 2 and 7 y

BACKGROUND: Observational studies in adults suggest that a diet with a high glycemic index (GI) or glycemic load (GL), a high intake of sugary foods, or a low fiber intake may increase the risk of overweight. OBJECTIVES: We aimed to examine prospectively whether dietary GI, GL, added sugar intake, or fiber intake between age 2 and 7 y are associated with the development of body composition. If so, we aimed to ascertain whether these associations are modified by meal frequency. DESIGN: Linear mixed-effect regression analyses were performed in 380 participants of the DOrtmund Nutrition and Anthropometric Longitudinally Designed (DONALD) Study for whom 4-6 weighed 3-d dietary records and anthropometric data were obtained between ages 2 and 7 y. RESULTS: Changes in dietary GI, GL, or added sugar intake between ages 2 and 7 y were not associated with concurrent changes in percentage body fat (%BF, as estimated from skinfold thicknesses) or body mass index SD scores. An increase in fiber intake was related to a concurrent decrease in %BF between ages 2 and 7 y only in children who consumed <6 meals/d as toddlers (beta +/- SE from fully adjusted model: -0.26 +/- 0.09%BF per 1-SD increase in fiber intake, P = 0.005), whereas children with a higher meal frequency had no concurrent change (0.07 +/- 0.07%BF per 1-SD increase in fiber intake, P = 0.3). CONCLUSIONS: Dietary GI, GL, or added sugar intake between ages 2 and 7 y does not appear to influence the development of body composition. Potential benefits associated with increasing fiber intake throughout childhood may be limited to toddlers with a lower meal frequency.     

Trends in dietary fiber intake in the United States, 1999-2008

BackgroundIntake of dietary fiber has been recommended for many years as part of the guidelines from the American Heart Association, the Institute of Medicine, and other groups. The recommended Adequate Intake for dietary fiber for adults is 25 to 38 g/day (14 g/1,000 kcal/day).ObjectiveTo determine the average daily intake of dietary fiber among adults during the past decade and, specifically, to document progress toward national goals.DesignCross-sectional weighted data from the National Health and Nutrition Examination Survey among adults aged 18 years and older.Participants/settingData were collected from noninstitutionalized adults aged 18 years and older using a nationally representative, complex, multistage, probability-based survey of people living in the United States that was conducted by the National Center for Health Statistics.Main outcome measuresDaily dietary fiber intake by members of the US population based on 2-year groupings of the continuous survey from 1999 to 2008.ResultsMean daily dietary fiber intake for 1999-2000 was 15.6 g/day, for 2001-2002 intake was 16.1g/day, for 2003-2004 intake was 15.5 g/day, for 2005-2006 intake was 15.8 g/day, and for 2007-2008 intake was 15.9 g/day. Participants with obesity (body mass index ≥30) consistently reported lower fiber intake than did individuals with normal weight or overweight (14.6 to 15.4 g/day and 15.6 to 16.8 g/day, respectively; P<0.0001). Mexican Americans had significantly higher intake in 1999-2000 than non-Hispanic whites (18.0 vs 16.1g/day; P<0.05), but Mexican Americans' intake did not increase over time (17.7 g/day in 2007-2008). Non-Hispanic blacks had fiber intake of 12.5 g/day at baseline that increased modestly to 13.1 g/day by 2007-2008.ConclusionsDaily fiber intake generally has not progressed toward national goals during the past decade, but there are some differences according to health and social factors. Additional clinical practice and public health strategies are needed.     

A priori and a posteriori dietary patterns at the age of 1 year and body composition at the age of 6 years: the Generation R Study

Dietary patterns have been linked to obesity in adults, however, not much is known about this association in early childhood. We examined associations of different types of dietary patterns in 1-year-old children with body composition at school age in 2026 children participating in a population-based cohort study. Dietary intake at the age of 1 year was assessed with a food-frequency questionnaire. At the children’s age of 6 years we measured their body composition with dual-energy X-ray absorptiometry and we calculated body mass index, fat mass index (FMI), and fat-free mass index (FFMI). Three dietary pattern approaches were used: (1) An a priori-defined diet quality score; (2) dietary patterns based on variation in food intake, derived from principal-component-analysis (PCA); and (3) dietary patterns based on variations in FMI and FFMI, derived with reduced-rank-regression (RRR). Both the a priori-defined diet score and a ‘Health-conscious’ PCA-pattern were characterized by a high intake of fruit, vegetables, grains, and vegetable oils, and, after adjustment for confounders, children with higher adherence to these patterns had a higher FFMI at 6 years [0.19 SD (95 % CI 0.08;0.30) per SD increase in diet score], but had no different FMI. One of the two RRR-patterns was also positively associated with FFMI and was characterized by intake of whole grains, pasta and rice, and vegetable oils. Our results suggest that different a priori- and a posteriori-derived health-conscious dietary patterns in early childhood are associated with a higher fat-free mass, but not with fat mass, in later childhood.     

Fiber Intake of Older Adults

This study evaluated reported dietary fiber and mineral intakes of 45 healthy adults, 60 years and over, using two sets of three-day dietary food records. The average age of the participants was 73 years, with an age range of 60 to 87 years. Mean reported dietary fiber intake was 18.3 g per day. Percentage of dietary fiber from different food sources was; breads and cereals 33%, vegetables 22%, fruits 21%, beans and legumes 7%, nuts and seeds 3%, combination foods 7% and others 7%. Significant positive correlations were observed between dietary fiber intake and copper, magnesium, selenium, potassium, iron and zinc intakes. Those participants with dietary fiber intakes of 20 g/day or more had significantly higher intakes of copper, magnesium, selenium, potassium and iron compared to those who had dietary fiber intakes less than 20 g/day, although mean kilocalorie intakes were not significantly different. Thus, increasing grain products, vegetables and fruits to increase dietary fiber intake, also may improve the mineral intakes of older adults.     

Dietary intake and the metabolic syndrome in overweight Latino children

Little is known about the relationship between diet and metabolic health in Latino children, a population at increased risk for diabetes. The present study evaluates diet composition and the metabolic syndrome in a cross-sectional sample of 109 overweight Latino children aged 10 to 17 years with a family history of type 2 diabetes. Dietary intake was assessed by two 24-hour recalls. Associations between nutrients and features of the metabolic syndrome were examined using multiple linear regression and analysis of covariance. Log cholesterol intake was positively associated with log systolic blood pressure (beta=0.034, P=0.017) and log soluble dietary fiber intake was inversely associated with log waist circumference (beta=-0.069, P=0.036). Log soluble fiber intake was significantly higher in participants with 0 features compared to those with 3+ features of the metabolic syndrome (P=0.046), which translates to 5.2 g vs 4.1 g soluble fiber daily. No other significant associations were found between dietary variables and either the individual features of the metabolic syndrome or the clustering of metabolic syndrome components. Increases in soluble fiber through the daily consumption of fruits, vegetables, and beans may improve metabolic health in Latino children.     

What Do We Know about Dietary Fiber Intake in Children and Health? The Effects of Fiber Intake on Constipation,Obesity, and Diabetes in Children

The effect of dietary fiber intake on chronic diseases has been explored in adults but is largely unknown in children. This paper summarizes the currently existing evidence on the implications of dietary fiber intake on constipation, obesity, and diabetes in children. Current intake studies suggest that all efforts to increase children's dietary fiber consumption should be encouraged. Available data, predominantly from adult studies, indicate significantly lower risks for obesity, diabetes, and constipation could be expected with higher dietary fiber consumption. However, there is a lack of data from clinical studies in children of various ages consuming different levels of dietary fiber to support such assumptions. The existing fiber recommendations for children are conflicting, a surprising situation, because the health benefits associated with higher dietary fiber intake are well established in adults. Data providing conclusive evidence to either support or refute some, if not all, of the current pediatric fiber intake recommendations are lacking. The opportunity to improve children's health should be a priority, because it also relates to their health later in life. The known health benefits of dietary fiber intake, as summarized in this paper, call for increased awareness of the need to examine the potential benefits to children's health through increased dietary fiber.     

2型糖尿病患者膳食纤维摄入量与肠道菌群的关联——基于一项随机对照试验的数据再分析

目的 分析2型糖尿病患者膳食纤维摄入量与肠道菌群的关联,发现相关差异菌群,为改善患者日常饮食,调节肠道菌群,促进健康提供依据。方法 采用横断面研究设计,以参与一项健康素养和体力活动干预随机对照试验的356名2型糖尿病患者为研究对象。总热能和膳食纤维摄入量基于3 d 24 h膳食调查数据和中国食物成分表计算。对粪便菌群DNA进行16S rDNA V4高变区测序,采用Qiime2软件进行生物信息学分析。多变量线性回归模型(multivariate analysis by linear models, MaAsLin)方法和广义线性模型用于获得与膳食纤维摄入量有关的肠道菌群。结果 研究对象的膳食纤维摄入量处于较低水平,中位摄入量(四分位间距)仅7.4(5.5, 9.7) g/d。未发现膳食纤维摄入量高低与肠道菌群Alpha多样性有关,但基于Jaccard距离矩阵(PERMANOVA P=0.016)计算的Beta多样性在两组间差异有统计学意义。低膳食纤维摄入组有较高丰度的梭杆菌属(Fusobacterium)、柯林斯菌属(Collinsella)和普雷沃菌属(Prevotella),而高膳食...     

Studies on protein and energy requirements in China

Protein and energy requirements of different population groups in China have been studied. Together with the data from other countries, Recommended Dietary Allowances were established and were revised in 1962 and again in 1976. The average BMR of 19 male subjects aged 20–30 was 37.8 kcal/min/m² of body surface. The average energy expenditure of 16 male college students was 2420 kcal/day, and the energy absorbed was 2976 kcal/day. The energy expenditure of 6 female students was 2170 kcal/day. There was a wide variation of energy expenditure among workers and peasants according to the intensity of work. From dietary surveys on the senior middle school boys, the energy intake of adolescents was 2618–3938, which is near or above the RDA. The calorie intake of 8 infants in a metabolic study was 100–200 kcal/kg/day. From nitrogen balance studies, the daily protein requirements of Chinese young adults and infants under 6 months of age consuming soybean milk substitute were determined to be 1.0 g/kg, and 4.5 g/kg respectively. Dietary surveys showed that the protein intake of children with 2–3 years of age was about 30–40 g/kg/day.The traditional Chinese diet with high plant protein and crude fiber content may be beneficial in preventing coronary heart disease. However, the question of absorption and utilization of protein and energy requires further investigation. The effect of meal frequency in relation to the health of the student has been studied, but further study is necessary. Based on our experience over the last 30 years, food rationing determined by age and physical activity appears to be the most efficient method of fighting hunger and malnutrition.     

Health, laxation, and food habit influences on fiber intake of older women

Fifty-nine elderly women interviewed about their food behavior, health, and laxation showed mean dietary fiber (DF) intakes of 14 gm. per day (range 3 to 34 gm. per day). DF intakes were lower for those in poor health and those with a tendency to constipation who took laxatives than for those who did not take laxatives or had no tendency to constipation. Factors not related to DF intake included age, participation in a meal program, living arrangements, understanding of fiber, and functional health. Cooked vegetables and bread added most to DF intakes.     

Squeezing Fact from Fiction about 100% Fruit Juice

Total fruit intake in the United States is ~1 cup equivalent per day, or one-half of the 2010 Dietary Guidelines for Americans recommendation for adults. Two-thirds of the fruit consumed is whole fruit and one-third is 100% juice. The nutritional value of whole fruit, with the exception of fiber and vitamin C, may be retained with appropriate juice production methods and storage conditions. One-hundred percent fruit juice consumption is associated with a number of health benefits, such as improved cardiovascular health and decreased obesity, although some of these and other potential benefits are controversial. Comprehensive analyses of the evidence by the Academy of Nutrition and Dietetics in 2014, the US Dietary Guidelines Advisory Committee in 2010, and the Australian Dietary Guidelines of 2013 concluded that 100% fruit juice is not related to adiposity in children when consumed in appropriate amounts for age and energy needs. However, some reports suggest the consumption of fruit juice contributes to unhealthful outcomes, particularly among children. A dietary modeling study on the best ways to meet the fruit intake shortfall showed that a combination of whole fruit and 100% juice improved dietary density of potassium and vitamin C without significantly increasing total calories. Notably, 100% juice intake was capped at amounts consistent with the 2001 American Pediatric Association guidance. The preponderance of evidence supports the position that 100% fruit juice delivers essential nutrients and phytonutrients, provides year-round access to a variety of fruits, and is a cost-effective way to help people meet fruit recommendations.     

学龄期儿童膳食多样化与微量营养素摄入量的关联性分析

目的 探究学龄期儿童膳食多样化与微量营养素摄入量的关系,为指导儿童科学饮食促进儿童健康提供科学依据。方法 课题组于2009年在江苏省射阳县建立关于儿童生长发育研究的出生队列,选取随访的442名6~7岁儿童作为研究对象。采用24 h膳食回顾调查评估儿童膳食状况,根据《中国食物成分表》估算微量营养素的摄入量并计算儿童膳食多样化评分(DDS和DDS₁₀)和食物多样性评分(FVS);同时评估儿童体格发育指标,应用多变量回归模型分析膳食多样化和微量营养素摄入量与儿童肥胖的关系。结果 研究地区儿童膳食微量营养素摄入量低于该年龄组的中国居民膳食营养素参考摄入量。三种儿童膳食多样化评分均值低于国内其他研究(DDS:6.69±1.59; DDS₁₀:4.94±1.51;FVS:16.79±7.53)。儿童膳食多样化评分与大部分微量营养素摄入状况均呈显著性正相关 (DDS:β=0.14~1.28; DDS₁₀:β=0.17~1.58 ; FVS:β=0.04~0.25,P<0.05),但膳食多样化评分与儿童肥胖无显著相关性(P>0.05)。结论 膳食多样化评分能从一定程度反映儿童微量营养素摄入的状况。倡导饮食多样化的膳食模式对促进儿童健康成长具有重要意义。     

Low-dietary fiber intake as a risk factor for recurrent abdominal pain in children

OBJECTIVE: To evaluate dietary fiber intake in children with recurrent abdominal pain. DESIGN: Cross-sectional study with control group. SETTING: Outpatients of the Pediatric Gastroenterology public health clinic of the Darcy Vargas Children's Hospital, Brazil. SUBJECTS: Forty-one patients with recurrent abdominal pain were evaluated and 41 children, as a control group. INTERVENTIONS: Macronutrients and fiber intake evaluation by the Daily Food Intake method. Two tables of fiber composition in foods were used. RESULTS: According to the Brazilian table the mean intake of fiber (g/day) by the children of the recurrent abdominal pain groups with chronic constipation or not, and the control group was, respectively, 18.2, 16.6 and 23.7 for total fiber (P=0.001), 7.5, 6.9 and 9.5 for soluble fiber (P=0.001) and 10.7, 9.7 and 14.1 for insoluble fiber (P=0.002). According to the AOAC table, the recurrent abdominal pain group with chronic constipation or not (10.6 and 9.9 g/day) also had lower intake of total fiber than the control group (13.4 g/day) (P=0.008). The intake of fiber was lower than the minimum recommended value (age+5 g) and statistically associated (P=0.021) with the recurrent abdominal pain group (78%) in comparison with the control one (51.2%). The odds ratio was 3.39 (95% CI, 1.18-9.95). CONCLUSION: fiber intake below the minimum recommended value is a risk factor for recurrent abdominal pain in children.