Constipation is a common problem in childhood, accounting for 25% of visits to pediatric gastroenterology clinics and 3% of referrals to teaching hospital clinics (1). The pathophysiology of constipation in children is not completely known. Several mechanisms have been associated with constipation, such as the vicious cycle of a painful bowel movement followed by withholding of stools, genetic factors, disturbances of intestinal motility, and dietary habits (1-4). The fact that constipation is uncommon in societies in which the dietary fiber content is high has been used as a justification to consider low-fiber diets as a predisposing factor and a high-fiber diet as an important measure for the treatment of constipation (5,6).
Data about children's dietary fiber intake are scarce, and there have been few attempts to correlate fiber consumption with the occurrence of constipation (7-9). To our knowledge, there is only one article that describes fiber intake in a small group of American children with chronic constipation (10).
In this case-control study, we evaluated the fiber intake of children with chronic constipation using two tables of fiber composition of foods, a Brazilian table that contains values of soluble and insoluble fiber (11) and a table based on the method of the Association of Official Analytical Chemists (AOAC) (12). The fiber intake was also correlated to the recent recommendation of dietary fiber intake for children (13).
The study was conducted at the outpatient clinic of Pediatric Gastroenterology at the Universidade Federal de São Paulo, Escola Paulista de Medicina (UNIFESP-EPM), São Paulo, Brazil). The study population consisted of 52 children between the ages of 2 and 12 years who had chronic intestinal constipation. Patients whose mothers had made dietary modifications as a form of treatment for constipation were not included in the study. Chronic intestinal constipation was defined by painful or difficult elimination of hard stools during 3 or more months, regardless of the time between bowel movements, soiling, and blood in the stool. Depending on the clinical characteristics of the disease, the diagnosis of functional, chronic constipation was confirmed using radiologic, histologic, and/or anorectal manometric assessment when clinically indicated.
The control group consisted of 52 children who were observed at the Pediatric Primary Care Unit who fulfilled the following criteria: no pain or difficulty during defecation, bowel movements at least once a day of soft stool without soiling, no recurrent abdominal pain, and no history of chronic constipation in the past. These children, free of acute or chronic diseases that could result in anorexia or other alterations in dietary habits, were age and gender matched with the cases of chronic constipation.
A specific questionnaire was designed to collect information related to the duration of exclusive breast-feeding. A 24-hour dietary recall (14) was administered by a nutritionist. The data were analyzed using the Nutrition Support Program (version 2.5) developed by the Center for Health Statistics at UNIFESPEPM to calculate the daily intake of nutrients. The percent adequacy of calories was obtained by dividing the total calories by the ideal weight for height and comparing this value with that recommended for age according to the Recommended Dietary Allowances(15). Weight was measured with the children unclothed, using a balance scale measuring up to a maximum of 150 kg in increments of 100 g. A vertical rule was used for measuring height. After removing the shoes, children stood on a flat surface by the scale with feet parallel and heels, buttocks, shoulders, and back of head touching the upright. The headpiece of the measuring device was gently lowered, making contact with the top of the head (16).
The quantity of protein, carbohydrate, and lipids in the diet was expressed in percentage of total caloric intake.
The dietary fiber intake was estimated using two reference tables for fiber content in foods. The Brazilian table differentiated between the grade of different fractions of dietary fiber in foods consumed in Brazil (11), using the method of Van Soest (17) for insoluble fibers modified in samples rich in starch and the method of McCready and McComb (18) to determine soluble fibers. In the second table, the fiber intake was estimated according to the fiber content determined by the AOAC, which provides the amount of total fiber in foods (12).
The percentage of recommended dietary intake of fiber was calculated by taking into consideration the actual daily fiber intake estimated by the AOAC table and the minimum fiber intake recommended by the American Health Foundation (AHF), daily expected minimum intake equal to 5 g of fiber plus age in years (age + 5 g) (13). The odds ratio was calculated as an estimate of relative risk of low dietary fiber for chronic constipation using the cutoff of age + 5 g as the minimum acceptable daily fiber intake.
Statistical analysis was performed using the statistical program Epi-Info, ver. 6.0, provided by the Centers for Disease Control (Atlanta, GA, U.S.A.) and the World Health Organization.
The median (25th and 75th percentiles) duration of constipation was 40 months (range, 21-81). Soiling was reported for 44.2% (23/52) patients with chronic constipation. Constipation first occurred during the first year of life in 51.9% (27/52) of the patients, between 1 and 5 years of age in 30.8% (17/52), and after 5 years in 15.3% (8/52).
Age, sex, time of exclusive breast-feeding, total food intake, and general data are presented in Table 1. Statistical analysis showed that the duration of exclusive breast-feeding was greater in the control group than in the case group. The food questionnaires showed a statistically significant difference in the number of meals and the percentage of total caloric intake contributed by carbohydrates, proteins, and fats between the two groups.
Daily fiber intake is presented in Table 2. The total fiber intake of the constipated children was lower than that of the control group according to both tables of fiber in foods. The Brazilian table showed that the lower consumption of total fiber was attributable to a low intake of insoluble fiber, whereas the intake of soluble fiber was not significantly different between the two groups.
The higher estimated intake of total fiber according to the Brazilian table compared with that estimated by the AOAC table was statistically significant. The median fiber intake according to the Brazilian table was approximately 1.5 times greater than that calculated with the AOAC table. The Spearman correlation coefficient between the two tables of fiber content showed a strong correlation in both groups: chronic constipation r = +0.82, p < 0.001; control r = +0.86, p < 0.001.
The median of the percentage of recommended dietary intake of fiber is also presented in Table 2. The chronic constipation group had a statistically significant lower percentage adequacy of fiber intake for age than the control group. The proportion of children consuming less than the minimum recommended fiber intake was significantly greater in the case group than in the control group, 75.0% (39/52) and 42.3% (22/52), respectively, (X2 = 11.46; p = 0.001). The odds ratio was 4.1 (95% confidence limits: 1.64-10.32), showing that there is a greater risk of constipation in children with a fiber intake below recommended levels.
The major food sources of dietary fiber, taking into consideration the total intake of both groups studied, were bread (33.7%), rice (21.2%), beans (17.1%), fruits (12.0%), vegetables and legumes (7.9%), potato (2.8%), and other foods (5.3%). Besides the quantity of fiber intake, the only difference between the groups was the higher frequency of consumption of legumes and vegetables in the 24-hour dietary recall in the control group (41/52, 78.8%) than in the constipation group (25/52, 51.9%; X2 = 7.2; p = 0.007).
The general characteristics of our patients with chronic, functional constipation are similar to those described in the literature (1-3), especially in relation to the age at the commencement of constipation, length of time with constipation before seeking specialized treatment, and frequency of soiling.
Our results show that the duration of exclusive breastfeeding was lower in the constipated group than in the control group, suggesting that exclusive breast-feeding may provide some protection against the development of constipation. However, this information should be interpreted with caution because there was a rather long period between the cessation of breast-feeding and the administration of the questionnaire. Further studies are needed to investigate the relationship between weaning, the quantity of fiber in the diet after weaning, and the development of constipation in weaned children, because in many cases, constipation in children begins during the first year of life. Recent studies (19,20) have demonstrated a relationship between chronic constipation and cow's milk intolerance, and breast-feeding may indirectly prevent some cases of constipation in infants by preventing cow's milk intolerance.
The 24-hour dietary recall indicated that the caloric adequacy and the amount of milk ingested were not statistically different between the two groups. The constipated group consumed a significantly greater number of daily meals; however, the numerical difference probably does not have clinical relevance. In terms of percentage of total calories, the constipation group consumed a greater percentage of carbohydrates and a lower percentage of lipids and proteins, than did the control group (Table 1). The Bogalusa population study of American children verified that a greater intake of fiber is associated with a greater role of carbohydrates in the total caloric intake, while the lower intake of fiber is associated with a greater intake of lipids (9). These data differ from those obtained in our study, in which daily quantities of carbohydrates consumed by both groups were not significantly different. The control group consumed more calories derived from proteins and lipids and also consumed vegetables and legumes more frequently than did the constipated group.
Both tables of fiber content of foods (11,12) used in this study showed that children with chronic constipation ingested less fiber than the control group (Table 2). The median intake of fiber according to the Brazilian table was 1.5 times greater than that estimated with the AOAC table. In contrast, the correlation coefficient showed a strong correlation between the values obtained with these two tables, indicating that the differences may be caused by laboratory methods used in the analyses of fiber in the food samples. The AOAC table uses the Prosky gravimetric enzymatic process (21) for fiber analyses, while the Brazilian table uses a combination of gravimetric and neutral detergent methods, providing the quantity of insoluble fiber (cellulose, hemicellulose, and lignin) and soluble fiber (pectin and propectin). In our study, in addition to the fact that Brazilian table overestimated the intake of fiber relative to the AOAC table, it also showed that the constipation group consumed less fiber, mainly insoluble fiber, than did the control group, which could have a major effect on the diminished intestinal transit time, more so than soluble fiber (22). The difference between the Brazilian and the AOAC tables also indicates that it is fundamental that studies that recommend a specific intake of fiber must specify which table should be used.
According to the AOAC table, the median intake of fiber by the control group, 12.6 g/day, was similar to that observed in children in the United States who consume approximately 12.4 g/day (9). The AHF proposed that the daily minimum intake of fiber in grams for children ages 3 through 20 years should be equal to the age in years + 5 g and that the maximum intake of fiber should be age + 10 g. Thus, we calculated the percentage adequacy of fiber intake in relation to the minimum recommended for age, based on the AOAC table, and found that 75.0% of the constipation group had a fiber intake below the minimum recommendation. This finding, along with an odds ratio of 4.1, indicates a low-fiber diet to be a risk factor for chronic constipation in children. Nonetheless, the large overlap between the two groups for fiber intake indicates that other factors could contribute to the pathophysiology of chronic constipation in children. Based on the data presented in this study, the minimum recommended daily intake of fiber (age + 5 g) was efficient for defining the risk of constipation.
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