Effects of rice-based and wheat-based diets on bowel movements in young Korean women with functional constipation

Constipation is a common digestive disorder, and insufficient dietary fiber intake is a major contributor. While fiber can improve bowel movements and reduce colonic transit time, high-dose fiber supplements may cause gastrointestinal discomfort and are not universally effective, especially in severe constipation. Whole grains provide natural dietary fiber and have been associated with improved bowel function compared with refined grains. However, direct comparisons of rice-based diets (brown and white rice) and wheat-based diets on bowel movements are limited. This study aims to compare and evaluate the effects and safety of rice-based diets (brown rice-based and white rice-based) versus a wheat-based diet on bowel health and bowel movements among young women with functional constipation.

Prior research indicates that fiber supplements (e.g., resistant maltodextrin, beta-glucan, psyllium, inulin, cellulose, konjac glucomannan) can increase bowel movement frequency but may not consistently improve stool consistency and can cause adverse gastrointestinal symptoms. Whole grains (e.g., brown rice, whole wheat, rye, barley, oats) generally contain more fiber than refined grains and have been shown to increase fecal weight, soften stools, and reduce colonic transit time. Despite multiple studies on fiber and bowel function, there is a paucity of studies directly comparing the effects of different cereal fibers and limited evidence comparing rice-based versus wheat-based diets on bowel movements. This gap motivates the present study focusing on natural dietary fiber intake through common staple foods rather than fiber supplements.

Design: Open-label, randomized, controlled, parallel-group dietary intervention over 4 weeks. Participants: 39 young Korean women (19–30 years) meeting Rome III criteria for functional constipation without recent diarrhea; excluded were those with relevant comorbidities, GI surgery/diseases, significant lab abnormalities, hypersensitivities to test foods, probiotic consumption, recent use of GI motility drugs, high alcohol intake, pregnancy/lactation, or participation in other studies. Ethics: IRB-approved (CBNUH_CTCF2_IRB 2012–01–002), registered at ClinicalTrials.gov (NCT01933100), conducted per the Declaration of Helsinki and ICH-GCP. Randomization: 1:1:1 to brown rice-based diet (BRD, n=13), white rice-based diet (WRD, n=13), or wheat-based diet (WD, n=13). Intervention diets: Provided three meals daily at the study center for 4 weeks using a 14-day rotating menu. Estimated daily energy: 2100 kcal (carbohydrate 55–70%, protein 7–20%, fat 15–25%). Target fiber intake about 30 g/day; average provided per analysis: BRD 39 g/day fiber, WRD 34 g/day, WD 34 g/day. Macronutrient targets were similar across groups; BRD used 100% brown rice as staple; WRD used equivalent white rice; WD emphasized wheat-based staples (bread, noodles, cereals, dumplings, sandwiches) with sides (salads, pickles, nuts, dairy, juices). Milk/dairy excluded from rice-based diets except in flour-based foods where applicable. Compliance: Subjects instructed to consume only provided test diets and maintain baseline physical activity; daily diet records captured any additional intake and leftovers to assess compliance. Outcome measures: Primary—Total colonic transit time (TCTT) measured at baseline (Week 0) and Week 4 using radiopaque marker method (Metcalf et al.). Participants ingested capsules containing 20 markers daily for 3 days prior to each X-ray; markers counted in right colon, left colon, recto-sigmoid; CTT (hours) calculated as number of markers × 1.2. Blinded reading by subject ID. Secondary—(1) Number of bowel movements (daily diary over 4 weeks); (2) Fecal weight (g/day); (3) Fecal short-chain fatty acids (lactic, butyric, propionic acids); (4) Fecal enzyme activities (urease, β-glucosidase, β-glucuronidase). Safety: Adverse events monitoring; vitals; ECG; hematology and biochemistry (WBC, RBC, hemoglobin, hematocrit, platelets, γ-GT, AST, ALT, total bilirubin, total protein, BUN, creatine kinase, albumin, total cholesterol, triglyceride, glucose; urinalysis) at baseline and Week 4. Statistical analysis: Intention-to-treat (ITT) population. Baseline homogeneity via chi-square and Wilcoxon rank-sum tests. Efficacy outcomes analyzed using linear mixed-effects models with Bonferroni post hoc tests to compare groups; within-group changes assessed by paired t-tests. Nutrient intake compared via ANOVA with Bonferroni correction. Significance set at p<0.05. Sample size: Pilot trial with 39 planned participants due to limited prior data in this population.

- Participants: 39 randomized; 4 withdrew (2 WRD, 2 WD); 35 completed and were analyzed. Baseline characteristics (age, anthropometrics, vitals, bowel movement frequency, stool weight) were similar across groups. - Primary outcome (TCTT): After 4 weeks, there was a statistically significant difference among the diet groups (p=0.028). BRD significantly reduced TCTT compared with WRD (−16.5 ± 8.1 vs +6.8 ± 2.1 hours; p=0.028). WD also significantly reduced TCTT compared with WRD (−17.1 ± 11.9 vs +6.8 ± 2.1 hours; p=0.022). Within-group, BRD showed significant decreases in left colon CTT (p=0.030) and total colon CTT (p=0.032). - Secondary outcomes: The study reports that BRD and WD increased the number of bowel movements compared with WRD, consistent with improvements in bowel function; detailed numeric secondary outcome data were not provided in the excerpt. - Safety: No specific safety concerns are reported in the provided text; routine monitoring was conducted.

The study directly addresses whether staple-grain dietary patterns affect bowel function in young women with functional constipation. Diets emphasizing whole grains (brown rice) and wheat-based staples with higher fiber content improved total colonic transit time relative to a diet based on refined white rice, indicating that the type and fiber content of staple grains materially influence bowel motility. These findings support dietary modification toward higher-fiber staple foods as a practical, food-based approach to managing functional constipation, potentially preferable to fiber supplements for some individuals given tolerability considerations. The observed reductions in TCTT and increases in bowel movement frequency suggest clinically meaningful improvements in bowel habits in this population.

In young Korean women with functional constipation, both a brown rice-based diet and a wheat-based diet improved bowel function compared with a white rice-based diet, as evidenced by reduced total colonic transit time and increased bowel movement frequency over 4 weeks. These results support recommending higher-fiber staple grains, such as brown rice or wheat-based foods, to alleviate functional constipation. Future research should include larger, more diverse populations, longer intervention durations, and comprehensive evaluation of secondary outcomes (stool characteristics, microbiota, metabolites) to clarify mechanisms and generalizability.

- Pilot, small sample size with 39 randomized and 35 completing, limiting statistical power and generalizability. - Open-label design may introduce performance/reporting bias. - Short intervention duration (4 weeks) limits assessment of long-term effects and sustainability. - Homogeneous participant group (young women only) restricts applicability to other ages, sexes, or clinical subtypes of constipation. - Differential fiber intakes among diets (e.g., BRD higher fiber than WRD/WD) could confound effects attributable to grain type versus total fiber amount.