Rye flour: a modest help for constipation, with a few catches

By meta-analysis of trials in adults with chronic constipation, rye bread raised stool frequency by 0.43 bowel movements a week over white bread (95% CI 0.03 to 0.83), and that estimate rests on 2 small trials with 48 people. It is the strongest result on this page, and it is a small one. The EU register points the same way. Its only authorized claim for rye is that rye fiber contributes to normal bowel function, and it refused a claim that rye flour has a low glycemic index. A refusal means the evidence fell short at assessment. It is no finding about rye either way. Most of the other trials used rye foods rather than flour on its own. In 242 adults with overweight on a calorie-restricted diet for 12 weeks, those eating high-fiber rye products lost 2.4 lb (1.08 kg) more than those eating refined wheat (95% CI 0.36 to 1.80).

The numbers, per 3.5 oz (100 g)

How much is actually in it

• Rye flour in USDA Foundation data carries 13.68 g fiber, 0.338 mg copper and 16.6 ug selenium per 3.5 oz (100 g), with fiber ranging 8 to 23.8 g across four USDA records. — Food composition data, US (n = 4 records; Foundation samples n=8) Evidence E source
  Fiber 13.68 g (records 8-23.8; Foundation samples 7.33-15.6), copper 0.338 mg, selenium 16.6 ug per 3.5 oz (100 g)
• The UK table lists rye flour at 319 calories, 14.1 g AOAC fiber and 2.43 mg iron per 3.5 oz (100 g), but only 2 ug selenium against 16.6 ug in the US record. — Food composition data, UK vs US Evidence E source
  Per 100 g: energy 319 calories, protein 7.3 g, AOAC fiber 14.1 g, iron 2.43 mg, magnesium 85 mg, zinc 2.3 mg, folate 60 ug, selenium 2 ug (USDA 16.6 ug)
• Among 35 analyzed foods, whole wheat rye flour held the most nystose, 0.5010 g per 3.5 oz (100 g), a fructan-type prebiotic. — 35 foods bought in Italian supermarkets, 2024 (n = 35 foods) Evidence C source
  Nystose 0.5010 g/100 g, 1F-beta-fructofuranosylnystose 0.1080 g/100 g in whole-meal rye flour

What your body absorbs

• In a 12-week RCT in Swedish women eating 7.1 oz (200 g) wholegrain rye bread a day, the low-phytate version did not raise iron status, and ferritin fell 12% from 32 to 27 ug/L. — females of reproductive age, 200 g/day blanched or dephytinized wholegrain rye bread for 12 weeks (n = 102 randomized, 55 completed) Evidence B source
  Low-phytate group: ferritin 32 to 27 ug/L (-12%), total body iron 6.9 to 5.4 mg/kg; no gain vs high-phytate bread

What cooking and storage change

• In 23 adults, unfermented wholegrain rye crispbread gave a 13% lower insulin response over 230 min than fermented rye crispbread and 17% lower than wheat crispbread. — healthy volunteers aged 27-70, standardized breakfast (n = 23) Evidence B source
  Insulin 0-230 min: uRCB -13% vs RCB (P<0.002), -17% vs WCB (P<0.001); glucose response no difference; fullness +16%
• Laboratory data suggest sourdough fermentation raises total arabinoxylans in rye bread and some starter strains cut its fructans. — rye sourdoughs fermented 24 and 48 h and breads baked from them Evidence D source
  Higher total arabinoxylans and water solubility; S. boulardii gave the largest fructan reduction

What it does to your health

• In a 12-week RCT in 242 adults with overweight on a calorie-restricted diet, high-fiber rye foods led to 2.4 lb (1.08 kg) more weight loss than refined wheat. — adults aged 30-70 with BMI 27-35, hypocaloric diet with rye or wheat products for 12 weeks (n = 242) Evidence B source
  -2.4 lb (-1.08 kg) body weight (95% CI 0.36 to 1.80) and -0.54% body fat vs wheat; CRP 28% lower
• By meta-analysis of trials in adults with chronic constipation, rye bread raised stool frequency by 0.43 bowel movements a week over white bread. — adults with chronic constipation; rye bread comparison from 2 studies (n = 48) Evidence A source
  MD +0.43 bowel movements/week (0.03 to 0.83)
• In 28 overweight men over 4-week periods, high-fiber rye foods raised fecal butyrate 36% versus low-fiber cereal foods, while the 46-49% fall in postprandial insulin was reported for rye and wheat foods together. — overweight men aged 40-65, 90 g/day cereal foods, three 4-week periods (n = 28) Evidence B source
  Postprandial insulin -46 to -49%, glucose -16 to -19% (rye and wheat); butyrate +26% vs wheat, +36% vs low fiber
• In adults with overweight, a day of wholegrain rye meals lowered day-long glucose iAUC by 30% compared with refined wheat meals. — healthy adults with BMI 27-35, clinic intervention days (n = 29) Evidence B source
  Day-long glucose iAUC -30% (P < 0.001); glycemic variability SD -2.3 mg/dL (-0.13 mmol/L)

Safety, limits and interactions

• In women with constipation, fiber-rich rye bread for 3 weeks eased defecation but increased gastrointestinal symptoms compared with low-fiber toast. — healthy women with self-reported constipation, 3-week intervention (n = 59) Evidence B source
  Transit time -0.7 (CI -1.1 to -0.2); GI symptoms +1.6 (CI 0.7 to 2.4) vs low-fiber toast
• Swedish rye flour sampled over 15 years averaged 0.017 mg/kg cadmium, lower than wheat flour at 0.029 mg/kg. — Swedish flour samples 1983-1997 (n = 30 rye flour samples) Evidence C source
  Cd mean 0.017 mg/kg dry wt (rye) vs 0.029 (wheat flour); rye Cu, Fe, Mn, Zn fell over time
• A single case report describes arterial ergotism in a farmer who inhaled ergotamine-containing dust while preparing rye flour. — one 42-year-old farmer (n = 1) Evidence D source
  Lower-leg arterial occlusion; reversed within 4 months of avoiding flour dust

What people believe that the data does not support

• The EU refused a claim that rye flour has a low glycemic index, and a sourdough rye bread glycemic claim was also non-authorized. — EU health claim applications Evidence E source
  Status non-authorized
• The only authorized EU claim for rye concerns its fiber: rye fiber contributes to normal bowel function. — EU health claims Evidence E source
  Status authorized

Who this works differently for

• Observational data link Danish infant diets richer in rye flour and far lower in gliadin with much lower childhood celiac disease than in Sweden. — 771 children investigated for celiac disease in Sweden and Denmark, 1972-1989 (n = 771) Evidence C source
  Celiac cases 155 Swedish vs 24 Danish; gliadin at 8 months 4,400 vs 100 mg

The bottom line

What is in the flour depends on the grind. USDA Foundation data give 13.68 g of fiber per 3.5 oz (100 g), and four USDA records run from 8 to 23.8 g, because dark and light rye flours differ widely in bran share. The UK table lists 319 calories, 14.1 g AOAC fiber and 2.43 mg iron per 3.5 oz (100 g). Selenium is where the two tables part: 16.6 ug per 3.5 oz (100 g) in the US record against 2 ug in the UK one, since selenium in grain follows the soil it grew in. One Italian supermarket sample of whole wheat rye flour held 0.5010 g of nystose, a fructan, per 3.5 oz (100 g), the most of 35 foods analyzed. Removing phytate did not help iron. In a 12-week randomized trial, Swedish women of reproductive age ate 7.1 oz (200 g) of wholegrain rye bread a day. The low-phytate bread gave no gain in iron status over the ordinary one, and ferritin in the low-phytate group fell 12%, from 32 to 27 ug/L. Only 55 of the 102 women randomized finished. Fermentation changes the result. In 23 adults, unfermented wholegrain rye crispbread gave a 13% lower insulin response over 230 minutes than fermented rye crispbread and 17% lower than wheat crispbread, with no difference in glucose, after a single breakfast. Laboratory data suggest sourdough fermentation raises total arabinoxylans in rye bread, with no human outcome measured. In 29 adults with overweight, 21 of whom completed, a day of wholegrain rye meals lowered day-long glucose by 30% against refined wheat meals. In 28 overweight men eating cereal foods for 4-week periods, rye foods raised fecal butyrate 36% over low-fiber options. The 46 to 49% fall in post-meal insulin in that trial was reported for rye and wheat foods together. The cautions come from four cards. In a 3-week trial in 59 women with self-reported constipation, fiber-rich rye bread eased defecation and also raised gastrointestinal symptom scores by 1.6 (CI 0.7 to 2.4) against low-fiber toast, in an industry-sponsored study. Rye contains gluten proteins and is not safe in celiac disease. A Swedish and Danish comparison of 771 children found far fewer celiac cases where infant diets used more rye flour and much less gliadin, an observational pattern that does not make rye safe for anyone with the disease. Swedish rye flour sampled from 1983 to 1997 averaged 0.017 mg/kg cadmium, lower than wheat flour at 0.029 mg/kg, in old data from one country. One case report describes arterial ergotism in a farmer who breathed in ergot-containing dust while preparing rye flour, a workplace exposure with no bearing on eating it.

Sources

1. ryef-daily-r6-01 · food composition table
   Flour, rye (FDC 2512375, Foundation, Cereal Grains and Pasta): fiber 13.68 g per 100 g (4 records 8-23.8; Foundation samples 7.33-15.6 n=8); copper 0.338 mg (0.213-0.56); selenium 16.6 ug (14.4-18; Foundation samples 0-56.6 n=8)
   USDA FoodData Central Foundation Foods, Flour, rye, FDC 2512375 · checked 2026-09-28
2. ryef-daily-r6-02 · food composition table
   CoFID 2021 11-897 Flour, rye (group AA): Energy 319 kcal, Protein 7.3 g, AOAC fibre 14.1 g, NSP 11.8 g, Iron 2.43 mg, Magnesium 85 mg, Zinc 2.3 mg, Folate 60 ug, Selenium 2 ug per 100 g
   CoFID 2021, Public Health England, code 11-897 · checked 2026-09-28
3. ryef-daily-r6-03 · food analysis
   Kestose was detected mostly in wheat bran (0.7300 g/100 g), while nystose and 1 F-β-fructofuranosylnystose were in whole-meal rye flour (0.5010 g/100 g and 0.1080 g/100 g, respectively).
   Nutrients, 2025 · checked 2026-09-28
4. ryef-daily-r6-04 · RCT
   In the low-phytate bread group (n = 24) there were significant within-group decreases in both ferritin (mean 12%; from 32 ± 7 to 27 ± 6 μg/L, geometric mean ± SEM, p < 0.018) and total body iron (mean 12%; from 6.9 ± 1.4 to 5.4 ± 1.1 mg/kg, p < 0.035).
   Eur J Nutr, 2019 · checked 2026-09-28
5. ryef-daily-r6-05 · RCT, crossover
   uRCB induced 13% (P<0.002) and 17% (P<0.001) lower postprandial insulin response between 0-230 min compared with RCB and WCB respectively.
   PLoS One, 2015 · checked 2026-09-28
6. ryef-daily-r6-06 · lab study
   The present study demonstrated that the treatments applied contributed to an increased total content of arabinoxylans in the breads, and the inoculation of the sourdoughs with the potential probiotic strains improved their solubility in water.
   Molecules, 2023 · checked 2026-09-28
7. ryef-daily-r6-07 · RCT
   After 12 weeks the participants in the rye group had lost 1.08 kg body weight and 0.54% body fat more than the wheat group (95% confidence interval (CI): 0.36; 1.80, p < 0.01 and 0.05; 1.03, p = 0.03, respectively).
   Clin Nutr ESPEN, 2021 · checked 2026-09-28
8. ryef-daily-r6-08 · meta-analysis
   Rye bread resulted in higher stool frequency than white bread (MD: +0.43 BM/week, [0.03-0.83], n = 48).
   Aliment Pharmacol Ther, 2024 · checked 2026-09-28
9. ryef-daily-r6-09 · RCT, crossover
   Postprandial plasma insulin was decreased by 46-49% (P = 0.0001) and postprandial plasma glucose by 16-19% (P = 0.0005). Rye foods were associated with significantly (P = 0.0001) increased plasma enterolactone (47% and 71%) and fecal butyrate (26% and 36%), relative to wheat and low-fiber options, respectively.
   Am J Clin Nutr, 2003 · checked 2026-09-28
10. ryef-daily-r6-10 · RCT, crossover
    Day-long glucose incremental area under the curve was 30% (P < 0.001) lower after the rye-based diet, and glycemic variability was measured as SD reduced (-0.13 mmol/L, P = 0.04).
    J Nutr, 2025 · checked 2026-09-28
11. ryef-daily-r6-11 · RCT
    The rye bread shortened total intestinal transit time (mean difference, -0.7; CI(95), -1.1 to -0.2; P=0.007), increased faecal frequency (0.3; CI(95), 0.1 to 0.5; P=0.001), softened faeces (-0.3; CI(95), -0.4 to -0.2; P<0.001) and made defecation easier (-0.4; CI(95), -0.5 to -0.2; P<0.001), but also increased gastrointestinal symptoms (1.6; CI(95), 0.7 to 2.4; P<0.001) compared to the low-fibre toast consumed in the LGG and control groups.
    Eur J Clin Nutr, 2006 · checked 2026-09-28
12. ryef-daily-r6-12 · monitoring study
    The Cd level in wheat and rye flours (mean 0.029 and 0.017 mg/kg dry wt, respectively) correlated significantly with time (p < 0.05) in 2 phases, tending to increase during the first half of the period and decrease during the latter half.
    J AOAC Int, 2001 · checked 2026-09-28
13. ryef-daily-r6-13 · case report
    Enquiry of the patient discovered that he had been exposed to ergotamine-containing milling dust in the preparation of rye flour.
    Dtsch Med Wochenschr, 1998 · checked 2026-09-28
14. ryef-daily-r6-14 · regulatory register
    POL-HC-7900 Rye flour Rukkijahu iseloomustab madal glükeemiline indeks. Clarification provided Rye flower is characterised by low glycaemic index. Non-authorised
    EU Register on nutrition and health claims, claim POL-HC-7900, snapshot 2026-09-21 · checked 2026-09-28
15. ryef-daily-r6-15 · regulatory register
    POL-HC-6459 Rye fibre Rye fibre contributes to normal bowel function Authorised
    EU Register on nutrition and health claims, claim POL-HC-6459, snapshot 2026-09-21 · checked 2026-09-28
16. ryef-daily-r6-16 · cohort
    The Danish infant diet differed significantly from the Swedish in containing a larger amount of the lower gluten-containing rye flour.
    J Pediatr Gastroenterol Nutr, 1995 · checked 2026-09-28