Wheat: the fibre holds up, the sourdough story does not

Two things about wheat are as solid as nutrition gets, and neither is what wheat gets argued about. Every extra 30 grams of whole grain a day tracks with 8 percent less cardiovascular disease and 6 percent less death from any cause, across studies holding 1.6 million people. The reviewers rate that body of evidence high on the NutriGrade scale, which says the cohorts agree with one another and the estimate is precise. That is a different measure from the badge on the row below, and it does not turn a cohort into a trial or an association into a cause. Refined grain tracks with none of those outcomes in either direction, and the evidence behind every refined grain figure is rated low. Read the unit carefully, though. That is whole grain counted together, and it does not single out wheat. The second solid thing is your bowel, and it was measured on wheat itself. Each extra gram of wheat fibre a day adds about four grams to the daily stool. Among people whose gut transit already takes more than 48 hours, each extra gram takes about three quarters of an hour off it. One thing you will not find further down, and it is the thing most worth having. We hold no evidence card on coeliac disease and none on wheat allergy, so this page says nothing about either. If wheat makes you ill, that is a question for a doctor before it is a question for a food page, and our silence here is a hole in our own work rather than an answer.

The numbers, per 100 g

How much is actually in it

• Wheat bred to carry more zinc did not raise anyone's blood zinc. — healthy adult men aged 18 to 51 (n = 36) Evidence B source
  Plasma zinc concentrations did not change at any point; from the end of the run-in to the end of the biofortified period, FADS2 activity rose from 0.020 to 0.025 (P = 0.02) and FADS1 activity fell from 6.37 to 5.53 (P = 0.01); glutathione and DNA strand breaks did not change, and the 25 mg supplement altered none of the endpoints

What your body absorbs

• Taking the phytate out of wholegrain bread made women's iron stores worse, not better. — healthy Swedish females of reproductive age, 102 allocated and 55 completing (n = 102 allocated, 55 completed) Evidence B source
  The change over time in total body iron stores differed significantly between groups (p < 0.035); in the low-phytate group of 24 women ferritin fell by a mean 12% from 32 ± 7 to 27 ± 6 µg/L (p < 0.018) and total body iron by 12% from 6.9 ± 1.4 to 5.4 ± 1.1 mg/kg (p < 0.035), while the high-phytate group showed no significant change
• Every trial behind the sourdough and iron story carried a high risk of bias or close to it. — the eight included human intervention studies (n = 8 studies) Evidence B source
  Of the six parallel-group trials, five were rated high risk of bias and one raised some concerns, with the worst domains being deviations from intended interventions and missing outcome data; both crossover trials were rated high risk, mainly for randomisation, deviations from intended interventions and selective reporting; all studies were at low risk for outcome measurement

What cooking and storage change

• Sourdough raises iron availability from bread in the short term and has never been shown to raise anyone's iron. — 8 human intervention studies in healthy or iron-deficient participants (n = 8 studies) Evidence A source
  Acute postprandial studies raised non-haem iron bioavailability, with low-phytate white bread producing a greater two-hour rise in serum iron than high-phytate wholemeal bread, 59 against 30 µg Fe/100 mL, and exogenous phytase raising absorption by 50% for ferrous sulfate and 61% for iron bis-glycine chelate; long-term trials did not improve and in one case reduced ferritin and total body iron; phytate reduction combined with iron fortification did raise haemoglobin in anaemic children

What it does to your health

• Thirty grams of whole grain a day tracks with fewer heart events and fewer deaths, and refined grain tracks with neither. — participants in 68 studies across 24 articles, 46 on whole grains and 22 on refined grains (n = 1624407) Evidence C source
  Per 30 g per day more whole grain, relative risks were 0.98 (0.96 to 1.00) for stroke, 0.94 (0.92 to 0.97) for coronary heart disease, 0.97 (0.89 to 1.07) for heart failure, 0.92 (0.88 to 0.96) for cardiovascular disease and 0.94 (0.92 to 0.97) for all-cause mortality; for refined grains no significant influence was found on stroke, coronary heart disease, heart failure or cardiovascular disease; evidence quality was high for cardiovascular disease and all-cause mortality and low for every refined grain association
• Each extra gram of wheat fibre adds about four grams to a stool and shortens a slow transit by three quarters of an hour. — generally healthy populations across 65 intervention studies (n = 65 studies) Evidence B source
  Each extra gram per day of wheat fibre raised total stool weight by 3.7 ± 0.09 g/d (95% CI 3.50 to 3.84), dry stool weight by 0.75 ± 0.03 g/d and stool frequency by 0.004 ± 0.002 times per day; transit time fell by 0.78 ± 0.13 hours per extra gram per day among people whose initial transit exceeded 48 hours

What people believe that the data does not support

• Cutting fermentable carbohydrates helped people who thought gluten was their problem. — people with non-celiac gluten sensitivity, meaning a doctor had ruled coeliac disease out first, in three included articles, one of which contained two interventions (n = 3 trials) Evidence B source
  Clinical trials identified an association between decreased gastrointestinal symptoms and FODMAP restriction; the review concludes that current evidence still supports the gluten-free diet as first-line therapy while a FODMAP restriction can decrease symptoms in these individuals

The bottom line

The sourdough claim deserves a careful answer, because half of it is right. Fermentation breaks down phytate, phytate binds iron, and in short after-meal studies low-phytate white bread put more iron into the blood over two hours than high-phytate wholemeal did, 59 against 30 micrograms per 100 millilitres. No human study has gone on to show that this raises anybody's iron stores. The longer trials did not improve ferritin, and one of them made it worse. Over twelve weeks, women eating wholegrain bread with the phytate stripped out lost about 12 percent of their ferritin and 12 percent of their total body iron. The women eating the ordinary high-phytate loaf held steady. That trial used rye bread, so read it as a result about phytate rather than about wheat. The review that gathered all eight human studies rated five of the six parallel trials at high risk of bias and both crossover trials at high risk too. Breeding more of a mineral into the grain has hit the same wall. Men eating bread made from zinc-biofortified wheat for six weeks showed no change in plasma zinc at any point, and neither did a 25 milligram zinc supplement added on top. If bread makes you feel bad and a doctor has ruled out coeliac disease, the evidence points at the fermentable carbohydrates as well as at the gluten. Three trials in people with non-coeliac gluten sensitivity found gut symptoms dropping when those carbohydrates were cut, and the reviewers still put the gluten-free diet first in line. That is a small pile of evidence under a very common complaint, and it is worth a dietitian rather than a self-diagnosis.

Sources

1. wht-dr-07 · controlled feeding randomised trial
   RESULTS: Plasma zinc concentrations did not change throughout the study.
   J Nutr, 2021 · checked 2026-09-22
2. wht-dr-01 · randomised parallel-design intervention trial
   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 2026-09-22
3. wht-dr-03 · risk of bias assessment within a systematic review
   Specifically, five studies were rated as having a high risk of bias (36–40) and one study was judged to have some concerns (41).
   Front Nutr, 2026 · checked 2026-09-22
4. wht-dr-02 · systematic review of intervention studies
   Despite convincing mechanistic evidence that the sourdough-fermentation process in bread fabrication improves iron bioavailability, through reduction of phytate, no human studies address this research question with the appropriate control and study quality.
   Front Nutr, 2026 · checked 2026-09-22
5. wht-dr-04 · dose-response meta-analysis of prospective cohort studies
   Per 30-g increase in daily whole grain consumption, the RRs and 95% CIs of stroke, coronary heart disease (CHD), heart failure (HF), CVD, and all-cause mortality were 0.98 (0.96, 1.00), 0.94 (0.92, 0.97), 0.97 (0.89, 1.07), 0.92 (0.88, 0.96), and 0.94 (0.92, 0.97), respectively.
   Am J Clin Nutr, 2023 · checked 2026-09-22
6. wht-dr-05 · systematic review of intervention trials with weighted regression
   Weighted regression analyses demonstrated that each extra g/d of wheat fiber increased total stool weight by 3.7 ± 0.09 g/d (P < 0.0001; 95%CI: 3.50-3.84), dry stool weight by 0.75 ± 0.03 g/d (P < 0.0001; 95%CI: 0.69-0.82), and stool frequency by 0.004 ± 0.002 times/d (P = 0.0346; 95%CI: 0.0003-0.0078).
   World J Gastroenterol, 2015 · checked 2026-09-22
7. wht-dr-06 · systematic review of randomised clinical trials
   Nevertheless, current evidence supports the gluten-free diet still represents first-line therapy. However, a FODMAP restriction can decrease gastrointestinal symptoms in individuals with non-celiac gluten sensitivity.
   Br J Nutr, 2023 · checked 2026-09-22