White bread: what the swap trials found

White bread is the loaf most bread trials use as the comparison, so most of what we know about it comes from people eating something else instead. A meta-analysis of 22 randomised trials in 1037 adults, some healthy, some at cardiometabolic risk and some with type 2 diabetes, tested breads with more fibre, whole grain or added functional ingredients against "regular" bread, which was not always white. Over at least two weeks, fasting blood glucose fell by 0.21 mmol/L. The effect sat mainly in the people with type 2 diabetes, and the trials varied a lot, so this is a small change in one group rather than a promise for everyone. HbA1c, the marker of blood sugar over the past few months, showed no clear change. The long-term picture is unsettled. Cohort studies follow people for years and can show a link but cannot prove a cause. In them, white bread was not analysed on its own, only as part of refined grains. Refined grains had no clear link with type 2 diabetes (relative risk 0.95 per 3 servings a day). For heart disease, across more than 875,000 people, the estimate leaned towards higher risk (hazard ratio 1.08), but the range, from about 1% lower to 18% higher, includes no effect, and the authors point to probable confounding. Whole grains, in the same observational diabetes data, were linked with 32 percent lower risk. A medium 28 g slice gives about 76 kcal and 134 mg of sodium.

The numbers, per 100 g

How much is actually in it

• According to the NIH Office of Dietary Supplements, since January 1998 the FDA has required 140 mcg of folic acid per 100 g in enriched breads and other grain products to reduce neural tube defects, which applies to US enriched products and not to bread elsewhere. — US enriched grain products Evidence E source
  140 mcg folic acid/100 g, mean folic acid intake up about 190 mcg/day
• US commercial white bread has over twice the iron (3.36 vs 1.50 mg) and thiamin (0.507 vs 0.22 mg) and over three times the selenium (23.2 vs 7 µg) per 100 g of the UK average white loaf, which shows white bread is not one fixed profile. — Bread, white, commercially prepared (FDC 325871) vs Bread, white, average (CoFID 11-1145) (n = 1 record each) Evidence E source
  Per 100 g US vs UK: energy 270 vs 236 kcal, iron 3.36 vs 1.50 mg, thiamin 0.507 vs 0.22 mg, selenium 23.2 vs 7 µg, sodium 477 vs 400 mg, calcium 211 vs 155 mg, fibre 2.3 vs 2.9 g

What a real portion looks like

• A medium slice of white bread weighs 28 g in the US survey database (thin 24 g, thick 43 g), which at the US table values gives about 76 kcal, 13.8 g carbohydrate and 134 mg sodium per slice. — Bread, white (FNDDS 2707598) portions, nutrients from FDC 325871 Evidence E source
  Per 28 g slice: 76 kcal, 13.8 g carbohydrate, 2.6 g protein, 0.6 g fibre, 134 mg sodium, 0.94 mg iron

What your body absorbs

• In 15 young women, zinc absorption from a 100 g serving of white bread was about 39% at 1.2 mg zinc and 23-27% at 3 mg zinc, and adding folic acid at the usual fortification level did not change it. — 15 healthy women aged 22-33, single test meals 2 weeks apart (n = 15) Evidence B source
  Zinc absorption 38.8% vs 40.6% (low zinc, P = 0.74), 26.7% vs 22.7% (high zinc, P = 0.16) without vs with added folic acid
• In 14 women eating a 4-day diet, calcium added to white bread, rye bread and other foods (about 800 mg a day) did not lower non-heme iron absorption, which stayed at 5-7% against 7.4% on the low-calcium diet. — 14 women aged 21-34, four 4-day diets in random order, 13.2 mg iron a day (n = 14) Evidence B source
  Non-heme iron absorption 7.4% (basic diet) vs 5.2%, 6.7% and 5.1% with calcium sources, P = 0.34
• A systematic review of 8 human studies found that low-phytate white bread raised serum iron more over 2 hours than high-phytate wholemeal bread (59 vs 30 μg/100 mL), a short-term rise that is not the same as better iron stores, since longer trials found no ferritin gain, and no properly controlled human study shows that sourdough fermentation improves iron status. — 8 human intervention studies in healthy or iron-deficient people, 1970-2024 (n = 8 studies) Evidence B source
  2-h serum iron rise 59 vs 30 μg Fe/100 mL, long-term trials no ferritin gain

What cooking and storage change

• According to the NIH Office of Dietary Supplements, baked bread has 20-30% less thiamin than its raw ingredients. — General food supply Evidence E source
  20-30% thiamin loss on baking

What it does to your health

• Across 22 randomized trials in 1037 adults, healthy or at cardiometabolic risk or with type 2 diabetes, eating reformulated breads (more fibre, whole grain or functional ingredients) instead of "regular" or comparator bread, not always white, for at least 2 weeks lowered fasting blood glucose by 0.21 mmol/L, with trials varying widely and the effect concentrated in the type 2 diabetes subgroup, and HbA1c showed no clear change. — Healthy adults, adults at cardiometabolic risk and adults with type 2 diabetes, bread interventions of 2 weeks or longer (n = 1037) Evidence A source
  Fasting glucose MD -0.21 mmol/L (95% CI -0.38 to -0.03), HbA1c MD -0.14 (95% CI -0.39 to 0.10)
• In a crossover trial in 22 healthy adults, swapping 3 slices a day of white bread for one particular high-fibre bread for 2 weeks doubled total dietary fibre intake to 40 g a day and raised a lab measure of gut microbiome diversity in stool, which is not a health outcome in itself. — 22 healthy adults aged 22-55 in Melbourne, 3 slices a day for 2 weeks per phase, 4-week washout (n = 22) Evidence B source
  Fibre intake about 40 g/d vs about 20 g/d, Shannon diversity higher (p = 0.014), butyrate-producing capability p = 0.062
• In a meta-analysis of trials in adults with chronic constipation, rye bread gave 0.43 more bowel movements per week than white bread, from only 48 participants. — Adults with chronic constipation, 2 trials of rye bread vs white bread (n = 48) Evidence A source
  MD +0.43 bowel movements/week (95% CI 0.03 to 0.83)

What people believe that the data does not support

• In 121 people with type 2 diabetes, 100 g of whole wheat, wheat bran or rye bread raised blood glucose over 2 hours about as much as 100 g of white bread (P = 0.093), so a darker loaf is not automatically a lower-sugar one. — 121 adults with type 2 diabetes in Turkey, single 100 g bread test on consecutive days (n = 121) Evidence B source
  No significant difference in glycemic (p = 0.093) or insulinemic (p = 0.297) effect between four breads

What is still unknown

• In 9 longer trials, fibre-reformulated bread lowered LDL cholesterol by 0.16 mmol/L against control bread, but in 3 acute trials it did not lower blood glucose 120 minutes after eating and body measures did not change. — Adults in randomized and non-randomized controlled trials of reformulated baked goods, 12 bread studies pooled (n = 12 studies) Evidence A source
  LDL-C MD -0.16 mmol/L (95% CI -0.27 to -0.04), total cholesterol MD -0.20 mmol/L, 2-h glucose MD 0.63 mmol/L (95% CI -0.25 to 1.51)
• Pooled cohort data link refined grains, the group white bread belongs to, with no clear change in type 2 diabetes risk (relative risk 0.95 per 3 servings a day), while whole grains in the same observational data were linked with 32% lower risk, and white bread was not analysed on its own. — 16 prospective cohort studies, 6 on refined grains (n = 16 cohorts) Evidence C source
  Refined grains RR 0.95 (95% CI 0.88-1.04) per 3 servings/day, whole grains RR 0.68 (95% CI 0.58-0.81)
• Across 17 prospective cohorts with more than 875,000 people, the estimate for refined grain intake and cardiovascular disease leaned towards higher risk (hazard ratio 1.08, 95% CI 0.99-1.18), but the range includes no effect, the authors flag probable confounding, and white bread was not analysed on its own. — Adults in 17 prospective cohort studies, refined grains analysed as a distinct category (n = >875,000) Evidence C source
  CVD HR 1.08 (0.99-1.18), stroke HR 1.06 (0.92-1.23), heart failure HR 0.95 (0.77-1.16)

The bottom line

If you want more fibre, a high-fibre loaf is an easy lever, with limits on what we know. In one 2-week trial of 22 healthy adults, 3 slices a day of one particular high-fibre loaf in place of white bread took total fibre intake from about 20 g to about 40 g a day. That 40 g is the whole day's diet, not the bread alone. The same trial found higher gut microbiome diversity in stool samples, which is a lab measure and not a health outcome in itself. Colour alone will not tell you much about blood sugar, though. In 121 people with type 2 diabetes, 100 g of whole wheat, wheat bran or rye bread raised glucose over two hours about as much as 100 g of white bread. That was a single test meal on each bread, so it says nothing about eating them for months. For constipation, rye bread beat white bread by 0.43 bowel movements a week in a meta-analysis, which sounds solid until you see it pooled just 48 people. What is in your loaf depends on where you buy it. Per 100 g, US commercial white bread has 3.36 mg of iron against 1.50 mg in the average UK white loaf, and 23.2 against 7 micrograms of selenium. According to the NIH Office of Dietary Supplements, since 1998 the FDA has required 140 micrograms of folic acid per 100 g in enriched US breads and grain products, to reduce neural tube defects in babies. This applies to enriched bread in the US. Bread elsewhere may not be fortified, so check the label. Baking itself costs 20 to 30 percent of the thiamin in the raw ingredients. The table values on this page are for baked bread, so that loss is already counted. Adding folic acid did not change how much zinc 15 young women absorbed from white bread, and extra calcium did not lower iron absorption in 14 women. White bread, which is low in phytate (a plant compound that binds iron), raised blood iron over two hours more than high-phytate wholemeal bread. A two-hour rise in blood iron is not the same as better iron stores, and the longer trials in the review found no gain in ferritin, the marker of stored iron. The same review found no properly controlled human study showing that sourdough improves iron status over time. Some questions stay open. Fibre-enriched breads lowered LDL cholesterol by 0.16 mmol/L in 9 longer trials, yet in 3 short trials they did not lower blood sugar two hours after eating, and body weight and waist did not move. Our research has not yet covered safety for specific groups, such as people who must avoid gluten or limit salt, so this page has no safety section. That is a gap in this page. It is not evidence that white bread suits every diet. If a doctor has put you on a diet for a medical condition, follow that advice first.

Sources

1. wbr-b3-12 · agency fact sheet
   In January 1998, the U.S. Food and Drug Administration (FDA) began requiring manufacturers to add 140 mcg folic acid/100 g to enriched breads, cereals, flours, corn meals, pastas, rice, and other grain products [13] to reduce the risk of neural tube defects (NTDs).
   NIH Office of Dietary Supplements, Folate fact sheet for health professionals · checked 2026-09-23
2. wbr-b3-13 · reference dataset
   Bread, white, commercially prepared (FDC 325871) - Energy 270 KCAL - Iron 3.36 MG - Thiamin 0.507 MG - Selenium 23.2 UG - Sodium 477 MG - Calcium 211 MG - Fiber 2.3 G | Bread, white, average (CoFID 11-1145) - Energy 236 kcal - Iron 1.50 mg - Thiamin 0.22 mg - Selenium 7 ug - Sodium 400 mg - Calcium 155 mg - AOAC fibre 2.9 g
   USDA FoodData Central record 325871 and CoFID 2021 (Public Health England, OGL v3.0) code 11-1145 · checked 2026-09-23
3. wbr-b3-14 · reference dataset
   Bread, white (FNDDS 2707598) - 1 small or thin/very thin slice 24 g - 1 medium or regular slice 28 g - 1 large or thick slice 43 g | Bread, white, commercially prepared (FDC 325871) per 100 g - Energy 270 KCAL - Carbohydrate 49.2 G - Protein 9.43 G - Fiber 2.3 G - Sodium 477 MG - Iron 3.36 MG
   USDA FNDDS food 2707598 portions and FoodData Central record 325871 · checked 2026-09-23
4. wbr-b3-07 · RCT (crossover)
   Mean (+/-SD) zinc absorption did not differ significantly in relation to the folate content of the breads at either the low zinc content (38.8 +/- 13.5% and 40.6 +/- 16.5% for A and B, respectively; P = 0.74) or the high zinc content (26.7 +/- 9.3% and 22.7 +/- 6.6% for C and D, respectively; P = 0.16).
   Am J Clin Nutr, 2001 · checked 2026-09-23
5. wbr-b3-08 · RCT (crossover)
   No significant differences in nonheme-iron absorption were found between the BD and the BD supplemented with milk, calcium lactate, or the milk mineral isolate...P = 0.34
   Am J Clin Nutr, 2004 · checked 2026-09-23
6. wbr-b3-09 · systematic review of intervention trials
   low-phytate white bread produced a greater 2 h increase in serum iron than high-phytate wholemeal bread (59 vs. 30 μg Fe/100 mL)...no human studies address this research question with the appropriate control and study quality.
   Front Nutr, 2026 · checked 2026-09-23
7. wbr-b3-11 · agency fact sheet
   For example, bread has 20%–30% less thiamin than its raw ingredients, and pasteurization reduces thiamin content (which is very small to begin with) in milk by up to 20% [3].
   NIH Office of Dietary Supplements, Thiamin fact sheet for health professionals · checked 2026-09-23
8. wbr-b3-01 · meta-analysis of RCTs
   Compared with "regular" or comparator bread, consumption of reformulated intervention breads yielded lower fasting blood glucose concentrations (MD: -0.21 mmol/L; 95% CI: -0.38, -0.03; I2 = 88%, moderate certainty of evidence)...HbA1c (MD: -0.14; 95% CI: -0.39, 0.10; I2 = 56%, very low certainty of evidence)
   Adv Nutr, 2023 · checked 2026-09-23
9. wbr-b3-03 · RCT (crossover)
   High-fibre bread consumption increased total dietary fibre intake to 40 g/d, which was double the amount of fibre consumed at baseline or during the white bread intervention. Compared to white bread, the high-fibre bread intervention resulted in higher faecal alpha diversity (Shannon, p = 0.014)
   Nutrients, 2024 · checked 2026-09-23
10. wbr-b3-04 · meta-analysis of RCTs
    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 2026-09-23
11. wbr-b3-10 · RCT (acute)
    No significant difference was found in either glycemic or insulinemic effects between four types of breads when compared to each other (p=0.093 for glycemic effect and p=0.297 for insulinemic effect).
    Diabetes Res Clin Pract, 2008 · checked 2026-09-23
12. wbr-b3-02 · meta-analysis of controlled trials
    The Meta-analysis of n = 3 acute studies showed that fiber-reformulated bread did not reduce postprandial blood glucose at 120 min after consumption compared to control (MD, 0.63 mmol/L; 95% CI -0.25, 1.51; p = 0.16; I2 = 86%)...LDL-C (MD, -0.16 mmol/L; 95% CI -0.27, -0.04; p = 0.01; I2 = 50%)...No impact was observed on anthropometric indices
    Crit Rev Food Sci Nutr, 2025 · checked 2026-09-23
13. wbr-b3-05 · meta-analysis of cohort studies
    The summary relative risk per 3 servings per day was 0.68 (95% CI 0.58-0.81, I(2) = 82%, n = 10) for whole grains and 0.95 (95% CI 0.88-1.04, I(2) = 53%, n = 6) for refined grains.
    Eur J Epidemiol, 2013 · checked 2026-09-23
14. wbr-b3-06 · meta-analysis of cohort studies
    refined grain intake was not associated with risk of CVD (HR = 1.08, 95% CI, 0.99-1.18, I2 = 70%; 9 cohorts)...Probable confounding from unmeasured variables in studies included in the meta-analyses diminishes the overall quality of evidence.
    Trends Cardiovasc Med, 2024 · checked 2026-09-23