Does Bread Cause Constipation? What Our Data Says

2026-09-11 · By the Diet for Constipation data editorial team · 1,584 words

Does bread cause constipation? For the white loaf, this is the one food folklore blames that our data partly backs — white bread carries 2.3 to 2.7 grams of fiber per 100 grams in our database, roughly a third of what whole-wheat delivers, and a controlled feeding trial found people passed stools less frequently on a refined-grain diet than on a whole-grain one. But the verdict is not "bread is bad." The verdict is that refinement is the crime, and the bakery aisle splits cleanly along that line. Here is how the call breaks down.

Why the white loaf rates red

The mechanism is not mysterious, and unlike most food folklore it survives contact with the label. White flour is made by stripping the wheat kernel of its bran and germ — the two parts that carry almost all of the fiber. What survives the sifting is the starch-heavy endosperm, and the loaf it bakes into shows the damage plainly. In our 202-form bread database, standard commercial white bread lands at 2.7 grams of fiber per 100 grams. French and Italian loaves, made from the same refined flour, sit just below at 2.1 to 2.2. The sandwich whites rate worst of all: the bread in fast-food submarine sandwiches carries 0.7 to 1.6 grams per 100 grams, which is effectively a fiber-free delivery vehicle.

Two slices of standard white bread therefore contribute about 1.3 grams of fiber — against a daily target of 25 to 30. And bread is not a side dish in most diets; it is the base layer. Sandwiches, toast, burger buns, the basket at dinner — white bread tends to show up multiple times a day, and every appearance crowds out a slice of the whole-wheat version that would have tripled the fiber count. That combination — low fiber per bite, high frequency of appearance — is exactly the profile that earns a red rating on this site. It is the same opportunity-cost logic as our white rice verdict, with bread as the more entrenched habit.

What the evidence actually finds

Bread is unusual among folk-blamed foods: the perception data and the trial data point the same direction.

The perception survey with white bread at the top. A 2026 survey from King's College London put a list of 143 foods and beverages before 204 UK adults who reported constipation and asked which ones they believed made their symptoms worse (PMID 41562174). White bread led the entire list — named by 69% of respondents, ahead of refined sugary cereals (61%) and processed red meat (60%). Two details make this survey more than folklore-tallying. First, the same respondents ranked prunes (85%), dates (71%) and prune juice (70%) as top relievers — a ranking that clinical trials on kiwifruit, prunes and psyllium have since validated, so this cohort's collective instincts are not randomly wrong. Second, white bread was named far more often than chocolate, the classic folk villain — the blame tracks the refined-grain profile, not old wives' tales.

The feeding trial. Perception is still perception, so the stronger evidence is interventional. A Danish randomized cross-over trial put 50 adults on two eight-week diets in a fixed order with a washout between (PMID 37324737): a whole-grain diet providing at least 75 grams of whole grain per day, and a refined-grain diet providing under 10. Stool frequency at the end of the refined-grain period was significantly lower than at the end of the whole-grain period (p = 0.001), and the whole-grain period also increased the water content of the stool (p = 0.007). Fiber intake was the main thing that changed between the two diets — this is the closest a trial gets to feeding someone white bread instead of wholemeal and watching what happens.

The honest asterisks. Neither study tested bread as a single food: the survey captures belief, and the trial manipulated whole grain intake across the whole diet. No one has run a bread-only randomized trial, and no study has isolated white bread's effect on transit in isolation. What we can say honestly: the food people blame most is also the one whose removal of fiber is measurable on the label and whose refined-grain pattern is associated with slower output in a controlled trial. Most folk villains got that reputation without either.

The whole-wheat escape clause

The red call attaches to refinement, not to the grain. Swap the loaf and the same food group flips:

That is the entire verdict in one table: "bread" is not one food. A sandwich built on 2.3-gram white is a different decision from the same sandwich on 6-gram whole-wheat — same fillings, triple the fiber base. If you take one action from this page, make it this swap; nothing else in the bread aisle moves the number this far for the same effort.

The sandwich math: where bread actually costs you

Bread's red rating compounds through what it hosts. The classic lunch — a sub on white bread with deli fillings — pairs a 1-gram fiber base with fillings that rarely add any, which is why the sandwich whites in our database end up at 0.7 to 1.6 grams total. We made the same point from the other side in the eggs verdict: the white toast under the breakfast eggs was the real deficit in that plate. Bread is the base layer of both meals, and a base layer set at 2 grams fiber caps what the whole plate can reach.

Rebuild the base and the meal follows. The same sub on whole-wheat or a multigrain roll starts at 6 grams; add half a cup of the beans ranked on our legumes list and lunch alone clears a third of the daily fiber target. The full rebuild playbook lives in Best Foods for Constipation, and the loaves that earned their red ratings sit on Foods That Cause Constipation.

Where the white loaf ends up, and why

Red for the white loaf — the rare folk accusation that the label, the perception survey and a feeding trial all support — with a standing green escape clause for whole-wheat and multigrain. Bread is not the enemy; the sifted flour is. The full red-flag board and its escape routes live in Foods That Cause Constipation.

Sources

• USDA FoodData Central — bread, white, commercially prepared; bread, french or vienna (incl. sourdough); bread, Italian; bread, whole-wheat; bread, multi-grain; fast foods, submarine sandwich (white bread forms)
• Farsi DN, Whelan K, Dimidi E — Perceptions of the Role of Diet Among People With Constipation. J Hum Nutr Diet, 2026 (PMID 41562174)
• Procházková N, Venlet N, Hansen ML, et al. — Effects of a wholegrain-rich diet on markers of colonic fermentation and bowel function: a randomised controlled cross-over trial. Front Nutr, 2023 (PMID 37324737)
• Related: the eggs verdict — the zero-fiber partner on the breakfast plate, and where the white-toast deficit came from

→ All Food Verdicts

→ All Best Lists

This article is informational only, based on published research and USDA data. It is maintained by our data editorial team — not medical professionals. Nothing here is medical advice.