White bread has a high glycemic index, commonly cited between 71 and 75, and this site uses 73 as its working value. Whole wheat sandwich bread tests almost the same, which surprises most people. Bread isn't automatically off-limits: the number of slices decides the glycemic load, and fermentation and grain structure are what move the GI itself.
The numbers
The international GI tables average white wheat bread at 75 and whole wheat (wholemeal) bread at 74, and individual white bread tests in the University of Sydney GI database cluster around 71 to 75, depending on the product. Whole wheat sandwich bread lands in the same high range because fine milling breaks down the grain's structure even when the flour is technically "whole."
| Bread | Typical GI | Why |
|---|---|---|
| White sandwich bread | High (about 71 to 75) | Finely milled flour with minimal intact structure |
| Whole wheat sandwich bread | High (about 74) | Milling breaks the grain down even when it's technically whole |
| Sprouted grain bread | Medium (about 55 to 67) | Germinated grain; a modest step down from white bread |
| True wheat sourdough | Low (around 54) | Lactic acid fermentation reduces how available the starch is to digestive enzymes |
| Specialty grain bread with intact kernels | Low (about 53) | Whole kernels keep their structure, so starch digests more slowly |
Sources: white bread (75 ± 2), whole wheat (74 ± 2) and specialty grain bread (53 ± 2) from Atkinson, Foster-Powell and Brand-Miller, International tables of glycemic index and glycemic load values: 2008, Diabetes Care; sourdough and sprouted breads from tested entries in the University of Sydney GI database. GI bands: low 55 or below, medium 56 to 69, high 70 or above.
Two slices of white bread carry roughly 24 grams of available carbohydrate: 73 × 24 ÷ 100 ≈ 18, near the top of the medium band before anything goes on top. One slice has about 12 grams: 73 × 12 ÷ 100 ≈ 9, a low load. Run your own slices in the glycemic load calculator, and see where bread sits among other staples in our glycemic index and load chart.
Why white bread raises blood sugar quickly
Three things stack up. The flour is finely milled, so digestive enzymes reach the starch almost immediately. Baking gelatinizes that starch, swelling and loosening it so it breaks down faster still. And there's little intact fiber left to slow things down. The same story applies to most whole wheat sandwich loaves, which is why they test nearly the same. You'll see the same processing logic at breakfast on our oatmeal page, where instant oats outpace steel-cut for the same reason.
"Bad" is the wrong frame, though. White bread is a fast-digesting carbohydrate whose real effect depends on how many slices you eat and what goes on them. Protein, fat and vegetables in a sandwich slow the meal down; a sweet spread does the opposite.
Why sourdough and sprouted grain behave differently
Sourdough's lower GI comes from fermentation, not just the flour. Research on sourdough fermentation (Scazzina et al., Journal of Cereal Science, 2009, and later studies building on it) points to lactic and acetic acid produced during fermentation reducing how available the starch is to digestive enzymes and slowing gastric emptying, both of which blunt the blood sugar rise compared with bread leavened only with baker's yeast.
One caveat: "sourdough" on a label doesn't guarantee a lower GI. Tested sourdough entries in the Sydney database range widely, from the high 40s to the low 80s. Loaves made mostly with starter and a long fermentation are the ones that tested lower; a loaf with a little sourdough added for flavor can behave much like ordinary white bread. The ingredient list helps: a true sourdough lists starter or culture and little or no added yeast.
Sprouted and whole-kernel breads
Sprouted breads in the University of Sydney database test at about 55 to 67, in the medium band, so sprouted bread is a modest step down from white bread, similar to a true sourdough, not a low-GI food. "Sprouted" on a label means the grain was germinated before milling or mashing. "Whole kernel" or "grain" breads, where you can see intact seeds and grains in the slice, are a different category: the international GI tables average these specialty grain breads at about 53, because the unbroken kernels keep their structure.
Sourdough (GI around 54, low) works as a near drop-in replacement for a white sandwich loaf, if it's a true long-fermented loaf. Sprouted grain bread (about 55 to 67, medium) is a similar step. Dense breads with visible whole kernels are the bigger step down. If pasta is more your speed, our pasta page covers the same al-dente-and-swap logic for noodles, and our white rice page covers it for grains generally. See our low-GI swaps guide for bread, rice, and pasta together.
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