The primary nutrient in white bread is starch, with two slices providing more than 20 g of starch, according to the U.S. Department of Agriculture National Nutrient Database. White bread also contains protein and small amounts of fat and other sugars. To benefit from these nutrients, however, your body must digest the bread into components small enough to be absorbed.
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The process of starch digestion starts in your mouth, where amylase in your saliva snips the large starch molecules into sugars called glucose. As the white bread moves from your mouth to your stomach and finally to your small intestine, additional amylase secreted by your pancreas digests any remaining starch molecules until only glucose remains. The protein in white bread undergoes digestion in your stomach and small intestine, where stomach acid and protein-digesting enzymes cleave the protein molecules into single amino acids. The small amount of fat in white bread gets digested in your small intestine, where lipase splits each fat molecule into three fatty acids and a glycerol.
Following digestion, the cells lining your small intestine absorb the molecules of glucose, amino acids, fatty acids and glycerol released from the white bread you ate. Glucose is the most abundant product of white bread digestion, and this nutrient serves as a fuel source for all the cells of your body. Your brain and kidneys in particular rely on glucose to function properly. The amino acids from the protein in white bread – nearly 4 g in a 2-slice serving – help build and repair muscles and tissue, support your immune system and synthesize red blood cells. Although white bread contains less than 2 g of fat per 2 slices, this nutrient helps maintain your cell membranes and allows for normal growth and development once the digestion process releases its components to your cells.
If you suffer from celiac disease, the proteins in white bread may upset your digestive system. Wheat, the predominant ingredient in white bread, contains a protein known as gluten, and this protein gives bread dough its characteristic elasticity. However, celiac disease results in an allergic reaction to gluten where the protein molecules inflame and, in severe cases, damage the cells lining your small intestine. You should avoid white bread and any other source of gluten if you have celiac disease or have difficulty digesting gluten.
White bread contains just over 1 g of fiber in two slices, which is a very small amount. Although your body does not digest fiber, it is important to your digestive health. By providing bulk to waste products, fiber helps keep your digestion regular and lowers your risk of constipation. Whole grain bread is preferable to white bread as a source of fiber.