When bread is placed in the freezer and then heated or allowed to thaw before eating, its starch structure changes.
As a result, some of the starch becomes resistant starch, which the body digests more slowly than regular starch.
Because it digests more slowly, glucose or sugar also enters the bloodstream more slowly.
As a result, blood sugar may not rise as much after eating.
Source: National Institute Of Health – The impact of freezing and toasting on the glycaemic response of white bread
Freezing bread changes the structure of its starches. As the bread cools and then thaws, some starch becomes resistant starch, which is digested more slowly than regular starch. This slows the release of glucose into the bloodstream and may reduce the rise in blood sugar after eating. Research suggests that frozen and thawed bread can produce a lower glycemic response than fresh bread, although the exact effect varies by bread type and preparation.