The molecular structure of amylopectin (AP) governs the propensity of its chains to re-associate into crystalline arrangements after starch gelatinization. Amylose (AM) crystallization and AP re-crystallization (i.e. retrogradation) decrease starch digestibility. The aim of this work was to enzymatically elongate AP chains using an amylomaltase (AMM, i.e. 4-α-glucanotransferase) from Thermus thermophilus to promote AP retrogradation and to investigate the impact thereof on in vivo glycemic responses in healthy subjects. Participants (n = 32) consumed two oatmeal porridges (containing 22.5 g available carbohydrates) prepared with or without the enzymatic modification and stored at 4 °C for 24 h. Finger-prick blood samples were taken fasting and at intervals during 3 h following test-meal consumption. The incremental area under the curve (iAUC0-180) was determined. The AMM was very effective at elongating the AP chains at the expense of AM, resulting in increased retrogradation capacity upon storage at low temperature. Nevertheless, postprandial glycemic responses were not different after consumption of either the AMM modified oatmeal porridge or its unmodified counterpart (iAUC0-180 = 73 ± 30 vs. 82 ± 43 mmol min L-1, respectively; p = 0.17). Unexpectedly, promoting starch retrogradation by selectively modifying its molecular structure did not result in reduced glycemic responses, challenging the notion that starch retrogradation negatively impacts glycemic responses in vivo.