Refrigeration and reheating of cooked rice increases resistant starch content by more than 2.5-fold and reduces postprandial glucose and insulin responses.

Study: The impact of starchy food structure on postprandial glycemic response and appetite: a systematic review with meta-analysis of randomized crossover trials

Cooling cooked rice changes its starch in a way that makes your body absorb less sugar from it, which lowers your blood sugar spike after eating. The study found this really works.

Study: The Effect of Cooking and Cooling Chickpea Pasta on Resistant Starch Content, Glycemic Response, and Glycemic Index in Healthy Adults

Cooling and reheating chickpea pasta made it contain almost double the resistant starch and lowered blood sugar spikes after eating—exactly what the claim says happens with rice. Even though it’s pasta, not rice, the same cooling trick works the same way.

Study: Impact of Post-Cooking Storage on the Glycemic Profile of Boiled Rice: Integrating Glycemic Index, Resistant Starch, and Post-Technological Stability

Cooling cooked rice, especially freezing it for a long time, turns some of its starch into a type that your body can't digest easily, which helps keep your blood sugar from spiking after eating. This matches what the claim says, though the biggest effect happens with freezing, not just fridge cooling.

Study: Microwave reheating enriches resistant starch in cold-chain cooked rice: A view of structural alterations during digestion.

Cooling and then microwaving cooked rice makes some of its starch harder for your body to digest, which means it turns into less sugar in your blood. This study found that after cooling and reheating, about 30% of the starch became resistant to digestion.