What it is
A sugar alcohol fermented from corn glucose — bulk and sweetness with almost no calories, the base of most "keto" sweeteners.
Why your biology objects
Unlike the other polyols, erythritol is ~90% absorbed and excreted unchanged, so it skips the laxative effect — but recent work links circulating erythritol to heightened platelet aggregation and clotting risk. A molecule your metabolism has no evolutionary history with at these blood levels.