Reb M, or Rebaudioside M, is a rebaudioside (steviol glycoside) that is rebaudioside A in which the hydroxy groups at positions 2 and 3 of the beta-D-glucosyl ester moiety have both been converted to the corresponding beta-D-glucoside. Found in very low concentrations in the leaves of Stevia Rebaudiana, it is more than 200 times sweeter than sucrose (1). It is a new zero-calorie sweetener that can also be derived from the fermentation of sugar cane.
1. Prakash et al., Development of Next Generation Stevia Sweetener: Rebaudioside M.
Foods. 2014 Feb 27;3(1):162-175. doi: 10.3390/foods3010162.