Lactic acid (2-hydroxypropanoic acid) is an endogenous agonist of the hydroxycarboxylic acid receptor 1 (HCAR1) with an EC50 of 4.2-5 mM. This glycolysis product is utilized in research areas including immunometabolism and tumor biology, serving as a key metabolite for in vitro and in vivo studies of cellular energetics and signaling.