HCAR1 and neoplasm: The role of lactate, MCTs, and GPR81 in the tumor microenvironment is predominantly driven by the Warburg effect, i.e., enhanced glycolysis and lactate secretion of cancer cells even at normoxia, resulting in the tumor lactate levels reaching 10–20 mmol · L−1, while the normal concentration is around 1–2 mmol · L−1 [105].