We identify IFN-I as sufficient and necessary to induce wide-spread metabolic reprogramming in hepatocytes and specific activation of the hepatic tryptophan-kynurenine pathway via Tdo2. The resulting infection-associated drop of the serum tryptophan to kynurenine ratio seems to be highly compensated during viral infection in vivo. The gene discussed is TDO2; the disease is viral infectious disease.