Involved in the modification of secondary bile acids into iso-bile acids (3beta-bile acids) via epimerization of the 3-OH group through a 3-oxo-intermediate. Catalyzes the reduction of 12-alpha-hydroxy-3-oxo-5-beta-cholan-24-oate (3-oxo-DCA) and 3-oxo-5-beta-cholan-24-oate (3-oxo-LCA) to yield isodeoxycholate (isoDCA) and isolithocholate (isoLCA), respectively. Is also able to catalyze the reduction of 3-dehydrocholate (3-oxo-CA or 7alpha,12alpha-dihydroxy-3-oxo-5beta-cholan-24-oate) and 7-alpha-hydroxy-3-oxo-5-beta-cholan-24-oate (3-oxo-CDCA), into isocholate (isoCA) and isochenodeoxycholate (isoCDCA), respectively. Accepts both NADPH and NADH as cosubstrates. The conversion of the abundant bile acid deoxycholate (DCA) into isoDCA by the gut bacterium R.gnavus favors the growth of the keystone commensal genus Bacteroides, since isoDCA is less cytotoxic than its parent compound, DCA; iso-bile acids have thus a potential role in modulating gut community composition. 3-oxo-5beta-cholan-24-oate + H(+) + NADPH = isolithocholate + NADP(+) 12alpha-hydroxy-3-oxo-5beta-cholan-24-oate + H(+) + NADPH = isodeoxycholate + NADP(+) 12alpha-hydroxy-3-oxo-5beta-cholan-24-oate + H(+) + NADH = isodeoxycholate + NAD(+) 7alpha,12alpha-dihydroxy-3-oxo-5beta-cholan-24-oate + H(+) + NADPH = isocholate + NADP(+) 7alpha-hydroxy-3-oxo-5beta-cholan-24-oate + H(+) + NADPH = isochenodeoxycholate + NADP(+) kcat is 9430 min(-1) with 12alpha-hydroxy-3-oxo-5beta-cholan-24-oate as substrate. Belongs to the short-chain dehydrogenases/reductases (SDR) family.