Bile acids are surfactant molecules that form micelles to let the intestine absorb fats more easily. In the 1920s, before modern spectrometers existed, Wieland oxidatively broke cholic acid and related molecules into small pieces and then deduced their overall structure from the fragments. He was the first to show that bile acids possess a four-ring steroid nucleus with a straight side chain. This discovery laid the groundwork for research on cholesterol, steroid hormones, and eventually drugs for lipid disorders. Determining chemical structure is the first step toward understanding function and applying that knowledge to society.

Wieland established a stepwise oxidative degradation protocol using chromic acid and concentrated sulfuric acid, quantitatively analyzing the low-molecular acids released from cholic acid. From these data he concluded that bile acids are saturated tetracyclic compounds with a C17 steroid core (cholanic acid nucleus) and a C8 side chain, bearing stereospecific hydroxyl groups. By explaining their amphipathic nature at the molecular level, he hinted at roles of bile acids in lipid transport and signaling—topics now central to metabolomics. His degradation strategy inspired natural-product total synthesis and underlies the preparation of the well-known Wieland–Miescher ketone. From clarifying biosynthetic pathways to aiding the development of liver drugs, his work bridged fundamental chemistry and biomedical application.

Wieland cleaved cholic and deoxycholic acids under CrO3–H2SO4 conditions, quantified the resulting dicarboxylic acids, and thereby established both the linearity of the C24 side chain and the stereochemical positions of the hydroxyl groups. Bromination–dehydration followed by reductive manipulations proved the saturation of the A and C rings, positioning bile acids as cholanic acid derivatives with a tetracyclic C17 nucleus and a straight C8 side chain. The structural parameters he reported became the starting point for Ruzicka’s tetrahydronaphthalene concept and Windaus’s final elucidation of cholesterol. The eponymous Wieland degradation remains a staple in steroid analysis and natural-product retrosynthesis. His methodology has also been applied to mechanistic studies of bile-acid metabolic enzymes (e.g., CYP7A1, BAAT), offering a chemical rationale for site-specific oxidative cleavage steps.