Metabolic food reactions differ from other adverse reactions because toxicity occurs only when the food is eaten in excess and improperly prepared. Lima beans and related seeds cause cyanosis by which mechanism?

• A. Releasing hydrogen cyanide on contact with stomach acid
• B. Interfering with utilization of Iodine
• C. Mimicking mineralocorticosteroids
• D. Interfering with oxidation of fatty acids

Answer: (A)

Lima beans contain cyanogenic glycosides that release hydrogen cyanide when they come into contact with stomach acid. When the beans are eaten in excess or not prepared properly, these glycosides are hydrolyzed in the gut, releasing HCN. Cyanide then inhibits cytochrome c oxidase in the mitochondrial electron transport chain, blocking cellular respiration and causing tissues to experience hypoxia despite adequate blood oxygen. This is why toxicity from these foods occurs specifically with improper preparation or large doses. The other options describe different toxic mechanisms that don’t explain cyanide release from lima beans.