Why does thiamin deficiency cause beriberi?

Thiamin (vitamin B1) is essential for the metabolism of carbohydrates, where it functions as a coenzyme in its active form, thiamin pyrophosphate.12

When thiamin is absent, the body cannot properly process carbohydrates, leading to an accumulation of pyruvic acid and lactic acid in the tissues. These metabolic byproducts are believed to be the primary cause of the neurological and cardiac damage characteristic of [beriberi]. This condition manifests as nerve degeneration (neuritis) and heart failure, often resulting in symptoms like muscle weakness, limb numbness, and edema.12