Thiamine deficiency decreases cellular metabolism because:

Thiamine deficiency results in decreased energy production because TPP interferes with its coenzyme for pyruvate and alphaketoglutarate dehydrogenase.Chronic peripheral Neuritis, Beriberi & Wemicke Encephalopathy with Korsakoff's Psychosis.Thiamin requirements increase in excess intake of carbohydrates and its deficiency leads to decreased energy production