A 42-year-old man with a history of chronic weight loss and heat intolerance is referred to the metabolic clinic. He reports progressive fatigue, excessive sweating even at rest, and a resting metabolic rate 35% above predicted normal. Thyroid function tests and catecholamine levels are normal. Genetic testing reveals a heterozygous mutation in the UCP1 gene (uncoupling protein 1). Which investigation would be most appropriate to assess the functional consequence of this mutation on mitochondrial energy coupling?