The respiratory quotient (RQ) is a dimensionless number used in metabolic studies to measure the ratio of carbon dioxide produced to oxygen consumed during respiration, indicating which macronutrients are being metabolized for energy. An RQ of 0.7 suggests fat metabolism, 1.0 indicates carbohydrate metabolism, and values in between suggest a mix of both, while values above 1.0 can indicate anaerobic respiration or lipogenesis.