Carol eats 800 kilocalories a day to lose weight. She is 40 years old. Carol has been walking briskly 60 minutes daily for the past 2 months. In her younger days she lifted weights, but she has lost considerable lean body mass over the years because of inactivity. Recently, Carol was diagnosed with hypothyroidism, which lowers metabolism. Which of the following factors increases Carol's energy output and promotes weight loss?