ABSTRACT
“A calorie is a calorie” is a common refrain in nutrition science conveying the idea that energy intake, and not diet composition, is the dominant factor when it comes to weight loss. This concept has a long history and is often incorrectly identified with the first law of thermodynamics. However, it is physiology and not physics that determines how the body responds to differing diet composition in terms of energy expenditure, energy partitioning, and appetite control. In this chapter, I will describe the history of these ideas and summarize the main physiological mechanisms by which the body adapts its metabolism to changing dietary macronutrients. I will also discuss the importance of considering diet composition in a broad sense beyond its traditional focus on nutrients. It may be that non-nutrient factors of the diet are at least as important as macronutrient composition when it comes to influencing appetite and the control of energy intake.
