The RMR is largely determined by skeletal muscle mass, which represents a considerable source of energy demand because of energy consuming processes such as protein synthesis, ionic regulation, and heat generation. For non-athletes, RMR is often the largest component of energy expenditure (more than 50%), but for athletes in hard training, RMR may represent less than 1/3 of TDEE. This is not to say that RMR in non-athletes exceeds that of athletes though, as athletes have a higher mass-specific RMR. Diet-induced thermo genesis represents the cost of processing and storing nutrients and depends mainly on energy intake. Because DIT is difficult to measure reliably in humans, it is usually estimated as representing 10% of TDEE.
This expenditure can vary somewhat with the macronutrient composition of the diet, with diets high in protein increasing the DIT above 10% whereas diets high in fat lower it. Even with these slight variations, DIT is the lowest contributor to TDEE. Physical activity traditionally represents energy expenditure attributed to voluntary movement and is the most variable component of TDEE. Physical activity increases energy expenditure because, among other things, there are energy costs associated with actin/myosin cross-bridge cycling, ion pumping, hormone synthesis, etc. Of note is a relatively new concept termed non-exercise activity thermo genesis (NEAT), which has been coined to include very low intensity activity like fidgeting or simple standing, and is incorporated into measurements of physical activity
