What is the key indicator of energy expenditure in an individual?

The key indicator of energy expenditure in an individual is primarily linked to basal metabolic rate (BMR). BMR represents the amount of energy expended while at rest, and it accounts for the majority of an individual's total energy expenditure. BMR is influenced by various factors, including lean body mass, which is the non-fat portion of the body.

Lean body mass, which includes muscles, organs, and bones, has a higher metabolic activity compared to fat mass. Therefore, individuals with more lean body mass typically have a higher BMR, leading to greater energy expenditure at rest. This makes it an important factor when considering overall energy requirements and metabolism.

While caloric intake is relevant in determining energy balance (calories in vs. calories out), it is not a direct measure of energy expenditure itself, and fat mass does not contribute as significantly to energy expenditure compared to lean mass. Thus, lean body mass serves as a crucial aspect of understanding and estimating energy expenditure in an individual, making it the most appropriate choice here.