Resting energy expenditure (REE) is most affected by

• A. Body composition
• B. Physical activity
• C. Time of day
• D. Food intake

Answer: (A)

Resting energy expenditure is driven by the amount of metabolically active tissue in the body. Lean mass—including organs like the liver, brain, heart, and muscles—has high energy needs even at rest, so more lean mass directly raises REE. Fat mass contributes less to REE because adipose tissue has a lower metabolic rate. While time of day and recent meals can modulate overall energy expenditure, REE is measured after fasting to capture the baseline resting rate, which is determined mainly by body composition. Physical activity affects total daily energy expenditure and, over time, can increase REE if it builds lean mass, but the resting rate itself is set primarily by how much lean tissue a person has.