What primarily influences the body's basal metabolic rate?

The body's basal metabolic rate (BMR) is primarily influenced by body composition, specifically the proportion of lean mass to fat mass. Lean body mass, which includes muscles and organs, is metabolically active and burns more calories at rest compared to fat mass. As a result, individuals with a higher percentage of muscle relative to fat generally have a higher BMR, meaning they require more energy to maintain their bodily functions at rest.

While other factors such as physical activity, dietary intake, and sleep patterns can affect metabolism and overall energy expenditure, they do not have as direct or significant an impact on BMR compared to body composition. For instance, while increased physical activity can elevate energy needs, it doesn't change the intrinsic metabolic rate that is determined by the amount of lean tissue. Similarly, dietary intake can influence temporary energy balance, and sleep patterns can affect hormonal regulation, but body composition fundamentally shapes BMR by determining the baseline energy expenditure of the body.