Of the many factors that affect the basal metabolic rate (BMR), the most important factor is the person's lean body mass or muscle mass. Lean body mass refers to the weight of the body's tissues other than fat, including muscles, bones, and organs.
Muscle tissue is metabolically active and requires more energy to maintain compared to fat tissue. Therefore, individuals with higher muscle mass tend to have a higher BMR. This is because muscles contribute significantly to the body's energy expenditure even at rest.
Other factors that influence BMR include age (BMR tends to decrease with age), gender (males generally have a higher BMR due to higher muscle mass), genetics, hormonal factors, body size and composition, and activity level. However, lean body mass remains one of the most crucial factors affecting BMR, as it directly contributes to the body's energy requirements.