Which factor has the most significant impact on an individual's resting metabolic rate?

The most significant factor impacting an individual's resting metabolic rate is muscle mass. Muscle tissue is more metabolically active than fat tissue, meaning it requires more energy to maintain. Therefore, individuals with greater muscle mass generally have a higher resting metabolic rate because they burn more calories at rest compared to those with lower muscle mass.

While other factors such as sex, size, and age do play roles in determining resting metabolic rate, muscle mass has the most substantial and direct influence. For instance, while size can affect metabolism, it is largely related to the amount of muscle and fat tissue present. Similarly, sex can influence metabolic rates due to hormonal differences that affect body composition. Age, on the other hand, often correlates with muscle loss, which can decrease metabolic rate over time. However, the underlying reason these factors impact metabolic rate is ultimately linked to muscle mass. Thus, emphasizing the importance of maintaining or increasing muscle mass through resistance training can be an effective strategy for enhancing metabolic rate and overall energy expenditure.