Does more muscle mass rev one’s metabolism? On Sept. 13, 2024, exercise scientist Dr. Milo Wolf delved into the science of this issue, clarifying misconceptions and shedding light on the relationship between muscle mass and basal metabolic rate (BMR) — energy expended at rest.

Key Takeaways

A higher percentage of fat-free mass generally correlates with an increased BMR.

Skeletal muscle tissue is relatively less metabolically active than organ tissues. 

Athletes with higher fat-free mass often have larger organs, potentially developed alongside their muscle mass. The brain is an exception.

Larger organs can contribute to a higher BMR. 

[Related: New Study Finds the Human Metabolism Has Four Stages]

Understanding Metabolism

Metabolism involves all the chemical reactions within the body that convert food into energy. It includes vital functions like breathing, blood circulation, cell repair, and growth. Metabolism comprises the BMR and the energy burned during physical activity.

However, these tissues are relatively metabolically inactive compared to other organs. Wolf explains that the BMR of muscle tissue is around 15 to 30 times lower than that of organs. 

Muscle tissue is three times as metabolically active as fat tissue.

While the BMR of a kilogram of muscle is relatively low, we carry more skeletal muscle weight than any other organs, such as the liver, brain, or heart. 

The BMR Dilemma

People with high fat-free mass tend to have a lower BMR per unit of fat-free mass than sedentary individuals with low fat-free mass. This is attributed to an increased ratio of low metabolic rate tissues (muscle) to high metabolic rate tissues (brain). 

This shift results in a decreased metabolic rate per unit of fat-free mass. Most energy expenditure equations overlook this phenomenon and instead predict total daily energy expenditure as linearly increasing with additional fat-free mass. 

How Hypertrophy and Metabolism Works In Athletes

Research indicates that muscle hypertrophy may stimulate the growth of other organs. Or, at the very least, athletes consistently exhibit larger and heavier organs than non-athletes. (1) Athletes who carried 50 percent more fat-free mass also had larger internal organs. 

A study found that BMR per unit of fat-free mass remained relatively consistent across athletes of varying sizes in men and women. This is because muscle, liver, kidney, and heart mass increase linearly with total fat-free mass. (2)

Wolf concludes that the muscle mass gained via strength training likely increases the total daily energy expenditure in athletes compared to sedentary individuals. However, the primary factor behind this increase in BMR is likely the rise in organ sizes rather than the muscle mass itself.

References

Oshima, S., Miyauchi, S., Asaka, M., Kawano, H., Taguchi, M., Torii, S., & Higuchi, M. (2013). Relative contribution of organs other than the brain to resting energy expenditure is consistent among male power athletes. Journal of nutritional science and vitaminology, 59(3), 224–231. https://doi.org/10.3177/jnsv.59.224

Midorikawa, T., Kondo, M., Beekley, M. D., Koizumi, K., & Abe, T. (2007). High REE in Sumo wrestlers attributed to large organ-tissue mass. Medicine and science in sports and exercise, 39(4), 688–693. https://doi.org/10.1249/mss.0b013e31802f58f6

Featured image: @wolfcoach_ on Instagram

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