The body is an amazing organism and is made up of many different elements, including various tissues, bones, organs and fluid. The two that we seem to focus on the most, when it comes to exercise and our health, are muscle and fat. We exercise and monitor our nutritional intake to build one, muscle, while trying to lose the other, fat (also known as adipose tissue).
Skeletal muscle is the most abundant tissue in the body accounting for approximately 42% and 35% of body weight in men and women respectively. An average male who weighs 185 pounds would have about 78 pounds of lean muscle tissue while a female who weighs 140 pounds has approximately 49 pounds of lean muscle tissue. The remaining body weight, once muscle and fat are accounted for, includes water, mineral, bone and organ weight (the average human heart weighs about 10 oz. while the brain weighs about 3 lbs.). That same average male that where talking about may have, on average, about 25% body fat (or 46 pounds of fat) while that average female may have 30% body fat (or 42 pounds of fat).
One of the amazing things about muscle tissue is that it has the ability through regular, progressive exercise, to increase in size (known as muscle hypertrophy). Donnelly and colleagues have reported that strength training studies (lasting from 8 to 52 weeks) have shown increases of 2.2 to 4.5 pounds of muscle mass. On the other hand, fat tissue typically decreases in size when an exercise prescription is consistently followed. In addition to increasing in size, muscle can also get stronger and with additional training, improvements can be seen in endurance capacity, power output and force production as well.
Fat is stored in the body in the form of triglycerides and also stored in fat cells which are called adipocytes. According to Coyle, about 50,000 to 60,000 calories of energy are stored in fat cells throughout the body. Fat can also be stored within skeletal muscle cells. Protein stores in muscle can account for about 30,000 calories of energy. Muscle tissue can contribute approximately 20% of the body’s total daily energy expenditure compared to 5% for fat tissue.
The photo shows equivalent amounts of both muscle and fat (5 lbs.) but the same amount of muscle, which is more dense, takes up one-third less space compared to fat. Five pounds of muscle and fat may in fact weigh the same but that is where the similarities end. Muscle tissue, pound for pound, requires 3-4 times more calories to maintain compared to fat and is important in the process of energy metabolism. A pound of metabolically active muscle tissue requires 5-7 calories per pound while fat tissue is less metabolically active, needing about 2 calories per pound.
Finally, muscle plays an important role in the aging process. With advancing age we experience a loss of exercise capacity. This is due to first, to a decline in skeletal muscle mass and strength during aging and then a decrease in maximal oxygen uptake mainly due to a drop in maximal heart rate, according to Henning Wackerhage, PhD, a Senior Lecturer in Molecular Exercise Physiology at the University of Aberdeen.
Marieb, EN and Hoehn, K. (2010). Human Anatomy and Physiology (8th ed.). San Francisco: Benjamin Cummings.
Elia, M. (1999). Organ and Tissue Contribution to Metabolic Weight. Energy Metabolism: Tissue Determinants and Cellular Corollaries. Kinney, J.M., Tucker, H.N., eds. Raven Press. New York.
Donnelly, J.E., Jakicic, J.M., et. al. (2003). Is Resistance Training Effective for Weight Management Evidence-Based Preventive Medicine. 1(1): 21-29.
Wackerhage, H. (2014). Molecules, Aging and Exercise in Molecular Exercise Physiology. Routledge.
Coyle, EF. (1995). Fat metabolism during exercise. Sports Science Exchange, 8(6):59.