By Roberta H. Anding, M.S., Baylor College of Medicine and Texas Children’s Hospital

Edited by Kate Findley, The Great Courses Daily

Have you ever wondered why your friend could eat a ton and never gain weight, and then heard, “She has good metabolism,” as an explanation? Professor Anding explains how metabolism works and how you can even burn calories at rest. But this doesn’t let you off the hook completely—exercise still matters.

Basal Metabolic Rate

While monitoring caloric intake is important, a basic understanding of metabolism can provide a foundation for developing lifelong strategies for managing weight, losing weight, or preventing weight gain. To understand how this process works, you need to first determine your basal metabolic rate.

Basal metabolic rate is the number of calories required to keep your body functioning at rest for involuntary functions such as heart rate, digestion, respiration, and maintenance of blood pressure and body temperature. If you’re in a room that’s 70 degrees Fahrenheit (°F), for example, the amount of heat that you produce to keep your body at 98.6 °F requires energy.

To digest your food and breathe in and out also require a significant amount of calories. Basal metabolic rate is responsible for approximately 60 percent to 75 percent of the calories we need per day. 

Several factors influence your basal metabolic rate including gender, age, and voluntary movement, which is not exercise but rather small movements like fidgeting or walking from side-to-side while giving a presentation. Voluntary movement is also referred to as non-exercise activity thermogenesis, or NEAT.

Muscle Mass and Metabolism

The biggest influence on your metabolism, though, is your muscle mass. Men have a higher basal metabolic rate than women. This is because men, under the influence of testosterone, have more lean mass (which includes muscle mass, bones, and bodily fluid) than women. 

As we age, we lose about 3 percent of our muscle mass per decade due to loss of lean body mass, known as sarcopenia. Sarcopenia contributes to a poor quality of life as we age and increases the tendency to add body fat.

Many of us in the modern workforce have sedentary jobs and engage in leisure activities that largely involve sitting, such as reading, watching television, or surfing the internet. As you age, you lose that functional lean mass, thus lowering your metabolic rate. The older we get, without physical activity, we start to atrophy.

“I worked with a woman who was 5 foot 6 inches and 118 pounds,” Professor Anding said. “Most of you would probably think she was pretty lean. However … I would describe her as a skinny fat woman.”

When this woman’s body composition was measured, she had 40 percent body fat.

“I saw her trying to get out of a chair, and she had to rock and use momentum to get herself out of the chair,” Professor Anding said. “Why? Because she’d had significant loss of functional lean mass. She might have worn a size 6, but she was clinically obese.” 

The woman dieted frequently and had gone on one weight-reduction plan after another, but she did not exercise. She made the mistake, as many people do, of equating weight with health. The number on the scale can be deceptive as it does not take into account factors such as muscle mass or body fat percentage.

Increasing Muscle Mass

The importance of preserving lean muscle mass and reducing body fat has prompted the American College of Sports Medicine to have launched its campaign, Exercise Is Medicine. The campaign promotes aerobic exercise, such as walking, jogging, or playing golf, along with strength training. 

If you are only focusing on aerobic exercise, this does not encourage lean mass gains the same way as strength training. Many women shy away from strength training because they do not want to become big and bulky. However, this concern is largely unsubstantiated by science as most women don’t have enough testosterone to add the kind of lean mass where they would resemble a male bodybuilder.

When it comes to basal metabolic rate, muscle mass influences the rate at which you burn calories. Therefore, if you incorporate weight training into your exercise regimen, not only will you build up your strength and improve your body composition, but you can also afford to eat more calories than if you were simply engaging in aerobic exercise.

This article was edited by Kate Findley, Writer for The Great Courses Daily.

Professor Roberta H. Anding is a registered dietitian and Director of Sports Nutrition and a clinical dietitian at Baylor College of Medicine and Texas Children’s Hospital. She also teaches and lectures in the Baylor College of Medicine’s Department of Pediatrics, Section of Adolescent Medicine and Sports Medicine, and in the Department of Kinesiology at Rice University.