Does Exercise Affect Resting Metabolism?

Can you really boost calorie burning at rest?

by Chris Melby, Dr.P.H.

It is rare these days to pick up a health or fitness magazine from a grocery store shelf without our attention being drawn to an article proclaiming to have the latest information on the best way to exercise in order to boost metabolism. With the high risk for obesity in America, it would seem foolish to pass up reading an expose on newly discovered secrets about how to change our metabolism from a “warm glow” to a “raging fire.” Unfortunately, we are often exposed to considerable misinformation that can leave us frustrated when our implementation of these latest “secrets” falls short of all the metabolic benefits promised.

Components of Energy Expenditure 
Metabolism is a word that, for our purposes, describes the burning of calories necessary to supply the body with the energy it needs to function. There are three major ways we burn calories during the day:

• Resting metabolic rate (RMR),
• The thermic effect of food (TEF),
• Physical activity energy expenditure (PAEE).

RMR is the number of calories we burn to maintain our vital body processes in a resting state. It is usually determined by measuring your body’s oxygen utilization (which is closely tied to calorie burning) while you lay or sit quietly in the early morning before breakfast after a restful night’ssleep. RMR typically accounts for about 65-75 percent of your total daily calorie expenditure.

The TEF results from eating food, and is the increase in energy expended above your RMR that results from digestion, absorption, and storage of the food you eat. It typically accounts for about 5-10 percent of the total calories you burn in a day.

The last component, PAEE, accounts for the remainder of your daily energy expenditure, and, as the name suggests, is the increase in our calorie burning above RMR resulting from any physical activity. Included in PAEE is the energy expended in exercise, the activities of daily living, and even fidgeting. PAEE can vary considerably depending on how much you move throughout the day. For example, your PAEE would be high on a day that you participate in several hours of vigorous sports competition or exercise, while the calories you burn in physical activity would be quite low the next day if you choose to rest and recover.

Your total daily energy expenditure is the sum of these three components— if it is less than your energy intake, you will store most of the surplus energy, especially as body fat. If it is more than your energy intake, you will burn some body stores of energy to provide the needed energy not available from your food.

Is My Metabolic Rate Elevated Following Exercise? 
Your calorie expenditure obviously increases above your resting rate when you exercise, with the magnitude of this increase dependent on how long and hard you exercise. One frequently asked question is “Do we continue to burn “extra” calories after we finish exercising?” In other words, does our energy expenditure remain elevated above RMR for a period of time after we stop the exercise, and if so, does it contribute significantly to our total energy expenditure on the day we exercise? Research has clearly shown that energy expenditure does not return to pre-exercise resting baseline levels immediately following exercise. The amount of this post-exercise elevation of energy expenditure depends primarily on how hard you exercise (i.e., intensity) and to a lesser degree on how long you exercise (i.e., duration).

Endurance Exercise: Exercise of the intensity and duration commonly performed by recreational exercisers (e.g., walking for 30- 60 minutes or jogging at a pace of 8-10 minutes per mile for 20-30 minutes) typically results in a return to baseline of energy expenditure well within the first hour of recovery. The post-exercise calorie bonus for this type of exercise probably accounts for only about 10-30 additional calories burned beyond the exercise bout itself. In athletes performing high intensity, long duration exercise, the post-exercise energy expenditure may remain elevated for a longer period and could contribute significantly to total daily calorie burning. Ironically, such athletes are typically less concerned about this “extra” calorie burning and its implications for body weight regulation than are the recreational exercisers. The average person who does considerably less strenuous exercise will likely experience little meaningful contribution of this post-exercise bonus to their total daily calorie expenditure.

Weight Lifting Exercise: A number of recent studies suggest that vigorous weight lifting exercise may elevate calorie burning above usual resting values for several hours after exercise. However, the average person at the gym who rests for several minutes between sets of exercise will likely not experience a prolonged elevation of postexercise energy expenditure.

Does Exercise Training Increase My RMR?

Endurance Exercise: A number of studies have found endurance-trained athletes to exhibit higher RMR than non-athletes. It appears likely that the combination of high exercise energy expenditure and high energy intake in these athletes can temporarily, but not permanently, elevate their RMR when measured the next morning after exercise. However, there is little evidence that the amount of physical activity performed by recreational exercisers for the purpose of weight control and health promotion will produce any increases in RMR, with the possible exception of such exercise in older individuals. It is clear that for both athletes and non-athletes, young and old, the major impact of exercise on total daily energy expenditure occurs during the activity itself, and not from increases in RMR.

Weight Lifting Exercise: Some fitness enthusiasts have promoted the idea that because regular weight lifting can increase skeletal muscle mass, such exercise will dramatically increase RMR. However, it is estimated that each pound of muscle burns about five-10 calories per day while at rest, so you would have to bulk up quite a bit to increase your RMR. Most people who lift weights for health rather than for body building will not increase their muscle mass enough to have a major effect on RMR.

Summary 
Regular exercise has many benefits and plays an important role in increasing our daily energy expenditure. However, despite what we may read in many popular magazines, the increase in energy expenditure from exercise occurs primarily while we are exercising, rather than due to any sizeable exercise- induced elevations in our resting energy expenditure.

Copywrite belongs to American College of Sports Medicine, ACSM Fit Society® Page, Summer 2004, p. 4-5.