Kravitz on Calorie Burning

I wanted to post this because it clears up a lot of misconceptions on calorie burning.

By Len Kravitz, PhD

As personal trainers, you are continually seeking new and better programs to help clients attain their aerobic activity goals and maximize caloric expenditure in their endurance workouts. To help you better understand fat burning, caloric expenditure and exercise, here are answers to four controversial questions on this subject.

Question #1: How does high-intensity interval training (HIT) help burn more fat?

Answer: As exercise intensity increases, the body uses more carbohydrate as fuel. However, scientists feel that at the cellular level this overloading stimulus also involves some of the same molecular signaling messages that induce increases in muscle capillary density, mitochondria proteins (energy factory of cells), fatty-acid oxidation (burning) enzymes and other regulatory proteins (Burgomaster et al. 2008; Baar 2006). So, the connection between HIT and improved fat metabolism appears to be associated with adaptation changes that occur at the molecular level of muscle.

Question #2: How many more calories do you burn with the addition of each pound of muscle?

Answer: The scientific estimation is approximately 7 kilocalories per pound per day (Elia 1992). However, the key point is not so much that caloric yield increases from this additional muscle; rather, it is that the person becomes much more capable of working out longer and harder. It is this training effect that adds to the caloric deficit from exercise.

Question #3: Will you burn more calories from fat if you exercise first thing in the morning on an empty stomach?

Answer: The substrate that most effectively powers your workout is carbohydrate. Fat contributes, but carbohydrates in the form of glucose are the main exercise fuel. After a night’s sleep, the muscles are greatly depleted of glycogen (stored glucose) and therefore lack the energy substrate they need for exercise. In addition, the brain utilizes glucose for all of its fuel needs. Consequently, exercising first thing in the morning on an empty stomach can impair the muscles and some brain functions. Clients should have a light carbohydrate snack (e.g., fresh fruit, yogurt and trail mix) before working out, to properly “fuel up” and safeguard themselves from bodily harm.

Question #4: Why is caloric expenditure lower during upper-body exercise?

Answer: Upper-body exercise is generally complicated by the small muscle mass in the upper body relative to the lower body. This muscle mass is less effective at inducing the return of blood flow to the heart, thus decreasing the volume of blood pumped by the heart each beat. Also, for a given intensity, contraction of the upper-body musculature provides greater resistance to blood flow than occurs with lower-body exercise, resulting in a greater increase in blood pressure. These factors lead to a lower energy (caloric) expenditure from the upper-body muscles.

Determining caloric expenditure during exercise

At rest, the body expends energy to maintain the functions of life- sustaining cells. The heart’s continual pumping of blood demands energy, as does the continual ventilation (movement of air into and out) of the lungs. Maintaining a life-supporting environment within and around cells requires a constant breakdown of certain energy-releasing molecules. This energy is also used to form the molecules necessary for repairing cells, storing energy (glycogen and triglycerides), fighting infection and processing nutrients obtained from digestion. These energy-demanding functions form the body’s resting metabolic rate (RMR), which can vary from approximately 800 to 1,500 kilocalories (kcal), depending on body size and temperature, muscle mass, percent body fat, diet, health status and glandular function.

The body uses adenosine triphosphate (ATP) as a chemical means to perform cellular work. Exercise adds to caloric expenditure, as muscle contraction involves the need to repeatedly form and break down ATP. The energy that is released fuels the contraction of skeletal muscle, thereby adding to the energy demands of the body. During exercise the increase in caloric expenditure is predominantly due to the contraction of skeletal muscle. A moderate energy increase comes from the energy demands of the heart and ventilatory muscles in the lungs.

Len Kravitz, PhD, is the program coordinator of exercise science and a researcher at the University of New Mexico, where he has won the Outstanding Teacher of the Year award. In 2006, he was honored as the Can-Fit-Pro Specialty Presenter of the Year and as the ACE Fitness Educator of the Year. Last year he received the 2008 Can-Fit-Pro Lifetime Achievement Award.