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Harvard Commentaries
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Focus on Fitness Focus on Fitness
 

Burning Calories With Exercise


May 04, 2014

By Howard LeWine M.D.

Brigham and Women's Hospital


People choose to exercise for a variety of reasons: to feel more energetic during the day, because they enjoy the challenge of becoming fit, to get the health benefits, and because they want to look good. These are all fine motivations to get you started and usually help somewhat to keep you going. But on a day-to-day basis, most of us want the gratification of knowing how many calories we burned with that agony and sweat.

Metabolism at Rest and With Exercise

A completely sedentary person burns between 800 and 1,500 food calories per day, creating the energy for the daily obligatory functions of all the cells in the body. This called our basal metabolic rate, or BMR. A food calorie is actually equal to 1,000 chemical calories; that is why the abbreviation for a food calorie is Kcal.

Muscle cells, like most other cells in the body, can't directly convert calories from carbohydrate and fat into the energy needed to do the cells' work. The energy from food calories first needs to be transferred to ATP (adenosine triphosphate) molecules. Inside our cells, ATP molecules act like tiny power packs than can rapidly store energy, release it on demand, and then quickly get recharged.

Most types of cells in the body have a limited capacity to increase energy expenditure because the demands for energy are just not there. These cells might need to work a little harder if they need to fight an infection or correct a chemical imbalance, for example. On the other hand, muscle cells have a much greater ability to cycle ATP and thereby increase energy expenditure and calorie burning.

Researchers have looked at what factors contribute to calorie burning with exercise. The two most important contributors are the contracting skeletal muscles and the increased work of the heart and lungs. The frequency and power of muscle contractions are by far the biggest determinants of the number of calories burned during exercise. Faster heart and breathing rates also burn calories, but not nearly as much as movement of legs, arms, abdominal and back muscles.

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Three Ways Exercise Burns Calories

Exercise burns calories not just while you are doing the workout. In the immediate post-exercise period, you use energy and burn calories to bring your body back to its normal resting state. Heart rate and breathing rate come down; lactic acid gets cleared from inside cells and in the blood; oxygen and energy stores are replenished; and elevated body temperature is cooled. All of these processes use up calories.

A third way that exercise leads to higher daily caloric burn is a long-term effect. As you begin exercising regularly and building more muscle mass, you will increase your basal metabolic rate. You don't actually increase the amount of resting energy spent per pound of muscle — that doesn't change. But because you have more lean muscle weight, the total resting calorie burn is higher. In addition, there is some evidence that post-exercise calorie burn is also higher when you have more muscle mass. With light or no-resistance exercise, you won’t build much extra muscle. It takes resistance or strength training to get this third calorie-burning benefit from exercise.

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Measuring Calories Burned

If you are just curious about the number of calories an average person expends on any particular type and duration of exercise, there are plenty of tables online and in print that can estimate this for you. To get a personal calorie score, you can work out on an aerobic exercise machine that has an attached electronic calorie counter. These built-in electronic devices use formulas based on how fast you go and how much resistance you have dialed up. Some also ask for your body weight.

The most accurate counts of energy expended are done in special laboratories. The direct method of tracking calories burned measures how much heat is produced during exercise; however, technically it is quite difficult. More commonly, energy is measured by determining the amount of oxygen consumed to calculate the number of calories burned. Except for short bursts of anaerobic metabolism, cells use oxygen to create the energized ATP from food calories. This maintains basal metabolic rate and allows muscles to quickly and vigorously contract when exercising. Since there is a direct relationship between the amounts of oxygen consumed and calories burned, this is a straightforward calculation. Calories (kcal) burned per minute equals the liters of oxygen consumed per minute multiplied by 5.

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Getting Fit To Increase Calorie Burn

Athletes and trainers often gauge level of fitness according to a person's volume of oxygen consumed per minute at maximum exercise intensity, known as the VO2 max. To be even more accurate, the number should be adjusted for body weight, since larger people can reach higher rates of oxygen consumption. So VO2 max is recorded as milliliters of oxygen per kilogram per minute (ml O2/kg/min).

If you have not been exercising regularly, your muscle cells are not geared up to efficiently use oxygen to make ATP, and your heart and lungs aren't in shape to increase the uptake and transport of oxygen to those muscle cells. So your initial VO2 max will be on the low side. As you get fit, the muscle cells can create ATP more quickly from oxygen and your cardiac and respiratory systems strengthen to speed up the delivery of oxygen to the muscle cells. In general, the muscles gear up more quickly and usually have excess capacity to expend more energy and make more ATP if only the muscle cells could get more oxygen. The limiting factor is almost always how hard the heart or the lungs can work. This is true for both the very sedentary person and the competitive athlete.

Here is an example. A healthy 35-year-old male of average weight (170 pounds) who has not been sedentary would have a VO2 max around 2.5 liters per minute. To convert that to calories burned, multiply by 5. That equates to burning 12 calories per minute. However, this is the amount of calories burned at maximum intensity. An untrained person would be only to able to hold this level for one to two minutes. With increasing fitness, your VO2 max rises, and the amount of minutes you can spend at higher levels, closer to your VO2 max, also goes up.

That same 35-year-old male who begins a serious aerobic exercise program could bring that VO2 max up to 3.5 liters per minute, burning 18 calories per minute. Assuming he could hold exercise intensity at 80% of maximum for 45 minutes, he would burn 630 calories. And this would not include the extra calories burned post-workout as his body recovers. Add in some resistance training to increase muscle mass, and the total caloric burn rises a little more.

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Heart Rate and Energy Expenditure

Once you get fit and your muscles are using oxygen efficiently, you can use heart rate to calculate your oxygen consumption and calories burned. At this point in your training, the muscles in your body are primed to receive as much oxygen as your heart and lungs can deliver, which provides a relatively direct relationship between heart rate, maximum oxygen consumption and calorie burn per minute.

I recommend a heart-rate monitor. For about $50 you can get a model that is reliable and durable. It will show your current heart rate, and at the end of your workout, hit a button and it shows the amount of time you worked out and your average heart rate for the number of minutes you exercised.

First, you need to know your maximum heart rate (MHR). The simplest formula is 220 minus your age. For example, a 35-year-old has an MHR of 185. Based on your heart rate as a percentage of your personal MHR, you can use the table below to see your percent of maximum oxygen consumption (VO2 max). VO2 varies based on fitness level, sex and age. The table below uses a midrange VO2 max of about 3 liters of oxygen per minute:

Percent Max
Heart Rate
Percent
VO2 Rate
Food Calories
Burned Per Minute
50%
20%
3
60%
36%
5.4
70%
52%
7.8
80%
67%
10
90%
83%
12.5

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If You're Just Beginning an Exercise Program

Don’t think about calories burned if you're just starting to get serious about exercise. Your muscles are still learning how to efficiently burn energy, so tracking heart rate as reflection of energy expended will be less reliable. With daily exercise and slow, steady increase in intensity, your muscle cells will continue to improve their cellular energy burn. After a couple of months, this will plateau. You are now ready to count those calories you burn and watch the number rise as you are able to maintain a higher heart rate with stronger heart contractions, combined with more vigorous lung and chest-wall action.

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Howard LeWine, M.D., is chief editor of Internet publishing, Harvard Health Publications. He is a clinical instructor of medicine at Harvard Medical School and Brigham and Women's Hospital. Dr. LeWine has been a primary care internist and teacher of internal medicine since 1978.

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