Most of the energy you need at rest and during sustainable exercise comes from the oxidation of carbohydrates and fats in your diet. This process requires oxygen. The calories you burn can be estimated by measuring the oxygen you remove from the air you breathe. How hard you work during exercise influences this value.
Calories
A calorie is the basic unit of energy measurement. One calorie is the heat required to raise the temperature of 1 g of water 1 degree Celsius. This is a very small amount, so human energy expenditure is typically measured in 1,000-calorie increments called kilocalories. However, people use the term "calorie" for short when expressing this unit. At rest, you burn calories to maintain your vital functions. During exercise, you burn additional calories to satisfy the energetic demands of the work rate.
Oxygen
Oxygen is a gas that makes up 20.9 percent of atmospheric air. When you breathe, air enters your lungs so that the oxygen can be removed. This oxygen is transported in your blood to your tissues where it is used to produce energy. For every liter of oxygen you take in, you burn 4.7 to 5.05 calories, respectively, depending on whether you are breaking down stored fat or carbohydrates.
Resting Metabolic Rate
Your resting metabolic rate, or RMR, is the amount of energy you need in the waking state at rest. On average, you take in 3.5 mL of oxygen per kilogram of body mass each minute to satisfy your RMR. You burn 4.7 calories per liter of oxygen at rest because energy demands are low and stored fat is the primary fuel source. This means that a 175-lb. person takes in 0.3 L of oxygen and burns 1.3 calories each minute at rest.
Estimating Oxygen Consumption
Prediction equations have been developed to estimate the oxygen cost of various exercises. The American College of Sports Medicine's Guidelines for Exercise Testing and Prescription provides equations for walking, running, leg cycling, arm cycling and stair climbing. You can also estimate the oxygen you need for different tasks by using metabolic equivalents, or METS. METS are multiples of the RMR that allow any activity to be quantified according to a scale. The 2011 Compendium of Physical Activities provides MET values for many different activities. For example, yoga requires 2.3 METS, while vigorous uphill mountain biking requires 14. This means that when you perform yoga, you need 8 mL of oxygen per kilogram of body mass each minute, and when you mountain bike, you need 49.
Exercise
Your calorie burn during exercise depends on how hard you work. This influences both the oxygen cost of the activity and the caloric equivalent of the oxygen you take in. For example, when a 175-lb. person walks 3 miles per hour, she needs an additional 0.6 L of oxygen per minute to support the effort. This means that 0.9 L of oxygen is required. If this is relatively easy for her -- for example, if she is breathing comfortably and her muscles are not burning -- she will be satisfying her fuel requirements by using a mixture of approximately 50 percent stored fat and 50 percent stored carbohydrate. This equates to a caloric equivalent of 4.86 calories per liter and a calorie burn of 4.4 per minute. For this person, running at a challenging pace of 6 miles per hour would require using carbohydrate stores exclusively, a caloric equivalent of 5.05 calories per liter and oxygen intake of 2.6 liters per minute. This means she would burn 13 calories each minute sustaining that pace.
References
- "Exercise Physiology: Nutrition, Energy, and Human Performance Seventh Edition"; W.D. McArdle, et al.; 2010
- "ACSM's Guidelines for Exercise Testing and Prescription, Sixth Edition"; American College of Sports Medicine; 2000
- "Medicine and Science in Sports and Exercise"; 2011 Compendium of Physical Activities: A Second Update of Codes and MET Values"; B.E. Ainsworth, et al.; August 2011
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