Last Updated: October 19, 2017
We count them. We trim them. We fret over them. We work hard at burning them off, but we can’t even see them. Calories are simply a unit of energy, but they are a great mystery to many. They are not the enemy dieters often think they are. We need calories from food and drink to run our bodies the same way a car needs energy from gasoline and your refrigerator needs electric energy to keep your food cold.
To a scientist, a calorie is the quantity of heat (or energy) needed to boost the temperature of 1 ml of water by 1°C. A kilocalorie (kcal, often pronounced kay-cal) is the amount of heat required to raise the temperature of 1000 ml of water by 1°C. A kilocalorie is 1000 calories. Even though there is 1000-fold difference between these two values, calorie and kilocalorie are often used synonymously when discussing food and eating. Though it is commonplace, it is technically incorrect. When in doubt, use the word energy, as in: butter is rich in energy.
Calorie and kilocalorie will not be used to mean the same thing in this text. When you see food labels say that a serving of food is 100 calories, the real meaning is that the food has 100 kcals. Likewise, when we say that running a race burned 300 calories, we mean 300 kcals.
Your body weight reflects your energy balance. If you consume more calories than your body uses, you will gain weight. Likewise, you will lose weight if you consume fewer calories. Body weight is not, however, an indicator of nutrient adequacy or the nutritional quality of the diet.
There are three components to your metabolic rate: physical activity, resting metabolic rate, and the thermic effect of food. Physical activity - whether is it purposeful exercise such as jogging, activities of daily living such as typing and folding laundry, or simply unconscious fidgeting - is the only component of the three that you have much control over. The greatest component is the resting metabolic rate (RMR). It is responsible for 60-75 percent of the total calories burned each day. The digestion, absorption and storage of food - called the thermic effect of food (TEF) - is the smallest component. Let’s take a closer look at each of these components.
You burn calories all day and night even if you do nothing but sleep or watch TV. Most of the calories spent each day are for breathing, circulation, maintaining your body temperature, moving compounds in and out of cells, and other normal body processes you rarely need to think about. This is your resting energy expenditure (REE), a clinical measure of the RMR. The REE depends upon many things.
The basal metabolic rate (BMR) is the amount of energy needed to sustain metabolic activities while an individual is lying down and mentally resting in a temperature-controlled environment that prevents shivering or sweating. The individual should not have eaten or exercised for at least 12 hours. These conditions are difficult to meet, so scientists and practitioners typically measure the RMR instead. The RMR is frequently measured three or four hours after eating or exercising and with other less strict criteria. For these reasons, the RMR is higher than the BMR.
It takes energy to process the food you eat. Digestion of the food and the absorption, metabolism and storage of the nutrients account for approximately 10 percent of your total energy expenditure. The composition of your meal determines its TEF. Large meals have a greater TEF than small meals, and protein has a greater TEF than carbohydrate, which have a greater TEF than fat. In other words, eating protein “wastes” more calories than eating carbohydrate or fat. Thus, by increasing the protein content or your meal without increasing its calorie content, you can burn a few extra calories. The effect is not large, however. It has been estimated that by manipulating the macronutrient content of the diet, someone consuming 2000 kcals per day could burn approximately an additional 23 kcals daily.2
This is the most variable component of your daily energy expenditure. For most people, it accounts for approximately one-quarter of their total energy expenditure. It may be as little as 10 percent, however, in someone extremely inactive or bedridden and as much as 50 percent in athletes or heavy laborers.
Unlike your RMR, which is proportional to your LBM, the calories you burn in exercise are based on your body weight. For example, if a 100-pound person and a 200-pound person took a walk at the same speed and covered the same distance, the heavier person would use twice as many calories as his lighter walking companion. Sports specialists and researchers estimate the calorie cost of exercise in metabolic equivalents (MET). The metabolic cost of sitting quietly is 1.0 MET and is approximately 1 kcal/kg/hr. Using this value, other physical activities are assigned MET levels according to their intensity.3 Thus, energy expended by physical activity can be expressed in multiples of 1 MET. For example, walking on level ground at 3.0 mph has a MET value of 3.3, meaning it burns 3.3 times the energy of sitting quietly.
As you’ll see from the list below, different types of physical activities have different energy costs. For example, a person weighing 140 pounds (63.6 kg) burns approximately 63.6 kcals each hour she sits quietly. If she walks 3.0 mph on level ground (which has a MET value of 3.3) for one hour, she will burn approximately 210 kcals (63.6 x 3.3).
Source: Compendium of Physical Activities Tracking Guide 2000
While running, playing tennis, lifting weights and other planned exercise are big calorie burners, don’t underestimate the energy used for shifting and maintaining posture, wiggling your foot, tapping your fingers, brushing your teeth and other non-exercise activities. In a small study, researchers reported that sedentary lean individuals are upright in activity 152 minutes more per day than sedentary obese individuals. Such non-exercise activities account for about 350 calories daily4, about the same as three or four fun-size candy bars. So what’s the bottom line? Boycott sitting.
Direct calorimetry monitors the amount of heat produced by an individual in a highly sophisticated, small chamber. It is a good measure of the energy expended while in the chamber, but is not indicative of the energy expenditure of a free-living person. It is also very expensive. The doubly labeled water technique determines energy expenditure in free-living individuals who drink a known amount of water containing two stable isotopes. The subject’s energy expenditure is calculated by knowing the rate at which the water disappears from the body. This method is also costly. Indirect calorimetry estimates energy expenditure by measuring oxygen consumption and carbon dioxide production. Many registered dietitians and fitness centers offer indirect calorimetry to their clients for a moderate fee.
The simplest and cheapest way to estimate an individual’s calorie needs is to use one of several empirically derived equations. Each equation is based on groups of individuals, so you can expect differences between individuals. According to the Evidence Analysis Library of the Academy of Nutrition and Dietetics, the Mifflin-St. Jeor equation has the greatest accuracy of the equations assessed. (RMR here means resting metabolic rate, as usual. Weight is expressed in kilograms. Height is expressed in centimeters. Age is expressed in years.)
To estimate your daily energy expenditure, multiply the calculated RMR by an appropriate activity factor. Practitioners typically use an activity factor between 1.3 (sedentary individuals) and 1.9 (very active individuals).
This is such a common question. It seems like everyone is looking for the well-kept secret to blasting through calories without hitting the gym. Here’s what the science tells us.
So yes, you can manipulate your diet to boost calorie expenditure. But the effects are so small that the effort is better put toward healthy meal planning, food preparation and exercise.
Mahan KL and Escott-Stup S. Krause’s Food Nutrition and Diet Therapy, 11th Edition. Elsevier, 2004. ↩
Mattes RD. Dietary Approaches to Exploit Energy Balance Utilities for Body Weight Control, Chapter 26 in: Nutrition in the Prevention and Treatment of Diseases 2nd Edition. Elsevier, 2008. ↩
Dunford M, editor. Sports Nutrition: A Practice Manual for Professionals, 4th Edition. American Dietetic Association, 2006. ↩
Interindividual Variation in Posture Allocation: Possible Role in Human Obesity in January 2005 Science. http://www.sciencemag.org/cgi/content/full/307/5709/584 ↩
Obesity and thermogenesis related to the consumption of caffeine, ephedrine, capsaicin, and green tea in January 2007 The American Journal of Physiology - Regulatory, Integrative and Comparative Physiology. http://ajpregu.physiology.org/cgi/content/full/292/1/R77 ↩ ↩2
Effects of caffeine on energy metabolism, heart rate and methylxanthine metabolism in lean and obese women in Oct. 1995 American Journal of Physiology - Endocrinology and Metabolism. Abstract: http://ajpendo.physiology.org/cgi/content/abstract/269/4/E671 ↩
Green Tea Catechin Consumption Enhances Exercise-Induced Abdominal Fat Loss in Overweight and Obese Adults in Feb 2009 Journal of Nutrition. http://jn.nutrition.org/cgi/content/full/139/2/264 ↩