Understanding Calories

We count them, trim them, and fret over them. We work hard at burning them off but can't even see them. This sounds like a riddle, but don't worry — here, we'll demystify the often misunderstood calorie.

Last updated: Feb 25th, 2020
Understanding calories

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.

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Calorie vs. Kilocalorie

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.

Total Energy Expenditure

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 it is 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.

Resting Metabolic Rate

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.

  • Body composition. Lean body mass (LBM) is the greatest determinant of REE. It includes water, bone, skeletal muscles, and other organs such as the liver, brain and heart. These organs drive most of the REE. For example, the liver accounts for approximately 29 percent of your REE. Skeletal muscle at rest is a smaller portion of your REE, accounting for approximately 18 percent. Building muscle will then increase your metabolic rate, but it is not the metabolic furnace many people believe it to be. To make a large difference in REE, an individual would need to gain quite a lot of skeletal muscle. More calories are likely burned in building and maintaining the muscle than from the muscle's effect on the REE. In fact, the resting metabolic rate of athletes is only about 5 percent greater than non-athletes.
  • Body size. Generally, the greater the body's surface area, the greater the metabolic rate.
  • Age. The REE is greatest during periods of growth such as infancy, toddlerhood and adolescence. REE declines two to three percent per decade after early adulthood. The typical loss of LBM with aging does not account for the full drop in energy expenditure.
  • Gender. At the same age and weight, men usually have a higher REE because of the difference in body composition. Men usually carry more muscle than women.
  • Other factors. A women's menstrual cycle will increase her REE, explaining, in part, the dreaded weight creep that often comes with menopause. Having a fever or being in extreme temperatures also increases the REE.

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.

Thermic Effect of Food (TEF)

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.

Physical Activity

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. 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.

The Energy Cost of Physical Activities

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).

Activity and METs

  • Sitting quietly: 1.0
  • Bowling: 3.0
  • House cleaning, general: 3.0
  • Walking 3 miles per hour (20-minute mile): 3.3
  • Water aerobics: 4.0
  • Jogging, 5 miles per hour (12-minute mile): 8.0

Source: Compendium of Physical Activities Tracking Guide 2000

Non-Exercise Activities

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 daily, about the same as three or four fun-size candy bars. So what's the bottom line? Boycott sitting.

Measuring Energy Expenditure

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.

Prediction Equations to Estimate Resting Energy Expenditure

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.)

  • Men: RMR = 9.99 x weight + 6.25 x height - 4.92 x age + 5
  • Women: RMR = 9.99 x weight + 6.25 x height - 4.92 x age - 161

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).

Can You Increase Your Metabolic Rate?

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.

  • Pump up the protein. As discussed in the section on the thermic effect of food, digesting, absorbing and using protein does waste or use more calories than consuming either fat or carbohydrate. The effect is small. The extra calories burned daily are about the same as the calories you would get from eating a medium carrot.
  • Add spice. Capsaicin, the compound responsible for the burn of chili peppers, can bump up your metabolic rate and use a few more calories for a short time after meals by stimulating your sympathetic nervous system. It's not likely to help much, however. Study subjects were unable to comply with the large dose needed.
  • Get a caffeine buzz. In the short term, caffeine boosts your metabolic rate just like it boosts your heart rate. Long-term studies, however, show no weight loss effect.
  • Go green. The catechins in green tea might spike metabolic rate. A beverage containing green tea catechins and caffeine outperformed a beverage containing only caffeine in a 12-week weight loss study.
  • What doesn't work: eating frequently. Such a common misconception, but eating several mini meals doesn't offer any overall boost to your metabolic rate. Since the thermic effect of food is proportional to the calories consumed (roughly 10 percent), the calories burned will be similar no matter how many times you eat per day as long as your total calorie intake remains the same. To make dieting matters worse for many people, eating six mini meals compared to three larger ones just means six opportunities to overeat.

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.



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