An average adult consumes nearly a million calories a year. Despite this huge number, some healthy people are able to maintain a relatively stable body weight over years and decades without much conscious effort. When "calories in" equals "calories out," a state of energy balance is achieved and body weight remains constant.
Metabolism is the process of converting food to fuel for the body. Metabolic rate is the number of calories used to fuel the body. Therefore, your resting metabolism is the amount of energy your body uses at rest.
The Energy Balance Equation
The energy balance equation is driven by common laws of physics and thermodynamics:
- Energy cannot be created or destroyed.
Energy is either used or stored. When "calories in" are less than "calories out," a state of negative energy balance occurs and body weight decreases.
In contrast, when "calories in" are greater than "calories out," a state of positive energy balance occurs and body weight increases.
Being overweight is a result of being in a positive energy balance where "calories in" is greater than "calories out". This can be caused by:
- eating too much
- not enough exercise or activity
- having a low metabolic rate
Food provides calories (fuel) for the body in the form of carbohydrates, proteins, and fats. Any calories from food that are not used immediately for energy production are stored. Each source of calories is unique in the way it is used and stored by the body.
Carbohydrates are usually the main source of energy for the body. Carbohydrates are broken down into glucose (sugar) and stored in individual muscle cells in the form of glycogen. The body can only store limited amounts of carbohydrates as glycogen. Carbohydrates contains four calories per gram.
Proteins are used by the body to build and maintain body tissues and is rarely used by the body for energy. Like carbohydrates, the body can only store limited amounts of protein. Protein also contains four calories per gram.
Fat is the most calorie-dense of the nutrients. Unlike carbohydrate and protein, the body has an almost unlimited capacity to store fat and body fat stores act as the long term fuel reserves to prevent against starvation. A common misunderstanding is that you can only increase body fat by eating too much fat but studies show that an abundance of fuels, especially carbohydrates, also leads to increased body fat storage. Fat contains nine calories per gram.
Watching the number of calories in the food you eat helps manage your "calories in," but to achieve energy balance you also need to know your "calories out." The body uses (burns) calories in three ways: Physical Activity, Digestion of Food, and your Resting Metabolism.
The body uses more calories for physical activity than it does at rest. Physical activity includes exercise as well as all other activity that engages muscles for movement. Because most physical activity is voluntary, it is the most variable part of "calories out" in the energy balance equation. The number of calories burned during physical activity and exercise varies from individual to individual. Calories for physical activity may represent less than 20 percent of "calories out" for a sedentary person or more than 30 percent of "calories out" for someone who is very physically active. Calories for physical activity depend upon the type, intensity (how hard you exert yourself), and duration (minutes) of the activity.
Digestion of Food
The body also uses calories to digest, absorb, transport, and store food. The technical name for this process is called the "thermic effect of food". Typically, the thermic effect of food represents only about 10 percent of the "calories out."
Resting metabolism provides the energy the body needs for pumping blood through the body, inhaling and exhaling air, maintaining body temperature, sending and receiving nerve impulses, thinking, and making important chemicals in the cells. Resting metabolism occurs in a continual process. Resting metabolism is the largest component (typically 60 to 70 percent) of "calories out" in the energy equation.
Balancing the Energy Balance Equation
Most people maintain a steady energy balance over time. You may eat a little more or a little less on any given day. You may do a little more or a little less physical activity. Your weight may go up or down by a pound or two, but for the most part, you maintain a balance. When the balance shifts, your weight changes. The key to gaining or losing weight is balancing the energy balance equation.
The Role of Oxygen, VO2
Just like an automobile engine, your body needs oxygen to mix with fuel to produce energy. Your lungs (carburetor) and heart (fuel pump) deliver oxygen to the individual muscle cells and combine with fuels (fat, carbohydrates) for the production of energy. One of the by-products (exhaust) of this energy creation is carbon dioxide. You breathe in oxygen and you breathe out carbon dioxide.
At lower exercise intensities, your aerobic system uses fats and some carbohydrates as fuel along with a moderate amount of oxygen. Of these fuels, only carbohydrates have the capacity to be used as fuel without oxygen, or anaerobically. As the intensity of your exercise increases and you reach the capacity of your aerobic system, to bring oxygen into your body and you shift progressively to your anaerobic system. Your anaerobic system primarily uses carbohydrates (in the form of blood sugar or stored glycogen) as a fuel source and produces an increased amount of carbon dioxide exhaled.
For example, if you are walking up a few flights of stairs, as you get to perhaps the third flight, you begin to switch from your aerobic system to your anaerobic system and will notice an increased demand for oxygen and you will breathe harder and more rapidly. If you continue to climb the next flight of stairs, you will notice a burning sensation (accumulation of lactic acid) in your leg muscles and as this lactic acid accumulates in the muscle, your body attempts to rid itself of this condition by buffering it with bicarbonate in the blood. This buffering process produces additional carbon dioxide in the blood which causes you to breathe even harder. This point is typically referred to as your anaerobic or ventilatory threshold.
A metabolic assessment analyzes the volume of oxygen consumed (VO2) and the volume of carbon dioxide produced (VCO2) in a controlled setting to determine the type of fuels your body is using, or your "metabolic profile". A Resting Metabolic Rate (RMR) assessment measures the amount of energy used at rest. The RMR is then adjusted by an activity factor to produce the amount of calories you burn in a typical day. Your RMR can be used to identify your caloric intake needs for a weight loss program (e.g. 2000 calories per day).
An exercise metabolic assessment measures the VO2 and VCO2 along with your heart rate during exercise with a gradual increase in intensity until you reach a point sufficient to collect the desired exercise "metabolic profile". Data such as heart rate, oxygen consumed (VO2 Max), and Anaerobic Threshold (AT) are determined and these are used to develop training program for you. Target heart rates are scientifically determined by your metabolic profile during exercise and can be incorporated into a fitness or weight loss training program by your exercise professional.
Metabolic assessments are better than estimates because the program is based on your unique response to exercise. Your exercise professional will have more information from the assessments on how your body is working which leads to more effective nutritional planning and exercise programming.
Risk for Heart Disease and Death
Especially from cardiovascular disease. People with an above average VO2max are at a reduced risk of premature death, especially from cardiovascular disease. Because lack of cardiorespiratory fitness is recognized as a major risk factor for cardiovascular disease, it should be measured objectively and regularly, just as we measure body weight, blood pressure, and blood cholesterol, triglycerides, and glucose.
An eight-year study conducted at the Cooper Institute for Aerobics Research (Dallas, Texas) found that men and women who were moderately or highly fit were at a far lower risk of dying during the study period than those who were unfit. Although the highly fit study participants were at the lowest risk, the difference between them and the moderately fit individuals was not very striking. The benefit of being fit was evident for all study participants regardless of other risk factors they might have, such as smoking, high blood pressure, high cholesterol, overweight, or family history of heart disease.
These findings suggest that even a modest improvement in cardiorespiratory fitness level among the most unfit people is likely to produce a substantial health benefit. Low cardiorespiratory fitness is now recognized as a major risk factor for premature death from cardiovascular disease and all causes. In fact, becoming fit (if you are unfit) has a similar benefit in terms of reducing your risk of premature death as stopping smoking if you are a smoker.