What Is Energy Metabolism?

Energy can be defined as the capacity for doing work. Metabolism is the sum of processes by which the body handles a particular substance. When scientists study energy metabolism, therefore, they are studying the processes that handle energy in the body.

What Is a Calorie?

The energy value of foods or beverages is expressed in terms of the kilocalorie. If a food is burned and the heat produced is measured, the quantity of heat produced expressed in kilocalories represents the gross energy value of the food. The gross energy value of food does not represent the energy available to the body. No food is completely utilized, since some of the energy is excreted in urine and feces. When corrections are made to account for this loss of excreted energy, the corrected energy value of foods is designated “metabolizable” energy. Energy values listed in food tables represent metabolizable energy. Although it is common to call the energy value of food “calories,” it is more accurate to use the term “kilocalories.”

Is a Calorie Really a Calorie?

The energy content of foods varies depending primarily on the amount of protein, fat, and carbohydrate contained in the specific food. The amounts of metabolizable energy in a gram of protein, fat, and carbohydrate are 4.0, 9.0, and 4.0 kilocalories, respectively. Although these nutrients are metabolized differently within the body, a kilocalorie from carbohydrate is the same as a kilocalorie from fat or protein.

What Is Energy Balance, or How Can I Maintain a Stable Body Weight?

After adjusting energy intake (from food and beverages) for energy excreted in human wastes, one can determine the calories absorbed by the body—or metabolizable energy. In order for a person to maintain a stable body weight, metabolizable energy must be equally balanced with energy expenditure—that is, the calories expended to perform physical and metabolic work.

When metabolizable energy is greater than energy expenditure, the excess energy is stored in the body as protein, carbohydrate, or largely as fat. If energy expenditure is greater than metabolizable energy, then energy is mobilized from body energy stores. In fact, this storage and mobilization of energy is an ongoing cycle. During the day we store energy as either glycogen or fat; during sleep we mobilize energy to meet the metabolic work needs of the body. When a person is in energy balance, this daily rhythm is such that weight varies by only 2-3 pounds around a particular weight. However, when a person is in positive energy balance, then energy is being stored and weight increases. In order to lose weight, therefore, it is necessary to decrease energy intake below energy expenditure or to increase energy expenditure beyond energy intake.

What Is Basal Metabolic Rate?

The amount of energy expended by a fasting person completely at rest (but awake) prior to getting out of bed for the day is defined as basal metabolic rate or BMR. For a sedentary person, BMR accounts for about 60-70 percent of daily energy expenditure; the remaining 30-40 percent is from physical activity and from body heat produced after a meal. Physical activity is responsible for as much as 50-60 percent of total energy expenditure in people who include frequent aerobic exercise into their lifestyle.

How Is Energy Expenditure Measured?

In the research or hospital setting, energy expenditure is determined by examining inhaled and exhaled air and measuring the person’s oxygen consumption and carbon dioxide production. The ratio between oxygen consumption and carbon dioxide production is called the respiratory quotient or RQ. Daily energy expenditure in kilocalories is calculated from an equation that accounts for the number of units of oxygen and carbon dioxide exchanged by a person under specific conditions. This technique for measuring energy expenditure is called indirect calorimetry because it measures heat production (calories) indirectly from respiratory gas exchange.

In the United States, a number of laboratories have built room-sized indirect calorimeters. Volunteers stay in the room for a total of 24 hours. During this time they are served meals and snacks, and they have access to a TV, VCR, telephone, desk, chair, bed/couch, and toilet facilities. While this technique accurately determines energy expenditure, the physical activity of the volunteers is typically lower than their normal daily physical activity because they are confined to the room calorimeter.

Scientists have recently turned to a technique called doubly labeled water to estimate the energy expenditure of free-living human volunteers. This technique requires volunteers to consume small amounts of water that contain the nonradioactive isotopes of hydrogen and oxygen—deuterium and oxygen-18. Each volunteer’s rate of excretion of deuterium and oxygen-18 is measured in daily spot urine samples for 14 days to estimate carbon dioxide production and oxygen consumption. From these two determinations, energy expenditure can be determined in individuals who have been free to go about their daily activity. This technique has successfully estimated individual energy expenditure and is currently being used in many laboratories around the country.

How Is Energy Stored in the Body?

Like a combustion engine, the body utilizes fuel (food and drink) for energy and corn-busts the fuel to give energy, water, and carbon dioxide. In a car excess fuel (gasoline) is stored in the gasoline tank; in humans, excess fuel is stored in the body. When energy intake exceeds energy requirements, the energy is stored within the body as glycogen and fat.

Glycogen is the storage form of carbohydrate in the body. Both liver and muscle are capable of synthesizing glycogen and breaking it down when energy is needed for muscle and liver function or other purposes. Since glycogen is a large molecule, it cannot be stored within the muscle or liver in great quantities. Energy is stored in the body primarily as fat.

When fat is stored, it is stored in adipose tissue by two processes: hypertrophy and hyperplasia. The primary form of storage during early childhood is hyperplasia, an increase in the number of adipose tissue cells. During puberty and adulthood, adipose tissue cells change in size with weight loss or weight gain. If weight is gained, they increase in size by hypertrophy, the process of enlarging adipose cells to accommodate additional fat. If weight is lost, then the fat is mobilized from the adipose tissue cells and the cells decrease in size.

In early childhood, energy is stored and mobilized to facilitate growth and development. Therefore it is important that the overall energy balance be positive. That is, more energy must be taken in than expended in the child’s daily activity to provide energy for growth. Children increase in weight until the end of adolescence, when they reach their adult weight.

The goal for adults is to maintain this healthy adult weight and not increase it. In order to maintain weight, adults must be in energy balance; that is, their energy intake must be closely matched with energy expenditure. When energy intake exceeds energy expenditure, a person is said to be in positive energy balance and gains weight.

Within the scientific community there is currently much discussion as to the exact definition of the term “ideal body weight.” The USDA-Health and Human Services 1990 Dietary Guidelines are one of many sources that suggest healthy body weights.

Am I an “Apple” or a “Pear”?

During the last 10 years, researchers in Europe and America have gathered data in large populations of men and women to demonstrate the importance of the location of fat on a person’s body. They found that the risk for disease (cardiovascular disease, hypertension, and diabetes) and premature death increases in obese populations that have greater deposits of fat around their abdomen (or waist) than around their hips. This distribution of fat is typical of obese males and is named “android” or “apple.” Women typically have more fat deposited around the hips and buttocks than around the abdomen. This distribution pattern is named “gynoid” or “pear.”

To determine whether you are an “apple” or a “pear,” you must first know your waist-tohip- ratio (WHR). This ratio is determined by dividing the waist circumference measurement by the hip circumference measurement. Men having a WHR greater than 1.0 and women having a WHR greater than 0.85 are consider “apples.” Conversely, men having a WHR less than 1.0 and women having a WHR less than 0.85 are consider “pears.” In people who are significantly overweight or obese, fat distribution has great significance. “Apples” have increased risk for cardiovascular disease, hypertension, and diabetes.

Most of this research has been conducted in Europeans or in Americans of European descent, and it is not certain at this time if these generalizations hold true for African-Americans, Hispanic-Americans, Asian-Americans, or Native Americans. One ARS study found that African-American women have greater amounts of fat on their upper bodies than European-American women. Further research is being conducted in ARS and other laboratories to see if these generalizations concerning disease risk are applicable to the whole American population.

Recent research studies have found that within the abdominal region, location of fat can be predictive of disease and premature death. Fat located just below the skin is called subcutaneous fat, while fat surrounding vital organs is called visceral fat. While most people with a high WHR have larger deposits of visceral fat than those with a low WHR, some people with high WHR have their fat located more subcutaneously. In epidemiological studies, large deposits of visceral fat in the abdominal region (near the waist) have been associated with greater risk for disease and premature death.

Can I Change My WHR or Fat Deposition Pattern?

In ARS and other research studies, magnetic resonance imaging (MRI) has been used to determine the location of fat within the abdominal region and also to monitor fat loss during a weight-reducing regimen. When a person loses weight, fat is lost from all over the body. However, the largest amount of adipose tissue will be lost from the regions of the body having the largest adipose tissue deposits at the beginning. This means that if one has large fat deposits on the buttocks and hips, these regions will lose the most fat. At the end of a weight-losing regimen, these regions of fat may still be large, but the amount of fat will be reduced. While it may be difficult to change the body’s fat patterning or WHR with weight loss, some studies report small changes. The benefit of weight loss for the person with large amounts of visceral fat is the decrease in visceral fat and therefore a presumed decrease in risk for disease and premature death.

How Can I Assess My Body Composition?

While there are many sophisticated methods of determining one’s body composition in the laboratory, the familiar “pinch-an-inch” test (pinching fat tissue between the thumb and forefinger) is probably the easiest way to determine the presence of fat deposits on one’s body. While it may seem imprecise, it has some merit.

Is It True That the More Weight One Loses, the Harder It Is To Continue Losing Weight?

Research studies at ARS have not supported the common belief that it becomes harder to lose weight the longer one diets. In fact the decrease in energy expenditure or metabolic rate seen in dieting individuals can be explained by the decrease in energy intake and the decrease in lean tissue that is obligatory with weight loss. Perhaps the single most important predictor of one’s energy requirement is physical activity, such as walking, running, swimming, cycling. Frequently this type of physical activity decreases during a weight-loss regimen.

What Determines a Person’s Energy Requirement?

The amount of energy a person requires to maintain energy balance is called the energy requirement. Many factors influence one’s energy requirement. Energy metabolism can be affected by any or all of these factors, making the study of energy requirements very complex.

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