Nutrition 331: Nutrition for Health

Study Guide: Unit 8

Metabolism of Nutrients and Energy Balance


In this unit, we examine the energy value of foods and the energy expenditure of humans. We discuss the challenges of assessing body weight and determining whether a person is underweight or overweight. Finally, we discuss the effects on the body of feasting, fasting, and following a carbohydrate-restricted diet. In Unit 11 we will deal with the consequences of excess weight and safe weight-loss strategies.


After completing this unit, you should be able to

  1. explain how the body deals with an energy intake above or below requirements.
  2. define food energy value, and state the physiological fuel values of protein, carbohydrate, fat, and alcohol.
  3. define basal metabolism, basal metabolic rate (BMR), voluntary activities, and the thermic effect of food.
  4. state the energy value of one pound (about 0.5kg) of body fat.
  5. discuss the influences of BMR, voluntary activities, and the thermic effect of food on energy expenditure.
  6. explain the anthropometric measures listed below, and discuss their strengths and weaknesses.
    • body mass index (BMI)
    • percent of body fat
    • waist measurement
  7. calculate BMI and define underweight, normal weight, overweight, and obese.
  8. discuss the health hazards associated with being underweight.
  9. explain the metabolic events that occur during feasting and fasting.

Section 1 Energy Balance and Food Energy

Measuring Food Energy

Gross food energy values are typically measured in a bomb calorimeter, which determines the energy in foods by measuring the heat released after complete burning of a food sample.

In the body, however, energy extracted from food is less than gross food energy value. As a result of incomplete digestion, absorption, and metabolism, a small amount of the energy value of the food is lost and excreted in the feces or the urine. Gross energy values can be adjusted to represent the energy actually available to the body. Such values are called physiological fuel values or energy nutrients. You have already learned the physiological fuel values (in kilocalories per gram) for carbohydrates (four), lipids (nine), proteins (four), and alcohol (seven).

Energy values in food composition tables are generally derived from the physiological fuel values based on the chemically determined contents of carbohydrates, lipids, proteins, and alcohol (if present) in foods.

Reading Assignment

Review Table 1.3 on page 7, “Calorie Values of Energy Nutrients.”

Read pages 376–379, “The Body’s Energy Balance.”

Energy Balance

To balance the energy equation, the energy expended must equal the intake of energy from carbohydrates, lipids, proteins, and alcohol (if present). If energy input is greater than energy output, the result is weight gain; conversely, if energy output is greater than energy input, the result is weight loss. Consequently, the cornerstones of healthy weight loss are increasing activity and decreasing food intake. The principle is simple but the lifestyle change is challenging. Healthy weight-loss strategies will be discussed further in Unit 11.

Measuring Human Energy Expenditure

The textbook indicates that energy expenditure can be estimated by measuring energy intake when weight is stable (p. 377, first paragraph under heading “How Many Calories Do I Need Each Day?”). Other methods of estimating energy expenditure are described next.

Direct calorimetry is the measurement of energy as heat. For humans, an expensive, specially designed chamber can be used to determine the total amount of heat released by the body. Energy released as heat is measured in kilocalories.

Indirect calorimetry measures oxygen consumption and carbon dioxide production. It requires a portable respiration apparatus that is simpler, less expensive, and more mobile than the equipment required to measure total heat released by the body. Indirect calorimetry allows for the measurement of a wider range of physical activities than direct calorimetry, making it the preferred measurement tool.

Factors Influencing Energy Expenditure

The three major categories of energy expenditure are basal metabolism, voluntary muscular activity, and the thermic effect of food.

Basal metabolism, also known as basal metabolic rate (BMR), is defined as the minimum energy required to maintain vital life processes (see textbook, p. 377). Vital life processes are summarized in the margin of page 377. Many people are surprised to learn that basal metabolic rate (BMR) accounts for the largest source of daily energy expenditure.

Factors that affect the BMR are listed in Table 9.3 on page 378 of the textbook. As the textbook notes, BMR can be elevated with exercise. Certain types of exercise build lean muscle mass, and the more lean body tissue, the higher the BMR—which is why exercise is emphasized in maintaining BMR for weight control (p. 377, final paragraph).

Voluntary muscular activity (or physical activity) is the most variable category that can influence an individual’s total energy output. It is also the most difficult to measure, and can account for errors in determining energy requirements. This problem occurs because there are virtually endless possibilities for types of activities. Also, the intensity or speed of activity varies greatly among people. Body weight is another important factor: heavier people use more energy than lighter people to perform the same activity.

The thermic effect of food, or diet-induced thermogenesis, is the energy required to digest, absorb, transport, metabolize, and store nutrients from food. You can relate the energy expended for this purpose to the rise in body temperature after a meal is consumed. The thermic effect of food depends on the carbohydrate, fat, and protein components in the diet, and the rate at which they are metabolized. The process of storing dietary fat in adipose tissue requires 3% of the ingested energy. By contrast, storing dietary carbohydrate as body fat requires 23% of energy intake, and storing it as glycogen requires only 7%. In a mixed diet, an average of about 10% of total energy intake is estimated to be used to process food.

The most accurate ways of determining energy requirements are direct and indirect calorimetry, which measure actual energy expenditure. Another fairly accurate way is by estimating average energy intake from food when body weight and activity are stable. In Assignment 1, you are asked to assess the stability of your body weight and to estimate the volume of activity you undertake. Another way of estimating your energy requirement is the “Estimated Energy Requirement” formula included in both Diet Analysis+ and the text (Fig. 9.6, p. 379).

If your body weight is stable and the disease risks for your body weight are low, you are probably consuming an appropriate number of calories. You may wish to talk to your tutor about appropriate energy intake if you are overweight or underweight.

Section 2 Body Weight

Reading Assignment

Read pages 380–382, “Body Weight versus Body Fatness,” and pages 370–375, “The Problems of Too Little or Too Much Body Fat.”

In Unit 1, we discussed anthropometric measures briefly, as one of the four components of a complete nutritional assessment. Now, we shall take a closer look at the four most common measurements used in assessing body weight and body composition.

Height and Weight Tables

For decades, the method for determining overweight and underweight has been to compare personal weights to “ideal” body weights given in height and weight charts. However, the relationship between weight and health can be difficult to assess from height and weight tables. One of the main limitations of height and weight charts is their inability to differentiate fat weight from lean muscle weight. Other methods provide a more accurate assessment of the risks for disease in the Western world.

Body Mass Index (BMI)

Body mass index is another anthropometric measure based on the relationship of body weight to height. Though it is an indirect measure, in general, BMI has a high correlation with body fat. It is a measure of proportional weight in relation to health, and allows a wider range of acceptable weights for heights than height and weight charts. The risk of health problems increases above or below a BMI range of 18.5 to 24.9 (see Table 9.2, p. 374). Note: For the purposes of this course, Obese Class I, II, and III are all considered obese. The formulas for calculating BMI are

Metric:  wt kg ht m 2

Imperial:  wt ( lbs ) ht  ( in ) 2 ×703 .

The use of the BMI is suitable for people between ages 20 and 65 years of age. It is not suitable for infants, children, adolescents, pregnant or breastfeeding women, or muscular athletes. People with a high proportion of muscle mass are heavier than average because muscle is denser (i.e., heavier) than fat. Researchers have developed BMI standards for children and adolescents. A BMI chart for children and adolescents is presented on page Z at the end of the textbook. For seniors, the normal BMI range may begin slightly above 18.5 and extend into the overweight (25.0–29.9) range.

Skinfold (Fatfold) Measurement

Unlike the height and weight tables or the BMI, skinfold measurement is a direct measure of body fatness. In skinfold measurement, the thickness of a fold of skin is measured at several different body sites using a special caliper (see Fig. 9.8, p. 382). The two sites that are most commonly used are the triceps skinfold (back of the arm, midway between the top of the shoulder and elbow) and the subscapular skinfold (on the back, parallel to the shoulder blade). Since each site has a different thickness, each measure must be compared with the appropriate standard. It is important to have skinfold measurements taken by a skilled professional as it is easy to take the measurements incorrectly. Skinfold measurements may be inconvenient to use in certain health care settings.

An overall average of fat percentages from different skinfold sites can be used to estimate the percent total body fat. The textbook provides guidelines on acceptable ranges for percent body fat (p. 382, “How Much Body Fat is Ideal?”). A limitation for interpreting body fat percentage is the lack of clarity of the relationship between body fat and the risk of various diseases. For example, at what percentage of body fat do men or women have an increased risk for heart disease? At what percentage should they be called obese or underweight?

Waist Measurements

Health professionals are now using waist circumference as an indicator of health risks from high body fat levels. For men, waist circumferences of greater than 102cm (40 inches) are correlated with a higher risk of disease; for women, risk increases when waist circumference exceeds 88cm (35in).

The waist circumference is measured at the narrowest part of the trunk, at the navel (see Fig. 9.9, p. 382). The waist circumference provides an indication of fat stored in the abdominal area, also called visceral fat (see Fig. 9.4, p. 373). Apple-shaped people have higher risks for chronic diseases such as type 2 diabetes, heart disease, hypertension, and breast cancer. Fortunately, abdominal fat is easier to lose through diet and exercise than fat stored in the hip area.

Waist circumference is a relatively easy measurement to take, and research has confirmed its correlation with chronic diseases. Unlike BMI, a waist circumference measure can differentiate between a muscular person and one who carries excess body fat. A disadvantage of waist circumference is that people may not accurately measure their own waists. With a little training, this may be overcome. Waist circumference measures will not accurately reflect risk for pregnant women.

Note: Risk is a descriptor used when weight may be a concern for health. The higher the risk, the more likely that health problems may develop, although there are no guarantees.


An adult can be termed underweight if his or her BMI is below 18.5. Being underweight poses a health risk. Underweight can be as serious a medical problem as overweight, especially in adolescent girls. Underweight women may manifest amenorrhea (cessation of menstruation) and may become infertile. Those who do conceive risk giving birth to unhealthy infants. Because underweight people often have a history of a low food intake, they may suffer from malnutrition. This condition, in turn, may reduce resistance to infectious disease. Underweight people may also suffer from physical weakness and reduced resistance to cold temperatures. They are at a disadvantage if they suffer from a wasting disease such as cancer. In fact, people with cancer often die, not from the cancer itself, but from starvation.

Lifetime prevalence estimates for the most common eating disorders are as follows: anorexia nervosa had occurred in 0.9% of women and 0.3% of men, respectively; bulimia nervosa in 1.5% and 0.5%; and binge eating disorder in 3.5% and 2.0% (Hudson et al., 2007). Approximately five percent of women between the ages of 14 and 24 have some kind of eating disorder. You may want to read the section titled “Controversy 9: The Perils of Eating Disorders,” on pages 411–418 of the textbook, as well as the section titled “What Strategies Are Best for Weight Gain?” on pages 404–405. You will not be tested on this material.

The causes of underweight are as many and varied as the causes of obesity. They include

  • consumption of food of insufficient quality or insufficient in quantity.
  • poor absorption and utilization of food consumed.
  • wasting diseases, such as tuberculosis, AIDS, and cancer, which increase basal metabolic rate.
  • excessive physical activity, as in the case of athletes in intensive training.
  • psychological or emotional disorders.

Section 3 Feasting and Fasting

Reading Assignment

Read pages 390–394, “How the Body Loses and Gains Weight.”

Metabolism of Nutrients

Metabolism is what the body does with nutrients following digestion and absorption. Depending on the body’s needs, the three energy-yielding nutrients (carbohydrates, proteins, and lipids) can be used to build body compounds (anabolic metabolism), or can be broken down to produce energy, carbon dioxide, and water (catabolic metabolism).

Food energy is trapped within chemical bonds that hold molecules of glucose and fatty acids together. During the catabolism of nutrients, chemical bonds are broken, and most of the associated energy is transferred to the chemical bonds of adenosine triphosphate (ATP).

The metabolism of nutrients is regulated by specific enzymes, coenzymes (many of which are B vitamins), cofactors (many of which are trace minerals), and hormones. Although the vitamins and minerals involved do not yield energy, they are essential for the liberation of energy.

Muscles, depending on their activity, use varying mixtures of fuels. Fat is the primary fuel used during rest and prolonged activity. Glucose is the major fuel used during short bursts of activity; it is also the source of fuel for the brain and nervous system. When there is insufficient energy or carbohydrate in the diet, such as during fasting or starvation, the body must create glucose from protein in the diet or from muscle. This process is called gluconeogenesis.

Fat is not easily converted into glucose. Only the glycerol component of triglycerides (5% of the molecule) can be converted to glucose.

As explained in Unit 7, the primary role of amino acids is to synthesize body proteins. Amino acids cannot be stored in the body. Instead, they are broken down, the nitrogen is removed, and the rest of the molecule enters energy metabolism. Proteins are used for energy when there is insufficient energy or carbohydrate in the diet (discussed earlier), when there is excess protein, or when not all of the essential amino acids are present.

All three energy nutrients in excess of need can be converted to body fat.


Surplus carbohydrates are stored as glycogen in the liver and muscles. However, the body only has a limited capacity to store glycogen in this way. Once glycogen stores are filled, the overflow glucose is converted to fat. Surplus lipids and protein are also converted to body fat for storage. Alcohol can also contribute energy or body fat. Alcohol is metabolized by the liver to generate energy.


Refer to Figure 9.11 on page 392, “Feasting and Fasting” during this discussion. If people choose not to eat, we say they are fasting. If they have no choice (as in a famine), we say they are starving. There is, however, no metabolic difference between the two situations.

The textbook gives a concise description of the metabolic effects at progressive stages of fasting (p. 391). You should recognize that the catabolism of body proteins from muscle and lean tissue starts when glycogen stores are depleted and no carbohydrates are available, which happens after several hours of fasting or low-carbohydrate dieting.

Ketone bodies are normal metabolites of the body. At low levels of production, ketone bodies are metabolized. However, the production of ketone bodies accelerates with fat catabolism during fasting or low-carbohydrate dieting. The lack of carbohydrate stimulates fatty acids to break down faster than the body can handle them. Ketone bodies are formed, some of which can be used to fuel particular brain cells and the nervous system. When ketone bodies reach a high concentration in the blood and urine, the condition is known as ketosis. Ketosis also happens in the case of untreated diabetes mellitus, when body cells are literally starved of glucose. A characteristic ketone body is acetone, some of which is excreted through the lungs, giving a rather sweet odour called acetone breath. The smell is similar to that of nail polish remover. Mild ketosis can cause appetite depression, an increase in urine output, and sometimes nausea. Severe ketosis can cause acidosis (a decline in the pH of the blood) and a loss of water from body tissues (water is needed for ketone excretion). Consequently, sodium and potassium are also depleted, blood pressure drops, and death can occur as a result of the collapse of the circulatory system.

Review Questions

For additional review questions, see the textbook, page 418, “Self-Check” questions 2, 3, 5, 6, 7, 8, 9, and 10.

Midterm Examination

You should now be ready to write the midterm examination. If you have not already done so, please arrange to write the midterm according to the instructions given in your Student Manual. Ensure that you specify that you are writing the midterm exam, not the final exam.