Unit 7
Metabolism of Nutrients and Energy Balance

After carbohydrates, lipids, and proteins have been digested and absorbed, their breakdown products are transported to the liver via the bloodstream, and then to the body cells for metabolism. The term metabolism comes from the Greek word metaballein, meaning “to change.” As defined by Whitney and Rolfes, metabolism refers to all chemical reactions that go on in living cells. The metabolism of energy-yielding nutrients involves two main processes: anabolism and catabolism. Anabolism is the building of complex body compounds from simpler substances; catabolism is the breaking of large molecules into smaller constituents. When anabolism exceeds catabolism in the body, growth, or weight gain, occurs. Conversely, if catabolism exceeds anabolism, loss of tissue substance, or weight loss, occurs. A balance between these two processes is necessary to maintain body tissues and body weight.

This unit provides an overview of the four major pathways for energy production and examines how the basic units of carbohydrates, lipids, and proteins enter these pathways. It analyzes the effects of feasting, fasting, a low-carbohydrate diet, and a protein-sparing fast on body metabolism. In the last section of the unit, we define the three major categories of energy expenditure and demonstrate how to estimate energy expenditure; we also review the common methods of assessing body weight and body composition and discuss the problems associated with overweight or underweight. Finally, we analyze the proper approach to weight control and identify criteria for evaluating weight-loss diets.

This unit consists of three sections:

7.1—Metabolism of Nutrients
7.2—Energy Balance
7.3—Weight Control

Objectives

After completing this unit you should be able to

1. explain the role of adenosine triphosphate (ATP) (the chemical characteristics) in the body.
2. discuss the three main pathways involved in glucose catabolism: glycolysis, the tricarboxylic acid (TCA) cycle, and the electron transport chain.
3. briefly describe the catabolism of glycerol and fatty acids in energy production, and explain why lipids are a rich source of energy but a poor source of glucose.
4. briefly describe the catabolism of amino acids in energy production, and explain how the body excretes excess nitrogen.
5. describe the metabolism of carbohydrates, lipids, and proteins when energy needs have been met (while food energy is plentiful).
6. describe the metabolic and physiological effects of fasting and low-carbohydrate dieting.
7. define ketosis, and explain the condition that leads to its development.
8. distinguish between direct and indirect calorimetry and between gross energy values and physiological values of foods.
9. calculate the energy content of food and the percentage of energy derived from each macronutrient.
10. identify the type of diet that provides the greatest satiety.
11. define the three major categories of thermogenesis (energy expenditure), calculate basal metabolic rate (BMR), discuss the factors that affect BMR, and explain how energy expenditure can be estimated.
12. describe the following common measurements used in assessing body weight or body composition: body mass index, skinfold measurement, and waist circumference.
13. calculate a person’s BMI and interpret its significance.
14. identify the health hazards associated with overweight and underweight.
15. discuss the factors that cause overweight and obesity.
16. describe the recommended approach to weight control.
17. identify the shortcomings of a low-carbohydrate diet.