Unit 7
Metabolism of Nutrients and Energy Balance

After carbohydrates, lipids, and proteins have been digested and absorbed, their breakdown products are transported via the bloodstream, then enter 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.

In this unit, we provide an overview of the four major pathways for energy production, and examine how the basic units of carbohydrates, lipids, and proteins enter these pathways. We analyze the effects on body metabolism of feasting, fasting, a low-carbohydrate diet, and a protein-sparing fast. In the last portion of the unit, we define the three major categories of energy expenditure, and demonstrate how energy expenditure can be estimated; 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 the criteria for evaluating weight-loss diets.

This unit consists of the three sections listed below.

• Section 7.1  Metabolism of Nutrients
• Section 7.2  Energy Balance
• Section 7.3  Weight Control