Eating is essential to life. Many of us look at eating as not only a necessity but also a pleasure. Since childhood, you may have been told to start the day with a good breakfast to give you the energy to get through most of the day. You most likely have heard about the importance of a balanced diet, with plenty of fruits and vegetables. But what does this all mean to your body and the physiological processes it carries out each day? You need to absorb a range of nutrients for your cells to have the building blocks for metabolic processes that release energy to carry out their daily jobs, manufacture new proteins, cells, and body parts, and recycle materials.^[1]

This chapter will take you through some of the chemical reactions essential to life, the sum of which is referred to as metabolism. The focus of these discussions will be anabolic reactions and catabolic reactions. You will examine the various chemical reactions that are important to sustain life, including why you must have oxygen, how mitochondria transfer energy, and the importance of certain “metabolic” hormones and vitamins.^[2]

Metabolism varies, depending on age, sex, activity level, fuel consumption, and lean body mass. Your own metabolic rate fluctuates throughout life. By modifying your diet and exercise regimen, you can increase your lean body mass and metabolic rate. Factors affecting metabolism also play essential roles in controlling muscle mass. Aging is known to decrease the metabolic rate by as much as 5 percent per year. Additionally, because males tend to have more lean muscle mass than females, their basal metabolic rate (metabolic rate at rest) is higher; therefore, males tend to burn more calories than females. Lastly, an individual’s inherent metabolic rate is a function of the proteins and enzymes derived from their genetic background. Thus, your genes play a big role in your metabolism. Nonetheless, each person’s body engages in the same overall metabolic processes.^[3]