Weight Loss and Body Composition

The body is made up of different components (water, muscle, fat, bone, nerve tissue, tendons, and so on), and each has a different density. From a functional standpoint, tissues are grouped together into those that are mainly fat (fat mass), which is mainly anhydrous (has little water associated with it), and those that have little fat (fat-free mass), which is hydrous (has a great deal of water associated with it). The fat-free mass is also commonly referred to as lean mass, although this is viewed by many to be an inaccurate description because the fat-free mass includes a great deal of water (greater than 65 percent). Because techniques for estimating body composition are widely available, bone mass is now also included as a third commonly assessed component of body composition. In this book the components of body composition are referred to as fat mass (the body tissue that is mainly fat) and fat-free mass (the body tissue that is mainly free of fat, including muscle and bone).

Fat mass is composed of essential fat and storage fat. The essential fat is a required component of the brain, nerves, bone marrow, heart tissue, and cell walls that we cannot live without. Approximately 12 to 15 percent of total body weight in adult females is essential fat, the majority of which is associated with reproductive function and includes the additional fat associated with breast tissue. Because males do not have this reproductive function, their essential-fat levels are considerably lower. Storage fat, on the other hand, is an energy reserve that builds up in adipose tissue underneath the skin (subcutaneous fat) and around the organs (interabdominal fat). It is common for healthy men and women to have a storage-fat level that contributes 11 to 15 percent to total body weight. Combining the essential-fat and storage-fat components, normal body fat percentage for males is approximately 15 percent (3 percent essential; 12 percent storage), while normal body fat percentage for females is 26 percent (15 percent essential; 11 percent storage).¹

Women with extremely low body fat percentages are at risk of developing reproductive system difficulties, commonly manifested as irregular menstrual periods (see table 12.1). Oligomenorrhea and amenorrhea are associated with increased fracture risk and low estrogen production, which increases the risk of osteoporosis (a bone disease associated with low bone density). It appears that a body fat percentage of 17 to 22 percent is needed to maintain a normal menstrual cycle in most women.²

Women who develop an excessively low body fat percentage typically exercise excessively for the amount of energy they consume, or they have an eating disorder. The female athlete triad, a condition prevalent in many female athletes, includes the interrelated presence of an eatingdisorder; amenorrhea; and low bonedensity, osteopenia, or osteoporosis(figure 12.1).

Fat-free mass is mainly water and protein but also includes small levels Osteoporosis of minerals and stored carbohydrate Amenorrhea (glycogen). The main constituents of fat-free mass include skeletal muscle, the heart, and other organs. Although total body weight is approximately 60 percent water, the water content of the fat-free mass is 70 percent. This can be compared with the water content of the fat mass, which is below 10 percent.3 Athletes typically have a higher fat-free mass and a lower fat mass than do nonathletes.

Figure 12.1 The female athlete triad.

Weight

Weight Issues