WHAT IS BODY COMPOSITION, AND WHY IS IT IMPORTANT?
The human body can be divided into fat-free mass and body fat. As defined in Chapter 2, fat-free mass comprises all the body’s nonfat tissues: bone, water, muscle, connective tissue, organ tissues, and teeth.
Body fat is incorporated into the nerves, brain, heart, lungs, liver, mammary glands, and other body organs and tissues. A certain amount of body fat is necessary for the body to function. It is the main source of stored energy in the body; it also cushions body organs and helps regulate body temperature. This essential fat makes up about 3-5% of total body weight in men and about 8-12% in women (Figure 6.1). The percentage is higher in women due to fat deposits in the breasts, uterus, and other gender-specific sites.
Most of the fat in the body is stored in fat cells, or adipose tissue, located under the skin (subcutaneous fat) and around major organs (visceral fat or intra-abdominal fat). People have a genetically determined number of fat cells, but these cells can become larger or smaller depending on
Answers (Test Your Knowledge)
1. True. Regular physical activity provides protection against the health risks of overweight and obesity. It lowers the risk of death for people who are overweight or obese as well as for those at a normal weight.
2. c. All four are risk factors for diabetes, but overweight/obesity is the most significant. It’s estimated that 90% of cases of type 2 diabetes could be prevented if people adopted healthy lifestyle behaviors.
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3. All four. Very low levels of body fat, and the behaviors used to achieve them, have serious health consequences for both men and women.
figure 6.1 Body composition of a typical man and woman, 20-24 years old. source: Adapted from Brooks, G. A., et al. 2005. Exercise Physiology: Human Bioenergetics and Its Applications, 4th ed. New York: McGraw-Hill. how much fat is being stored. The amount of stored fat depends on several factors, including age, gender, metabolism, diet, and activity level. The primary source of excess body fat is excess calories consumed in the diet that is, calories consumed beyond what are expended in metabolism, physical activity, and exercise. A pound of body fat is equal to 3500 calories, so an intake of just 100 calories a day in excess of calories expended will result in a 10-pound weight gain over the course of a year. Excess stored body fat is associated with increased risk of chronic diseases like diabetes and cardiovascular disease, as described later in this chapter.
Overweight and Obesity Defined
When looking at body composition, the most important consideration is the proportion of the body’s total weight that is fat the percent body fat. For example, two women may both be 5 feet, 5 inches tall and weigh 130 pounds. But one woman may have only 15% of her body weight as fat, whereas the other woman could have 34% body fat. Although neither woman is overweight by most standards, the second woman is considered overfat. Too much body fat (not just total weight) has a negative effect on health and well-being. Just as the amount of body fat is important, so is its location on your body. Visceral fat is more harmful to health than subcutaneous fat.
Overweight is usually defined as total body weight above the recommended range for good health as determined by