Summary of Chapter 6 – Energy Balance and Body Composition
Too little physical activity encourages body fat accumulation. Any food can make you fat if you eat too much of it. A net excess of energy is almost all stored in the body as fat in adipose tissue. Alcohol both delivers kcalories and encourages storage of body fat. Fat from food is particularly easy for the body to store as adipose tissue. Protein is not stored in the body except in response to physical activity; otherwise it is present only as working tissue. If not needed to meet energy needs, excess protein can be converted to fat. In short, excess energy from carbohydrate, fat, protein, and alcohol leads to storage of body fat.
When fasting, the body makes a number of adaptations: increasing the breakdown of fat to provide energy for most of the cells, using glycerol and amino acids to make glucose for the red blood cells and central nervous system, producing ketones to fuel the brain, and slowing metabolism. All of these measures conserve energy and minimize losses. Over the long term, a diet moderately restricted in energy promotes primarily fat loss and the retention of lean tissue.
A person takes in energy from food and, on average, spends most of it on basal metabolic activities, some of it on physical activities, and about 10 percent on the thermic effect of food. Because energy requirements vary from person to person, such factors as age, gender, and weight must be considered when calculating energy spent on basal metabolism, and the intensity and duration of the activity must be taken into account when calculating expenditures on physical activities.
Clearly, the most important criterion of appropriate fatness is health. Current standards for body weight are based on the body mass index (BMI), which describes a person’s weight in relation to height. Health risks increase with a BMI below 18.5 or above 24.9. Central obesity, in which excess fat is distributed around the trunk of the body, presents greater health risks than excess fat distributed on the lower body. Researchers use a number of techniques to assess body composition, including waist circumference and skinfold measures.
The health risks of obesity are many and serious. Guidelines for identifying the health risks of overweight and obesity are based on a person’s BMI, waist circumference, and disease risk profile. Obesity also incurs social, economic, and psychological risks.