Chapter 9 Summary



Download 8.83 Kb.
Date23.06.2017
Size8.83 Kb.
#21468
Chapter 9 Summary
The term energy balance concerns the relationship between energy intake and energy expenditure. When positive energy balance occurs, energy intake exceeds energy expenditure and body weight generally increases. When a person is in negative energy balance, energy intake is less than energy expenditure, and body weight decreases. Adipose tissue serves as the body’s primary energy reserve. The number and size of adipocytes determine its mass. When body fat increases, adipocytes can increase in size (hypertrophy) and number (hyperplasia). When body fat decreases, adipocytes decrease in size but not in number.
Energy intake is influenced by both physiological and psychological factors. Hunger is the basic physiological need for food, whereas satiety is the physiological response to having eaten enough. Both are controlled by neuropeptides produced in the brain and by neural and hormonal signals from the gastrointestinal tract, pancreas, and adipose tissue. Psychological factors such as the appearance, taste, and aroma of food can also influence the desire to eat, regardless of hunger or satiety. The desire for food is called appetite, whereas the term food craving refers to the desire for a specific food.
The body expends energy to maintain basal metabolism, for physical activity, and to process food. These components make up a person’s total energy expenditure (TEE). Basal metabolism accounts for most of the TEE, and is affected primarily by body size and composition. Energy expended for physical activity is variable, accounting for 15 to 30% of TEE. Thermic effect of food (TEF) accounts for about 10% of the TEE and reflects the energy expended to process nutrients after eating. Smaller components of the TEE include adaptive thermogenesis and nonexercise activity thermogenesis (NEAT). Adaptive thermogenesis is the energy expended to adapt to temperature changes. NEAT is the energy expended on fidgeting and to support posture. A variety of methods are used to estimate TEE. Direct calorimetry measures the body’s heat loss, whereas indirect calorimetry measures the respiratory gases oxygen and carbon dioxide. Other estimates of TEE are based on the metabolism of stable isotopes of hydrogen and oxygen in the body. Mathematical formulas called the Estimated Energy Requirements (EERs) can also be used to estimate TEE.
Overweight is excess weight for a given height, whereas obesity is an abundance of body fat in relation to lean tissue. Indices such as height–weight tables and body mass index (BMI) are used to assess body weight. Height–weight tables list “ideal weights” for adults. BMI is based on the ratio of weight to height squared and is considered a good indicator of body fat. Measures of body composition include hydrostatic weighing, dual-energy x-ray absorptiometry (DEXA), bioelectrical impedance, near-infrared interactance, skinfold thickness, computed tomography (CT), magnetic resonance imaging (MRI), and isotope dilution. Body fat centralized within the abdomen is called android adiposity, whereas body fat located in the thighs and hips is gynoid adiposity. Body fat distribution is typically assessed by measuring waist and hip circumference.
Many factors have contributed to the increasing prevalence of obesity in the United States. For example, average energy intake in adults has increased, and flavorful energy-dense foods are widely available and affordable. Furthermore, most Americans do not exercise regularly. Thus the combination of increased energy intake and decreased energy expenditure increases the likelihood of weight gain. Although lifestyle, socioeconomic, and cultural factors influence body weight, genetics greatly affects our susceptibility to obesity as well. Scientists have long suspected that internal signals regulate body weight by adjusting energy intake and expenditure, and developed the “set point” theory to explain this phenomenon. Research later confirmed that the hormone leptin plays a role in regulating body weight. However, most obese people produce adequate amounts of leptin. This suggests that leptin may instead protect against starvation, rather than preventing obesity.
The brain receives information about energy intake and the body’s energy reserve via nerves, circulating nutrients, and hormones. This complex signaling system strives to maintain energy balance by making adjustments in energy intake and/or energy expenditure. The brain releases catabolic and anabolic neuropeptides that promote weight loss and weight gain, respectively. Signals from the GI tract play a role in regulation of short-term food intake and include gastric stretching and GI hormones. Circulating concentrations of glucose, fatty acids, and amino acids also influence hunger and satiety. The majority of GI hormones inhibit food intake with the exception of ghrelin, which stimulates hunger. The hormones leptin and insulin play a role in long-term regulation of energy balance by suppressing the release of anabolic neuropeptides in the brain. When leptin and insulin concentrations decrease, anabolic neuropeptides are released, resulting in overall increased energy intake and decreased energy expenditure.
Stringent dieting alone rarely results in long-term weight loss. Instead, maintaining weight loss requires lasting lifestyle changes, which include eating healthy foods and regular exercise. A healthy weight-loss and weight-maintenance program sets reasonable goals, encourages intake of low–energy-dense, nutrient-dense foods, and promotes regular physical activity. A realistic weight-loss goal is to decrease body weight by 5 to 10%, not exceeding 1 to 2 pounds each week. Choosing reasonable serving sizes is also important for weight loss and weight management. In addition, exercise helps prevent weight gain and improves overall health.
Some experts recommend low-carbohydrate diets for weight loss, whereas others recommend low-fat diets. Proponents of low-fat diets believe that less fat in the diet leads to consumption of fewer calories and to greater weight loss. However, reductions in fat intake have accompanied increased prevalence of obesity in the United States. In addition, compared to low-fat diets greater short-term weight loss is achieved on low-carbohydrate diets, but long-term differences have not been found. Advocates of low-carbohydrate diets believe that high carbohydrate intake can cause insulin levels to rise, leading to weight gain. In addition, limiting carbohydrates causes the body to go into a ketotic state, which results in decreased appetite. Low-carbohydrate diets have been criticized for being too high in fat and protein; however, not all low-carbohydrate diets are high in protein. Concerns regarding the lack of fruits, vegetables, and whole grains have been also expressed. A positive aspect of weight loss associated with low-carbohydrate diets is the loss of body fat without substantial loss of lean tissue.

Download 8.83 Kb.

Share with your friends:



The database is protected by copyright ©ininet.org 2024
send message
    Main page
increasing prevalence
physiological response