Join Date: Sep 2003
Location: Los Angeles
Obesity develops when energy intake exceeds energy expenditure over a long period. Despite the seeming simplicity of the energy balance equation, the factors that influence energy balance are numerous, varied, and complex. These factors involve genetic, metabolic, environmental, behavioral, cultural, and socioeconomic influences.10 Ultimately, all factors that contribute to obesity influence energy intake or energy expenditure.
Certain genetic syndromes are associated with extreme obesity, including leptin deficiency, Prader-Willi syndrome, and Laurence-Moon-Biedl syndrome. These genetic defects are rare and are not responsible for most cases of obesity and extreme obesity. Clearly, our genetic makeup has not changed during the past few decades as the prevalence of obesity has increased dramatically. However, genetic alleles may have developed throughout evolution that promote food consumption and weight gain, which could increase survival during periods of food scarcity or famine. A prevailing view is that the current increase in obesity is the result of a permissive environment that has allowed greater expression of genetic predisposition.
Numerous metabolic factors influence energy regulation, perhaps in a feedback model. These factors include hormones, peptides, nutrients, uncoupling proteins, and neural regulatory substances from the gut, liver, brain, and even fat cells.11 The scientific understanding of these factors and the complexity of how they interrelate are in their infancy. Similarly, how these factors influence the development of extreme obesity and specifically how the feedback model
unravels in this situation are even less well understood. Genetic factors probably play a role in some of these regulatory processes.
Data reported by the Centers for Disease Control and Prevention suggest that mean energy intake increased by 168 kcal/d in American men and 335 kcal/d in American women between 1971 and 2000.12 However, variances in methodology and bias from self-reporting could affect the validity of these estimates. Dietary factors that have been identified as contributing to increased energy intake and obesity include larger portion size, increased intake of refined carbohydrates
including soft drinks, greater variety of foods consumed (except vegetables and fruits), eating away from home, and increased intake of fast food, salty snack foods, and pizza.13-16 Other factors such as smoking cessation and pregnancy may also contribute to an individual's susceptibility to gain weight. However, such factors generally are associated with modest amounts of weight gain and are unlikely to contribute to the development of extreme obesity.
Low levels of physical activity, including both exercise and nonexercise activity, are associated with an increased risk of obesity.17 A sedentary lifestyle probably contributes to the high prevalence of obesity in the United States. More than 60% of the US population does not perform regular physical activity, and approximately 25% are completely sedentary.18 Total daily physical
activity appears to have declined in recent years,19 and this decline may have contributed to the increase in obesity.20,21 A detailed study of physical activity found that obese subjects were sedentary (sitting) 2 1/2 hours more per day than were lean subjects, which translates into a decreased energy expenditure of about 350 kcal/d attributable to this behavior alone.22
The built environment is probably a strong contributor to decreased energy expenditure. During the past few decades, technological innovations such as elevators, escalators, computers, remote controls, and other labor-saving devices have produced a small but cumulative decrease in energy expenditure. The amount of time spent watching television has correlated positively with obesity
in both children and adults and with the risk of type 2 diabetes mellitus in adults.23,24 Although the relative contributions of environmental factors responsible for decreasing physical activity and increasing energy intake have not been fully elucidated, their collective effect appears to be powerful and pervasive in facilitating the development of obesity.
Why some people stop gaining weight after a moderate increase in BMI and others go on to develop extreme obesity is unknown. In fact, gaining weight tends to promote resistance to further weight gain. A major determinant of resting metabolic rate is the amount of lean tissue. As people gain weight, approximately 20% to 35% of excess weight is lean tissue to help support fat tissue.
Therefore, as weight increases, resting metabolic rate increases, ie, increasing obesity is associated with increasing metabolic rate. For example, a 40-year-old woman who is 67 inches tall and has extreme obesity (BMI, 45 kg/m2) has a calculated resting metabolic rate of 2030 kcal/d. In comparison, a person of the same age and height but of normal weight (BMI, 23 kg/m2) has a calculated resting metabolic rate of only 1420 kcal/d. This means that for an obese
individual to continue to gain weight, energy intake must be high enough to overcome the greater resting metabolic rate.
Obesity increases the risk of premature mortality. An estimated 112,000 to
365,000 deaths yearly in the United States are due to obesity.35-38 differences among these estimates are due to variations in the methods and assumptions used in each study, including adjustment for confounding factors, age of the data, and use of self-reported or measured values for BMI.35 As the degree of obesity increases, mortality increases.25 Compared to persons with a BMI of 23.5 to 24.9 kg/m2, women with extreme obesity have a 2-fold higher risk of all-cause mortality, and men have a more than 2-fold higher risk.39 Extreme obesity has been estimated to shorten life by 5 to 20 years, depending on sex, age, and racial/ethnic group.40 Although life expectancy continues to increase in the US population,41 some investigators suggest that the increase in life expectancy will soon cease because of the rising prevalence of obesity and that for the first time in history, children may not live as long as their parents. The relative effect of obesity on mortality is greater in whites than in blacks, greater in men than in women, and greater in young than in older adults. Extremely obese white men aged 20 to 30 years are estimated to lose 13 years of life compared with their normal BMI counterparts, and extremely obese white women in this age group lose 8 years.40 The elevated risk of mortality is primarily due to the increased incidence of cardiovascular disease and cancer associated with higher degrees of obesity.39 An estimated 14% of all cancer deaths in men and 20% of those in women are due to obesity.43 The relative increased risk of obesity-associated mortality diminishes with age, and most population-based studies have not found a statistically significant increase in besity-associated mortality in adults who are older than 75 years.44
DISCRIMINATION AND QUALITY OF LIFE
Obesity is associated with social discrimination, even among medical professionals.45,46 Obese persons are less likely to be hired, receive equal wages, or be promoted compared with their normal-weight counterparts.45 Education and health care are other areas in which discrimination is recognized. For example, obese persons are less likely to receive appropriate preventive care, including screening tests for cervical and breast cancer.47 Extremely obese individuals report reduced quality of life.48 This may be due to obesity-related medical complications or challenges in everyday activities such as buckling seat belts; fitting into seats in airplanes, theaters, and buses; maintaining good hygiene; urinating accurately (men); and even cutting toenails.49 After bariatric surgery-induced weight loss, extremely obese patients perceive a marked improvement in their quality of life and a decrease in discrimination.50-52
Last edited by jutang; 07-19-2007 at 11:29 PM..