Obesity is a state of excess adipose tissue mass. Obesity is therefore more effectively defined by assessing its linkage to morbidity or mortality. INTRODUCTION: Body weights are distributed continuously in populations, so that choice of a medically meaningful distinction between lean and obese is somewhat arbitrary. Measurement Although not a direct measure of adiposity, the most widely used method to gauge obesity is the body mass index (BMI), which is equal to weight/height2 (in kg/m2) . Other approaches to quantifying obesity include anthropometry (skin-fold thickness), densitometry (underwater weighing) BMIs for the midpoint of all heights and frames among both men and women range from 19-26 kg/m2; at a similar BMI, women have more body fat than men. Based on data of substantial morbidity, a BMI of 30 is most commonly used as a threshold for obesity in both men and women. Large-scale epidemiologic studies suggest that all-cause, metabolic, cancer, and cardiovascular morbidity begin to rise when BMIs are 25, suggesting that the cut-off for obesity should be lowered. A BMI between 25 and 30 should be viewed as medically significant and worthy of therapeutic intervention, especially in the presence of risk factors that are influenced by adiposity, such as hypertension and glucose intolerance. The distribution of adipose tissue in different anatomic depots also has substantial implications for morbidity. Specifically, intraabdominal and abdominal subcutaneous fat have more significance than subcutaneous fat present in the buttocks and lower extremities. This distinction is most easily made clinically by determining the waist-to-hip ratio, with a ratio >0.9 in women and >1.0 in men being abnormal. Many of the most important complications of obesity, such as insulin resistance, diabetes, hypertension, hyperlipidemia, and hyperandrogenism in women, are linked more strongly to intraabdominal and/or upper body fat than to overall adiposity The mechanism underlying this association is unknown but may relate to the fact that intra abdominal adipocytes are more lipolytically active than those from other depots. Etiology of Obesity Though the molecular pathways regulating energy balance are beginning to be illuminated, the causes of obesity remain elusive. In part, this reflects the fact that obesity is a heterogeneous group of disorders. At one level, the pathophysiology of obesity seems simple: a chronic excess of nutrient intake relative to the level of energy expenditure. However, due to the complexity of the neuro endocrine and metabolic systems that regulate energy intake, storage, and expenditure, it has been difficult to quantitate all the relevant parameters that is food intake and energy expenditure over time in human subjects. Role of Genes versus Environment Obesity is commonly seen in families, and the heritability of body weight is similar to that for height. Inheritance is usually not Mendelian, however, and it is difficult to distinguish the role of genes and environmental factors. Specific Genetic Syndromes A number of complex human syndromes with defined inheritance are associated with obesity . Although specific genes are undefined at present, their identification will likely enhance our understanding of more common forms of human obesity. In the Prader-Willi syndrome, obesity coexists with short stature, mental retardation, hypogonadotropic hypogonadism, hypotonia, small hands and feet, fish-shaped mouth, and hyperphagia. Most patients have a chromosome 15 deletion. Pathologic Consequences of Obesity Obesity has major adverse effects on health. Obesity is associated with an increase in mortality, with a 50-100% increased risk of death from all causes compared to normal-weight individuals, mostly due to cardiovascular causes. Mortality rates rise as obesity increases, particularly when obesity is associated with increased intra abdominal fat . It is also apparent that the degree to which obesity affects particular organ systems is influenced by susceptibility genes that vary in the population. Insulin Resistance and Type 2 Diabetes Mellitus Hyper insulinemia and insulin resistance are pervasive features of obesity, increasing with weight gain and diminishing with weight loss . Insulin resistance is more strongly linked to intra abdominal fat than to fat in other depots. Reproductive Disorders Disorders that affect the reproductive system are associated with obesity in both men and women. Male hypo gonadism is associated with increased adipose tissue, often distributed in a pattern more typical of females. Cardiovascular Disease The Framingham Study revealed that obesity was an independent risk factor for the 26-year incidence of cardiovascular disease in men and women including coronary disease, stroke, and congestive heart failure The waist/hip ratio may be the best predictor of these risks. Pulmonary Disease Obesity may be associated with a number of pulmonary abnormalities. These include reduced chest wall compliance, increased work of breathing, increased minute ventilation due to increased metabolic rate, and decreased functional residual capacity and expiratory reserve volume. Gallstones Obesity is associated with enhanced biliary secretion of cholesterol, super saturation of bile, and a higher incidence of gallstones, particularly cholesterol gallstones. Cancer Obesity in males is associated with higher mortality from cancer, including cancer of the esophagus, colon, rectum, pancreas, liver, and prostate; obesity in females is associated with higher mortality from cancer of the gallbladder, bile ducts, breasts, endometrium, cervix, and ovaries. Bone, Joint, and Cutaneous Disease Obesity is associated with an increased risk of osteoarthritis, no doubt partly due to the trauma of added weight bearing and joint mal alignment. The prevalence of gout may also be increased . Among the skin problems associated with obesity is acanthosis nigricans, manifested by darkening and thickening of the skin folds on the neck, elbows, and dorsal interphalangeal spaces.
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