State characterized by an excess of body fat. Clinical measures of obesity are achieved using methods including skinfold thickness, underwater weighing, total body potassium, bioelectrical impedance, computer imaging (for example COMPUTERIZED AXIAL TOMOGRAPHY OF MAGNETIC RESONANCE IMAGING scans) and the Quetelet or BODY MASS INDEX (BMI). Measures of adiposity may provide either whole-body composition or specific organ values (see Heymsfield et al., 1995). However, for convenience, body mass index [wt (kg)/ht (m)2] and standard height and weight tables are typically consulted to assess level of obesity relative to the general population. Normal weight is defined as BMI between 20 and 24.9, overweight from 25 to 29.9, obese from 30 to 40 and morbid obesity greater than a BMI of 40. Standard weight tables identify normal or ideal weight according to the population average for gender, height, and age with overweight as a percentage of ideal between 110 and 120% and obese as greater than 120% of ideal. Problems with these definitions arise when applied to indivi duals since some tables do no accurately reflect population norms (many are from North America and apply less well elsewhere, and some are from insurance companies) and they fail to take account of changes in body weight as a function of AGEING. Also, a BMI greater than 30 can occur in athletes who have a significant proportion of body composition as fat-free mass. Nevertheless, in the general population morbidity and mortality rates increase as a function of degree of overweight.
The prevalence of obesity is increasing in Western countries. In Great Britain from 1980 to 1987, the proportion of overweight or obese adults (aged 16–64) increased from 39% to 45% of men and from 32% to 36% of women. In the USA the prevalence of frank obesity increased between 1980 and 1995 from 25% to 34%. The medical consequences associated with obesity include DIABETES, HYPERTENSION, hypercholesterolemia, hypertriglyceridemia, cardiovascular disease, gallbladder disease, arthritis, and respiratory disease. However, regional adiposity is a better predictor of risk to health than BMI. ENERGY stores are determined by the balance between the amount of energy expended (in physical activity, THERMOGENESIS and resting METABOLISM) and the amount of energy consumed. Thus, during a static state of weight maintenance, energy intake is equal to energy expenditure, whereas during weight loss or weight gain, negative or positive energy balance is achieved by changes in the rates of energy intake to energy expenditure. For example, long-term energy restriction coupled with increased physical activity should result in negative energy balance and therefore weight loss. However, this relationship is influenced by genetic endowment and differential rates of oxidation and storage of different nutrients.
Studies of the aetiology of obesity have focused on both metabolic abnormalities and on evidence of overeating which might underlie pathogenesis. Psychological approaches to the treatment of obesity focus on changing eating behaviour and increasing physical activity. Clear evidence that the obese overeat relative to the non-obese has not been forthcoming. However, in part this may be due to methods of measuring food intake both in the laboratory where the demand characteristics of the study might produce artificially lower intakes and in free living conditions, where diet records, dietary recall and weighed intakes might result in under-reporting.
Classic studies by Stanley Schachter in the 1960s proposed that obese consumers differ from normal weight consumers in their responsiveness to external factors in determining when and how much to eat rather than responding to internal cues such as changes in blood glucose levels (see INTERNALITY). However, studies by Judith Rodin later concluded that EXTERNALITY is not determined by BODY WEIGHT, rather external responsiveness occurs across body weight categories. In addition, work by C.Peter Herman and his colleagues further refined assumptions about obese/normal weight differences by focusing on RESTRAINED EATING. Thus, differences in eating behaviour may be accounted for by the degree to which consumers restrict their eating below a desired level. Restrained eating can be associated with successful dieting and long-term normal weight maintenance, but it is also associated with episodes of overeating and lack of restraint. It has been suggested that chronic dieting, overeating and long-term weight fluctuations may characterize a sub-population of the obese and that there is no evidence of an obese eating style. Obesity involves excess body fat and is linked to increased risk of ill health. Behavioural therapies for obesity focus on reducing energy intake and increasing exercise, however, the success of such treatments may be influenced by genetic predisposition, metabolic inputs as well as the psychological effects of chronic dieting.
Heymsfield S.B., Allison D.B., Heshka S. & Pierson R.N. (1995) Assessment of Human Body Composition. In Handbook of Assessment Methods for Eating Behaviors and Weight-Related Problems, ed. D.B.Allison, pp. 575–560, Sage: Thousand Oaks CA.
MARION M.HETHERINGTON
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