Overweight and Obesity

 

Overweight and Obesity

Overweight and obesity are increasing problems in the US and throughout the Western world and have significant health implications. In the US, data from the Third National Health and Nutrition Examination Survey (NHANES III) conducted in 1988–1994 showed that 58% of people aged 20 years and above were either overweight (body mass index [BMI] 25.0–29.9 kg/m2) or obese (BMI ≥30 kg/m2). The prevalence of overweight was higher for men than for women (39.9% vs 25.7%), whereas obesity prevalence was higher among women than men (25.5% vs 19.9%). In the most recent NHANES data for 1999–2000, the prevalence of overweight and obesity combined had risen to 64%. Trends in obesity prevalence over the last 40 years showed little change from 1960 to 1980, then a marked increase to the present day (men 1980, 13%; 1991, 21%; 1999–2000, 28%; women 1980, 17%; 1991, 26%; 1999–2000, 34%). The increasing prevalence of overweight has also been observed in children and adolescents. In children (6–11 years), the proportion overweight (BMI >95th percentile) increased from 4% in 1965 to 13% in 1999, while for adolescents (12–19 years), the percentage rose from 5% in 1970 to 14% in 1999.

Overweight and obesity are associated with increased rates of mortality and morbidity. Mortality rates increase for both men and women throughout the range of moderate and severe overweight. Among obese individuals, the risk of death from all causes is 50–100% greater than for those of normal weight (BMI 20–25 kg/m2); most of the increased risk is due to cardiovascular causes.Estimates put the number of deaths attributable to obesity in the US at 300 000 per year. Overweight and obesity are known to increase the risk for a number of diseases, including diabetes, cardiovascular disease (e.g. coronary heart disease [CHD] and cerebrovascular disease), hypertension and certain cancers. In addition, they are associated with abnormal metabolic changes such as insulin resistance and dyslipidaemia, which are themselves risk factors for cardiovascular disease (CVD) and diabetes. The considerable overall impact of overweight and obesity on health is therefore not surprising, with CVD representing the major cause of mortality in the developed world: in 2000, heart disease accounted for almost 30% of all deaths in the US.

Adiposity and the Effect of Fat Distribution

The distribution of fat (adipose tissue) within the body is recognised as a key factor influencing the effect of increasing weight on health. Several studies have demonstrated a link between abdominal adiposity and overall mortality, with visceral adiposity particularly related to an increased risk of disease. Visceral adiposity is associated with an increased risk for dyslipidaemia and glucose intolerance. The changes in lipid parameters observed with visceral adiposity increase the risk for CVD. The association between visceral adiposity and increased insulin resistance is a key factor contributing to increased risk for glucose intolerance and dyslipidaemia. Differences in visceral adiposity accounted for much of the variation in insulin resistance seen between individuals in a study of African Americans with type 2 diabetes. Reductions in visceral adiposity in non-diabetic obese individuals were the best predictor of improved insulin sensitivity in a weight loss intervention study. Other aspects of regional adiposity may also have an effect on insulin resistance. In the lower extremities, intramuscular adipose tissue is strongly correlated with insulin resistance, whereas subcutaneous adiposity is only weakly associated.

Recent research has begun to address the pathophysiological mechanisms that link adiposity to insulin resistance (reviewed by Goldstein[20]). Adipose tissue secretes a number of factors, including free fatty acids (FFA), peptides and cytokines, which can adversely affect insulin action and may have a detrimental effect on beta cell function. Secretion of these factors is influenced by overall adiposity and by fat distribution, with visceral adiposity appearing more pathogenic. Increased visceral adiposity is associated with increased release of FFA from adipose tissue. Prolonged exposure to elevated FFA levels can directly reduce the response of skeletal muscle and liver to insulin action through activity on insulin receptor signaling pathways. Elevated FFA levels also appear to compromise pancreatic beta cell function, reducing insulin secretion.

Read also the The ObesityEpidemic

The Metabolic Syndrome

It has been known for several decades that intra-abdominal fat or visceral fat is a risk factor for CVD and diabetes. The terms ‘syndrome X’, the insulin resistance syndrome and the metabolic syndrome have been used to describe a set of commonly co-occurring conditions that include obesity (particularly abdominal obesity), insulin resistance, impaired glucose tolerance, disturbances in uric acid and lipid metabolism, hypertension, and prothrombotic and proinflammatory states, which can increase the risk of CVD. The core features of the metabolic syndrome, focusing on five key risk factors. The US Public Health Service has aimed to increase awareness of the metabolic syndrome, with the NCEP Expert Panel defining this to include obesity, dyslipidaemia, elevated blood pressure, insulin resistance (with or without glucose intolerance), and prothrombotic and proinflammatory states. The current NCEP clinical guidelines for its identification. Obesity and overweight, physical inactivity and genetic factors can all contribute to the metabolic syndrome. The metabolic syndrome is closely associated with insulin resistance, although specific pathophysiological mechanisms regulating the emergence of the metabolic syndrome remain complex and incompletely understood. Approximately 23% of the overall US population meet the NCEP definition for the metabolic syndrome, and approximately 47 million Americans are affected. Approximately 60% of BMI-calculated obese (≥30 kg/m2) US men and 50% of obese US women are affected, underscoring the public health impact of this condition. Among those with the syndrome, prevalent features are central obesity in 38% of individuals, low HDL-cholesterol in 36% and hypertension in 34%. Hypertriglyceridaemia occurs in about 30% of these patients, and insulin resistance in 12%. The metabolic syndrome increases the risk for CHD at any given level of LDL-cholesterol, making it a target for therapeutic intervention. In a study conducted in Sweden and Finland, individuals with metabolic syndrome had a 3-fold increased risk of CHD and stroke. NCEP guidelines recommend that root mediators of the syndrome (i.e. overweight and lack of physical activity) should be addressed in addition to the reduction of cholesterol levels. Weight reduction and increased activity are also effective at reducing insulin resistance, while additional medication should be used to treat high blood pressure and the prothrombic state. Despite the widespread occurrence of metabolic syndrome throughout the Western world and recent campaigns to raise awareness of this health problem, and the potential relationship between antipsychotic-induced weight gain and the development of this problem, a recent survey revealed a low awareness among US psychiatrists of the risk of metabolic syndrome with second-generation antipsychotic therapy. Although relatively higher percentages of psychiatrists associated second-generation antipsychotic treatments with a risk of weight gain/obesity (59%) and diabetes (51%), both of which are major features of metabolic syndrome, only 3% mentioned metabolic syndrome.

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