Morbidity and Mortality Associated With Obesity

Obesity is common and is associated with increased rates of mortality and morbidity. The effect of obesity on mortality risk has been addressed by a large number of studies over the past 20 yr. There are unfortunately a number of important methodological issues that have made it difficult to establish a clear relationship between these variables. For example, smokers weigh less than nonsmokers but have increased mortality related to their smoking, not their reduced weight. People with undiagnosed cancer may lose weight and have an increased risk of mortality that, again, is not related to their reduced weight. In an effort to correct for these factors, many modern studies exclude smokers and first 2- to 5-yr mortality (this period begins when BMI/weight is first measured) when looking at the effects of obesity on mortality. When this has been done, there remain some questions about whether mortality rates are increased in overweight individuals (BMI 25-30), but it is clear that obesity(BMI > 30) is associated with increased mortality. In one study, those with a BMI > 30 had a 70% greater risk of dying than lean individuals (2). Another study estimated that a man 20 to 30 yr old with a BMI > 45 would lose 13 yr of life expectancy owing to his excess weight (3). Obesity is felt by many to now rival cigarette smoking as a potentially modifiable contributor to mortality (4).

Many previous analyses, however, used older data sets obtained at a time when the screening and treatment of cardiovascular disease risk factors and diabetes were less aggressive than is currently the case. In a recent controversial analysis, Flegal et al. used NHANES data to make new estimates of excess deaths associated with underweight, overweight, and obesity (5). This analysis did not "correct" for the presence of comorbid conditions such as treated hypertension or treated hyperlipidemia. The results demonstrated that those with a BMI between 25 and 30 had a lower mortality rate than those with either a lower or higher BMI. Those with a BMI greater than 35 clearly had increased mortality. It appeared that the increased risk of mortality associated with obesity was higher in earlier cohorts, suggesting that current treatments for comorbid conditions may be improving the health of obese people. In a related study, Gregg et al. examined longitudinal trends in the management of cardiovascular disease risk factors in the NHANES cohorts (6). The authors looked at the prevalence of hypertension, hyperlipi-demia, and diabetes in people of varying BMIs over time. If a person had, for example, a blood pressure that was <140/90 mmHg because of treatment with antihypertensive medications, they were considered to not have hypertension for purposes of this analysis. A similar approache was used for hyperlipidemia. This study found that whereas hyper tension and hyperlipidemia were more common in obese than in lean individuals at all time points, because of more aggressive screening and treatment, an obese person in 2000 was less likely to have high blood pressure or hyperlipidemia than a thin person in 1960. The changes were dramatic and represented marked increases in the use of antihypertensive and lipid-lowering medications over the past 40 yr. The one exception to this pattern was in the area of diabetes. The prevalence of diagnosed diabetes has increased dramatically over the past 40 yr and is, in fact, more common now among obese individuals than it was in 1960. This is in part because the average obese person in 2000 weighed more than the average obese person did in 1960. The situation appears to be that whereas obesity has increased in prevalence, more aggressive treatment of hyperlipidemia, hypertension, and diabetes have minimized the effect that this rising prevalence has had on mortality. One might ask, though, if this is the public health strategy that we want to pursue for the next 40 yr.

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