Obesity Epidemic And Health And Cost Consequences

The proportion of the US population that is considered obese has seen a marked increase in the past two decades. Data based on the most recent National Health and Nutrition Examination Survey (NHANES) (1999-2000) (8), which uses objectively measured height and weight, indicate that 30.5% had a BMI over 30 (6), more than twice

Obesity Population Distribution
Fig. 1. The population distribution of BMI is shifting. Source: Author's calculation based on the Behavioral Risk Factor Surveillance System (BRFSS).

the rate in early 1980s (14.5% based on the second NHANES, conducted in 1976-1980) (8,9). Moreover, none of the major sociodemographic groups of the population has been immune to the epidemic: rapid weight gain has been found in both genders, all racial/ ethnic groups, and all educational levels (5,10); the entire population weight distribution is moving to the right. Figure 1 is a depiction of the shift of the BMI (based on self-reported weight and height) distribution among the US adult population from the early 1990s to the early 2000s.

There is another, possibly more disturbing, phenomenon. The distribution is becoming flatter—i.e., a smaller proportion of the population is now in the center of the distribution (which is shifting as well), and a larger proportion further out in the right tail, i.e., in the "obese" range. This picture does little to reveal what is going on at the more extreme end of the weight distribution—i.e., in the "severe obesity" range.

Does severe obesity simply parallel the general trend in obesity? Or is there something fundamentally different about clinically severe obesity? Two conflicting opinions exist about the trends in clinically severe obesity. Clinicians tend to consider clinically severe obesity a rare pathological condition that is not affected by behavioral changes in the general population. This view would suggest that severe obesity changes little over time and that the number of patients with these extraordinary health problems and care needs remains roughly constant, even as there are more moderately obese individuals. Epidemiologists tend to lean toward the opposite view, namely that severe obesity is part of the general population distribution and small increases in the population BMI would have proportionally larger effects in the extreme tail (11). Which of those views better describes reality is an empirical question, but the answer has major ramifications for health care systems (12).

Figure 2 shows the time trend in obesity prevalence by severity of obesity, adjusted for sociodemographic changes to isolate the unique trend in obesity rates. In addition to the standard "obese" category, defined as having a BMI greater than 30, the groups of primary interest here are the more extreme categories: BMI greater than 35, BMI greater than 40, BMI greater than 45, and BMI greater than 50.

Growth Prevalence Obesity
Fig. 2. Prevalence growth by severity of obesity. Source: Ref. 12.

Between 1986 and 2000, the prevalence of BMI over 40 quadrupled from about 1 in 200 adult Americans to 1 in 50; the prevalence of BMI over 50 increased by a factor of 5, from about 1 in 2000 to 1 in 400 (12). In contrast, obesity defined as a BMI of over 30 roughly doubled during the same time period, from about 1 in 10 to 1 in 5. The rate of increase for the BMI greater than 30 group is significantly lower than for the BMI greater than 40 group, which in turn is significantly lower than that for the BMI greater than 50 group. This trend has continued since those calculations were first published; from 2000 to 2004, we calculate that the prevalence of a BMI over 40 has increased to 2.7%, or more than 1 in 40 compared to 1 in 50 only 4 yr earlier; the prevalence of a BMI over 50 has increased to 1 in 300 (0.33%). These rates are all based on self-reported height and weight, which substantially underestimate higher weight categories.

The immediate consequence is that the economic burden of obesity in the health care system will increase at a much faster rate than the growth rate of moderate obesity. On average, an obese adult incurs health care costs that are about one-third higher than an otherwise similar normal-weight adult (13). Yet when we distinguish weight categories among Americans 54 to 69 years old, a BMI of 35 to 40 is associated with twice the increase in health care expenditures (about a 50% increase) relative to normal weight compared with a BMI of 30 to 35 (about a 25% increase); a BMI of over 40 doubled health care costs (approx 100% higher costs) compared with those with normal weight (14).

A key factor behind the large differences in health care costs by weight is the prevalence of major chronic conditions associated with obesity. Figure 3 shows how quickly the prevalence of diabetes increases with BMI. We measure diabetes here by self-reported, doctor-diagnosed diabetes or borderline diabetes, which obviously excludes what is likely to be a sizable number of undiagnosed cases.

Under 25-30 30-35 35-40 40+ 25

BMI Group

Fig. 3. Prevalence of diabetes among adults by body mass index. Source: Authors' calculation based on BRFSS 2004.

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