Trends and Correlates of Class 3 Obesity in the United States From 1990 Through 2000 FREE

The prevalence of obesity, defined as a body mass index (BMI) of 30 or more, has markedly increased during the last 3 decades in the United States^1- 3 and other countries.^4- 6 Between 1976-1980 and 1999, for example, the prevalence of obesity increased from 13% to 27% among US adults.^1,7

There are few epidemiologic investigations, however, of more extreme BMIs. Based on differences in treatment and health risks, obesity has been categorized as class 1 (BMI, 30-34.9), class 2 (BMI, 35-39.9), and class 3 (BMI, ≥40).^8- 11 Persons with class 3 obesity, also termed morbid or extreme obesity, are potential candidates for antiobesity surgery¹² and have a 2-fold higher risk for all-cause mortality than persons with BMIs of 30 to 31.9.¹³

The prevalence of class 3 obesity increased from 1% to 3% between 1960-1962 and 1988-1994 and is highest among black women.^1- 2 The objectives of the current study are to determine if trends have continued through 2000 and to examine various correlates of class 3 obesity.

We analyzed data from the Behavioral Risk Factor Surveillance System (BRFSS), a multistage survey that uses random-digit dialing to obtain a representative sample of adults (≥18 years old) in each state.¹⁴ Several steps were taken to ensure quality control in BRFSS, and the current analyses are based on data collected from 1990 (44 states) through 2000 (50 states).

The questionnaire focuses on behaviors associated with disease risks,¹⁵ and the data have been used to examine secular trends in overweight and obesity (cutpoints of 27-32).^3,16 Because height was recorded in inches and weight in pounds, these units are used throughout the text; BMI was calculated as the weight in kilograms divided by the squared height in meters.

More than 1 million persons participated in the BRFSS between 1990 and 2000. Approximately 4% of participants did not report weight or height, and the current analyses are limited to non-Hispanic whites, non-Hispanic blacks, and Hispanics. Other race/ethnic groups (approximately 4%) are excluded, as are pregnant women (approximately 2%) and persons (n = 364) with extreme values of weight or height (eg, weight >560 lb). Yearly totals for the current analyses ranged from 75 600 (1990) to 164 250 persons (2000).

All analyses accounted for the unequal selection probabilities.¹⁷ Statistical testing is not emphasized in this large sample, but SEs were calculated using SUDAAN to account for the design.¹⁷

Trends in various BMI categories are examined according to sex, race/ethnicity, age, educational achievement, and height. The independent relation of these characteristics to class 3 obesity (and possible interactions) was examined in logistic regression analyses.¹⁷ Differences in the BMI distributions in 1990-1991 vs 2000 were examined using a percentile comparison plot.¹⁸

The mean BMI increased from 24.9 (1990) to 26.5 (2000), and Figure 1 illustrates trends in 5 BMI categories. Although the prevalence of all BMI categories greater than 25 increased during the study period, the most striking increases were in the extreme BMI categories, with class 3 obesity increasing from 0.78% to 2.2%.

The prevalence of class 3 obesity according to various characteristics is examined in Table 1. (Because of the smaller number of persons interviewed in 1990, data for 1990 and 1991 have been combined.) In each year, the prevalence of class 3 obesity was approximately 2-fold higher among women than men, with the highest prevalence among black women (6% in 2000). Among men, there was little difference in class 3 obesity by race/ethnicity in 1990-1991, but the prevalence in 2000 was highest among blacks (2.4%). Although secular increases were seen in all age groups, the largest proportional increase (0.4% to 1.2%) was among 18- to 29-year-olds. The prevalence of class 3 obesity also increased within all categories of educational achievement, with the prevalence highest among persons who did not complete high school.

Additional analyses indicated that although mean levels of weight increased by 9 to 12 lb during the 11 years, these weight increases varied only slightly by height. We therefore examined the prevalence of class 3 obesity within various height categories (Table 1, bottom). Among both men and women, the prevalence of class 3 obesity was about 2-fold higher among short persons than among taller persons (eg, <67 vs ≥74 inches among men).

Logistic regression analyses indicated that each characteristic in Table 1, including year of study, was independently associated with the prevalence of class 3 obesity. For example, educational achievement was inversely associated with class 3 obesity among 30- to 69-year-olds in each race-sex group, with the prevalence highest (12% in 2000) among black women who did not complete high school (Table 2). Additional analyses indicated that the association with educational achievement was stronger among women than men.

All percentiles of BMI were higher in 2000 than in 1990-1991, but the increases were larger at high BMI levels (Figure 2). For example, although the 10th percentile of BMI increased by 0.6 units, the 95th percentile increased by 3.2 units (36.9 − 33.7 = 3.2).

Most studies of secular trends in obesity have focused on mean BMIs or on BMIs of 30 or higher, but our results indicate that the prevalence of BMIs of 40 or higher increased almost 3-fold between 1990 and 2000. These trends will greatly increase the risk for various diseases and premature mortality.^13,19 Approximately 75% of adults with class 3 obesity have at least one comorbid condition, such as high blood pressure or diabetes mellitus.²⁰ Furthermore, a BMI of 40 or higher is associated with a 2-fold higher risk for all-cause mortality than are BMIs of 30 to 31.9.¹³

Despite these consequences, relatively little is known about the distribution of class 3 obesity. We found the highest prevalence among black women and among persons with low levels of educational achievement. Although an inverse association between social class and less extreme obesity has consistently been found among women, a review²¹ of articles before 1990 concluded that the association among men was inconsistent. Our observations suggest that the (inverse) association among men may have become stronger during the last decade. In agreement with this possibility, several recent studies^22- 24 have found that obesity (BMI >30) is inversely associated with social class among men. However, the sex differences before 1990 may also reflect the greater stigmatization of obesity among women.

An unexpected finding was the high prevalence of class 3 obesity among shorter adults. Although it is generally assumed that height and BMI are uncorrelated, an inverse association (r = −0.10) has been reported,²⁵ and others^26- 27 have found a relatively high prevalence of obesity among short adults. It is possible that the association between BMI and height may be influenced by characteristics, such as dietary intake and physical activity, that vary only slightly by height.

In agreement with the trends observed between 1966-1970 and 1988-1994²⁸ is our finding that increases in BMI were most striking at high BMIs. Although all BMI percentiles increased between 1990 and 2000, the median increased by 1.2, whereas the 95th percentile increased by 3.2. We also found that 18- to 29-year-olds showed a large proportional increase in class 3 obesity, possibly reflecting the increases in childhood BMI (particularly at the upper percentiles) that have occurred since 1975.^28- 29

An important limitation of the current study is the use of self-reported rather than measured weight and height. The BMIs based on self-reported and measured data are highly correlated (r>0.95), and self-reported data have been used in cohort studies^13,30- 31 and in studies of secular trends.^3,5,16 However, because height is overreported and weight is underreported, BMIs based on self-reported data are biased downward.^32- 35 This bias increases at higher BMIs,^32- 34 and the sensitivity of self-reported data to detect a BMI of 30 or higher ranges from 63% to 74%.^32- 33,36 This underreporting likely accounts for the approximately 50% lower prevalence of class 3 obesity that we observed in 1995 (1% for men and 1.6% for women) than the estimates of 2% to 4% reported by others.^1- 2 However, if these biases remained fairly constant during the study, our observed trends and subgroup differences would parallel those calculated from measured data. The proportional increase in class 3 obesity that we observed between 1990 and 2000 is similar to those reported (based on measured data) from 1976-1980 to 1988-1994¹ and from 1985 to 1995.²

Although the optimum BMI remains uncertain,³⁷ the trends in class 3 obesity will result in substantial increases in morbidity and premature mortality. Additional studies are needed to elucidate future trends and to identify other characteristics that may be associated with class 3 obesity, such as repeated weight increases during pregnancy among women with more than 3 children.^38- 39 Because weight loss is difficult to maintain, the prevention of obesity should be emphasized.