Figure 1 presents a comparison of the population distributions of BMI measured using time of survey and maximum values. Comparison of the two distributions reveals a greater density at higher BMI values using maximum values (Figure 1).

Figure 1 Distribution of body mass index at time of survey and at maximum among US never-smoking adults ages 50-84. Distributions are unweighted. Source: National Health and Nutrition Examination Survey. Full size image

Descriptive statistics of the study sample, consisting of US adults ages 50-84 who never smoked, are presented in Table 1. Mean age at survey was slightly over 64 years. At the time of the survey, 20% and 12% of adults were in the obese class 1 and obese class 2 categories, respectively. When obesity status was assessed using maximum BMI, the percent obese class 1 and obese class 2 climbed to 27% and 19%.

Table 1 Characteristics of US never-smoking adults ages 50-84 Full size table

Table 1 also shows the population distribution across 10 categories defined using information on BMI at maximum and at time of survey. The majority of individuals (70%) were at their maximum BMI at the time of survey; 17% of individuals were in the normal BMI category both at time of survey and at their maximum BMI, and 26%, 15%, and 12% were overweight, obese class 1, and obese class 2 at both values. The remaining 30% of the population lost weight between their BMI at maximum and time of survey. The majority of individuals in this subpopulation transited between the overweight and normal (10%) or obese class 1 and overweight categories (11%). A small proportion of the population experienced more significant weight loss, with about 2% of individuals going from obese class 2 to normal or overweight and another 2% going from obese class 1 to the normal category.

Cox proportional hazards models predicting mortality for each of the two categorical measures of BMI are presented in Table 2. The results show a much stronger relationship using maximum values. In the specification using BMI at time of survey, the hazard ratios for obese class 1 and obese class 2 were only moderately associated with mortality and were not significant (obese class 1: 1.18 [95% confidence interval (CI), 0.91-1.54); obese class 2: 1.31 [95% CI, 0.95-1.81]). However, in the model using maximum BMI, both categories of obesity were strongly and significantly related to mortality (obese class 1: 1.67 [95% CI, 1.15-2.40]; obese class 2: 2.15 [95% CI, 1.47-3.14]).

Table 2 Hazard ratios for mortality from all causes according to body mass index at time of survey and body mass index at maximum Full size table

Kaplan Meier survival curves by category of BMI also reveal more substantial differences in survival across BMI categories using maximum values. A notable difference between the two sets of results is the improved survival of individuals in the normal BMI category when maximum values are used.

Table 3 again shows the hazard ratios for BMI at maximum and at time of survey (these results appear in the final row and column of the table). However, Table 3 has two additional elements. First, it includes age-standardized mortality rates (expressed as deaths per 1,000 person-years) associated with categories of BMI at maximum and at time of survey. Second, it shows age-standardized mortality rates and hazard ratios for each combination of BMI at maximum and time of survey. This information is arrayed in a matrix with the rows identifying categories of BMI at time of survey and columns identifying BMI at maximum. Cells below the diagonal are empty because BMI at time of survey is always equal to or less than BMI at maximum.

Table 3 Age-standardized all-cause mortality rates (per 1,000 person-years) and hazard ratios for mortality from all causes according to combinations of body mass index at time of survey and body mass index at maximum Full size table

The lowest mortality rates are generally along the diagonal of the matrix corresponding to persons with stable or increasing weight. Those with the lowest mortality rates were individuals of normal weight at their maximum and survey values (7.17 [95% CI, 4.58-9.76]) (measured by deaths per 1,000 person-years), followed by individuals who were overweight (7.84 [95% CI, 6.06-9.61]) or obese class 1 (12.55 [95% CI, 8.12-16.98]) at both their maximum and survey values. Mortality rates were consistently higher in subgroups above the diagonal of the matrix—individuals who lost weight between their BMI at maximum and time of survey. The population subgroups with the highest mortality rates were those that exhibited the most weight loss, including those that went from obese class 2 to normal and overweight and individuals that went from obese class 1 to normal weight. Although the mortality rates were very large in the groups that lost the most weight, the proportion of the population in these groups was small. Only about 2% of individuals transited from obese class 2 to normal or overweight between measurements (Table 1).

Table 3 also shows that the mortality rate for normal weight individuals was higher when the category is constructed using BMI at time of survey compared to BMI at maximum (10.41 [95% CI, 7.92-12.91] versus 7.17 [95% CI, 4.58-9.76]). This is consistent with findings from Figure 2 of improved survival among those in the normal category when using BMI at maximum versus BMI at time of survey.

Figure 2 Kaplan Meier curves for categories of BMI at time of survey and at maximum. Categories of BMI are normal weight (18.5-25.0 kg/m2); overweight (25.0-29.9 kg/m2); obese class 1 (30.0-34.9 kg/m2); and obese class 2 (35.0 kg/m2 or greater). The sample includes persons ages 50-84 who never smoked. Entry years are 1988-2004 with mortality follow-up through 2006. Estimates are weighted and account for complex survey design. Source: National Health and Nutrition Examination Survey. Full size image

Examination of mortality rates for combinations of BMI at maximum and time of survey reveals the source of the discrepancy. Using BMI at maximum, the normal category only includes stable normal-weight individuals. The mortality rate in this group (7.17) was lower than for any other group in Table 3. In contrast, the normal category defined using BMI at time of survey combines the low-risk stable-weight individuals with high-risk individuals that have experienced weight loss. Thirteen percent of individuals classified as normal using time of survey values were at one point in their lives either overweight or obese (Table 1). Mortality rates among groups that lost weight were substantially greater: 14.18, 16.53, and 66.56 for individuals that were overweight, obese 1, and obese 2 in their past and normal weight at time of survey. The contamination of the normal weight category when it is defined using BMI at time of survey explains why the mortality risks of overweight and obesity grew stronger after substituting maximum BMI for BMI at time of survey in Table 2.

Table 4 shows population attributable fractions for overweight and obesity based on BMI at survey and at maximum. Category-specific and overall PAFs are given. Using BMI at survey, an estimated 5.42% of deaths were attributable to the combination of overweight and obesity, whereas using maximum BMI, the attributable risk was substantially greater, at 32.58%.