Obesity and Mortality Risk
Obesity and Mortality Risk
Little research has addressed the heterogeneity and mortality risk of BMI trajectories in older populations. The present study focused on BMI trajectories past 51 years of age in the original participants in the HRS who were born between 1931 and 1941. People who were overweight at 51 years of age and remained overweight through age 77 years had the lowest mortality risk. People who were in the class II/III obese category at age 51 years and gained weight through age 77 years had the highest mortality risk. Compared with the overweight stable trajectory, the class I obesity upward and class II/III obese upward trajectories were significantly associated with 30% and 147% increases in mortality risk, respectively, without controlling for confounding health factors. The hazard ratios decreased after controlling for these confounding factors. The deleterious effects of these 2 trajectories are greater among people with no preexisting chronic illnesses or ADL limitations and those who reported their health as good or better at baseline. This is consistent with several studies that have found that obesity leads to a higher mortality risk among healthy people.
The differences between the overweight stable and overweight obesity trajectories were not statistically significant. This finding suggests that in people who are overweight at 51 years of age, small weight gains do not lower the probability of survival. By contrast, weight gain in obese people (either class I or class II/III obese) increases their mortality risk. These findings indicate the associations of weight gain with mortality risk depend on baseline BMI status. Many previous studies have found that weight gain was associated with a higher mortality risk in overweight/obese individuals; however, we found that weight gain does harm obese individuals but does not harm not overweight individuals. These inconsistencies may result from prior studies using arbitrary cutoff points for weight change or assuming a linear function of the weight-change effect across BMI status, which may have yielded overdeterministic results. Weight loss, even a small one (a decrease of approximately 1 BMI unit), in a person in the normal weight category 51 years of age can potentially have a significant deleterious effect on health. Many previous studies found that even small weight losses can exert a harmful effect on survival, regardless of the initial BMI level.
The associations of BMI trajectory with mortality are stronger than the associations of initial BMI status alone. Table 4 presents mortality risks by baseline BMI status. Persons in the underweight and class II/III obese categories had increased mortality risk compared with the reference category (overweight). Normal weight and class I obesity were not associated with significant increases in mortality risk. These findings are consistent with those from the analysis by Mehta and Chang. The effect sizes of these BMI statuses (normal weight, class I obese, and class II/III obese) were smaller than the corresponding effect sizes of BMI trajectories in Table 2. The Bayesian information criterion statistic suggests the model has a better fit when using BMI trajectories than when using BMI status, which supports previous studies in which it was concluded that weight change is more predictive of mortality than is initial weight status alone.
This study has several limitations. First, the BMI measures were constructed from self-reported weight and height and are therefore subject to potential bias. However, self-reported and clinically measured height and weight are strongly correlated, although the extent of this correlation among HRS respondents is unknown. Moreover, differential biases in weight reporting may have accumulated over time and further biased our estimates. However, there is no reason to assume that the bias in weight and height reporting varied across any of the 6 trajectories. Therefore, using self-reported weight and height should not have introduced substantial bias to our analysis. Second, we were not able to trace the BMI trajectories to the earlier stages of the life course. It may be important to investigate whether BMI trajectories in early and middle adulthood display similar heterogeneity and whether this heterogeneity has similar implications for mortality risk.
Third, we were not able to differentiate between intentional and unintentional weight changes, particularly weight losses. However, prior studies have found that intentional weight loss has, at best, weaker detrimental effects on mortality and not the anticipated protective effect. Moreover, we have controlled for a wide range of underlying health problems and functional limitations that may lead to unintentional weight change, thereby estimating the net effect of weight change. Fourth, although BMI is the most commonly used measure of adiposity, it has been criticized as not being able to directly measure body fat and muscle composition or distinguish between central and peripheral adiposity. Although some datasets (e.g., the dataset from the National Health and Nutrition Examination Survey IV, 1999–2004) have data from more accurate and direct measures of body composition, such as dual energy x-ray absorptiometry, they do not track long-term changes in these measures.
Improving upon prior studies, we investigated the association of dynamic BMI trajectories with mortality risk. In one previous study, investigators examined BMI trajectories over time instead of over the life course in the HRS, but they detected less heterogeneity in BMI trajectories (for a more detailed comparison, please refer to Web Appendix 2). We found people in the overweight stable trajectory had the lowest mortality risk, followed by people in the overweight obesity, normal weight upward, class I obese upward, normal weight downward, and class II/III obese upward trajectories. The lower mortality risk among people in the the overweight trajectories is consistent with the view that extra body weight, including lean tissue mass and fat mass, may provide protection against nutritional and energy deficiencies, metabolic stresses, the development of wasting and frailty, and loss of muscle and bone density caused by chronic diseases such as heart failure.
Mortality risks attributable to the class I obese upward trajectory and the class II/III obese upward trajectory were 3.0% and 4.2%, respectively, compared with the overweight stable trajectory. In total, approximately 7.2% of deaths after 51 years of age in the 1931–1941 birth cohort were due to obesity upward trajectories. These estimates are larger than those of Mehta and Chang (5.1% and 4.7% for obese females and males, respectively), who used baseline BMI measures with reference to overweight status in the same dataset and same cohort of respondents. This comparison again demonstrates that BMI trajectories are more predictive of mortality risk than are initial BMI statuses. Our estimates are not directly comparable to those obtained by Allison et al., Mokdad et al. or Flegal et al. because of the different age groups in the samples. Their studies included adults of all ages, whereas ours focused on people 51 years of age or older. Because of the age-dependent nature of the BMI-mortality link (i.e., a stronger correlation among younger adults), we might have observed an even larger association for the 1931–1941 birth cohort if we could take into account the risk of dying before the age 51 years. Our study suggests that trajectories of increasing obesity past 51 years of age pose a substantive threat to future life expectancy increases.
Discussion
Little research has addressed the heterogeneity and mortality risk of BMI trajectories in older populations. The present study focused on BMI trajectories past 51 years of age in the original participants in the HRS who were born between 1931 and 1941. People who were overweight at 51 years of age and remained overweight through age 77 years had the lowest mortality risk. People who were in the class II/III obese category at age 51 years and gained weight through age 77 years had the highest mortality risk. Compared with the overweight stable trajectory, the class I obesity upward and class II/III obese upward trajectories were significantly associated with 30% and 147% increases in mortality risk, respectively, without controlling for confounding health factors. The hazard ratios decreased after controlling for these confounding factors. The deleterious effects of these 2 trajectories are greater among people with no preexisting chronic illnesses or ADL limitations and those who reported their health as good or better at baseline. This is consistent with several studies that have found that obesity leads to a higher mortality risk among healthy people.
The differences between the overweight stable and overweight obesity trajectories were not statistically significant. This finding suggests that in people who are overweight at 51 years of age, small weight gains do not lower the probability of survival. By contrast, weight gain in obese people (either class I or class II/III obese) increases their mortality risk. These findings indicate the associations of weight gain with mortality risk depend on baseline BMI status. Many previous studies have found that weight gain was associated with a higher mortality risk in overweight/obese individuals; however, we found that weight gain does harm obese individuals but does not harm not overweight individuals. These inconsistencies may result from prior studies using arbitrary cutoff points for weight change or assuming a linear function of the weight-change effect across BMI status, which may have yielded overdeterministic results. Weight loss, even a small one (a decrease of approximately 1 BMI unit), in a person in the normal weight category 51 years of age can potentially have a significant deleterious effect on health. Many previous studies found that even small weight losses can exert a harmful effect on survival, regardless of the initial BMI level.
The associations of BMI trajectory with mortality are stronger than the associations of initial BMI status alone. Table 4 presents mortality risks by baseline BMI status. Persons in the underweight and class II/III obese categories had increased mortality risk compared with the reference category (overweight). Normal weight and class I obesity were not associated with significant increases in mortality risk. These findings are consistent with those from the analysis by Mehta and Chang. The effect sizes of these BMI statuses (normal weight, class I obese, and class II/III obese) were smaller than the corresponding effect sizes of BMI trajectories in Table 2. The Bayesian information criterion statistic suggests the model has a better fit when using BMI trajectories than when using BMI status, which supports previous studies in which it was concluded that weight change is more predictive of mortality than is initial weight status alone.
This study has several limitations. First, the BMI measures were constructed from self-reported weight and height and are therefore subject to potential bias. However, self-reported and clinically measured height and weight are strongly correlated, although the extent of this correlation among HRS respondents is unknown. Moreover, differential biases in weight reporting may have accumulated over time and further biased our estimates. However, there is no reason to assume that the bias in weight and height reporting varied across any of the 6 trajectories. Therefore, using self-reported weight and height should not have introduced substantial bias to our analysis. Second, we were not able to trace the BMI trajectories to the earlier stages of the life course. It may be important to investigate whether BMI trajectories in early and middle adulthood display similar heterogeneity and whether this heterogeneity has similar implications for mortality risk.
Third, we were not able to differentiate between intentional and unintentional weight changes, particularly weight losses. However, prior studies have found that intentional weight loss has, at best, weaker detrimental effects on mortality and not the anticipated protective effect. Moreover, we have controlled for a wide range of underlying health problems and functional limitations that may lead to unintentional weight change, thereby estimating the net effect of weight change. Fourth, although BMI is the most commonly used measure of adiposity, it has been criticized as not being able to directly measure body fat and muscle composition or distinguish between central and peripheral adiposity. Although some datasets (e.g., the dataset from the National Health and Nutrition Examination Survey IV, 1999–2004) have data from more accurate and direct measures of body composition, such as dual energy x-ray absorptiometry, they do not track long-term changes in these measures.
Improving upon prior studies, we investigated the association of dynamic BMI trajectories with mortality risk. In one previous study, investigators examined BMI trajectories over time instead of over the life course in the HRS, but they detected less heterogeneity in BMI trajectories (for a more detailed comparison, please refer to Web Appendix 2). We found people in the overweight stable trajectory had the lowest mortality risk, followed by people in the overweight obesity, normal weight upward, class I obese upward, normal weight downward, and class II/III obese upward trajectories. The lower mortality risk among people in the the overweight trajectories is consistent with the view that extra body weight, including lean tissue mass and fat mass, may provide protection against nutritional and energy deficiencies, metabolic stresses, the development of wasting and frailty, and loss of muscle and bone density caused by chronic diseases such as heart failure.
Mortality risks attributable to the class I obese upward trajectory and the class II/III obese upward trajectory were 3.0% and 4.2%, respectively, compared with the overweight stable trajectory. In total, approximately 7.2% of deaths after 51 years of age in the 1931–1941 birth cohort were due to obesity upward trajectories. These estimates are larger than those of Mehta and Chang (5.1% and 4.7% for obese females and males, respectively), who used baseline BMI measures with reference to overweight status in the same dataset and same cohort of respondents. This comparison again demonstrates that BMI trajectories are more predictive of mortality risk than are initial BMI statuses. Our estimates are not directly comparable to those obtained by Allison et al., Mokdad et al. or Flegal et al. because of the different age groups in the samples. Their studies included adults of all ages, whereas ours focused on people 51 years of age or older. Because of the age-dependent nature of the BMI-mortality link (i.e., a stronger correlation among younger adults), we might have observed an even larger association for the 1931–1941 birth cohort if we could take into account the risk of dying before the age 51 years. Our study suggests that trajectories of increasing obesity past 51 years of age pose a substantive threat to future life expectancy increases.
