Short-term weight change, compared with stable weight, is a risk factor for mortality among older Americans. The direction and magnitude of risk, however, depends on the direction and magnitude of the change itself, as well as on the initial BMI. In this study, weight loss was associated with increased mortality among normal and overweight people, as well as mildly obese people up to a BMI of ~32–33, which includes the lower range of class I obesity (30–34.9). These findings suggest that weight loss itself, or the practices used to lose weight, may be harmful even if one is overweight or mildly obese. Large weight gains may also be associated with increased mortality, although only among people who have already reached Class II obesity or above (BMI > 35). Lastly, we found no evidence that small weight gains change mortality risk at any level of initial BMI.
To minimize confounding from unintentional weight loss due to illness, we controlled for diagnosed conditions and self-rated health before weight change, as well as changes in these factors during the weight change period. We also conducted sensitivity analyses excluding those with pre-existing conditions, poor health at baseline, smokers, and those who died within a short period from the start of the follow-up. The observed weight loss-mortality association could still, however, be due to undiagnosed conditions. Hazard ratio estimates adjusted for unobserved confounding suggest that in order to dissipate the weight loss effects, the unmeasured confounder would need to have both high prevalence and a very large effect on mortality. Existence of such an unknown confounder seems unlikely.
To summarize, large and small weight losses (compared with having a stable weight) are indicators of increased mortality among 50–70 year old people who were normal, overweight, or mildly obese at baseline. Weight gain, on the other hand, may also be associated with excess mortality, but only among people who are already obese and if the change is large. The observed interaction between weight loss and initial BMI may partly explain the discrepant findings in previous studies on the magnitude of the effects of weight loss on mortality,1–15
as the magnitude of the effect observed in any given study may strongly depend on the baseline BMI distribution of the study subjects.
The modifying effect of initial BMI in the weight loss-mortality association may be due to differential balancing of the benefits of a lower weight status and harmful effects associated with weight loss or weight loss practices. It is well-known that many weight loss methods such as smoking, fasting, and diet drugs can have harmful effects. Prior studies have shown that among middle-aged and older people, overweight is not associated with excess mortality.16,20,28–30
Recent literature also suggests that obese class I (BMI 30–35) may not be associated with increased mortality, compared with normal weight.16,17,20
Hence, we would not necessarily expect people losing weight from initial BMI levels between 25 and 35 to benefit in terms of mortality, and if weight loss practices are unhealthy, one might expect increases in mortality. Among older people, only obesity levels over BMI 35 are consistently associated with increased mortality. Thus, losing weight from these high levels could potentially result in no effect or a decrease in mortality. For normal weight people, weight loss may increase mortality as BMI approaches underweight levels, which are known to be associated with higher mortality.16,17,20
The above reasoning also applies when interpreting the finding that weight gains may be associated with increased mortality only if initial BMI is well into the obese range: if having a BMI between 25 and 35 is not associated with excess mortality when compared with normal weight, gaining weight from an initial BMI level of 25–30 may not be associated with excess mortality. If initial BMI is close to the range where BMI is positively associated with mortality (BMI ~35), then gaining weight may increase mortality risk.
As noted above, we conducted several sensitivity analyses suggesting that undiagnosed conditions are unlikely to be responsible for the whole weight loss-mortality association. It is nevertheless possible that such conditions may be responsible for a part of the effect. Moreover, in cases where death and weight loss are both being driven by underlying illness, the modifying effect of initial BMI in the weight loss-mortality association would support the “obesity paradox” hypothesis,31
indicating that when such conditions occur, being obese may be protective.
This study has limitations. First, BMI was constructed from self-reported height and weight. Self-reported and clinically measured height and weight are known to be highly correlated,32
and the correlation for weight may be as high as 0.98 among older persons because of diminished cultural pressures to be thin.33,34
Second, the data did not allow us to study causes of death. As recent research has shown that the BMI-mortality association varies by cause of death,35
the effect of weight change may also vary by cause of death. Prior research has found that weight loss may be associated with increased cardiovascular and coronary heart disease mortality1,14,36
and non-cancer mortality,4
but with decreased diabetes-related mortality.15
In a recent study on long term weight loss from age 20, the effect of weight loss on mortality was mainly driven by causes other than cancer and cardiovascular disease, but the “other causes” category was not further explored.9
Further research could consider more detailed cause-specific mortality. Third, we did not have direct information on whether weight losses were intentional or unintentional. We did, however, include extensive controls for underlying health conditions, as well as changes in these conditions, to adjust for sources of unintentional weight loss. Indeed, to the extent that changes in health status are a product of weight loss and on the causal pathway between weight loss and death, these controls may constitute an “over-adjustment,” lending a conservative bias to our estimates. Lastly, our results were also robust to several different sensitivity analyses designed to address the issue of confounding from underlying illness.
Despite these limitations, our findings have important implications. We found weight loss to be associated with increased mortality among both overweight and obese persons up to a BMI ~32. If weight loss from these BMI levels is potentially harmful, public health policy should focus on prevention rather than “treatment” of overweight and obesity. In the U.S., 62% of those who considered themselves somewhat overweight and 52% of those who considered themselves a little overweight reported that they were trying to lose weight.37
Even small weight losses were associated with increased mortality, and patients, especially those who are not obese, should be educated on the potentially harmful effects of weight loss from unhealthy diets or other weight-loss behaviors. More research is needed on the health effects of various weight loss strategies (dieting, exercise, eating disorder behavior) used by the general population; simply knowing whether the loss is intentional may not be enough.
While our results failed to show overall benefits to mortality from weight loss, this is not inconsistent with research showing that weight loss decreases cardiovascular risk factors such as hypertension and hyperlipidemia,38,39
and that weight loss may be associated with decreased mortality in certain sub-populations, such as overweight diabetics.5,15
We find, however, no evidence that in the general population aged 50–70 weight loss, when compared to having a stable weight, would decrease mortality for overweight persons or mildly obese persons with BMI up to about 32. Among the morbidly obese, large weight loss achieved by bariatric surgery has been shown to decrease mortality.40
This is in line with our estimate of 0.81 (95% CI = 0.50–1.30) for the hazard ratio for large weight loss from an initial BMI of 40.
In summary, our findings suggest that for older persons, weight loss is not associated with a decrease in all-cause mortality and may even be associated with increased mortality among normal, overweight, and mildy obese people. Weight gains, large and small, on the other hand, seem to be harmless among normal and overweight people, and potentially harmful only among those who are well into the obese range. Given the only a small proportion of the older people is obese and gaining weight, and a relatively large proportion is losing weight from the normal or overweight range, weight losses might warrant more attention than weight gains.