In this new article, Dowd and Zajacova consider the question of cumulative dose of obesity over the lifecourse, and how this might impact on later life functional disabilities. They analysed data from over 7000 adults ages 60–79 from the 1999–2010 NHANES survey, with the outcome being a self-report of difficulty or severe difficulty with any of eight functional tasks. The exposure was self-reported 25-year old BMI (does anyone really know that?).
Predictably, there was an association between 25-year-old obesity (BMI > 30 kg/m2) and functional limitations, and this was greatly diminished in magnitude after controlling for current BMI. For severe limitations, for example, the covariate-adjusted OR for age 25 obesity was 2.72 (95% CI: 2.13–3.46), and this diminished to 1.32 (95% CI:1.00–1.75) after adjustment for current BMI. The authors interpret the persistent 25-year old obesity effect as an indication of a role for lifecourse burden of obesity, which certainly makes some sense.
There are quite a few potential concerns here, but I think that many of them become readily apparent as soon as you try to draw a DAG and ask what kind of effect we are trying to estimate. With respect to the 25-year old obesity OR, the late life BMI measure is not a confounder, since it is downstream of the exposure, and so must be an intermediate. Therefore, we are essentially asking if early adult obesity has a direct causal effect on functional impairment that is not relayed through late life BMI.
Setting aside questions about the casual interpretation of BMI as an exposure, one has to think about the fact that the normative tendency is to increase weight into the 50s and 60s, at which point a substantial number of people start to lose weight again. There are many factors associated with loss of weight among the elderly, none of which bode well for functional health:
Predictably, there was an association between 25-year-old obesity (BMI > 30 kg/m2) and functional limitations, and this was greatly diminished in magnitude after controlling for current BMI. For severe limitations, for example, the covariate-adjusted OR for age 25 obesity was 2.72 (95% CI: 2.13–3.46), and this diminished to 1.32 (95% CI:1.00–1.75) after adjustment for current BMI. The authors interpret the persistent 25-year old obesity effect as an indication of a role for lifecourse burden of obesity, which certainly makes some sense.
There are quite a few potential concerns here, but I think that many of them become readily apparent as soon as you try to draw a DAG and ask what kind of effect we are trying to estimate. With respect to the 25-year old obesity OR, the late life BMI measure is not a confounder, since it is downstream of the exposure, and so must be an intermediate. Therefore, we are essentially asking if early adult obesity has a direct causal effect on functional impairment that is not relayed through late life BMI.
Setting aside questions about the casual interpretation of BMI as an exposure, one has to think about the fact that the normative tendency is to increase weight into the 50s and 60s, at which point a substantial number of people start to lose weight again. There are many factors associated with loss of weight among the elderly, none of which bode well for functional health:
The relation between the covariate and the outcome is therefore quite problematic. They are likely to share an unmeasured common ancestor from the table above, such as illness. But as shown in the same table, another common cause of weight loss is functional impairment, the outcome variable! Moreover, although a single effect estimate is reported, I am sceptical that the process is comparable for a thin person at 25 years who is SET to be thin or fat in late life, versus a fat person at age 25 who is SET to be thin or fat in late life. This implies a likely interaction between the two BMI measures, which is not considered by the authors.
Were the follow-up at age 50, loss of weight would be considerably more rare, raising questions of positivity. By ages 70+, weight loss is common, but also highly informative. And don't even get me started on attrition bias here.
Were the follow-up at age 50, loss of weight would be considerably more rare, raising questions of positivity. By ages 70+, weight loss is common, but also highly informative. And don't even get me started on attrition bias here.