Surrogate BMI approach supports overweight mortality risk
medwireNews: A novel approach of using children’s BMI as a surrogate for their parents’ weight confirms that overweight and obesity increase people’s mortality risk.
In line with some previous epidemiological studies, David Carslake (University of Bristol, UK) and colleagues found a paradoxical protective effect of mild obesity when they looked at associations between people’s BMI and cardiovascular, cancer, and all-cause mortality.
As reported in the International Journal of Epidemiology, there was a U-shaped association for all three outcomes, in both women and men, with the lowest risk seen for people with BMIs of around 21 to 25 kg/m2.
One possible reason for this is that the associations are distorted by people who lose weight as a result of ill health, so the team replaced people’s BMI with that of their children, as a surrogate, thereby avoiding the problem of reverse causation (ie, ill health altering weight).
The researchers assumed that children’s BMI would be a good surrogate because of shared genetic and environmental factors that would create concordance between parental and offspring BMI, although they caution that this analysis “may be more vulnerable to socioeconomic and behavioural confounding than a conventional analysis of mortality and BMI.”
The findings come from the Norwegian population-based HUNT surveys, which yielded 32,452 mother–child pairs and 27,747 father–child pairs. When the team substituted the parents’ BMIs with their children’s, the U-shaped associations with mortality disappeared and became closer to linear.
Positive associations (between high BMI and mortality) strengthened and negative ones (between low/normal BMI and mortality) weakened, giving overall stronger relationships. For example, the risk for mortality from any cause among women increased by a significant 4% per standard deviation increase in their own BMIs but by 26% per increase in their children’s BMIs.
Carslake and team say their findings “confirm the conventional view that overweight is detrimental to health.”
However, in a press statement, Tom Sanders (Professor Emeritus of Nutrition and Dietetics, King's College London, UK), who was not involved in the study, stressed that previous research has found a J-shaped association between BMI and mortality, with an increased non-cardiovascular mortality risk at very low BMIs that was not seen in the current analysis.
“My main concern is that this paper should not be interpreted as suggesting that being lighter in weight than currently recommended is healthy,” he said, adding: “While we have a major epidemic of obesity, we have a parallel epidemic of younger people trying to achieve unhealthily low body weights.”
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