Thursday, January 10, 2013

Is extra weight good for your health?

Researchers and statisticians have long sought a simple association between weight and good health.  This evaluation became easier when ideal body weight tables were abandoned in favor of a classification system based on body mass index (BMI), a single number found by dividing weight in kilograms by height in meters squared (made easier yet with the use of a smartphone app!).  Allowing for proportionately greater weights in individuals with large muscle mass, rising BMIs correlate with increasing fat mass.  On average, per conventional wisdom, as BMIs climb through normal to overweight and beyond, so does the risk of future disease.

So how many of us are normal in a BMI sort of way? According to recent US data, roughly one-third of adults have bypassed normal into overweight (BMI 25-30) and an additional one- third weigh in at obese (BMI 30 and above).  Epidemiologists warn that the obesity epidemic is increasing the incidence of chronic disease. 

Statisticians from the CDC conducted a meta-analysis (combining data from many studies) of  nearly 3 million subjects from 97 separate studies that looked at all-cause mortality over time as related to BMI(1). Relative to normal BMI, the researchers compared the risk of dying in those overweight (BMI 25 to 30) and obese (BMI 30 and above). The researchers used hazard ratios (HR) to express their findings, a measure of how often a particular event happens in one group compared to how often it happens in another group during the course of a study.  They found that the overweight and the grade 1 obese were slightly less likely to die than those in the normal BMI range (HR=.95).  On the other hand, those with BMIs at or above 35 had a calculated HR of 1.29, nearly a third again more likely to check out early than those weighing less.

Clearly, there’s a lot more at work here than whether or not one hauls extra weight through life’s journey.  An editorial that accompanies this article cites a host of confounding factors including sex, age, race, fat distribution, and cardiorespiratory fitness to name a few.  The article itself has a 138 item bibliography, most echoing the same conclusion, namely that overweight or slightly obese is not necessarily a long-term health risk but big-time obesity is big-time trouble over time.
Did we really need another study for this?

1.       Flegal, KM et al. Association of All-Cause Mortality With Overweight and Obesity Using Standard Body Mass Index Categories:. JAMA. 2013;309(1):71-82. doi:10.1001/jama.2012.113905.

2.       Heymsfield, SB and Cefalu, WT. Does Body Mass Index Adequately Convey a Patient's Mortality Risk? JAMA. 2013;309(1):87-88. doi:10.1001/jama.2012.185445.

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