Wednesday, January 2, 2013

What We Know About Obesity and Mortality

To begin 2013 I would like to do a series of posts related to weight, overweight and obesity on mortality and brain disorders.

This morning a new meta-analysis was published in JAMA examining the relationship between weight category and all cause mortality.  This study is receiving a significant degree of attention as it challenges a commonly held belief that even mild obesity is linked to earlier death.

The study was conducted by a research team at the Centers for Disease Control, the University of Ottawa in Canada and the National Institutes of Health.  A key focus in this analysis of all relevant published research is examining the effect of the lowest level of obesity (those with a BMI between 30 and 35).  Normal weight is considered to be in the 20 to 25 BMI range, overweight between 25 and 30.  BMI rates above 35 are considered severe obesity.

So for perspective, let's look at the cutoffs for a BMI of 25, 30 and 35 for two heights (5 foot 6 inches and 5 foot 10 inches)

  • BMI 25- 155 pounds/175 pounds
  • BMI 30- 185 pounds/210 pounds
  • BMI 35- 215 pounds/245 pounds

So and individual who is 5 feet six inches tall would be in the overweight category from 155 to 185 pounds, mildly obese between 185 and 215 pounds and moderately to severely obese over 215.  Likewise you can substitute the relevant weight for a person 5 foot 10 inches tall by using the second weights in the above table.

I have summarized the hazard ratio estimates from the current study in the chart below.  The hazard ratio is set a 1.0 for those in the normal weight range and relevant category mortality hazard ratios are compared by BMI.  The hazard ratios are estimated from combining all relevant data of published research studies on this topic.

 The mortality hazard estimates actually are lower for the overweight and mildly obese categories in this meta-analysis.  However, only the 25-30 BMI overweight category reaches statistical significance compared to normal weight categories.  The hazard ratios suggest that for me at 5 foot 10 inches, I would need to be over 245 (BMI of 35) pounds to significantly increase my mortality risk.

This study suggests we might want to dial back a bit the public health concerns about milder amounts of obesity.  There is  no doubt that greater levels of obesity (BMI over 35) contribute significantly to earlier risk of death.  For those with lower levels of obesity, weight loss may be less important than regular aerobic exercise, eating a healthy diet and not smoking in promoting a longer life.

For those interested in calculating their own BMI, I recommend this easy web-based calculator provided by the National Heart Lung and Blood Institute.

Photo of great blue heron in flight is from the author's files.

Data from the chart comes from the original manuscript cited below.  Interested readers can access the JAMA free full manuscript by clicking here

Flegal KM, Kit BK, Orpana H, Graubard BI (2013). Association of All-Cause Mortality With Overweight and Obesity Using Standard Body Mass Index Categories JAMA, 309 (1), 71-82

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