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A new study reveals something important about obesity and health costs

A new paper in the journal PharmacoEconomics estimates that obesity is costing the US $315 billion in health-care costs alone. But those costs aren't distributed the way people normally think.

As physician Aaron Carroll writes, the way we talk about obesity suggests that "if everyone in the US would just lose a little weight, we’d save massive money." This paper shows that's not true. Rather, "the relationship of medical care costs over BMI [body mass index] is J-shaped" — which is to say, the costs are concentrated in the small percentage of the population that's most obese.

To put this in dollar terms, someone with a BMI of 30 who loses 5 percent of her bodyweight will save $69 in health-care costs each year. Someone with a BMI of 35 who loses 5 percent of his bodyweight will save $528. Someone with a BMI of 40 who loses 5 percent of her bodyweight will save $2,137.

Technically, all these people are obese. But so far as health-care spending goes, not all obesity is equal, or even close to it.

"If our goal is to reduce the cost to the US for obesity, we should likely focus on that small group of people, who are also likely the hardest to reach and influence," writes Carroll. "Trying to get lots of people who have a BMI of 30-35 to lose some weight might make them feel better, but won't influence health care spending in the US much at all."

This is a principle that recurs often in health care: A massive amount of the costs are concentrated in a very small fraction of the population. In Medicare, for instance, 5 percent of the patients account for 50 percent of the costs.