If overweight is healthier, what's "normal" about normal weight?

A new study of nearly 12,000 Canadian adults over a twelve year period was published in the Obesity journal a few days ago. As media reports are saying, the study has found that being overweight leads to a longer life than being in the "normal" weight range.

The riskiest weights are to be underweight (BMI less than 18.5) or morbidly obese (BMI over 35).

The healthiest weight turns out to be what doctors currently class as being overweight - BMI 25-30.

If your weight falls into the "normal" range of 18.5-25 or obese (30-35), you're likely to live longer than the underweight or morbidly obese but not as long as the moderately overweight.

It isn't a new finding - existing research backs it up. It also makes sense in the context of a population in which the adults are both getting fatter and living longer than ever before.

Which rather leads us to ask what we mean by "normal" weight anyway.

It doesn't mean the weight most people fall into. It isn't even the most common weight category. Two out of five people in the UK (including myself) fall into the healthy "overweight" category and a further one in five are obese.

If being "normal" weight is both less common and less healthy that being "overweight", wouldn't it make sense to re-work the classifications.

Currently a lot of effort - millions of pounds in the UK alone - is invested into encouraging overweight people to get down into the normal category. Are we spending that money to shorten people's lives?

If we're going to stick with Body Mass Index (BMI) as a measure (and that's a whole other debate), might it not make more sense to have something like:

• BMI less than 18.5: morbidly underweight
• BMI 18.5-24.9: underweight
• BMI 25-29.9: normal
• BMI 30-34.9: overweight
• BMI 35-39.9: obese
• BMI 40+: morbidly obese
  

That would better reflect both the real distribution of weight and the health impact.

The researchers are saying this isn't a reason for people who weigh less to suddenly decide to splurge a hundred quid at their local Burger King. That sounds right to me: other evidence points to our natural weight as being largely genetic so forcing your weight up by over-eating may not be beneficial.

You can (for a price) access the full study here.

Here's the abstract in full, copied from the Obesity journal website:

Although a clear risk of mortality is associated with obesity, the risk of mortality associated with overweight is equivocal. The objective of this study is to estimate the relationship between BMI and all-cause mortality in a nationally representative sample of Canadian adults. A sample of 11,326 respondents aged 25 in the 1994/1995 National Population Health Survey (Canada) was studied using Cox proportional hazards models. A significant increased risk of mortality over the 12 years of follow-up was observed for underweight (BMI <18.5;>35; RR = 1.36, P <0.05). rr =" 0.83," rr =" 0.95,">0.05). Our results are similar to those from other recent studies, confirming that underweight and obesity class II+ are clear risk factors for mortality, and showing that when compared to the acceptable BMI category, overweight appears to be protective against mortality. Obesity class I was not associated with an increased risk of mortality.