BMI Mortality

The upper curve shows the relationship between body mass index (BMI) and mortality expressed as relative risk of death compared to those with a BMI in the range 22.5 to 25. The data indicates quite robustly that being in the overweight category is ‘protective’. The data point at BMI=18 includes data for all people with lower BMI and the data point at BMI=35 includes data for all people with higher BMI. The lower curve shows the distribution of BMI in the population used for the study.

Friends, I am concerned about my weight. In particular I am concerned about whether being overweight (i.e. with a body mass index or (BMI) in the range 25 to 30) is genuinely bad for me, or whether it just makes me feel bad.

I have looked at this issue before and expressed my puzzlement at how ‘normal’ ever came to be defined as having a BMI in the range 20 to 25, when as far as I could tell, it has never coincided with the central range in the population.

My puzzlement appears to be vindicated by research which shows that the relative risk of death – mortality – is lower for people who are overweight compared with people in the ‘normal range’. These conclusions are backed up by other studies. But even so, it is important to understand how the research was done in order to appreciate what it really tells us.

The research followed 11,834 individuals in Canada from 1994/5 to 2006/7, and saw how mortality was affected by their BMI at the start of the study in 1994/95. Let me stress this. It saw how the BMI statistic in 1994/5 affected the rate at which they died in the subsequent 12 years. This large population included men and women, smokers and people who never smoked, and people in all age groups.

Could the ‘BMI effect’ have been protective in young people, but harmful in older people? This might make sense, since young people are less likely to die in any case. This might have masked the effect that I would have expected to see: that being overweight was harmful. The researchers controlled for that and looked at how the relative risk of death varied with BMI categories for various sub-populations within the group. Surprisingly – to me at least – the effect was seen in all categories.

BMI Mortality versus age

BMI Mortality versus age for different subpopulations within the study. The risk of death is relative to those with BMI in the ‘normal’ range. The lowest and highest BMI points include data for all individuals at lower or higher BMI. I have missed out the confidence indicators (error bars) because they make the graph too confusing.

Mortality and Morbidity. This report recorded the BMI of a population at a point in time, and studied correlations between the BMI and the rate at which people died in the following 12 years: This is called the risk of mortality. It did not record whether the individuals concerned became ill or unwell, and did not study how their BMI affected their chance of becoming unwell – that is called the risk of morbidity. I do not have data to hand but I would be pretty sure that being in the ‘overweight’ category (as defined by a BMI in the range 25 to 30) would be a significant risk factor for diseases such as cardio-vascular disease and type II diabetes.

This data was taken amongst a population which, for possibly the first time in human history, has enjoyed essentially unrestricted access to food for several generations. This is an astonishing cultural achievement. If the data really does fall in this way for other populations – including that in the UK – it will be very interesting to understand why that occurs. It will also be interesting to hear doctors argue that ‘chubby’ people like me should lose weight – something which will increase my risk of death!

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4 Responses to “Overweight?”

  1. The incredible lightness of being wrong « Protons for Breakfast Blog Says:

    […] BMI caused increased mortality – an increased risk of death. As I mentioned in a previous article, this appears to be not true. In fact being ‘overweight’ (BMI 25 to 30) appears to […]

  2. Are you sure you want to lose weight? « Protons for Breakfast Blog Says:

    […] have written about this before (here), but this new study by Flegal et al (available free here) confirms this result a fortiori . […]

  3. Kathryn Margaret Cook Says:

    Hi Michael, I’m writing an article for Spoon University about body shaming. I was hoping I’d be able to use one of these graphs! I’d link back to this article of course. Do you mind if use it?

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