Posts Tagged ‘Obesity’


July 1, 2012
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!

Weight and Obesity: updated

January 20, 2011
The normal distribution of Body mass Index and what I think one would rationally expect to call normal, and overweight

The normal distribution of Body mass Index and what I think one would rationally expect to call normal, and overweight

It’s that happy time of month when Scientific American lands on my doorstep and allows me to consume something relatively highbrow, but accessible. And this month they featured an article on Obesity. I am a 51 year old male, 1.75 metres in height (5′ 9” for you oldies out there) and weighing 82 kg. So my  body mass index – my weight (in kilograms) divided by my height (in metres) squared (=82/1.75 x 1.75) is 26.8.  So I am officially overweight. So as I venture into heart attack territory, the issue of obesity is a matter of personal interest. The article is easy to summarise – to loose weight you need to change your habits – gimmicks or willpower based diets aren’t sustainable. But the article featured one extraordinary graph that shocked me profoundly. It is a graph of the distribution of the BMIs of americans from 1976 to 1980. The question is this –  what would you call ‘normal’?


The curve representing the distribution of Body Mass Index amongst the American population measured from 1976 to 1980 is shown in the figure at the head of this article. This kind of curve represents the distribution of many properties of a population, such as height. I think most scientists would  say that if you were within the band middle band of the above graph – your BMI would be ‘normal’ and it would be perverse to class people within this central band as ‘overweight’. But that is exactly what the usual medical classification of overweight does. The medical classification looks like this:

Medical classification of terms 'normal' 'overweight' and 'obese' in terms of Body Mass Index

Medical classification of terms 'normal' 'overweight' and 'obese' in terms of Body Mass Index

Now this seems to me to be perverse. This distribution is the ‘natural’ distribution of weight amongst a population. This classification defines nearly 50% of people to be overweight! Saying this is a problem and implying that those who are overweight or obese by this classification should loose weight and thus move into the ‘normal’ zone is bizarre. It’s like insisting that everyone should be above average intelligence. I find this curve really shocking. There could only be one possible justification for it: if having a Body Mass Index even slightly above average caused a pronounced increase in liklihood of health problems, then this could make sense. Does it? No.

Health : UPDATED

When I wrote this article I resorted to Wikipedia, and used data from the article on obesity which I thought had just the data I wanted. The data is sadly complex but is summarised in the graph below and shows essentially that there is a slight excess mortality in the ‘overweight’ group – BMI between 25 and 30 – but only just above the statistical significant levels.

Data on relative mortality as a function of Body Mass Index for white american males

Data on relative mortality as a function of Body Mass Index for white american males

Please note that the data are taken from this reference and were derived by examination of 1.4 million americans whose ages ranged from 19 to 84 with a median age of 58. They were re-visited after 10 years and it was seen whether or not they were dead! Note that the abstract does not describe how the general trend for more elderly people to be heavier was accounted for – clearly a critical correction since the elderly are overwhelmingly more likely to die. Note also that the Wikipedia figure curtails the y-axis so that the tiny changes look larger, and neglects to plot the confidence intervals of each data point.

However on reflection I realise that this is not the data I want. What I want are answers to two questions

  • Firstly: Was the definition of ‘normal’ (BMI in the range 19 to 25) derived from a previous time when this reflected the normal distribution of weights?
  • Secondly: What I want to know is the impact of being overweight NOW – aged 51. I doubt very much that this has much effect on my current mortality, but I could believe that it could affect me in years to come. So there is probably a risk factor – or excess mortality – which grows the longer I spend overweight. As an illustration, an overweight 25 year old probably has no excess mortality in their twenties or thirties, but if he or she lives to be 60 and stays overweight, then it is possible that all those years of overweightness could have some consequence.
  • Try as I might – and I have looked for hours –  I have not yet found the data I want.  I will be sure to let you know when I do.

Summary: UPDATED

From what I have read and seen so far, there is not a jot of evidence only marginal evidence for ascribing any negative health outcomes to men (actually white american men) in the overweight category of BMI between 25 and 30. Indeed the classification of people with BMI in this range as being ‘overweight’ or in any way abnormal is bizarre. I have not considered diseases associated with overweight, most notably diabetes, but I am looking for the data on that as well!

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