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:
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.
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.
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!