Weight and Obesity: updated

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!

Tags: , , ,

7 Responses to “Weight and Obesity: updated”

  1. Andrew Says:

    Perhaps you are being overly anxious about the ‘relative’ definition, and the fact is that ‘absolutely’, half the population are just overweight.

  2. Andrew Says:

    I should add – ‘overweight’ as far as someone significant is concerned who would like us all to be leaner generally.

  3. Dave Says:

    Commenting on the start of this section: I don’t see the word “normal” as useful Obesity is a medical term, not relative to who you group yourself with. If my lung capacity was normal compared to asbestos miners I would be concerned! Taking the bmi/mortality data at face value; if you are much over 25 you might want to think about doing something to reduce it. If you are much over 30, then look on the bright side, you should get a better pension if you buy an annuity as any actuary will put you down as having reduced life expectancy. Having decided on your medical terms, if you then apply the values to a population distribution you find that USA has a weight problem. If you applied the same values to for example Japan you might find it was “normal” to have a lower bmi.

  4. The effect of excess BMI « Protons for Breakfast Blog Says:

    […] Dave Lowe is one of those colleagues who makes working at NPL such a pleasure. Having read my blog about the excess mortality associated with BMIs greater than 25 , he dropped me a line to let me know of a paper in the Lancet. The paper (Body-mass index and […]

  5. Health and Efficiency « Protons for Breakfast Blog Says:

    […] overweight as I am, I attended the gym yesterday and amongst other endurances, I spent 5 minutes on a rowing machine. […]

  6. Rex Larsen Says:

    The definition of normal BMI is based upon data gathered in some of the early NHANES studies in the 1970’s. So yes, it was normal data from a different time period. And although you may say that there is only a small increase in mortality for increases in bodyweight, that does not account for the increases in medical costs and morbidity (disability, suffering and the like). BMI however because of it’s nature is not an accurate health assessment tool and should not used to diagnose obesity in an individual. Body fat percentage has a much stronger correlation with negative health outcomes.

  7. Overweight? « Protons for Breakfast Blog Says:

    […] 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. […]

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s

%d bloggers like this: