Science and Weight Loss – Day 4 the BMI

By Michael Edmonds 01/02/2012 4

No gym today – it is good to have a break every few days, though I must admit my mood can be a little off without my endorphin fix.

89.6 kg today

Anyway the BMI or Body Mass Index is an often used system to determine whether someone is normal weight or under/overweight. However, it is a fairly blunt instrument which has some weak points.

There are plenty of BMI calculators available online – my Iphone even comes with an application to calculate it and graph it if I want it to.

The calculation is based on weight and height with age and gender often being factored in as well. An outline of some of the maths involved can be found here.

Typically a BMI result of between 18.5 and 25 is considered “normal” with lower than 18.5 considered underweight. 25 to 30 is considered overweight, while above 30 is considered obese. Though these values and ranges can vary slightly from website to website.

According to the application I am using at the moment my BMI is 27.2, firmly in the overweight category.

As I mentioned earlier, while the BMI is a widely used tool, it is also a fairly blunt instrument. For example, it cannot differentiate between muscle and fat, hence most bodybuilders are classed as overweight or even obese even when their body fat composition is below 5%. Indeed, some health insurance companies will refuse to cover bodybuilders due to their high BMI.

Unfortunately, I can’t use the excuse of excess muscle to explain my 27.2 BMI 🙁

According to my calculations to get a BMI of 25 I should weigh exactly 82 kg. However, given this is 7.6 kg away and that I’m not convinced of the reliability of the BMI I’m going to set an initial target of 86 kg, see how long this takes and then reassess my plans.

4 Responses to “Science and Weight Loss – Day 4 the BMI”

  • It annoys me a little when I see people complaining about the invalidity of BMI, on the grounds that bodybuilders or All Blacks are erroneously considered obese.

    I think it’s clear that (a) elite athletes are a tiny fraction of the population, and (b) they probably are very aware of their own health and weight, and thus don’t need to care about their BMI.

    I would be more interested if critics of BMI could point to a significant fraction of the population that is being mislead…

  • It’s also worth noting that there’s not much evidence that a high BMI increases your risk of poor health outcomes on its own. It’s things like blood pressure and excess stomach fat that cause the problems, and these are correlated with extra weight. My understanding is that if you’re fit and active, then carrying an extra 7kg is probably not doing you much harm, provided it’s not all around your waist.

  • John,
    perhaps the mention of bodybuilders/All Blacks is not the best example of the limitations of BMI, although it is unfortunate for those who are turned down health insurance based on a number without the insurance companies thinking more clearly about what that number means.
    I think Amy’s comment more effectively points out its limitations – that it is possible to be fit while still being classed as overweight.
    And because the BMI focuses on height and weight alone it does not consider other aspects of fitness e.g. levels of activity, cardiac health etc.

    Like all tools the BMI has it’s uses, but one must be aware of its limitations as well,

  • I think the value of the BMI is as a good indicator of being overweight, rather than a precise and universal measure of such. And as such, it is merely one source of information that can be used as a guide.

    Pretending that if an All Black has the same BMI as you means you’re not overweight, is likely to be wandering over into denial territory for most people who aren’t top athletes.