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What is BMI

There are so many numbers associated with health and wellbeing, and more so with weight.

So I decided to do a little bit of digging to find out what the numbers meant.


Body Mass Index or BMI

BMI or Body Mass Index has been used for many years as a rough estimate of how your weight relates to your height.

It's calculated by the simple formula : Weight in Kg/(Height in Meters x Height in Meters)

To illustrate I weigh 95Kg and I'm 184cm (or 1.84M) tall.

So my BMI would be  calculated


or 30Kg/M².   

So I'm boarder line obese.

Here's a neat little app to calculate it for you .


Or if you're old school


BMI does have it's limitations.

The biggest one is that it doesn't necessarily match up well with Body Fat %.

Take for instance Loni Uhila, Prop Forward for the Hurricanes.

He's 122 Kg and 180cm tall.

His BMI is 112/ (1.80 x 1.80) = 38.

So theoretically he's classed as obese according to the BMI scale.

There are other ways of telling if someone is underweight, overweight, or at a healthy weight. Most of them focus on body fat percentage.

There are two "types" of body fat.   Cutaneous fat (pronounced q-tay-knee-us) and visceral fat (pronounced vis-er-al).   Cutaneous  fat sits just below the skin. It's the layer of fat that helps keep us warm. Visceral fat sit around the organs in your abdominal cavity; stomach, liver, intestines.  Basically your guts.

The Calliper Method

The calliper method uses a set of callipers to take readings from around the body and using a set of tables tells you roughly what % body fat you have.

This method is quick, and takes very little training to get reproducible results.   On the downside there can be significant differences in readings if measurements are taken at slightly different places.   It also tends to focus on cutaneous fat over visceral fat.

Waist Circumference

Another quick and easy measurement. Just run a tape measure around you waist, just above your belly button.  For women if this measure is more than 88cm, or more than 102cm for men, you're considered overweight.

Waist-Hip Ratio

As it says Waist-Hip ratio takes two measurements and divide one by the other. It's pretty easy to do, and doesn't require buying any expensive kit.  It's also relatively good at predicting health, and likelihood of developing serious health conditions. Particularly heart disease, and diabetes.

In short if waist/hip is less than 0.8 for women, or 0.9 for men your probably at a healthy weight.

For women if the ratio is between 0.8 and 0.84 or between 0.9 and 0.99 for men you're considered over weight.

If the ratio is greater than 0.85 for women or 1.0 for men you're probably obese.

So for me; I have a waist measurement of 107cm, and a hip measurement of 104.

So my waist/hip ratio is 107/104 =1.02

Bioelectrical Impedence

This method is based on the idea muscle conducts electricity much more effectively than fat.  So it's possible to put a small electric current through someone and calculate their % body fat.  The person being measured stand barefoot on a machine with a metal plate which send the current up one leg and down the other and measures the difference.

The advantages of this method are that it's quick, painless, non-invasive, and is automated so human error is minimised.

The downside is that is not quiet as accurate as some people might like.  It tends to over-estimate body fat in skinny people and over-estimate it in obese people.

Body Fat Percentage

Here's a table that give you an idea about Body Fat Percentage ranges

Description |  Women  |   Men

Essential fat| 10–13%   |  3–5%

Athletes       |14–20%    | 6–13%

Fitness         |21–24%    | 14–17%

Average       |25–31%    |18–24%

Obese         |32%+         |25%+


But which one to use?

As you can see there are a number of ways of assessing if you are in a healthy weight range or not.  All have their pluses and minuses. There are others we haven't talked about too.

When you trade off accuracy and how easy they are to use I think the Waist-Hip Ratio is probably "better".   We have a Bioelectrical Impedence machine at CookStPharmacy, which I'll be using tomorrow.

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Confessions of an overweight Pharmacist Part 3 I want a chocolate biscuit!