BMI – What is it & is it Accurate?

I have had a few people ask me about the BMI or Body Mass Index. This gets mentioned in the media quite a bit, and I think sometimes they just assume people know exactly what it is!

Quite simply, your BMI is your weight in Kg’s divided by your height in meters, squared.

So, if your weight is 70kg and your height is 1.70m your BMI is 24 – i.e you times               1.70 x 1.70 = 2.9        so 70 divided by 2.9 = 24

This is designed to measure your total amount of body fat and should only be used for adults aged 18+ once you have measured your index compare it to this scale:-

Under 20            –     Underweight

20 – 25              –     Normal

26 – 30              –     Overweight

30 – 35              –     Obese

35+                    –     Severely Obese

However, it is important to note that this is a general measure and not always accurate as it does not differentiate between muscle mass & fat so exceptions to the rule would be:

  • Bodybuilders, who have extra muscle mass, but low bodyfat
  • Athletes, again have a much lower bodyfat percentage
  • Ethnic people, naturally have a leaner physique
  • Pregnant women & children
  • Elderly, as they tend to loose muscle mass with age

So, use this as a guide, you normally know in yourself if you are overweight, but if you’re not sure consult your healthcare practitioner. There are other ways to check your health status..

  • Waist measurement, these figures have been released by the World Health Organisation (WHO) & used by the Australian Government in a series of ads to raise awareness of Obesity. They are:-

Men –      <94cm  increased risk of disease

<102cm  greatly increased risk

Women –    <80cm  increased risk of disease

<88cm  greatly increased risk

These figures are important as it is thought that people carrying extra fat around their abdomen are at a greater risk of say heart disease, stroke, high blood pressure or type II diabetes than people with it around their hips/thighs..which leads us to the..

  • Waist / Hip ratio – a simple way to check your distribution of fat. You divide your waist circumference (at just above the belly button) & your hips at their widest part (the greater trochanters – where the hip bones stick out!), then compare them to these figures..

Women –     >0.7 healthy range

Men –     >0.9 healthy range

  • BMR  Basal Metabolic Rate, this simply is the number of calories you require to sustain basic life at rest (e.g breathing, heart beat, generating body heat), so it does not include any activity, even moving. There are numerous complicated equations used to measure this, and some clinics use machines with electrodes to measure it, but again as it is based on height, weight, age it will not be 100% accurate and only used as a measure. So broadly speaking, to lose weight, you would aim to burn more calories than you take in, by means of moderating your diet and exercising.

My advice would be to use these tools as a general indicator of your health, but you must note that there are many other factors involved,(bodyfat%, medical conditions, stress, temperature, genetics, thyroid), always check with your Doctor if unsure.

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