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Sunday, March 18, 2018
Article written by David Brooks

What is Body Mass Index (BMI)

Body mass index (BMI) is used to estimate your total amount of body fat. It is calculated by dividing your weight in kilograms by your height in metres squared (m2). The value for the BMI relates quite closely to your amount of body fat. Differences in BMI between people of the same age and sex are usually due to body fat. However, there are exceptions to this rule (see below).

How to calculate your BMI BMI is an approximate measure of the best weight for health only. To calculate your BMI, you need to know:

Your weight in kilograms

Your height in metres.

You can use our handy BMI calculator (see below)

What does your BMI value mean? Once you have measured your BMI, you can determine your healthy weight range. If you have a BMI of:

Under 18 - you are very underweight and possibly malnourished.

Under 20 - you are underweight and could afford to gain a little weight.

20 to 25 - you have a healthy weight range for young and middle-aged adults.

23 to 28 - you have a healthy weight range for older adults.

26 to 30 - you are mildly overweight.

Over 30 - you are very overweight or obese.

Some things to remember BMI does not differentiate between body fat and muscle mass. This means there are some exceptions to the rules:

1)Muscles - body builders and people who have a lot of muscle bulk will have a high BMI but are not overweight.
2)Physical disabilities - people who have a physical disability and are unable to walk may have muscle wasting. Their BMI may be slightly lower but this does not necessarily mean they are underweight. In these instances, it is important to consult a dietitian who will provide helpful advice.
3)Height - for people who are shorter (for example Asian populations), the cut-offs for overweight and obesity may need to be lower. This is because there is an increased risk of diabetes and cardiovascular disease, which begins at a BMI as low as 23 in Asian populations.
4)BMI calculations will tend to be an overestimate for high performance athletes and pregnant women
5)BMI calculations will underestimate the amount of body fat for elderly people.
6)BMI will also not reliably give an accurate number for people with eating disorders like anorexia nervosa or people with extreme obesity.

Being overweight or underweight can affect your health The link between being overweight or obese and the chance you will become ill has not been completely proven. The research is ongoing. However, when data from large numbers of people are analysed, statistically it is found that there is a greater chance of contracting various diseases if you are overweight. For example, the risk of death rises slightly (by 20-30 per cent) as BMI rises from 25 to 27. As BMI rises above 27, the risk of death rises more steeply (by 60 per cent).

Being active is very important If you are overweight (BMI over 25) and physically inactive, you may develop:

-Cardiovascular (heart and blood circulation) disease

-Gall bladder disease

-High blood pressure (hypertension)



-Certain types of cancer, such as colon and breast cancer.

Don't allow yourself to be underweight If you are underweight (BMI less than 20), you may be malnourished and develop:

-Compromised immune function

-Respiratory disease

-Digestive disease



-Increased risk of falls and fractures.

Where to get help

Consult your doctor about ways to lose weight.

Things to remember

- BMI is an approximate measure of your total body fat.

- Being underweight or overweight can cause health problems, especially if you are also inactive.


Please note. We are not a medical authority and this information is for reference only. Use this information at your own risk. The information has been collected and edited from reliable sources and is purely for informational purposes only. Please consult your doctor with any health concerns.
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