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).
Differences in BMI between people of the same age and sex are usually due to body fat. However there are exceptions to this rule, which means a BMI figure may not be accurate.
BMI calculations will overestimate the amount of body fat for:
* Body builders
* Some high performance athletes
* Pregnant women.
BMI calculations will underestimate the amount of body fat for:
* The elderly
* People with a physical disability who are unable to walk and may have muscle wasting.
BMI is also not an accurate indicator for people with eating disorders like anorexia nervosa or people with extreme obesity.
BMI is not the best measure of weight and health risk. A person’s waist circumference is a better predictor of health risk than BMI.
BMI and children
The healthy weight range for adults of a BMI of 20 to 25 is not a suitable measure for children.
For adults who have stopped growing, an increase in BMI is usually caused by an increase in body fat. But as children grow, their amount of body fat changes and so will their BMI. For example, BMI usually decreases during the preschool years and then increases into adulthood.
For this reason a BMI calculation for a child or an adolescent must be compared against age and gender percentile charts.
The new BMI charts for children have been developed by the US Centre for Disease Control.
The charts are useful for the assessment of overweight and obesity in children aged over two. However they should be used only as a guide to indicate when make small lifestyle changes, and when to seek further guidance from a doctor or an Accredited Practising Dietitian (APD).
Calculating 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.
What your BMI means
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.
* 26 to 30 – you are overweight.
* Over 30 – you are obese.
For older people over the age of 74 years, general health status may be more important than being mildly overweight. Some researchers have suggested that a BMI range of 22-26 is acceptable for older people.
Some exceptions to the rule
BMI does not differentiate between body fat and muscle mass. This means there are some exceptions to the BMI guidelines.
* Muscles – body builders and people who have a lot of muscle bulk will have a high BMI but are not overweight.
* 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.
* 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.
Being overweight or underweight can affect your health
The link between being overweight or obese and the chance you will become ill is not definite. The research is ongoing. However, when data from large groups of people are analysed, statistically there is a greater chance of developing 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).
Risks of being overweight and physically inactive
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)
* Diabetes
* Osteoarthritis
* Certain types of cancer, such as colon and breast cancer.
Risks of being underweight
If you are underweight (BMI less than 20), you may be malnourished and develop:
* Compromised immune function
* Respiratory disease
* Digestive disease
* Cancer
* Osteoporosis
* Increased risk of falls and fractures.
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.
* Your waist circumference is a better predictor of health risk than BMI.
