- Introduction to the body mass index (BMI)
- What does your BMI mean?
- Why are BMIs useful?
- Risks associated with extreme BMIs
- What are the limitations of the BMI?
- Other measures of obesity
The body mass index (BMI) is a physical measurement used to assess an individual’s total amount of body fat. The BMI was invented by Belgian polymath Adolphe Quetelet in the 1800s, and consequently is sometimes known as the Quetelet index. The BMI is calculated by dividing your weight in kilograms (kg) by your height in metres squared (m2). It is expressed as kg/m2. Calculate your BMI using the calculator below, then compare your result to the BMIs in Table 1.
This information will be collected for educational purposes, however it will remain anonymous.
The BMI scores give an indirect measure of body fat. Depending on the BMI value calculated you may be underweight, normal weight, overweight or obese. The cut off values are as follows:
|Severely obese (also referred to as Obese Class 1)|
|Morbidly obese (also referred to as Obese Class 2)|
|Super obese (also referred to as Obese Class 3)|
The BMI is a simple, inexpensive screening tool used to identify possible weight problems for both adults and children. A BMI measurement is useful to assess who needs further testing to identify health risks such as heart disease. Individuals at risk will need further assessment. Assessments may include skin fold thickness test, diet, physical activity level, family history and other appropriate health screenings.
Being either overweight (with a BMI of 25 or above) or underweight (with a BMI lower than 18.5) can affect your health.
Being overweight increases your chances of developing disease. An overweight individual whose BMI rises from 25 to 27 is 20–30% more at risk of death. As an individuals BMI rises above 27, the risk of death increases more steeply by almost 60%. If you are overweight and have low physical activity, the risks of developing disease increase.
Diseases associated with being overweight include:
- Cardiovascular diseases (i.e. stroke, heart attack);
- High blood pressure (hypertension);
- Some forms of cancer (i.e. breast, colon and endometrial cancers);
- Menstrual abnormalities;
- Gall bladder disease;
- Sleep problems (e.g. obstructive sleep apnoea);
- Respiratory problems (i.e. asthma).
If you are underweight you may be malnourished and in addition you may develop the following nutritional deficiencies:
- Compromised immune function with increased susceptibility to infections;
- Respiratory diseases;
- Digestive diseases;
- Increased risk of falls and fractures.
Limitations associated with the BMI are:
- BMI varies by age, sex and race. Hence one’s BMI can only be compared to those of the same sex, age and race.
- BMI does not differentiate between muscle and fat and will therefore underestimate in some and overestimate for others (e.g. An athlete may have a high BMI because of greater amount of muscle rather than fat).
- Disabled or elderly people have less muscle mass and therefore will have a lower BMI. This does not necessarily mean that their weight is normal or underweight.
- Pregnant women will also have a higher BMI because of increased weight associated with pregnancy, but not necessarily due to increased fat. BMI will overestimate body fat in this case.
- BMI does not differentiate between body fat distribution. Fat around the waist (“apple” body shape) is more dangerous than that around the hips (“pear” body shape), but this will not be picked up by the BMI.
Consult your healthcare giver for other physical measures that may need to be used along with BMI for assessing health risks associated with obesity.
|Waist circumference||Waist circumference (WC) is a good indicator of abdominal fat and can be used to indicate health risks. It is measured by putting an unstretched tape measure around the narrowest level of your waist over light or no clothing.
|Hip circumference||Hip circumference (HC) is measured by putting a tape around the widest part of your hip area over minimal clothing. HC is not useful on its own; usually it is used as a ratio with WC as above.|
|Waist–hip ratio||Waist to hip ratio (WHR) is the ratio of your waist circumference to your hip circumference. For females the normal WHR is about 0.80, while for men it is 0.95.|
|Waist–height ratio||Waist to height ratio is the ratio of your waist circumference to your height.|
|For more information on nutrition, including information on types and composition of food, nutrition and people, conditions related to nutrition, and diets and recipes, as well as some useful videos and tools, see Nutrition.|
|For more information on obesity, health and social issues, and methods of weight loss, as well as some useful tools, see Weight Loss.|
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