BMI (Body Mass Index)

• Introduction to the body mass index (BMI)
• What does your BMI mean?
• Why are BMIs useful?
• Risks associated with extreme BMIs
  • Overweight
  • Underweight

• What are the limitations of the BMI?
• Other measures of obesity

Introduction to the body mass index (BMI)

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.

What does your BMI mean?

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:

Table 1: BMI cut-off values

Why are BMIs useful?

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.

Risks associated with extreme BMIs

Being either overweight (with a BMI of 25 or above) or underweight (with a BMI lower than 18.5) can affect your health.

Overweight

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:

• Gastrointestinal disorders;
• Cardiovascular diseases (i.e. stroke, heart attack);
• Diabetes;
• High blood pressure (hypertension);
• Osteoarthritis;
• Infertility;
• 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).

Risks of being underweight

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;
• Cancer;
• Osteoporosis;
• Increased risk of falls and fractures.

What are the limitations of the BMI?

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.

Other measures of obesity

More information

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.

Reference

1. News-Medical.Net. BMI unreliable when assessing obesity and risk of heart disease. News-Medical.Net; 2007. Available from: http://www.news-medical.net.
2. CDC. About BMI for Adults; 2006. Available from: http://www.cdc.gov.
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7. Pouliot M, Despers J, Lemieux S, Moorjan S. Waist circumference and abdominal sagital diameter: best simple anthropometric indexes of abdominal visceral adipose tissue accumulation and related cardiovascular risk in men and women. Am J Cardiol. 1994 [cited mar 1];73(7):460-8.
8. Schneider J, Glaesmer H, etal. Accuracy of Anthropometric indicators of Obesity to predict cardiovascular Risk. The J. of clinical Endocrinology 2007;92(2):589-594.
9. Kunesova M, Hainer V, Hergetova H, Zak A. Simple anthropometric measurements - relation to body fat mass, visceral adipose tissue and risk factors of atherogenesis. 1995;96(3):257-67.
10. Seidell JC, Perusse L, Despres J-P, Bouchard C. Waist and hip circumferences have independent and opposite effects on cardiovascular disease risk factors: the Quebec Family Study. Am J Clin Nutr 2001 [cited September 1, 2001];74(3):315-321. Available from: http://www.ajcn.org/cgi/content/abstract/74/3/315
11. Mukuddem-Petersen J, Snijder MB, van Dam RM, Dekker JM, Bouter LM, Stehouwer CDA, et al. Sagittal abdominal diameter: no advantage compared with other anthropometric measures as a correlate of components of the metabolic syndrome in elderly from the Hoorn Study. Am J Clin Nutr 2006 [cited November 1, 2006];84(5):995-1002. Available from: http://www.ajcn.org/cgi/content/abstract/84/5/995
12. Lean M. Waist Circumference as a measure for indicating need for weight management. BMJ 1995;311:158-161.
13. Esmailzadeh A, Mirmiran P, Azizi F. waist-hip ratio is a better screening measure for cardiovascular risk factors than other anthropometric indicators in Tehrannian adult men. Intntl J. of Obesity 2004;28:1325-1332.
14. NHMRC. Clinical Practice Guidelines for the Management of Overweight and Obesity in Adults; 2003. Available from http://www.health.gov.au/internet/main/publishing.nsf/Content/obesityguidelines-guidelines-adults.htm

Diseases which use this investigation:

• Anorexia Nervosa (Self-Starvation, Malnutrition, Severe Weight Loss, Extreme Weight Loss)
• Bulimia Nervosa
• Malnutrition
• Metabolic Syndrome
• Obesity

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Article Dates:

Modified: 15/7/2010

Created: 16/2/2007