An Introduction to BMI
Body Mass Index (BMI) is a physical measurement used to assess an individual’s total amount of body fat. BMI is calculated by dividing your weight in Kilograms (kg) by your height in metres squared (m2). BMI 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
|Category||BMI range – kg/m2||BMI Prime||Mass (weight) of a 1.8 metres (5 ft 11 in) person with this BMI|
|Severely Underweight||less than 16.5||less than 0.66||under 53.5 kilograms (8.42 st/118 lb)|
|Underweight||from 16.5 to 18.5||from 0.66 to 0.74||between 53.5 and 60 kilograms (8.42 and 9.45 st/118 and 132 lb)|
|Normal||from 18.5 to 25||from 0.74 to 1.0||between 60 and 81 kilograms (9.4 and 13 st/130 and 180 lb)|
|Overweight||from 25 to 30||from 1.0 to 1.2||between 81 and 97 kilograms (12.8 and 15.3 st/180 and 210 lb)|
|Obese Class I||from 30 to 35||from 1.2 to 1.4||between 97 and 113 kilograms (15.3 and 17.8 st/210 and 250 lb)|
|Obese Class II||from 35 to 40||from 1.4 to 1.6||between 113 and 130 kilograms (17.8 and 20.5 st/250 and 290 lb)|
|Severely Obese||from 40 to 45||from 1.6 to 1.8||between 130 and 146 kilograms (20 and 23 st/290 and 320 lb)|
|Morbidly Obese||from 45 to 50||from 1.8 to 2.0||between 146 and 162 kilograms (23.0 and 25.5 st/320 and 360 lb)|
|Super Obese||from 50 to 60||from 2.0 to 2.4||between 162 and 194 kilograms (25.5 and 30.5 st/360 and 430 lb)|
|Hyper Obese||above 60||above 2.4||
above 194 kilograms (30.5 st/430 lb)
Why are BMIs Useful?
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.
What are the Risks Associated with Having an Extreme BMI?
Being either overweight (with BMI of 25 or above) or underweight (with BMI lower than 18.5) can affect your health.
Risks of Being Overweight: Being overweight increases your chances of developing diseases. An overweight individual whose BMI rises from 25 to 27 is 20-30% more at risk of premature death. As an individual’s BMI rises above 27, the risk of death increases more steeply-by almost 60%. If you are overweight and have low physical activity, the risk of developing diseases increases. Listed below is a list of diseases associated with being overweight.
- Gastrointestinal disorders
- Cardiovascular diseases: stroke, heart attack
- High blood pressure (Hypertension)
- Some forms of cancer: breast, colon and endometrial cancers
- Menstrual abnormalities
- Gall bladder disease
- Sleep problems: obstructive sleep apnoea
- Respiratory problems: 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
- Osteoporosis ( weakening of the bones)
- Increased risk of falls and fractures
What are the Limitations of BMI?
Listed below are limitations associated with BMI:
- BMI varies by age, sex and race, hence one’s BMI can only be compared to those of same sex, age and race.
- BMI does not differentiate between muscle and fat and will, therefore, underestimate in some and overestimate for others: For example, 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 BMI 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 and hence BMI will overestimate body fat in this case.
- BMI does not differentiate between body fat distribution and, since fat around the waist (“apple” body shape) is more dangerous than that around the hips (“pear” body shape), this will not be picked up by BMI.
- Do consult your health care 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
The table below displays other measures of obesity.
Table 2: Other Measures of 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 – 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 – height ratio is the ratio of your waist circumference to your height|
- News-Medical.Net. BMI unreliable when assessing obesity and risk of heart disease. News-Medical.Net; 2007. Available from: www.news-medical.net.
- CDC. About BMI for Adults; 2006. Available from: www.cdc.gov.
- NIH. Aim for a healthy weight; 2007 [cited 2007 March]. Available from: www.nhlbi.nih.gov/health/public/heart/obesity/lose_wt/risk.htm.
- Dencker M, Thorsson O, Linden C. BMI and objectively measured body fat and body fat distribution in prepubertal children. Clinical Physiology and functional imaging 2007 [cited March 2007];27:12-16.
- Snijder MB, van Dam RM, Visser M, Seidell JC. What aspects of body fat are particularly hazardous and how do we measure them? Int. J. Epidemiol. 2006 [cited February 1, 2006];35(1):83-92. Available from: www.ije.oxfordjournals.org
- Gill T, Chittleborough C, Taylor A, Ruffin R, Wilson D. Body mass index, waist hip ratio, and waist circumference: which measure to classify obesity. Centre for Population Studies in Epidemiology, Dept. of Human Services, Adelaide 2003;48(3):191-200.
- 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.
- 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.
- 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.
- 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: www.ajcn.org/cgi/content/abstract/74/3/315
- 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: www.ajcn.org/cgi/content/abstract/84/5/995
- Lean M. Waist Circumference as a measure for indicating need for weight management. BMJ 1995;311:158-161.
- 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.
- 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
*Disclaimer: Results may vary for each person.