The body mass index (BMI) is a calculated number, used to compare and analyse the health effects of weight on human bodies of all heights. It was developed by the Belgian polymath Adolphe Quetelet in the course of working out his system of "social physics", between 1830 and 1850 (and is therefore also known as the Quetelet Index). It is equal to the weight, divided by the square of the height:
Typically, the weight is in kilograms and the height in metres, and the unit kg/m2 is often left out. (Using imperial units this is 703.07 times the weight in pounds, divided by the square of the height in inches.)
A statistical device
The Index is primarily a statistical tool, designed for public health studies, which enables the investigation and comparison of any medical data set in which the height and weight of subjects were recorded, to determine whether obesity was a correlate in health outcomes. It should be noted that the BMI serves as an estimate of adiposity in a sample data set. If standard bioelectrical impedance data were routinely included in such data sets (detailing actual adiposity), the BMI would become obsolete. But since height and weight are routinely recorded in data sets for a wide variety of investigations, across many countries, while the more precise measures are not, the BMI has offered public health statisticians by far the broadest, if fuzzy, overview of the correlations between overweight and health stress.
BMI in practice
The BMI has been used to define the medical standard for obesity measurement in several countries since the early 1980s, and is the measure employed in World Health Organization obesity statistics. In the late 1990s and early 2000s the BMI became more familiar to a wider public through government-sponsored public health projects, intended to encourage fitness and healthy eating.
Despite the limitations of the BMI as a statistical estimate of adiposity, 21st Century public health campaigns tend to promote the BMI as a ‘rule-of-thumb’ guide to optimum weight for an individual of a given height, with the convenience of being easily calculable at home. This use of the BMI may cause confusion. Better tools for diagnosing overweight are available to health professionals, eg the skinfold test, by which a ‘pinch’ of skin is accurately measured to record the thickness of the subcutaneous fat layer, typically over the triceps. For those cases where there is doubt about overweight, skinfold and bioelectrical impedance measures are necessary. For cases where overweight or underweight is evident to the naked eye, the value of the BMI is limited. Some evidence exists that people with a BMI that would classify them as slightly overweight (25-27 for average Caucasians) actually live longer, healthier lives on average. This may have historical reasons, as the BMI’s classification stems from a time when nutritional deficiencies were more common than obesity. Besides, as shown above, the BMI is only of limited practical use and relevance except in more extreme cases, where it again quickly becomes redundant as extreme obesity, for example, is plainly visible without the need for calculations. The BMI should, therefore, not be used as an absolute measure, but rather as one of several guiding lines.
Human bodies rank along the index from around 15 (near starvation) to over 40 (morbidly obese). This statistical ‘curve’ is usually described using more familiar categories, for easier comprehension by health professionals; eg, severe underweight, underweight, optimum weight, pre-obese, obese, morbidly obese. The exact index values used to determine weight categories vary from authority to authority, but in general a BMI less than 18.5 is underweight and may indicate malnutrition, an eating disorder, or other health problem, while a BMI greater than 25 is overweight and above 30 is considered obese. These range boundaries apply to adults over 20 years of age.
Since BMI is merely a statistical estimate of adiposity, it is possible to have a BMI above 30, yet not be obese. A bodybuilder, for example, can have a BMI above 30 because of a high percentage of muscle mass. Health recommendations made to such an individual cannot be based on their BMI. Similarly, elder patients with very low muscle and bone mass may be clinically obese while scoring within optimum BMI ranges. Both extreme cases serve as a reminder that the BMI is not designed for use as a clinical diagnostic tool, but is rather an estimating tool, used by public health statisticians.
The U.S. National Health and Nutrition Examination Survey of 1994 indicates that 59% of American men and 49% of women have BMIs over 25. Extreme obesity — a BMI of 40 or more — was found in 2% of the men and 4% of the women.
Body mass index calculations are not just for adults—they can also be used to identify the growing number of overweight children. BMI for children aged 2 to 20 years is calculated just as it is for adults, but it is classified differently. Instead of set thresholds for underweight and overweight, it is their BMI percentile that is important. For children, a BMI that is less than the 5th percentile is considered underweight and above the 95th percentile is overweight. Children with a BMI between the 85th and 95th percentile are considered to be at risk of becoming overweight.
Recommended BMI thresholds
Given the reservations detailed above concerning the limitations of the BMI as a diagnostic tool for individuals, the following are common definitions of BMI thresholds:
• Underweight: less than or equal to 20 (<=20)
• Ideal: 20–24.9
• Overweight: 25–29.9
• Obese: 30–39
• Extremely obese: 40 or more (>=40)
Note: These recommended distinctions along the linear scale may vary from time to time and country to country, making global, longitudinal surveys problematic. In 1998, the US NIH brought US definitions into line with WHO guidelines, lowering the normal/overweight cut-off from BMI 27.8 to BMI 25. This had the effect of redefining approximately 30 million Americans, previously "technically healthy" to "technically overweight". The WHO uses the term "pre-obese" where the USA uses "overweight". It also recommends lowering the normal/overweight threshold for South East Asian body types to around BMI 23, and expects further revisions to emerge from clinical studies of different body types.
The new cut-off BMI index for obesity in Asians is 25 compared with the traditional WHO figure of 30. An Asian adult with a BMI of 23 or greater is now considered overweight and the ideal normal range is 18.5-22.