Article: Body mass index

The body mass index (BMI) or Quetelet Index is a statistical measure of the weight of a person scaled according to height. It is used as a simple means of classifying inactive individuals of an average body composition according to their body fat content. As a rough guideline for adults a BMI of less than 20 implies underweight, over 25 is overweight, and over 30 is obese. It is calculated by taking the weight of the individual in kilograms and dividing by the square of the height in metres. It was originally developed between 1830 and 1850 by the Belgian polymath, Adolphe Quetelet during the course of developing "social physics". The BMI is a controversial measurement as it is not intended to make a medical diagnosis, but instead to broadly categorise populations for purely statistical purposes, and its accuracy in relation to actual individual body fat composition can be distorted by such factors as fitness level, muscle mass, bone structure, and ethnicity.

Calculation of the index

An example of a somewhat older body mass index chart.

BMI is defined as the individual's body weight divided by the square of the height, and is almost always expressed in the unit kg / m², which is therefore often left out. Hence, using the units in parentheses the BMI value can be calculated as

or (a version adapted for Imperial units):

In physiology the word “weight” means the same as “mass”. The reason height is squared rather than cubed or raised to some other power is simply that, taken over people of different height, the resulting index correlates reasonably well with degree of underweight or overweight. No law of physics or physiological growth is implied.

Generally, a BMI chart displays calculated BMI as a function of weight (horizontal axis) and height (vertical axis) using “contour lines” for different values of BMI or colors for different BMI categories.

BMI categories

BMI 19.3 Male, borderline underweight

A frequent use of the BMI is to assess how much an individual's body weight departs from what is normal or desirable for a person of his or her height. The weight excess or deficiency may, in part, be accounted for by body fat (adipose tissue) although other factors such as muscularity also affect BMI (see discussion below and overweight).

Human bodies rank along the index from around 15 (near starvation) to over 40 (morbidly obese). This statistical spread is usually described using categories: eg, severe underweight, underweight, optimum weight, pre-obese (or overweight), 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 problems, 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.

Thresholds

Given the reservations detailed below concerning the limitations of the BMI as a diagnostic tool for individuals, the following are common definitions of BMI categories:

• Starvation: less than 15 (<15)
• Anorexic: less than 17.5 (<17.5)
• Underweight: less than 18.5 (<18.5)
• Ideal: greater than or equal to 18.5 but less than 25 (≥18.5 but <25)
• Overweight: greater than or equal to 25 but less than 30 (≥25 but <30)
• Obese: greater than or equal to 30 but less than 40 (≥30 but <40)
• Morbidly Obese: greater than or equal to 40

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. There are differing opinions on the threshold for being underweight in females, doctors quote anything from 18.5 to 20 as being the lowest weight, the most frequently stated being 19. A BMI nearing 15 is usually used as an indicator for starvation and the health risks involved, with a BMI <17.5 being one of the DSM criteria for the diagnosis of anorexia nervosa.

Different ages

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 compared with children of the same gender and age that is important ^[1]. 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.

Recent studies in England have indicated that females between the ages 12 and 16 have a higher BMI than males by 1.0 kg/m² on average ^[2].

International variations

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 U.S. National Institutes of Health brought U.S. 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". 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...

For Asians,the new cut-off BMI index for obesity is 27.5 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 18.5-22.9. Singapore BMI Cut-offs

Uses

Statistical device

The Body Mass Index is generally used as a means of correlation between groups related by general mass and can serve as a basic means of estimating adiposity. However, the duality of the Body Mass Index is that, whilst easy-to-use as a general calculation, it is limited in how accurate and pertinent the data obtained from it can be. Generally, the Index is suitable for recognising trends within sedentary or overweight individuals because there is a smaller margin for errors ^[3].

This general correlation is particularly useful for consensus data regarding obesity or various other conditions because it can be used to build a semi-accurate representation from which a solution can be stipulated, or the RDA for a group can be calculated. Similarly, this is becoming more and more pertinent to the growth of children, due to the majority of their exercise habits. ^[4]

The growth of children is usually documented against a BMI-measured growth chart. Obesity trends can be calculated from the difference between the child's BMI and the BMI on the chart. However, this method again falls prey to the obstacle of body composition: many children who are generally born, or grow as an endomorph, would be classed as obese despite body composition. Clinical professionals should take into account the child's body composition and defer to an appropriate technique such as densiometry.

Clinical practice

BMI can be calculated quickly and without expensive equipment. However, BMI categories do not take into account factors such as frame size and muscularity. ^[5] and the categories do not distinguish what proportions of a human body's weight are muscle, fat, bone and cartilage, or water weight.

Despite this, BMI categories are generally regarded as a satisfactory tool for measuring whether sedentary individuals are "underweight," "overweight" or "obese." It has been used by the WHO as the standard for recording obesity statistics since the early 1980s. In the United States, BMI is also used as a measure of underweight, owing to advocacy on behalf of those suffering with eating disorders, such as anorexia nervosa and bulimia nervosa.

Individuals who are not sedentary - especially athletes - as well as children, the elderly, the infirm, and individuals who are naturally endomorphic or ectomorphic (i.e., people who don't have a medium frame) are ill-fitted to assessment using the BMI. Or to state the problem more accurately, the BMI measurements at which these people may be underweight, overweight or obese are different from for sedentary mesomorphs whose ages are between about 20 and 70.

In athletes, the problem is that muscle is denser than fat. Most professional athletes are "overweight" or "obese" according to their BMI^[6] - unless the number at which they are considered "overweight" or "obese" is adjusted upward. In children and the elderly, differences in bone density and, thus, in the proportion of bone to total weight can mean the number at which these people are considered underweight should be adjusted downward.

In all cases, methods for actually measuring body fat percentage are always preferable to BMI for measuring healthy body size.

Problems

As a general rule, developed muscle contributes more to weight than fat and the BMI does not account for this. Therefore a person with more muscle mass such as a body-builder will seem to be overweight. Likewise it could be stipulated that some long-distance or endurance athletes would be classified as ectomorphic (underweight, anorexic or starvation), this type of athlete tends to have low body fat and well developed slow twitch muscle, which does not contribute greatly to muscle mass. These individuals could be widely regarded as the perfect composite for their particular sports. Due to these limitations, body composition for athletes would not be calculated using the body mass index, and instead the body fat would be determined by such techniques as skinfold measurements or underwater weighing. In parallel to this, sportsmen or women from sports such as Rugby, where size and muscle are often of importance, could be listed as overweight, due to a large amount of muscle. At the extreme, many bodybuilders would be classified as morbidly obese, such as Markus Rühl who has a BMI of 46.

Another issue is that competitive athletes often know very accurately what their actual height and weight are, while the general public has tendencies toward over-estimating their height, and under-estimating their weight. The BMI standards, as a public health tool, take this tendency into account. This can lead to athletes having a higher reported BMI than a lay person of the same height and weight.

Also, there is often an assumption that athletic performance equals good health. This assumption is often false. For instance, many of the players in the NFL qualify as obese by BMI standards, when they actually fall on the far end of the curve for body size, and could not easily fit into the healthy categories, despite being very physically active and having normal body fat percentages. This does not seem to protect them from health issues associated with obesity, such as increased risk of heart disease and diabetes.