Body mass index (BMI), also called the Quetelet Index, is a calculation used to determine an individual's amount of body fat.
BMI gives healthcare professionals a consistent way of assessing their patients' weight and an objective way of discussing it with them. It is also useful in suggesting the degree to which the individual may be at risk for obesity-related diseases.
BMI is a statistical calculation intended as an assessment tool. It can be applied to groups of people to determine trends or it can be applied to individuals. When applied to individuals, it is only one of several assessments used to determine health risks related to being underweight, overweight, or obese.
Calculating BMI requires two measurements: weight and height. To calculate BMI using metric units, weight in kilograms (kg) is divided by the height squared measured in meters (m). To calculate BMI in imperial units, weight in pounds (lb) is divided by height squared in inches (in) and then multiplied by 703. This calculation produces a number that is the individual's BMI. This number, when compared to the statistical distribution of BMIs for adults ages 20–29, indicates whether the individual is underweight, average weight, overweight, or obese. The 20–29 age group was chosen as the standard because it represents fully developed adults at the point in their lives when they statistically have the least amount of body fat. The formula for calculating the BMI for children is the same as for adults, but the resulting number is interpreted differently.
Although the formula for calculating BMI was developed in the mid-1800s, it was not commonly used in the United States before the mid-1980s. Until then, fatness or thinness was determined by tables that set an ideal weight or weight range for each height. Heights were measured in one-inch intervals, and the ideal weight range was calculated separately for men and women. The information used to develop these ideal weight-for-height tables came from several decades of data compiled by life insurance companies. These tables determined the probability of death as it related to height and weight and were used by the companies to set life insurance rates. The data excluded anyone with a chronic disease or anyone who, for whatever health reason, could not obtain life insurance.
Interest in using the BMI in the United States increased in the early 1980s when researchers became concerned that Americans were rapidly becoming obese. In 1984, the national percentage of overweight individuals was reported in a major assessment of the nation's health. Men having a BMI of 28 or greater were considered overweight. This BMI number was chosen to define overweight because 85% of American men ages 20–29 fell below it. A different calculation, not BMI, was used for women in the report.
In 1985, the term overweight was redefined as a BMI equal to or greater than 27.8 for men and equal to or greater than 27.3 for women. No BMI was selected to define underweight individuals. This definition of overweight was used in reports on obesity until 1998. In 1998, the United States National Institutes of Health revised its weight definitions to bring them in line with the definitions used by the World Health Organization. Overnight 30 million Americans went from being classified as normal weight to being classified as overweight. Overweight is now defined for both men and women as a BMI of 25 or more. At the same time, an underweight classification was added, as was the classification of obese for individuals with a BMI greater than or equal to 30.
All adults age 20 and older are evaluated on the same BMI scale as follows:
Some researchers consider a BMI of 17 or below an indication of serious, health-threatening malnourishment. In developed countries, a BMI this low in the absence of disease is often an indication of anorexia nervosa. At the other end of the scale, a BMI of 40 or greater indicates morbid obesity that carries a very high risk of developing obesity-related diseases such as stroke, heart attack, and type 2 diabetes.
The formula for calculating the BMI of children ages 2–20 is the same as the formula used in calculating adult BMIs, but the results are interpreted differently. Interpretation of BMI for children takes into consideration that the amount of body fat changes as children grow and that the amount of body fat is different in boys and girls of the same age and weight.
Instead of assigning a child to a specific weight category based on their BMI, a child's BMI is compared to other children of the same age and sex. Children are then assigned a percentile based on their BMI. The percentile provides a comparison between their weight and that of other children the same age and gender. For example, if a girl is in the 75th percentile for her age group, 75 of every 100 children who are her age weigh less than she does and 25 of every 100 weigh more than she does. The weight categories for children are:
The BMI was originally designed to observe groups of people. It is still used to spot trends, such as increasing weight in a particular age group over time. It is also a valuable tool for comparing body mass among different ethnic or cultural groups, and can indicate to what degree populations are undernourished or overnourished.
When applied to individuals, the BMI is not a diagnostic tool. Although there is an established link between BMI and the prevalence of certain diseases such as type 2 diabetes, some cancers, and cardiovascular disease, BMI alone is not intended to predict the likelihood of an individual developing these diseases. The National Heart, Lung, and Blood Institute recommends that the following measures be used to assess the impact of weight on health:
BMI is very accurate when defining characteristics of populations, but less accurate when applied to individuals. However, BMI is widely used because it is inexpensive and easy to determine. Calculating BMI requires a scale, a measuring rod, and the ability to do simple arithmetic or use a calculator. Potential limitations of BMI when applied to individuals are:
BMI is a comparative index and does not measure the amount of body fat directly. Other methods do give a direct measure of body fat, but these methods generally are expensive and require specialized equipment and training to be performed accurately. Among them are measurement of skin-fold thickness, underwater (hydrostatic) weighing, bioelectrical impedance, and dual-energy x-ray absorptiometry (DXA). Combining BMI, waist circumference, family health history, and lifestyle analysis gives healthcare providers enough information to analyze health risks related to weight at minimal cost to the patient.
Childhood obesity is an increasing concern. Research shows that overweight children are more likely to become obese adults than normal-weight children. Excess weight in childhood is also linked to early development of type 2 diabetes, cardiovascular disease, and early onset of certain cancers. In addition, overweight or severely underweight children often pay a heavy social and emotional price as objects of scorn or teasing.
Both the American Academy of Pediatrics (AAP) and the United States Centers for Disease Control and Prevention (CDC) recommend that the BMI of children over age two be reviewed at regular intervals during pediatric visits. Parents of children whose BMI falls above the 85th percentile (at risk of being overweight and overweight categories) should seek information from their healthcare provider about health risks related to a high BMI and guidance on how to moderate their child's weight. Strenuous dieting is rarely advised for growing children, but healthcare providers can give guidance on improving the chid's diet, eliminating empty calories (such as those found in soda and candy) and increasing the child's activity level in order to burn more calories and improve fitness.
BMI often provides misleading information for professional and elite athletes. A given volume of muscle weighs more than a similar volume of fat, so individuals with low body fat and high muscle mass weigh more than individuals of the same body size who have less muscle and more fat. The BMI scale was designed to provide an indication of body fat for average individuals in the general population. This means that it frequently provides inaccurate information for individuals who are not very similar to the average population in body makeup.
A variety of other body fat tests are more accurate than BMI and are more appropriate for elite athletes. If BMI is relied upon even though it does not necessarily provide an accurate picture of body fat for athletes, there can be a variety of negative consequences. Most seriously, it can lead to increases in eating disorders. Elite athletes are already at a higher risk for eating disorders than the general population, especially in sports such as wrestling, gymnastics, and dance. Athletes, especially young female athletes, who are already at risk can misinterpret BMI to indicate that they are overweight, when in fact they have very little body fat. This can lead to increased dieting, unrealistic body image, and eating disorders.
See also Age and exercise ; Calories ; Cardiovascular disease ; Eating disorders ; Obesity ; Waist circumference .
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Tish Davidson, AM