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Body Mass Index: Considerations for Practitioners
What is BMI?
Body mass index (BMI) is a measure of weight adjusted for height, calculated as weight
in kilograms divided by the square of height in meters (kg/m
). Although BMI is often
considered an indicator of body fatness, it is a surrogate measure of body fat because it
measures excess weight rather than excess fat. Despite this fact, studies have shown
that BMI is correlated to more direct measures of body fat, such as underwater weighing
and dual energy x-ray absorptiometry.
Why use BMI?
BMI is a simple, inexpensive, and noninvasive surrogate measure of body fat. In
contrast to other methods, BMI relies solely on height and weight and with access to the
proper equipment, individuals can have their BMI routinely measured and calculated
with reasonable accuracy.
Furthermore, studies have shown that BMI levels correlate with body fat and with future
health risks. High BMI predicts future morbidity and death. Therefore, BMI is an
appropriate measure for screening for obesity and its health risks.
Lastly, the widespread and longstanding application of BMI contributes to its utility at the
population level. Its use has resulted in an increased availability of published population
data that allows public health professionals to make comparisons across time, regions,
and population subgroups.
What are some issues to consider when using BMI for adults?
The clinical limitations of BMI should be considered. BMI is a surrogate measure of
body fatness because it is a measure of excess weight rather than excess body fat.
Factors such as age, sex, ethnicity, and muscle mass can influence the relationship
between BMI and body fat. Also, BMI does not distinguish between excess fat, muscle,
or bone mass, nor does it provide any indication of the distribution of fat among
individuals.
The following are some examples of how certain variables can influence the
interpretation of BMI:
• On average, older adults tend to have more body fat than younger adults for an
equivalent BMI.
• On average, women have greater amounts of total body fat than men with an
equivalent BMI.
• Muscular individuals, or highly-trained athletes, may have a high BMI because of
increased muscle mass.
What are some issues to consider when using BMI for children and adolescents?
The concerns associated with using BMI for adults also apply to children and
adolescents. Other factors, including height and level of sexual maturation, influence
the relationship between BMI and body fat among children as well.
In addition, the accuracy of BMI varies substantially according to the individual child’s
degree of body fatness. Among obese children (or a BMI-for-age greater than or equal
to the 95
th
percentile), BMI is a good indicator of excess body fat. However, among
overweight children (or a BMI-for-age between the 85
th
and 94
th
percentiles), elevated
BMI levels can be a result of increased levels of either fat or fat-free mass. Similarly,
among relatively thin children, differences in BMI are often due to differences in fat-free
mass.
How should different BMI levels for adults be interpreted?
For adults 20 years and older, BMI is interpreted by using standard weight status
categories that are the same for all ages and for both men and women. The standard
weight status categories associated with BMI ranges for adults are:
BMI Weight Status
Below 18.5 Underweight
18.5 – 24.9 Normal
25.0 – 29.9 Overweight
30.0 and Above Obese
Research has shown that individuals with a higher BMI are more likely to experience
obesity-related health problems. Although some debate continues about whether
different categories should exist for specific subgroups (such as Asians), these BMI
categories are used worldwide for all adults 20 years and older.
Because no single body fat measure clearly distinguishes health from disease or risk of
disease, BMI should serve as the initial screening of overweight and obesity for adults.
Health care providers should recognize that other factors, such as fat distribution,
genetics, and fitness level, contribute to an individual’s assessment of disease risk.
How should different BMI levels for children and adolescents be interpreted?
BMI is calculated the same way for adults and children, but the results are interpreted
differently. For adults, BMI classifications do not depend on age or sex. For children
and adolescents between 2 and 20 years old, BMI is interpreted relative to a child’s age
and sex, because the amount of body fat changes with age and varies by sex.
Percentiles specific to age and sex classify underweight, healthy weight, overweight,
and obesity in children. The BMI-for-age determined for an individual indicates the
relative position of the child’s BMI value among children of the same sex and age. BMI-
for-age categories and corresponding percentiles are:
Body Mass Index: Considerations for Practitioners
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