For anyone interesting in maintaining health and wellness, it's important to keep track of body fat. The human body functions best when body fat is within an optimal range--too much fat leads to a variety of health disorders, while too little body fat can interfere with normal function. Body mass index (BMI) is one way to keep track of body composition, which is the amount of fat tissue relative to lean tissue.
Calculation
BMI is a modified ratio of weight to height, calculated by the formula: weight (kg) divided by height (m) squared. For instance, an individual with a height of 1.83m and weighing 68kg would have a BMI of [68 / (1.83 x 1.83)] = 20.3. A second formula for BMI relies on height in inches and weight in pounds: weight (lb.) divided by height (in.) squared, quotient multiplied by 703. In establishing a baseline BMI, it's best to use an average weight calculated over three days. There are a number of BMI calculators available on the Internet.
Adult Assessment
BMI values correspond to weight status categories, as set forth by the National Institutes of Health (NIH) and other national and global health organizations. Values of less than 18.5 are taken to indicate underweight, while normal BMI values fall between 18.5 and 24.9. Individuals with BMI between 25 and 29.9 are considered overweight, while values of 30 and greater correspond to obesity. Because BMI is only one measure of body composition, the NIH recommends talking to a physician before embarking on a program to significantly increase or decrease BMI.
Child and Teen Assessment
For children and teenagers under the age of 18, there is a different mechanism for assessing the meaning of BMI. Based upon age and gender, BMI values fall into percentile rankings on standard growth charts. Children and teens falling below the fifth percentile are considered underweight, while those from the fifth to less than the 85th percentile are of normal weight. Overweight children and teens are those from the 85th to less than the 95th percentile, and those in greater than the 95th percentile are considered obese. Growth charts may be found on the Internet, but the NIH and other health organizations recommend discussing child and teen BMI values with a physician for assistance in analyzing results.
Utility
While there are many ways to track body composition and weight status, BMI is one of the most common. It is easy to calculate at home, requires no specialized equipment and is relatively stable across time of day and hydration level. The CDC recommends BMI to individuals and physicians as a mechanism of assessing fitness because of the high correlation between BMI and body fat percentage. Further, since high BMI is associated with diseases of overweight and obesity, including type 2 diabetes, arthritis and coronary heart disease, it is a useful metric in risk assessment.
Athletes
Because muscle weighs more than fat, BMI may not correlate with body fatness as strongly for highly muscular people as it does for the average individual. It is important to note that a high BMI does not represent a risk factor for diseases of obesity if it does not correlate well with body fat, and individuals with high BMI due to musculature who have low body fat are no more at risk than less muscular individuals of similar body fat. Athletes and anyone with significant musculature may wish to consider an alternate mechanism for determining and tracking body composition, such as calculation of body fat percentage from caliper measurements.
Warning
BMI may underestimate body fat in individuals with wasting diseases or muscular atrophy, or in the very inactive. Slight, small-framed women and the elderly are particularly likely to fall into this category, whereby BMI may indicate a normal (or even below normal) weight despite higher than desirable body fat. It is therefore important to remember that in these individuals, BMI may underestimate risk for diseases associated with obesity, as it is ultimately body fat (rather than weight-to-height ratio) that determines risk.
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