Body mass index, or BMI, is an indirect technique for body fat measurement that uses your height and weight as inputs in a standard testing formula. Health-care providers frequently use it as a cheap, relatively accurate screening method for identifying potential cases of obesity. Despite its acknowledged usefulness, BMI testing has several known limitations.
Basics
BMI testing uses a set formula to place your body fat level on an index that ranges from underweight to obese. If you have a normal amount of body fat, your result from this formula will place you between 18.5 and 24.9 on this index, according to MayoClinic.com. If you are overweight, your result from this formula will fall between 25.0 and 29.9. Obese individuals fall between 30.0 and 39.9, while morbidly obese individuals have a BMI result that equals or exceeds 40.0. Underweight individuals have a body fat level that falls below 18.5 on the BMI index.
Skewed Results
Several known factors can skew your results on a BMI test. Since BMI calculations plug numbers into a set formula, they can't distinguish weight attributed to body fat from weight attributed to lean muscle mass. For instance, men with a given BMI result typically have less body fat than women with the same BMI. Additionally, relatively young people with a given BMI result typically have less body fat than relatively older people with the same BMI. Also, if you are a trained athlete or regularly perform vigorous workouts, BMI testing can easily overestimate your fat levels and underestimate your lean muscle mass.
Health Risk Estimations
While BMI testing can estimate your body fat levels, it cannot be used to diagnose your actual state of health or health risks. If you receive a high BMI reading, your doctor will typically place your results in the context of the results from other health assessment tools, including a physical examination, your medical history, evaluations of your diet and level of physical activity and direct measurement of your skinfold thickness with devices called calipers.
Considerations
Because children are constantly growing, doctors don't rank their BMI results in a single all-ages category. Instead, they compare BMI results from children of the same age and gender and rank them by ordered, internal groupings called percentiles. In addition to skinfold thickness, common alternatives to BMI for body fat measurement include bioelectrical impedance analysis, underwater weighing and an X-ray-based technique called dual-energy absorptiometry, or DXA. Consult your doctor for more information on these approaches.
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