Body mass index is one of the methods doctors use to estimate your risk for health conditions associated with being overweight or obese. But the BMI measurement has its limitations. BMI is a ratio of your weight to your height, and helps estimate how much body fat you have. However, BMI is not the only indicator of health or disease risk.
Benefits
BMI measurements provide a relatively accurate estimate of body fat for many people at virtually no cost. You only need to accurately measure your height and weight and plug them into a formula to figure out your BMI, so BMI is also a simple and inexpensive way to estimate body fat.
Limitations
BMI doesn't actually measure body fat, so people who have more muscle than the average person, such as athletes, or less muscle, such as elderly people, have BMIs that don't reflect their true body fat levels. BMI also tends to be less accurate in identifying children with excess body fat, notes Baylor College of Medicine. Some children in the normal weight range have high levels of body fat, while others that weigh more than normal still have normal levels of body fat.
Alternatives
Using BMI in conjunction with other methods provides a more accurate estimate of your risk for obesity-related health conditions. These include waist circumference, blood pressure, triglycerides, blood sugar and blood cholesterol measurements. In children, some doctors may still use visual inspection, but since the average weight of children is increasing, this is a less accurate method. For a true measurement of body fat, underwater weighing and DEXA are accurate but expensive. Body fat calipers and bioelectrical impedance are less expensive but also less accurate body fat measurements than underwater weighing or DEXA.
Considerations
Although BMI is not a perfect estimate of body fat, it is a useful screening tool. If you have a BMI above 25, you may be overweight. Your doctor can do further tests to identify whether you have an increased risk for certain health conditions and tell you whether you should aim to lose at least a small amount of weight to lower your risk for these health conditions.
References
- National Heart Lung and Blood Institute: Assessing Your Weight and Health Risk
- Baylor College of Medicine; Weigh BMI Limitations When Evaluating Children; Summer 2000
- "Pediatrics"; The Use of BMI in the Clinical Setting; Stephen R. Daniels, MD, PhD; September 2009
- University of Virginia Health System: Body Mass Index
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