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Why Is BMI Important?

By Andrea Boldt ; Updated July 18, 2017

Your body mass index, or BMI, provides a quick way for your doctor to estimate your level of body fat. Although not infallible, it can clue him in as to whether you're at risk of chronic disease and other complications related to being overweight or obese. A high BMI makes it likely that you'll be subject to additional health screenings, including diet and physical activity evaluation, family history, blood tests and other appropriate evaluations.

Determining Your BMI

You can calculate your BMI yourself, using just your height and weight. Use the following formula to calculate it by hand:

BMI = your weight in pounds / (height in inches x height in inches) x 703.

Alternatively, use an online BMI calculator.

A healthy BMI is between 18.5 and 24.9. If you register below 18.5, you may be diagnosed as being severely underweight. A BMI of between 25 and 29.9 puts you in the category of overweight. If your BMI is 30 or greater, you may be diagnosed with obesity, depending on how muscular your build is.

What a High BMI Indicates

A BMI that places you in the higher range of overweight or obese tends to be a pretty accurate indication of body fatness. Carrying too much fat increases your risk of early death. High BMI levels usually correlate with an elevated risk of chronic disease, including type-2 diabetes, coronary artery disease, stroke, gallbladder disease, osteoarthritis, sleep apnea, chronic inflammation and depression.

BMI Can Misdiagnose

People who are extremely muscular, such as athletes and bodybuilders, may register a high BMI even though they are exceptionally healthy. Increased muscularity makes you weigh more than average for your height, so the equation results appear as if your BMI is too high. Your doctor should be able to visually assess your muscular condition and offer alternative methods for evaluating your risk of disease, such as blood tests.

BMI can also miss people who have a high body fat percentage but a normal weight. If you have 20 percent fat as a man, or 30 percent fat as a woman, but your BMI is in the normal range, you are still at risk of many of the same ailments associated with obesity. You have a condition known as normal-weight obesity. Sedentary people and older adults are at particular risk for this condition that's estimated to affect 30 million Americans, reports Today's Dietitian in 2011.

Alternative Measures of Fat

Waist circumference is a relatively easy and noninvasive way to perform a quick analysis of a person's health risk due to being overweight or obese. A waist size that's larger than 40 inches on a man or 35 inches on a woman indicates that you likely carry excessive belly fat, which puts you at a higher risk for weight-related diseases. Measuring your waist size may be a way to catch those who have normal-weight obesity.

Body composition measurements, including those taken with skinfold calipers or a body fat scale, can also give you a rough idea of the amount of body fat you carry in relation to lean body mass. More in-depth analyses -- such as DEXA scans and hydrostatic weighing -- are more accurate, but they're expensive and require specialized equipment.

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