BMI is a simple screening tool to estimate body composition. Body composition is important because of its impact on health. These days, many people have too much body fat, and some have too little lean muscle. Excess fat increases the risk of multiple diseases; and the risks go up markedly at the higher ranges of obesity. And too little muscle also creates problems, such as increased risk of injury from falls.
Determining & Classifying BMI
BMI = WT / HT2 where WT = body weight in kg and HT is height in meters. Weight classification ranges for BMI in adults 20 years and older are defined as follows:
BMI Classification
<18.5 Underweight
18.5 to <25 Normal or healthy weight
25 to <30 Overweight
30 and above Obese
Some Limitations of BMI
Again, BMI is a screening tool, not a direct measure of body composition. While BMI is a good predictor of body composition and of the risk of weight-related health issues for groups of people, individual results are sometimes misleading. For example, an athlete’s BMI might be in the overweight range, but his or her body fat might be normal or even unusually low. Or someone could have a BMI in the healthy range but be deficient in muscle mass.
Actions Based on BMI
Good general advice is to aim to be both fit and in the normal weight range. That said, people who are overweight or obese can improve their health prospects by even a modest weight loss of 5% — for example, a 200 pound person who achieves a goal of losing 10 pounds. Indeed, if overweight, the first measure of success is to stop gaining weight. Tracking your BMI (or weight or waist circumference) can help you set goals and monitor progress.
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