If you think you are overweight, measuring your body mass index (BMI) is a logical first step to determine if you actually are and, if so, how much weight you should try to lose. BMI is easy to calculate and a relatively accurate indicator of body fat and the health risks associated with excess body fat.
What is BMI?
BMI is a measurement of the health of your weight. It is calculated using the following formula: body mass (kg) ÷ stature (m^2). For example, if you weigh 160 lb. (72.7 kg) and are 5 feet, 8 inches tall (1.73 m), your BMI is 24.3 (72.7 ÷ 1.73^2 = 24.3). According to McArdle, Katch and Katch, BMI measurements below 18.5 are classified as underweight, between 18.5 and 24.9 as normal, between 25.0 and 29.9 as overweight and 30.0 or more as obese.
How is BMI increased?
You can increase your BMI by either increasing your body mass or decreasing your stature. Since your height does not significantly decrease during your lifespan, the most prevalent way to increase your BMI is to increase your body mass. Using the same example as above, if you increase your body mass from 160 lb. to 180 lb. (81.8 kg), your BMI increases from 24.3 to 27.3 (81.8 ÷ 1.73^2 = 27.3).
Is an increased BMI good or bad?
An increased BMI can be good or bad depending on your circumstances. If you are underweight as determined by BMI, increasing your BMI is desirable. Health problems such as decreased immune function, osteoporosis and menstrual irregularities (for females) are more likely if your BMI is below 18.5. Conversely, if you are classified as overweight or obese, you should take steps to decrease your BMI. Health risks such as diabetes and high blood pressure are associated with a higher than normal BMI.
Should you work to lower your BMI?
If you are overweight or obese as determined by BMI, you should consider working to decrease your BMI into the normal range. However, if you are muscular, your BMI may show that you are overweight or obese when, in fact, your weight is healthy. The BMI equation does not account for fat mass versus lean tissue mass (includes muscle mass) despite the fact that the health risks associated with an increased BMI are only prevalent if you are overweight or obese due to excess body fat. Therefore, if you feel your BMI does not accurately define the health of your weight, you should also measure your body fat percentage. Depending on your age, your body fat should not exceed 16 percent to 21 percent if you are male or 17 percent to 31 percent if you are female.
How can BMI be lowered?
If you have determined that you need to decrease your BMI, eliminating excess body fat is the most beneficial method. According to McArdle, Katch and Katch, to lose approximately 1 lb. per week, you should use 3500 calories more than you consume each week. To accomplish this you can exercise more, eat less, or both. Your exercise program should include resistance exercises to maintain your muscle mass while you are working to lose weight.
References
- "Sports and Exercise Nutrition (Third Edition)"; William D. McArdle, Frank I. Katch and Victor L. Katch; 2009
- LifeTips: Health Risks Associated with Being Underweight
- NowLoss: Body Fat Percentage Norms
Member Comments