Overview Of Body Mass Index

According to the Centers for Disease Control and Prevention (CDC), body mass index (BMI) is a simple screening method, often used by doctors and researchers, to indicate potential weight problems. While BMI does not directly measure body fat, it correlates with true measures of body fat such as underwater weighing. However, BMI does have limitations and should be considered with true body fat measurements, nutritional and physical activity assessments and genetic profiles for accurate diagnoses.

How to Calculate BMI

BMI is calculated the same way for adults, children and teens. It is calculated by dividing your weight in pounds by your height in inches squared and then multiplying by a conversion factor of 703. For example, if you weigh 200 pounds and are 70 inches (5 feet, 10 inches) tall, your BMI is 28.7 (200 ÷ 4900 --- 703 = 28.7).

BMI Classifications

BMI is divided into four general categories: underweight, normal weight, overweight and obese. For adults, you are considered underweight if your BMI is less than 18.5, normal weight if your BMI falls between 18.5 and 24.9, overweight between 25.0 and 29.9, and obese if your BMI is 30.0 or more.

BMI for Children and Teens

Although the BMI formula does not change for children and teens, the calculations are interpreted differently. Children and teens are classified using age- and sex-specific percentiles because their body fat changes with age and differs between females and males. Your child or teen is considered underweight if her BMI is in less than the 5th percentile, normal weight if her BMI falls between the 5th and 85th percentile, overweight between the 85th and 95th percentile, and obese if her BMI is in the 95th percentile or greater.

Limitations of BMI

BMI can be misleading, especially if you are muscular. Because BMI uses your general body weight and does not account for fat mass versus muscle mass, you may fall into the overweight or obese classifications when, in fact, your weight is fine. If you believe your BMI is inaccurate, you should have your body fat percentage measured; this will tell you if you truly need to lose weight.

How to Change Your BMI

If you are overweight or obese, you should take steps to lose weight to decrease your BMI into the normal range. Your energy balance is the most important factor to consider, according to exercise scientists William McArdle, Frank Katch and Victor Katch. This is simply the difference between the number of calories you burn and the number of calories you consume. To lose 1 to 2 pounds per week, you should try to eat between 500 and 1000 calories less than you burn each day. To accomplish this, you should increase your physical activity through exercise, eat less food or both.
Conversely, if you are underweight, you should consume more calories than you burn until your BMI is within the normal range. While you certainly should eat more calories to gain weight, you should also consider performing resistance training exercises to increase your muscle mass.