What Is Body Mass Index, or BMI?Body Mass Index, or BMI, is a formula that looks at your weight in comparison with your height, in order to help determine how healthy you are. It can be a useful tool among a variety of factors that should be examined together when assessing your health. Athletes and serious fitness fanatics make use of BMI in building fitness, along with intense physical activity and diet adjustments, like snacking on energy bars. You can calculate your own BMI score, using either metric measures, or pounds and inches, to see where you fall on the Adult BMI Chart. The result is your BMI. It places you in a category that gives an indication of your “degree of fatness.” The BMI formula takes your weight in kilograms (kg), and divides it by your height in meters, squared (m2).1 The result is a number that will fall within a range on the BMI charts, indicating that you are either:
- Underweight – with BMI less than 18.5
- Normal weight – with BMI between 18.5 to 24.9
- Overweight – with BMI at 25 to 29.9, or
- Obese – with BMI at 30 or more.1
The difference comes in how the readings are interpreted. Children and teens are calculated the same way, but the outcome is judged differently because of the greater differences in body fat between boys and girls.2 When a high BMI is obtained, indicating overweight or obesity, the health care provider should perform additional assessments, such as examining diet, level of physical activity, skinfold thickness measurements, and other options, to obtain a fuller picture of an individual's health.
If you are a highly developed athlete with greater muscle mass, chances are you follow a regimen of intense workouts, which demand high nutrition levels, as well as often being supplemented by nutrition bars. You may get a BMI reading that suggests you have more body fat than you do, when, in fact, you are not overweight. Your greater weight is possibly the result of high lean body mass, or bone and muscle, rather than high body fat.2 Why Is Obesity Such a Big Deal? The health care and medical communities are very focused on health consequences of obesity because they are so significant. Obesity puts people at risk for a greater incidence of diseases including: high LDL cholesterol, type 2 diabetes, high blood pressure, coronary heart disease and stroke, gallbladder disease, sleep apnea, osteoarthritis, some cancers, and many other factors. What Is a Healthy BMI? In general, BMI status from 25 to 29.9 is considered overweight, while 30 or more is considered obese. Athletes are generally assumed to have a higher BMI due to increased muscle mass, but they should be evaluated by a professional who will take into account all related health factors to get an accurate assessment of body fat. By contrast, aging adults who have lost muscle mass may get a low body fat reading that is inaccurate. Measuring Body Fat – A More Complete Picture There are limitations to the BMI method of assessing body composition, due to the factors described above, like differences in muscle mass. Obtaining a more accurate reading may require adding other methods that are considered acceptable by the medical and health communities.
- Skinfold Caliper Testing – Often used in combination with the BMI measurement, this test is widely and easily available. It can be performed by someone with the experience and training needed to get accurate assessments. As the name implies, it uses a set of calipers to pinch and measure areas of skin and fat underneath, in several specific locations around the body. The results are converted to a body fat percentage estimate. Experts suggest this method should be combined with a measurement of deep belly fat (not measureable with the skinfold calipers).
- Bioelectrical Impedance – This method sends a very slight electrical current (don't worry—you won't feel a thing) through the body, to measure the degree of resistance. Fatty tissue returns the electrical impulse more slowly than lean tissue, so a faster response rate indicates that you have a leaner body. You can keep a scale at home that measures body fat percentage in this way, and some personal trainers and gyms make them available. Experts note that the results can be affected by factors like your level of hydration and how recently you've had a meal, so it is best to use this method always at the same time of day, and to combine it with one or more other approaches, in order to obtain a fair comparison of results over time.3
- DEXA or Dual-Energy X-Ray Absorptiometry – A more scientifically accurate test, DEXA scans are x-rays that measure density of bone minerals, along with lean body and fat mass. The test is fast and painless, but the price is higher than other methods, and availability is limited to certain medical professonals.3
- Hydrostatic Weighing – Depending on your spirit of adventure and fondness for water, this may or may not be your method of choice. It measures body fat by submerging you completely under water and measuring your weight. This number is compared to your weight on dry land, and, together with the density of the water, gives a measure of body composition. Access and cost make it less appealing for the average person, or even the average health enthusiast.3