The body mass index, or BMI, uses height and weight as predictors of body fatness. A person's weight in pounds or kilograms is divided by his height in meters or inches, squared, giving a BMI number. This number is not only an indicator of body fat, but can be used to estimate the probability of various diseases relating to obesity, such as heart disease and diabetes. It's an inexpensive, easy and fairly accurate indicator of body fat that works particularly well on whole populations.
The basal metabolic rate, or BMR, is the number of calories your body burns at mental and physical rest to maintain basic body functions, such as heartbeat, breathing and digestion. The formula to determine BMR is called the Harris-Benedict Equation and is a calorie formula that uses your BMI, age and sex to determine your BMR. It is available on many websites.
Limitations
BMI and BMR tests are just a guide as many factors affect results. BMI does not take gender into account, and women have more body fat than men. Skeletal size and muscle mass are also variables not taken into consideration. Athletes with greater muscle mass will show a greater BMI number, but, in this case, it's muscle and not fat that raises the number. People with high muscle mass may be placed in a higher "fatness" category because of this. There is also a risk of underestimating fat deposits based on BMI in the more sedentary elderly population.
With BMR, the number tends to decrease with age, about 2 percent per decade. Conversely, stress raises BMR as does exercise and increased muscle mass.
Ethnicity
Ethnicity is not taken into account with BMI and some ethnic groups require adjustments. Some Asian populations experience greater metabolic risks, such as high blood pressure and diabetes, at a much lower BMI number.
Adults and Children
BMI is tested the same on adults and children, however, results are interpreted different. Growth charts are consulted when interpreting BMI for children. The amount of body fat changes with age, and body fat differs between boys and girls.
BMR includes a variable called the energy cost of growth. Children are continually requiring energy for synthesis of new tissue and this is factored into BMR.
Get Tested
There are calculations to self-test for BMI and BMR. BMI uses height and weight and a calculation. BMR uses the BMI number and then a calculation to determine your BMR. Both calculations are available online.
You should also take additional tests. After you test your BMI and then BMR, assess your waist to hip ratio. People with more weight at their waists have been shown to have greater risk of developing heart disease and diabetes. You simply measure your hip and waist and then consult an online calculator for your number.
Skin fold tests are also suggested, but they require a skin fold caliper. The three aforementioned tests will suffice and give you a fairly accurate portrait of your "fatness" level.
References
- Center for Disease Control: Adult BMI Calculator
- BMI Calculator: The Harris Benedict Equation Formula
- BMI Calculator: Waist to Hip Ratio
- The American Journal of Clinical Nutrition: Body mass index and waist circumference in midchildhood and adverse cardiovascular disease risk clustering in adolescence
- The American Journal of Clinical Nutrition: Body mass index and waist circumference in midchildhood and adverse cardiovascular disease risk clustering in adolescence
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