Determine Percent Body Fat or BMI

What is percent body fat? 
For the average adult, an optimal range of body fat has been identified. The optimal range differs based on gender and age. Percent body fat is the percentage of total body weight that is fat.

When is use of percent body fat appropriate? 
Body composition analysis is appropriate when there is access to the appropriate methods. Methods commonly used in the physician's office include bioelectrical impedance, skinfold measures and girth measurements.

What tools are used to measure body fat? 
A bioelectrical impedance machine assesses impedance information to estimate the amount of lean and fat tissue within the body. Skinfold calipers are used to measure folds of skin and fat taken from several specific sites on the body; the results are entered into an equation to estimate total body fat. Girth measurements are taken using a non-stretchable tape measure. Measurements at specific body sites are converted to a constant and then entered into a formula to predict percent body fat.

Do not use self-reported percent body fat, as it may be inaccurate.

Why use percent body fat? 
Knowing the percent body fat is a better measure of health and fitness than weight. Measuring weight alone does not give any indication of body composition. Excess body fat increases the risk of many diseases including type 2 diabetes, hypertension, cardiovascular disease and some cancers.

What is BMI? 
BMI is a relationship between weight and height that is used to assess your patient's health risk. It is a mathematical formula that correlates with body fat in adults and is expressed as weight in kilograms divided by height in meters squared (BMI = kg/m2).

When is use of BMI appropriate? 
BMI tables provided in the appendix are appropriate for use in adults between 19 - 70 years of age. These numbers are somewhat arbitrary, although BMI can be useful once growth has stopped in younger individuals and throughout adulthood.

Exceptions to use of BMI 
Do not use the BMI tables in this document as the basis for assessing treatment in patients who are competitive athletes and body builders, (i.e., in patients whose BMI is high due to relatively great muscle mass1) and/or in women who are pregnant and/or lactating. They are not intended for use in growing children or in elderly patients who are both frail and sedentary.

Why use BMI? 
Obesity is defined as an excess of body fat, but direct measurement of body fat is usually not practical. BMI indicates a patient's relative health risk. Most studies designed to evaluate health risk, including studies of both morbidity and premature mortality, rely on BMI values. For example, prospective studies have related BMI to the subsequent development of cardiovascular disease, cancer, and other morbidities, as well as premature death2-9.

How to determine your patient's BMI 
Start by measuring your patient's height (to the closest inch) and weight (to the closest pound). A protocol for measuring height and weight is provided in the appendix.

Do not use self-reported height and weight information, as it may be inaccurate10.

Next, determine your patient's BMI using the tables in the appendix. These tables, complete with metric conversions, apply to both male and female patients. All necessary calculations have already been done.


How often to assess percent body fat or BMI? 
Percent body fat or BMI should be determined at each visit. (If you are tracking BMI, you only need to weigh the patient; repeated height measurements are unnecessary more than once a year.) If you do not see a patient on a routine basis, an assessment every six months to a year is appropriate.

In some patients who are not being treated for weight reduction, a weight gain that changes percent body fat or BMI classification may occur rather quickly. Routine determination of your patient's percent body fat or BMI will help you intervene at the earliest opportunity.

It is also important to note percent body fat or BMI decreases in patients undergoing treatment, since this information signals you to provide valuable support of your patient's efforts and encourages continued progress.