Beyond Pinching An Inch

How to Measure and Interpret Your Body Composition

Why Weight Doesn’t Really Matter. . .

Relying solely on a scale to evaluate your eating habits and training program has its limitations. For example, a scale can’t differentiate fat gained by overeating from muscle added as a result of training. In addition, our weight fluctuates from day to day, and even throughout the day. Daily weigh-ins indicate only shifts in body fluids resulting from sweat losses, hormonal changes and the carbohydrate content of meals and snacks. In addition, skimping on calories and shunning healthy foods is no formula for successful running. Rather, it sets you up for poor training, frequent illnesses and injuries.

. . . And Body Composition Does

Assessing the percentage of your body weight that is fat provides more useful information. Excess body fat can be unhealthy, as it increases the risk of heart disease, diabetes, high blood pressure and some cancers. Very few performance runners have body fat levels high enough to present a health risk, but see your doctor if you have concerns.

To function properly, our bodies need to store at least 5% fat for men and 8% to 12% for women. Average body-fat levels observed in elite runners range from 5% to 12% in men and 8% to 15% in women. A body-fat level below these recommended amounts can signal overtraining or the possibility of an eating disorder. The acceptable levels for the general population are 15% to 22% for men and 20% to 32% for women.

Can running performances improve with fat loss? Only to a point.

"I’ve counseled many coaches who bring athletes in, particularly female runners, who they believe need to lose weight," says Levi Query, an exercise physiologist and runner in Portland, OR, with 18 years experience evaluating athletes’ body composition. "A runner with a stocky build or one who carries most of her weight on the hips and thighs can appear overweight but actually be quite lean and muscular."

Keep in mind that runners perform successfully at a wide range of leannesses. One study of 70 elite women distance runners found their average body fat was 16.8%, with a range of 6% to 35.8%. The goal is to determine a range at which you feel and perform best, not to chase after a specific value. For most runners, monitoring body composition a couple of times a year is sufficient.

Techniques for Measuring Body Composition

We can only estimate body fat, and even the best methods have errors in the range of 3%. In other words, if you’re told you have 12% body fat, you could have between 9% and 15%. The following methods are among the most popular.

Hydrostatic (underwater) weighing. This method involves being weighed underwater after expelling all the air from the lungs. The difference between your land weight and water weight is used to estimate body density, from which percent body fat is extrapolated. Considered the most accurate method, underwater weighing can be expensive, time-consuming (repeated dunkings are required) and uncomfortable, as you must remain underwater for up to 10 seconds after expelling your breath. Also, the formulas used may not be accurate for some athletes, such as older and non-Caucasian runners. Sports medicine centers, hospital-based wellness programs and universities with an exercise science program typically offer hydrostatic weighing.

Air displacement (Bod Pod). A new technology, this method requires that you sit in an enclosed capsule for about a minute while computer sensors measure the amount of air displaced by your body. Research findings indicate that the method is reliable and valid in healthy adults; however, additional studies are needed on competitive athletes. Although fast and easy, the method can be expensive and is not yet widely available. Check universities and athletic facilities serving professional and collegiate athletes.

Skinfold calipers. A tester uses calipers to pinch and measure the fat located right under the skin on several parts of the body, usually the abdomen, thigh, hip, and the back of the arm and shoulder. Simple, painless and inexpensive, this technique can provide accurate and reliable readings (if the measurement taker is skilled), although it tends to underestimate body fat in lean athletes. Serial measurements can monitor changes in body composition over time.

Bioelectrical impedance analysis (BIA). In this technique, a mild electrical current is passed through the body. Because muscles contain most of the body’s water and electrolytes, they conduct the current more quickly than fat stores; thus the slower the signal the more fat you have. Quick and painless, BIA is offered routinely at fitness clubs and health fairs. Be aware that it tends to overestimate body fat percentage in lean individuals, and that fluid changes, such as being dehydrated, can skew the measurements. For best results, avoid alcohol and caffeine for 24 hours, don’t exercise for 12 hours and don’t eat or drink for four hours prior to the test.

Interpreting Your Results

Chances are, you’re within a healthy range and don’t need to make any changes. As with weight, aim to be within a healthy range rather than obsessing over the exact number and fretting about slight variations over time. Susan Fox, director of the Human Performance Lab at Oregon State University, suggests that athletes monitor changes in body fat over a season, such as before and after a specific phase of training. She also notes that to make a valid comparison, you must use the same method each time.

If you fall significantly outside the healthy range, consult a registered dietician or sports medicine physician to help you achieve a more appropriate level.