How good is the ‘bod pod’?

Athletes from a variety of different sports feel the need to monitor their body composition. Resistance-trainers, for example, check weight changes and percentage body fat on a regular basis to ensure that the gains they make are in the form of lean mass rather than fat; endurance athletes are often concerned about potential gains in fat weight because it can reduce maximal aerobic capacity and impair movement efficiency; athletes in power sports like volleyball and tennis worry that excessive fat will interfere with their ability to jump explosively or get to the ball quickly and some athletes intent on losing weight need to make sure that they don’t lose muscle mass along with fat tissue. All such athletes would welcome a reliable and user-friendly method of assessing body composition.

Hydrostatic weighing is considered the ‘gold standard’ in this respect, but it is neither problem-free nor easy to use. The repeated underwater submersions (and corresponding maximal exhalations) required for this technique have proved troublesome for athletes, while many coaches find it hard to measure hydrostatic weights accurately, which requires a significant amount of technical expertise.

Skin-fold measurements, the commonly used alternative to hydrostatic weighing, is much easier to carry out but poses its own unique difficulties. For one thing, the accuracy of the method is highly dependent on the individual actually taking the measurements; a true change in body fatness over time may be missed entirely if the skin-fold readings are taken by different people or by the same person with sloppy technique. In addition, skin-fold measurements require ‘population-specific equations’ (formulae which take proper account of the unique characteristics of a particular sporting population – female swimmers, for example) to provide reliable estimates of percent body fat. There is no single equation that applies to the entire athletic world, and thus athletes, coaches, and sports medicine experts who rely on skin-fold measuring must be sophisticated about which mathematical formulae they use.

Because of these problems, athletes, coaches and trainers have searched for a way to evaluate percent body fat which requires little technical know-how and also keeps potential tester bias to a minimum. And for some people, the ‘Bod Pod’ Body Composition System seemed like the answer to their prayers. The Bod Pod uses a technique called air-displacement plethysmography (ADP) to estimate percent body fat quickly, non-invasively and easily – with no special expertise required by the tester.

But, although ADP has been found to be highly accurate in measuring the volume of inanimate objects, its use for determining the amount of fat in human subjects has been controversial. Some studies have indicated that ADP underestimates percent body fat in lean men and in young men and women in general. Other research has suggested that ADP might overestimate percent body fat by as much as 14% in lean women. Therefore, concern has arisen that ADP might have a particular gender or body-composition bias – or that it is simply incapable of providing reliable body-composition readings.

ADP shown to overestimate body fat…

To find out more about the accuracy and reliability of ADP (with specific reference to the Bod Pod device), researchers from Washington, DC and Michigan recently compared the technique with hydrostatic weighing and skin-fold measurement in 80 female college athletes – 32 track-and-field performers, 17 volleyballers, 17 softball players, four footballers and 10 rowers.

To obtain the Bod Pod measurement, each athlete sat in the Bod Pod chamber, wearing either Spandex shorts or a swim suit. Two measures of body volume were taken, each lasting about 35-45 seconds; if these first two measurements were more than 150 ml apart, a third was taken. The estimation of thoracic gas volume (critical for estimating body density) was determined by connecting the subject to the Bod Pod’s breathing circuit through an air filter and breathing tube; after 2-3 breathing cycles, the airway was momentarily closed, and the athlete gently ‘puffed’ three times during this time. Body weight was determined to the nearest 20g on a calibrated digital scale, and a special software programme then used the athlete’s weight, body volume and thoracic lung volume to calculate body density and percent body fat.

The hydrostatic weighing was carried out in an indoor pool. A scale suspended over the water was attached to a chair submerged in the water. Each athlete entered the pool and removed air bubbles from her swimsuit and body hair. While submerged and seated in the chair, the athlete exhaled maximally, and the trial ended when no more air could be exhaled and the two investigators taking the measurements had agreed on a weight within 20g. All the athletes performed a minimum of three trials until three measurements agreed within 20g, and the three heaviest weights were then averaged to calculate body density and percent body fat from standard mathematical formulae.

All the skin-fold measurements were taken by the same experienced scientist at each of the two institutions involved in the trial. Skin-fold thicknesses at the triceps muscle, suprailiac region, abdomen and thigh were determined with Lange skin-fold calipers, and percent body fat was then calculated from the appropriate, gender-specific equation outlined by the American College of Sports Medicine (1).

In the course of this study, the researchers unearthed some interesting differences in body composition between the athletes. The track athletes were by far the leanest, with an average body fat percentage of 15.7 (measured by hydrostatic weighing). The slimmest 75% of the track athletes averaged an even trimmer 13.6% body fat. By contrast, the women in every other sport checked in with average body fat percentages of 21 or more; the heftiest being the volleyball players and rowers, with average body-fat levels above 22%.

As it turned out, there was no significant difference between the hydrostatic-weight and skin-fold estimations of body fat, but percent body fat measured by the Bod Pod was significantly higher than by either of the others; (remember that hydrostatic weighing is considered the ‘gold standard’). This overestimation by the Bod Pod was a factor even when the athletes were divided into lean and normal subsets; the Bod Pod tacked on about 2% of extra body fat in the general population (from the 19% determined by hydrostatic weighing to 21%) and 3% in the leaner athletes (from 14 to 17%). The researchers concluded that the Bod Pod (and ADP in general) may not be valid tools for measuring percent body fat in female athletes, especially the leaner ones.

…and sometimes to err the other way

ADP has run into trouble in other studies, too. In one investigation carried out with fairly lean male college football players, ADP underestimated body fatness, yielding an average reading of 15.1%, compared with 17% for hydrostatic weighing (4). In another piece of research, ADP overestimated body fatness by 7% in women and underestimated it by 16% in men (8), and in a third study ADP underestimated fatness by almost 3% in men (9). In a very interesting investigation, in which the subjects were divided into lean, average and overweight groups, ADP did well with the average and overweight people but overestimated body fatness by 14% with the lean group, most of whom (81%) were female(6). There is thus a growing consensus that ADP is plagued by a gender bias – and that it doesn’t work well with lean athletes.

So what are the take-home lessons of this research? For one thing, college female athletes tend to have rather high levels of body fatness in a variety of different sports (other than track). Thus, many volleyball, softball and football players could, in theory, improve their jumping ability, alacrity in getting to the ball and general movement speed by reducing body fatness. Such reductions should be carried out sensibly, however, and without a corresponding loss in lean tissue. Regular strength training, reasonable amounts of ‘aerobic’ energy-burning exercise during training (as a complement to normal sport-specific work) and a diet rich in fruits and vegetables, adequate in protein and relatively low in fat should help to achieve this aim.

It is also important to note that skin-fold measuring worked well in this study, providing measurements of body fat as accurate as those obtained via hydrostatic weighing. Skin-fold measurements are, of course, much easier to carry out than hydrostatic weighing (getting that scale and submersible chair to the swimming pool is not always easy!) However, if you decide to use skin-fold assessments to monitor your change in body composition over time, make sure you use the same ‘tester’ (the person who takes your skin-thickness readings) each time, and be sure you use the correct formula to calculate your percent body fat (1). If you bear these points in mind, skin-fold assessment can work well for you, but Bod-Pod measuring of body fatness (and ADP techniques in general) can not be recommended for determining body composition accurately, especially if you are a relatively lean athlete.

Owen Anderson