Evaluating CMR

Indices of Total Adiposity – Densitometry (Hydrostatic Weighing and Air Displacement Plethysmography

Densitometry estimates body composition based on body density, which is the ratio of body mass to volume. Because fat is less dense than lean tissue, the lower the body density, the higher the percent body fat. Historically, body density was estimated using hydrostatic weighing and was once considered the gold standard measure for body composition. Hydrostatic weighing entails weighing an individual while they are submerged after a maximal exhalation. The major source of error in measuring body density is associated with the subject’s ability to maximally exhale when the measure is taken (46). In addition, hydrostatic weighing is labour intensive and requires high subject compliance, making this measure impractical for some diseased populations, the elderly, and young children (29). More recently, the advent of air displacement plethysmography—commonly known as the BodPod®—has offered an alternative method for assessing body density that requires much less effort by users and patients (47).

The body fat estimates of both air displacement plethysmography and hydrostatic weighing are similarly repeatable (coefficient of variation <5%). In theory, air displacement plethysmography and hydrostatic weighing should provide the same estimates of body fat, as they are based on the same principles of densitometry. However, studies that have compared air displacement plethysmography and hydrostatic weighing report that their body fat estimates vary considerably (~9-16%). However, there does not appear to be any systematic differences in their measures, with studies reporting that air displacement plethysmography can under or overestimate percent body fat compared to hydrostatic weighing (47). As such, measures by air displacement plethysmography and hydrostatic weighing are not interchangeable and more research will be needed to understand where the discrepancies between the measures lie. However, from a clinical standpoint, it is interesting to note that percent body fat as measured by air displacement plethysmography is reported to be no better than BMI or waist circumference in predicting health risk (48).

There are several techniques available to quantify total and regional adiposity. These range from relatively simple measures such as BMI, skinfolds, and BIA to more complicated measures such as densitometry. Currently, there is little evidence to suggest that these measures improve prediction of health risk beyond that of waist circumference alone.