Race-related differences in tissue dielectric constant measured noninvasively at 300 MHz in male and female skin at multiple sites and depths.

Background/purpose: We hypothesized that reported race-related differences in skin properties cause skin and skin-to-fat water differences among races that are measureable by skin tissue dielectric constant (TDC) values that depend strongly on water content. Our first aim was to test this hypothesis. Also, since inter-side TDC ratios are used to assess edema and lymphedema, the second aim was to test if TDC ratios are race-dependent. The third aim was to determine the extent to which TDC depends on total body water (TBW) and fat (TBF).

Methods: Tissue dielectric constant was measured to 1.5 or 5.0 mm depths bilaterally on chest, forearm and ankle in 100 young (19-39 years) adults with 10 male and 10 female per self-expressed race. Races were African-American, Asian, Asian-Indian, Caucasian and Hispanic groups. TBW and TBF were measured using bioimpedance.

Results: Tissue dielectric constant values decreased from chest to forearm to ankle (P<.001) independent of race with most values greater for males but with inter-arm TDC ratios independent of gender, site, depth, or race. For females, forearm TDC values differed among races (P<.01) with Asian and Asian-Indian values tending to be least. For males, chest TDC values differed among races (P<.01) mainly due to large African-American TDC values. For the composite group, TDC was strongly (P<.001) positively correlated with TBW and negatively correlated with TBF.

Conclusions: Tissue dielectric constant dependence on race of the type herein uncovered should be considered in assessing skin hydration comparisons that include different race or ethnic subjects. Further, the demonstrated relationship between TDC and body composition should be considered as an important covariate. However, despite these variations, the inter-arm TDC ratio remains robust as a potential indicator of unilateral tissue water changes.