Disparate Hydration of Adipose and Lean Tissue Require a New Model for Body Water Distribution in Man

Abstract

The intra and extracellular fluid spaces in adipose tissue were determined by ³H₂O, ⁸²Br, and ³⁵SO₄⁼ uptake, and by appropriate chemical methods, in surgical biopsy material from 16 patients undergoing elective laparatomy. Total water space for adipose tissue was 14 ± 1.4%; extracellular component was 11 ± 1.1% in mesenteric and subcutaneous depots. Use of these adipose tissue hydration constants, combined with measurements of total body water (TBW), extracellular water (ECW), total body potassium, and experimentally derived age-specific constants for lean body potassium content, permits development of a four compartment model for body water which considers intra and extracellular components separately for adipose (AT) and adipose-free (AFM) tissue masses. This model has the form TBW = 0.14 (AT) + x(AFM), and ECW = 0.11 (AT) + y(AFM) where x is the total hydration and y the extracellular hydration of the adipose-free mass. The equations can be solved for the ECW and ICW of the adipose-free mass, defined by its potassium content. In four normal subjects, x was measured as 0.80 ± 0.032, and y as 0.24 ± 0.017. Thus, the adipose free mass, compared with adipose tissue, has approximately six times the total water and twice the extracellular water content per unit weight.