High dietary sodium chloride consumption may not induce body fluid retention in humans

A commonly accepted hypothesis is that a chronically high-sodium diet expan ds extracellular volume and finally reaches a steady state where sodium int ake and output are balanced whereas extracellular volume is expanded. Howev er, in st recent study where the main purpose was to investigate the role o f natriuretic peptides under day-to-day sodium intake conditions (Heer M, D rummer C, Baisch F, and Gerzer R. Pflugers Arch 425: 390-394, 1993), our la boratory observed increases in plasma volume without any rise in extracellu lar volume. To scrutinize these results that were observed as a side effect , we performed a controlled, randomized study including 32 healthy male tes t subjects in a metabolic ward. The NaCl intake ranged from a low level of 50 meg NaCl/day to 200, 400, and 550 meq/day, respectively. Plasma volume d ose dependently increased (P < 0.01), being elevated by 315 +/- 37 mi in th e 550-meq-NaCl-intake group. However, in contrast to the increased plasma v olume, comparable to study I, total body water did not increase. In paralle l, body mass also did not increase. Mean corpuscular volume of erythrocytes , as an index for intracellular volume, was also unchanged. We conclude fro m the results of these two independently conducted studies that under the c hosen study conditions, in contrast to present opinions, high sodium intake does not induce total body water storage but induces a relative fluid shif t from the interstitial into the intravascular space.