Spleen contraction and Hb elevation after dietary nitrate intake

Abstract

Engan H, Patrician A, Lodin-Sundstr€om A, Johansson H, Melin M, Schagatay E. Spleen contraction and Hb elevation after dietary nitrate intake. J Appl Physiol 129: 1324–1329, 2020. First published October 8, 2020; doi:10.1152/japplphysiol.00236.2020.—Ingestion of dietary nitrate (NO-3 ) is associated with improved exercise tolerance and reduced oxygen (O2) cost of exercise, ascribed to enhanced mitochondrial efficiency, muscle contractile function, or other factors. Nitrate ingestion has also been found to attenuate the reduction in arterial oxygen saturation (SaO2) during apnea and to prolong apneic duration. The spleen serves as a dynamic blood pool expelling erythrocytes into the circulation during apnea, and NO-3 and nitric oxide donors may induce vasoactive effects in the mesenteric and splanchnic circulation. Our aim was to investigate the effect of ingestion of concentrated organic NO-3 -rich beetroot juice (BR) on spleen volume and spleen contraction during apnea, and the resulting hemoglobin (Hb) concentration. Eight volunteers performed two apneas of submaximal and maximal duration during prone rest ~2.5 h after ingesting 70 mL of BR (~5 mmol NO-3 ) or placebo (PL; ~0.003 mmol NO-3 ), on separate days in weighted order. Heart rate and SaO2 were monitored continuously and spleen diameters were measured every minute for triaxial volume calculation. Capillary Hb samples were collected at baseline and after the maximal apnea. Baseline spleen volume was reduced by 66 mL after BR ingestion (22.9%; P = 0.026) and Hb was elevated (+3.0%; P = 0.015). During apneas, spleen contraction and Hb increase were similar between BR and PL conditions (NS). The study shows that dietary NO-3 reduces spleen volume at rest, resulting in increased Hb. This spleen-induced Hb elevation following NO-3 ingestions represents a novel mechanism that could enhance performance in conditions involving exercise, apnea, and hypoxia. NEW & NOTEWORTHY This is the first study to examine changes of spleen volume and circulating Hb following dietary NO-3 supplementation. After dietary NO-3 ingestion, the spleen volume at rest was reduced and Hb was elevated. The spleen contains a dynamic red blood cell reservoir, which can be mobilized and facilitate oxygen transport during various types of physiological stress. This study has revealed an additional, previously unexplored mechanism possibly contributing to the ergogenic effects of dietary NO-3.