Beetroot Juice Supplementation Does Not Improve Mitochondrial Efficiency or ADP Sensitivity in Humans

Abstract

Ingestion of sodium nitrate (NaNO3) simultaneously reduces whole-body oxygen consumption (VO2) during sub-maximal exercise while improving mitochondrial efficiency, suggesting a causal link. Consumption of beetroot juice (BRJ) elicits similar decreases in VO2 but potential effects on the mitochondria remain unknown. Therefore we examined the effects of 6 day supplementation with BRJ (280 ml/d, ~26 mmol NO3-) in young active males (n=10) who had muscle biopsies taken Pre and Post supplementation for assessments of mitochondrial bioenergetics. Subjects performed 20 min of cycling (10 min at 50% and 70% VO2Max) 48 h prior to receiving biopsies, and on day 4 of supplementation. Mitochondrial respiration in permeabilized muscle fibres showed no change in oligomycin-leak respiration, maximal substrate-supported respiration, or ADP sensitivity (apparent Km). To compare the effects of BRJ to previous findings with NaNO3, 5 subjects had muscle taken for isolation of subsarcolemmal and intermyofibrillar mitochondria. Neither subpopulation showed an improvement in mitochondrial efficiency assessed by the oxygen consumed/ATP produced (P/O Ratio), or in mitochondrial coupling assessed by respiratory control ratio (RCR). Membrane potential determined fluorometrically using safranine-O during respiration was not different Post vs. Pre supplementation. In summary, in contrast to NaNO3, BRJ supplementation did not alter key parameters of mitochondrial function. This occurred despite a decrease in exercise VO2, suggesting that the ergogenic effects are not due to a change in mitochondrial coupling or efficiency. Supported by NSERC, Canada.