Andrew Jones and his group at the University of Exeter are certainly busy lately. I’ve reported on two of their papers on nitrate and exercise performance here and here. They just published another in the Journal of Applied Physiology.
In their last 2 papers, they used beetroot juice as the source of nitrate, leaving a little room that it was other compounds in the juice that were having an effect. The first study, done by another group (Larsen et al.; see my first post on the topic) used sodium nitrate and found a reduced oxygen cost during cycling. Since a similar effect was observed with beetroot juice, it was likely that nitrate was the active compound. This was verified in this latest study by developing a process to selectively remove the nitrate from beetroot juice (BRJ). Another goal was to determine if increasing nitric oxide with BRJ increase mitochondrial biogenesis (theorized as one of the mechanisms by the authors in their first paper). Finally, since their first study was on cycling and their second with knee-extensions, they used walking and running (as well as knee-extensions to estimate mitochondrial involvement) as exercises in this study.
In a double-blind, randomized, cross-over fashion, they gave 9 active males 0.5 L/day of BRJ (~6.2 mmol nitrate; brand: Beet it) or nitrate-depleted beetroot juice as placebo (PL) for 6 days (10 day washout). Unlike the previous studies, the subjects didn’t have to refrain from eating nitrate-rich foods throughout the study. Walking/running tests on days 4 and 5 were performed, and knee-extensions were performed on day 6.
Plasma nitrite increased significantly in the BRJ group compared to PL (105%), and this peaked prior to day 4. Systolic blood pressure decreased 4% in the BRJ (BRJ: 124 +/- 10mmHg, PL: 129 +/- 9mmHg), while diastolic blood pressure and mean arterial pressure did not change. This reflects the previous studies.
During moderate-intensity exercise:
BRJ reduced VO2 during the baseline walking by ~12%, as well as during the last 30 seconds of moderate-intensity running by ~7%. The amplitude of the pulmonary VO2 response was also reduced by ~4%, and the oxygen cost of running 1 km by ~6%. VCO2, pulmonary ventilation, resting energy expenditure, heart rate, and blood lactate concentration were no significantly altered.
During severe-intensity exercise:
In the severe trial, BRJ again reduced VO2 during walking, by ~14% compared to PL, and by ~7% in end-exercise VO2 (end of 6 minute severe-intensity runs). At failure, VO2 was ~6% lower in the BRJ group. The amplitude of the primary VO2 response was not significantly different at ~3%, and the amplitude of the VO2 slow component wasn’t either. These were both different results than the previous studies. Most relevant to “real world,” BRJ increased time to failure by ~15%: BRJ: 8.7 +/- 1.8 min. vs PL: 7.5 +/- 1.7 min vs Control: 7.5 +/- 1.7 min. And again, VCO2, pulmonary ventilation, resting energy expenditure, heart rate, and blood lactate concentration were not different.
Knee-extensions were used to measure phosphocreatine concentration [PCr]. [PCr] recovery kinetics were not different between BRJ, PL, and control. Of the muscle metabolites phosphocreatine, phosphate, ADP, and pH, none were significantly altered. Qmax was estimated using 31P-MRS like their previous study, but it was unchanged. Task to failure was longer after the 6 days of BRJ consumption compared to PL and control: BRJ: 8.5 +/- 0.8 min, PL: 8.2 +/- 0.9 min, control: 8.2 +/- 0.9 min.
In summary, this study established that it does indeed seem to be the nitrate in beetroot juice that has these consistent positive effects on exercise. This one found that again plasma nitrite concentration increased, systolic blood pressure decreased, the oxygen cost of walking, moderate-intensity, and severe-intensity running was reduced, and the time to failure in severe-intensity running and the incremental knee-extensions was extended. The lack of an effect on Qmax suggests that the effects of nitrate are unlikely to be mediated through an increase in mitochondrial biogenesis by nitric oxide.
Unlike the other studies, they found in this one that at sub-maximal exercise the dietary nitrate was able to reduce the oxygen cost (of walking). They note that it isn’t clear what this means, but the implications of improving task completion for elderly and patients may be important. I would assume that the small subject numbers may be one reason for the discrepancies.
Smaller effects on VO2 compared to the other studies are suggested to be because other sources of dietary nitrate wasn’t restricted in this study, unlike the others. The authors didn’t estimate nitrate intake though (I think this would be difficult as dietary sources can vary significantly). A stat I found interesting is that in 4 days of BRJ consumption, the ~6% reduction in energy cost during moderate exercise is a comparable improvement in running economy to 6-9 weeks of actual training. Running economy is associated with improved endurance running (I am unsure how strong the link is).
As noted, the authors suggest a lack of change in the amplitude of VO2 slow component (unlike their previous studies) may be because it is smaller during running compared to cycling, making it more difficult to measure changes, or because of the 6% reduction at the point of failure (they cite differences between “whole-body” exercise and isolated muscles).
They estimate that the ~15% improvement in time to failure during the intense exercise is equivalent to a ~1% reduction in the time to cover a set distance, which may be “meaningful.” An improvement occurred in all 9 subjects, so even with the small sample size, the odds of this happening at random are low.
It should also be noted that many of the same researchers from this group just published a study on L-arginine supplementation at 6 grams per day in what appears to be the same design and found very similar results to this one. This increased nitrite concentration suggests increased nitric oxide availability, further corroborating the results of the other studies. Arginine is also high in many foods (including leafy greens) so I may have to do a separate post on that.
By the time their next one comes out I will try to have a table ready with all of these results…
Lansley, K., Winyard, P., Fulford, J., Vanhatalo, A., Bailey, S., Blackwell, J., DiMenna, F., Gilchrist, M., Benjamin, N., & Jones, A. (2010). Dietary nitrate supplementation reduces the O2 cost of walking and running: a placebo-controlled study Journal of Applied Physiology DOI: 10.1152/japplphysiol.01070.2010