Dietary nitrate, muscle metabolism, and physical performance

I posted previously about 2 studies that suggest dietary nitrate enhances some measures of performance.  Another by one of the same groups (Bailey et al.) reproduces these findings and further explores mechanisms.

For a background of the existing speculated mechanisms, see the last post.  This study used a different technique: Phosphorus-31 magnetic resonance spectroscopy (31P-MRS), which assesses muscle metabolism noninvasively.  They chose this to examine muscle phosphocreatine (PCr) concentration, ADP concentration, and pH, along with pulmonary VO2 dynamics (reflects oxygen consumption), to estimate total ATP turnover rates and which fuel sources contributed at what extent from PCr hydrolysis, glycolysis, and oxidative phosphorylation.  They speculated that nitrate would reduce the O2 cost of exercise like established previously, but intramuscular PCr degradation would also reduce, suggesting phosphate cost of force production would also decrease, without a pH change.  Because of a sparing effect of PCr concentration, high-intensity exercise should be prolonged.

7 healthy males were asked like the other studies to refrain from high nitrate foods during the study.  After some initial baseline testing which established work rates, they were randomly assigned, in a double-blind, crossover fashion, 6 days of 5.1 mmol/day nitrate (0.5 liter organic beetroot juice; same product as previous study) or a placebo (low-calorie black-currant juice cordial with negligible nitrate).  There was again a 10-day washout between the periods.  The authors noted that this study was performed before their first study was published, so the subjects did not know then that beetroot juice might be ergogenic.

On days 4 and 5 of 6 of the supplementation periods, step exercise tests from baseline to low and high-intensity work rates tested VO2 dynamics and muscle activity.  Day 4 included a high-intensity rate until failure.   EMG data among others (e.g. blood pressure) were taken as well.

On day 6, the testing protocol from day 4 was repeated while undergoing 31P-MRS.

The subjects reported beeturia and red stools corroborating other studies (a good sign they were consuming it as well I suppose).

Subjects consuming the beetroot juice had greater nitrite concentrations, as expected, as well as a significantly lower average systolic blood pressure. 

For both low and high-intensity exercises, muscle activity as measured by iEMG were not different between groups, nor was heart rate after exercise. 

Low-intensity exercise (knee extension)

Like the previous studies show, this one also found that the nitrate group had a 25% reduction in the increase in pulmonary VO2 from rest to low-intensity, and VO2 and the end of exercise was reduced. Resting was not different.  End of exercise CO2 production (VCO2) was not different. VE (pulmonary ventilation) was not different, suggesting along with heart rate that nitrate exerts its effects only on the skeletal muscle, not from an indrect reduction of energy cost of cardiorespiratory processes.  Resting energy expenditure had a small but insignificant increase in the nitrate group, which may suggest a shift in substrate utilization toward more carbohydrate use through NO-mediated glucose uptake in the skeletal muscle cells, but this will have to be further studied.  Work efficiency was also greater in the nitrate group. 

The nitrate group had a 36% reduction in the amplitude of PCr degradation, and phospate concentration accumulation was reduced by 21%.  The amplitude of ADP concentration was reduced, while pH was not different between the groups.

Here are the estimated ATP-derived fuel sources (white bars = nitrate group, grey = placebo).  Reductions in total ATP, oxidative phosphorylation-derived ATP, and PCr-derived ATP were statistically significant:

High-intensity exercise (knee extension)

Primary component VO2 amplitude had a tendency to be lower in the nitrate group, which is in contrast to the previous study with cycling.  This may be related to the intensity, specific exercise, or body positioning.  The amplitude of the slow component was reduced by 52%.  End of exercise VCO2 were not different.  Work efficiency was greater in the nitrate group.

The concentration of PCr slow component amplitude was reduced by 59% in the nitrate group.

Here are the estimated ATP-derived fuel sources (white bars = nitrate group, grey = placebo).  Reductions in total ATP, oxidative phosphorylation-derived ATP, and PCr-derived ATP were statistically significant (consistent with the low-intensity group):


Exercise Tolerance

There was a 25% increased time to failure in the nitrate group in the exercise tolerance tests (nitrate: 734 +/- 109 seconds vs. placebo: 586 +/- 80 seconds), with all 7 subjects improving.  This tended to be correlated to the PCr concentration remaining.

Conclusions

Again, nitrate reduced the oxygen cost of exercise (at both exercise intensities), but this study found that PCr degradation is also reduced during both low and high-intensity exercise without a change in muscle pH.  Total ATP was also reduced with nitrate.  These are consistent with the previous hypothesis that nitrate improves the efficiency of exercise through reducing the ATP cost of producing force.  The tolerance of high-intensity exercise may be maintained by the preservation of PCr concentrations by nitrate, supported by other research.

Regarding the ATP determinations, the authors note that these estimations use a number of assumptions and may be subject to error, but along with a reduction in the VO2 suggests that PCr and oxidative metabolism contributions were reduced, suggesting an increased efficiency in force production rather than an increased mitochondrial P/O ratio (ATP synthesis at a given oxygen consumption). 

Interestingly, they also note that evidence of altitude training improving submaximal endurance efficiency may support these results as people living at high altitudes have much greater concentrations of nitric oxide products, including nitrite, so this might reduce their energy cost of activity.

In all, much of this study corroborated their other, and it will be interesting to see what they look at next.

 

A final note: in the last post I noted a study on creatine supplementation that changed oxygen uptake kinetics.  I failed to notice the lead author Andrew Jones from that one is also involved in these studies.  With brief overviews the designs are very similar to these 2 nitrate studies (the first used cycling, and this used knee-extensor).  Creatine has been well established to be an ergogen, it seems it and nitrate may have some overlap in their mechanism.  I may investigate this in a future post and email Dr. Jones for his thoughts.

Cautious Interpretations

This is the 3rd study showing dietary nitrate may effect markers or tests of endurance performance.  However, each study is very small in subject numbers and requires replication in different contexts as well as larger studies.  Also, as stated in the other post, it remains to be directly tested if dietary nitrate can improve time to failure at submaximal exercise (e.g. long distance endurance), and the real world implications of improving time to failure at severe exercise is probably limited.  As noted by the authors, if the goal is sprinting (going from one point to another as fast as possible), the improvement would likely be much smaller.

Nevertheless, this is interesting stuff and important in a number of ways.

Reference

Bailey SJ, Fulford J, Vanhatalo A, Winyard PG, Blackwell JR, Dimenna FJ, Wilkerson DP, Benjamin N, & Jones AM (2010). Dietary nitrate supplementation enhances muscle contractile efficiency during knee-extensor exercise in humans. Journal of applied physiology (Bethesda, Md. : 1985) PMID: 20466802