The new research was published in the Journal of the International Society of Sports Nutrition. It was the work of researchers associated with Indiana University Purdue University Indianapolis (IUPUI) and the Indiana University Bloomington.
Nitrate is one of the dietary ingredients endorsed by World Athletics as being generally regarded as boosting performance. The others are caffeine, creatine, B-alanine and bicarbonate. Dietary nitrate is found most prominently in beetroot juice and powder, but also can be found in pomegranate, watercress and red radish.
A previous meta analysis also published in JISSN found that nitrate supplementation may boost endurance. But those researchers concluded that the evidence was only of moderate quality.
In the view of the authors of the present meta analysis, the effects of nitrate supplementation on endurance exercise is nominal at best. But they said its effect on muscular power output is more promising and was worth delving into the studies to try to quantify this effect across a broad spectrum of research.
Uncertain mechanism of action
The authors of the present study said one issue with nitrate supplementation is the uncertainty about the mechanism of action. The common thought has been that nitrate supplementation increases the amout of nitric oxide (NO) available in the body.
“The resultant increase in NO bioavailability then decreases O2 demand and enhances performance during sustained exercise, seemingly by improving mitochondrial coupling and/or by reducing ATP turnover itself. Other studies, however, have failed to find any effects of NO3− supplementation on mitochondrial function in either mice or humans, leaving a decrease in ATP utilization, possibly at the cross-bridge level, as a more plausible mechanism,” they wrote.
The researchers said that looking at power output leaves aside the role of ATP as it is minimally involved in brief contractile activities. They noted that animal studies have shown that nitrate supplementation can increase the speed of muscular contractions while having minimal effect on total force.
Thus power, which is a function of force and velocity, is increased. When a 100 pound barbell is lifted off the floor to chest height over three seconds or over half a second, the same amout of work has been done. But in the second example, it was done with more power output.
Total of 268 subjects
For their meta analysis, the authors selected 19 studies with a total of 268 participants, weighted more heavily toward men (218 males, 50 females). All of the studies they included were double blind and placebo controlled and looking at peak or maximal neuromuscular power. The included studies also measured power output in major muscle groups when those muscles were not fatigued.
To judge the quality of the evidence, the authors judged the studies they included against a metric developed by the American Heart Association in 2017 and published in the journal Circulation.
Some of the weaknesses of the 19 studies included a lack of precision on the nitrate dose as well as use of a lower quality placebo. Some studies used colored water as a placebo, whereas the best (and presumably much more expensive) placebo is a beetroot juice from which the nitrate has been removed.
In any case, the researchers felt confident in their conclusion that nitrate supplementation can in fact boost peak power output and the effect was substantial, falling in the 5% range.
“Based on the currently available literature, this ergogenic effect is seemingly independent of subject age, sex, or the amount of muscle mass engaged in the activity but may be greater with acute vs. repeated dosing. Importantly, this dietary NO3-induced increase in power is sufficient to have important practical and clinical implications. Further research to determine the optimal supplementation regimen, target population, etc., is therefore imperative,” they wrote.
Source: Journal of the International Society of Sports Nutrition
18, Article number: 66 (2021)
Effect of dietary nitrate on human muscle power: a systematic review and individual participant data meta-analysis
Authors: Coggan AR, et al.