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In 1992, the prestigious scientific journal Science named nitric oxide (NO) the “Molecule of the year.” The reason it got this distinction was the new understanding of nitric oxide as an incredibly important signal, especially for causing the dilation of blood vessels. Because of the enormous clinical implications of this mechanism (think hypertension, other cardiovascular disease and even erectile dysfunction), three US scientists received the Nobel Prize for Physiology and Medicine in 1998. Part of the importance of NO stemmed from its ease of being targeted pharmacologically. In fact, it was later found that many popular drugs that were already being used (nitroglycerine for example) were actually targeting the NO pathway. Because of the importance of blood flow, and hence oxygen delivery, to endurance exercise performance it was only a matter of time before NO manipulation emerged as a potential site of performance enhancement.
It was recognized relatively early that NO was formed when an enzyme named nitric oxide synthase (NOS) oxidized L-arginine (an amino acid) to NO. The NO derivatives nitrite (NO2-) and nitrate (NO3-) were byproducts of this reaction. Only more recently was it understood that in addition to the reaction catalyzed by NOS, there were reactions in which nitrate could be reduced to nitrite and then further reduced to NO (a scientific aside - these are redox reactions in which there are oxidations and reductions based on electron transfer). Since some food sources, particularly leafy vegetables, are high in dietary nitrate, providing dietary nitrate through the diet became a potential way to increase NO availability and hence blood and oxygen delivery.
The first evidence of potential exercise benefits was provided by a study conducted in Sweden in which dietary nitrate in the form of sodium nitrate was able to reduce the oxygen cost of a submaximal exercise bout. In other words, the subjects were able to do the same amount of work when cycling albeit at a lower oxygen cost (increased efficiency). These results were expanded further to dietary sources of nitrate, such as beetroot juice, with similar findings. One important study used beetroot juice with nitrate removed as a placebo control and found that the performance benefits were dependent on nitrate being present. Even though it was thought that dietary nitrate could improve exercise performance by increasing oxygen delivery, these study showed that exercise performance increased by an alternative mechanism of increased muscular efficiency.
What followed was a series of investigations in a variety of exercise modes, at different exercise intensities, and with different populations from untrained to highly trained individuals. Under a variety of conditions, these studies have consistently shown an increase in exercise performance, as measured by time to exhaustion or time trial performance. Although the exact mechanisms are yet to be fully understood, it is thought that the improvement in exercise efficiency could be because of a reduced energy cost of muscle contraction and/or enhanced mitochondrial function. The laboratory of Andrew Jones at Exeter University in the UK has been particularly influential in this area of study.
Besides the mechanism of action, there is still one great unknown with dietary nitrate supplementation – do truly elite athletes benefit from its use. It is known that a great many elite athletes use beetroot juice to enhance performance. However, studies in elite athletes are few (elite athletes are by definition rare), and inconclusive. It is clear that those at the sub-elite level (the vast majority of us) can gain a performance benefit from dietary nitrate, but at what performance ability that might stop is yet unknown. It is thought that the truly elite are operating at such a high limit that the margin of potential benefit is small. However, even in the studies of elite athletes that have inconclusive results, there are some individuals that show a clear benefit, while others do not. Therefore, it is a classic case of “responders” and “non-responders” that may be determined by ones normal nitrate/nitrite levels.
Although this column is usually not one to support supplementation, at this point there is pretty clear evidence that dietary nitrate, like that found in beetroot juice, can improve exercise performance. There is additional evidence (not gone into here) that this could be even more beneficial at high altitude, which a consideration in Colorado (although maybe not Fort Collins, which is considered moderate altitude). The research on the topic is still emerging and new products will continue to hit the market since many are on the beetroot bandwagon. To help you make informed decisions, I have included a link to practical recommendations from Dr. Jones: Beetroot recommendations.