Nitric oxide (NO) supplements are very popular in the sports and bodybuilding community. The NO molecule has been found to play an important role in many functions in the human body including mitochondrial respiration (and hence energy metabolism), blood flow, vasodilation and implicated in my own research, neuronal functions and developments. Nitric oxide is synthesized via two physiological pathways, I won't bore you with the details, all you need to know here is that L-arginine acts as a main precursor of the first pathway whereas nitrate is the substrate used to produce NO by the second pathway. It was hypothesized that NO supplementation can enhance oxygen and nutrient delivery to active muscles and hence improve performance. Truth or myth, let's find out.
L-arginine works on the first NO pathway and it has been proposed that taking L-arginine can increase NO levels and subsequently exercise performance. However, oral L-arginine supplementation has been found to unable to elevate NO levels in the test subjects and did not improve strength performance (Alveare et al 2012, Applied Physiology, Nutrition and Metabolism). Other studies further suggested that the supplementation of L-arginine had no effects on the hormone and NO levels in the body and had no effect on performance (da Silva et al 2014, Food and Nutrition Research; Zajac et al 2010, Journal of Strength and Conditioning). There are some contradicting reports showing L-arginine has a somewhat measurable effect on hormone levels and exercise performance. However, the general consensus of the scientific community is that there is a lack of concrete evidence to support the claim that oral L-arginine supplementation has a positive effect on NO levels in the body and exercise performance.
Nitrate is converted into nitrite after ingestion and can be converted into nitric oxide when the body's oxygen availability is low. The supplementation of nitrate has been shown to lower oxygen demand during submaximal workout, and improves exercise efficiency (Larson et al 2007, Acta Physiologica). It was suggested by one study that nitrate should be consumed 2-3 hours prior to competition or training for maximum benefits (Jones et al 2012, Medicine and Sports Science).
One of the common flaws of the studies involving NO supplements and exercise performance is that only young males were used as subjects. To my knowledge, the effects of NO supplements on exercise performance in older males or women have not been yet been explored as of today. The effect of oral L-arginine supplementation is debated and the outcome is not conclusive. The use of nitrate supplements has shown to improve exercise performance in some and may be used for their ergogenic potentials.