Nitric oxide (NO) has many functions that influence our vascular system and immunity. Preliminary studies focused on the therapeutic usage of NO as a vasodilator and its benefits for individuals with vascular compromise. Research has revealed beneficial hemodynamic effects of arginine, an NO precursor, for patients with congestive heart disease . Arginine supplementation has also been shown to enhance exercise capacity in patients with stable angina . As a result, additional research has sought to evaluate NO precursors as a performance aid in healthy individuals and athletes. Topical NO has also been recently considered as a therapeutic agent in common tendinopathies.
NO is synthesized from arginine under enzymatic control of NO synthase (NOS). Three isoforms of NOS, including neuronal, inducible, and endothelial forms, exist and closely regulate NO production and concentration .
The two primary functions of NO include cell signaling and a nonspecific immune response. Its prototypical role in cell signaling is as a mediator in vascular dilation. Tight regulation of NO production to avoid cellular toxicity is strictly controlled through stimulus-induced calcium release. A calcium/calmodulin/NOS complex actively produces NO until calcium is reabsorbed . This regulation plays an important role in contractile activity of muscle fibers in addition to autoregulation of blood flow and glucose homeostasis .
NO also serves a major role in nonspecific host immunity similarly to superoxide. Macrophages contain the toxic properties that are released at sites of inflammation with resultant destruction of all local cells. This is a large differentiation from the specific cell-signaling response seen in vasodilatation .
Arginine supplementation has been the primary means of augmenting NO in individuals with disease and vascular compromise. Consequently, it has been a focus of studies in evaluating its effects on performance in athletes. Arginine is a nonessential amino acid involved in many physiologic functions including protein synthesis, detoxification of ammonia, and glucose metabolism.
A typical Western diet contains 3 to 6 g of arginine primarily derived from plant proteins. Following ingestion, the majority reaches the systemic circulation and is metabolized by the enzyme arginase and converted to ornithine or to NO via NOS [3,5]. Arginine most likely enhances protein anabolism indirectly via NO production. NO increases blood flow to muscles, augmenting the anabolic effects of other amino acids. Therefore, when supplementing subjects with additional amino acids, the benefits in postexertion protein synthesis may be enhanced [3,5].
The effect of exercise training on the enhancement of endothelial function has been well established. Repetitive exercise over weeks results in an up-regulation of endothelial NO activity. This is a not a localized, but a systemic response in endothelial function when a large muscle mass is regularly activated as in aerobic exercise. Although enhanced NO activity is implicated, the extent of its role in cardiovascular benefits is unclear. Furthermore, individuals with lower baseline or impaired endothelial function appear to be most responsive to training .
Performance is affected by multiple factors including nutrition, dietary supplementation, and adequate training. Performance enhancement in the endurance athlete through widely available ergogenic aids is continually changing. In general, confounding results from studies evaluating arginine supplementation has been influenced by multiple factors, including variability in dosage and administration of supplements as well as testing subjects with different athletic abilities.
The effect of arginine supplementation on endurance and exercise capacity was initially studied in animals with disease. Many preliminary studies focused on those with exercise intolerance or cardiovascular disease. When studying hypercholesterolemic (apolipoprotein E deficient) subjects, a significant increase in VO2max was found with L-arginine supplementation, as predicted . Further research in humans established benefits for compromised patients including enhanced functional capacity.
Some earlier studies on arginine aspartate supplementation in endurance athletes revealed little benefit to athletes. Colombani et al. , in a double-blind crossover study, evaluated 14 endurance-trained runners following daily supplementation for 2 weeks before a marathon. They found no significant difference in the accumulation of metabolic end products or respiratory exchange ratio. However, exercise training was not standardized among subjects.
The chronic effects of arginine supplementation in competitive, endurance athletes on performance have been studied with equivocal results. Abel et al. [9•] performed VO2max testing in 30 competitive cyclists and triathletes following daily supplementation of arginine aspartate for 1 month. Subjects were given a low or high dose of arginine or placebo. They found no statistically significant changes in peak performance among all groups. However, subjects did not take arginine immediately before exercise that may have positively affected performance. Furthermore, dosage was not adjusted for body mass index.
More recent research has focused on aerobic exercise and examined metabolic parameters as well as performance. Burtscher et al. , in a double-blind, placebo-controlled trial, evaluated 16 male collegiate students during submaximal exercise. Test subjects were given daily supplementation of arginine aspartate for 3 weeks and re-evaluated for cardiorespiratory responses and metabolic parameters. The study reported lower levels of blood lactate accumulation, and decreased oxygen uptake and carbon dioxide output in the arginine aspartate-treated group. These findings may result from the synergistic effect of both amino acids. The contribution of the NO pathway cannot completely account for their results. Limitations similar to other studies included various levels of baseline activity and lack of dosage adjustment for body mass index.
The effects of arginine supplementation may result in reduction of muscular fatigue through enhanced clearance metabolic end-products. In a double-blind, randomized crossover trial in eight recreational athletes performed by Schaefer et al. , subjects received 3 g of intravenous arginine 90 minutes before cycling to maximal power output. During exercise recovery, there was a statistically significant difference in blood lactate and ammonia levels compared with the placebo-treated athletes. Despite this finding, there was no significant effect on exercise-induced oxygen uptake and carbon dioxide production. Furthermore, performance enhancement could not be evaluated because exercise tests were standardized in a crossover design.
In summary, arginine may enhance performance in endurance athletes in combination with other supplements. Further studies with standardized oral supplementation and baseline activity level are needed to accurately evaluate performance.
The role of NO in enhancement of resistance training is as of yet unclear. Some studies have revealed promising findings for improvement of muscular resistance and fatigue. The exact mechanism for these preliminary findings is still unknown. Supplementation with arginine may increase resistance by enhancing NO-induced vasodilation. Conversely, exercise induced augmentation of NO may account for gains seen in resistance training.
It is well established that endurance-trained individuals, compared with sedentary controls, have enhanced blood flow to skeletal muscle for the same workload. However, local vasomotor reactivity in the skin following endurance training has not been well established. Boegli et al.  studied the microvascular effects following iontophoresis of acetylcholine and sodium nitroprusside on the forearm in endurance-trained and sedentary subjects. The subgroup of young subjects with at least 3 months of endurance training of greater than 4 hours per week displayed a significant increase in skin blood flow following sodium nitroprusside administration compared with the other groups. Older, trained athletes did not demonstrate a similar increase compared with sedentary controls. These findings may have implications for local development of muscle strength and resistance in selected populations.
Other studies have also evaluated the effects of arginine supplementation in noncompetitive healthy subjects. Santos et al.  measured muscular fatigue during continuous leg extension exercises with isokinetic dynamometry in 12 healthy men. Subjects did not regularly exercise prior to being tested. All subjects were tested in a control phase then subsequently tested following supplementation with arginine for 15 days. Results revealed a statistically significant difference in resistance to fatigue in all subjects following the second phase. When comparing the difference between the two phases, a 10% improvement in muscular resistance was found in over half of the subjects.
Topical NO therapy may be an option as an adjunct to rehabilitation for tendinopathies. Healing may be enhanced by NO stimulation of fibroblasts with subsequent collagen formation and remodeling. Other potential mechanisms include the local vasodilatory effect of NO with increased clearance of cellular mediators.
Extensor tendinopathy of the elbow may be responsive to many treatment strategies including relative rest, bracing, stretching, and strengthening through supervised rehabilitation. In spite of numerous treatment options, many cases remain resistant to typical regimens with presence of chronic symptoms. Paoloni et al.  evaluated the effects of topical glyceryl trinitrate in a 6-month prospective randomized trial. Both groups were formally instructed with a standard home rehabilitation program. At 2 weeks, the treatment group experienced a significant decrease in pain with activities compared with the control group. There was a not a significant effect with activity for the duration of the trial. Subjective outcomes at 6 months revealed a statistically significant difference in those reporting asymptomatic activities of daily living in the topical NO group compared with the control group.
Chronic supraspinatus tendinopathy may be responsive to transdermal NO therapy. Paoloni et al. [15•], in a prospective, randomized double-blinded trial, evaluated pain symptoms, muscular force, and range of motion following 6 months of transdermal therapy combined with standard rehabilitation. In those receiving daily transdermal patches, a significant decrease was found in pain symptoms at night and with activity following 6 months of treatment. Objective measures of supraspinatus strength improved significantly after 6 weeks. Limitations including using a home exercise program may have altered typical rehabilitation efficacy. Patients were also not followed after discontinuation of topical therapy. Therefore, the duration after cessation of treatment is unknown.
Topical application of glyceryl trinitrate has also been studied in patients with Achilles tendinopathy. In a randomized, double-blind placebo-controlled trial, Paoloni et al. [16•] evaluated 65 patients with chronic noninsertional Achilles tendinopathy for 6 months. Subjects were treated with either a glyceryl trinitrate patch or placebo in combination with standard rehabilitation. A significant improvement in pain with activity and at night was found in the experimental group after 12 weeks. Improvement in pain with activity remained significant for the duration of the trial. Limitations similar to previous studies were found including lack of assessment following treatment. Furthermore, an increased incidence of headaches in the NO-treated group, requiring acetaminophen, may have influenced the double-blind effect.
In summary, recent evidence has revealed potential benefits of topical glyceryl trinitrate therapy in reducing pain with activity. Common side effects of topical therapy including headaches, rash, and weakness may be a deterrent for many patients. Although some studies have found a similar incidence in headaches compared with control groups . Nonetheless, topical treatment may be an option in the future for patients with resistant symptoms or those unable attend physical therapy.
NO has been touted to have many beneficial effects. NO therapy in the form of arginine supplementation has been well established to benefit those with cardiovascular compromise or endothelial dysfunction, and recent studies have revealed possible benefits of enhanced performance for endurance athletes at various levels. Scientific evidence is lacking to conclude that a positive ergogenic effect exists. Major limitations of previous studies include small subject numbers and lack of standardization of previous activity level. Many studies have also used the supplement arginine aspartate making it difficult to evaluate the effect of the NO pathway because aspartate most likely acts synergistically with arginine. Similar problems exist when evaluating the effects of NO on resistance training. Recently, topical therapies aimed at targeting the NO pathway have been studied. Positive preliminary findings may indicate a future role in the treatment of tendinopathies. Future studies assessing both the therapeutic and ergogenic potential of NO are needed.
References and Recommended Reading
Papers of particular interest, published recently, have been highlighted as: • Of importance •• Of major importance
1. Koifman B, Wollman Y, Bogomolny N, et al.
: Improvement of cardiac performance by intravenous infusion of L-arginine in patients with moderate congestive heart failure.J Am Coll Cardiol
2. Ceremuzynski L, Chamiec T, Herbaczynska-Cedro T: Effect of supplemental oral L-arginine on exercise capacity in patients with stable angina pectoris.Am J Cardiol
3. Campbell BI, LaBounty PM, Roberts M: The ergogenic potential of arginine.J Int Soc Sport Nutr
4. Marletta MA, Spiering MM: Trace elements and nitric oxide function.Am Soc Nutr Sci
5. Paddon-Jones D, Borsheim E, Wolfe RR: Potential ergogenic effects of arginine and creatinine supplementation.J Nutr
6. Maiorana A, O'Driscoll G, Taylor R, Green D: Exercise and the nitric oxide vasodilator system.Sports Med
7. Maxwell AJ, Ho HV, Le CQ, et al.
: L-arginine enhances aerobic exercise capacity in association with augmented nitric oxide production.J Appl Physiol
8. Colombani PC, Bitzi R, Frey-Rindova P, et al.
: Chronic arginine aspartate supplementation in runners reduces total plasma amino acid level at rest and during a marathon run.Eur J Nutr
9.• Abel T, Knechtle B, Perret C, et al.
: Influence of chronic supplementation of arginine aspartate in endurance athletes on performance and substrate metabolism.Int J Sports Med
Randomized controlled trial evaluating VO2max following daily arginine supplementation in competitive cyclists.
10. Burtscher M, Brunner F, Faulhaber M, et al.
: The prolonged intake of L-arginine-L-aspartate reduces blood lactate accumulation and oxygen consumption during submaximal exercise.J Sport Sci Med
11. Schaefer A, Piquard F, Geny B, et al.
: L-arginine reduces exercise-induced increase in plasma lactate and ammonia.Int J Sports Med
12. Boegli Y, Gremion G, Golay S, et al.
: Endurance training enhances vasodilation induced by nitric oxide in human skin.J Invest Dermatol
13. Santos RS, Pacheco M, Martins R, et al.
: Study of the effect of oral administration of L-arginine on muscular performance in healthy volunteers: an isokinetic study.Isokinet Exerc Sci
14. Paoloni JA, Appleyard RC, Nelson J, Murrel GAC: Topical nitric oxide application in the treatment of chronic extensor tendinosis at the elbow.Am J Sport Med
15.• Paoloni JA, Appleyard RC, Nelson J, Murrell GAC: Topical glyceryl trinitrate application in the treatment of chronic supraspinatus tendinopathy.Am J Sport Med
A prospective, randomized, double-blind controlled trial evaluating pain and function in patients with supraspinatus tendinopathy following 6 months of transdermal NO therapy.
16.• Paoloni JA, Appleyard RC, Nelson J, Murrell GAC: Topical trinitrate treatment of chronic noninsertional Achilles tendinopathy.J Bone Joint Surg
A randomized, double-blind placebo-controlled trial evaluating pain in patients with chronic noninsertional Achilles tendinopathy during 6 months of transdermal NO therapy.