A Safe Supplement to Improve Exercise Performance

Man Mountain BikingElite athletes constantly strive to improve performance through training techniques, diet and supplementation. And in many cases, the performance improvements achieved by athletes through these “enhancements” have been shown to be effective for us lesser mortals who regularly work out, and even those who don’t.

When it comes to supplementation, there is much controversy surrounding performance-enhancing substances such as anabolic steroids, human growth hormones, and exotic concoctions like recombinant human erythropoietin. While these substances do seem to boost athletic performance, they also come with significant side effects, and unless prescribed by a physician to address a specific health concern, are not suggested for general use.

However, there are forms of dietary supplementation that are safe and effective for general use that have been shown to enhance exercise capacity and reduce exercise fatigue in a mix of study populations, including elite athletes, healthy adults who regularly exercise, healthy adults who don’t regularly exercise, and people with chronic cardiovascular and respiratory health conditions. The natural ingredients that fall in this latter category are ones that we consume in food or are produced naturally by our bodies and are known broadly as antioxidants.

During exercise, our bodies burn more energy than they do at rest in order to meet the increased demand we place on our muscles and vital organs to act. In part, the process to create energy in our bodies involves splitting oxygen molecules we’ve inhaled. One part of the oxygen is used in energy production while the other part of each oxygen molecule is left over. Leftover oxygen molecules are known as “pro-oxidants” or “free radicals,” and unless they are quickly matched and neutralized by “antioxidant” molecules that circulate through our bodies, the free radicals damage our cells.

Free radical damage during exercise induces fatigue, reduces muscle power and prolongs muscle recovery. Therefore, it has been theorized by sports nutrition researchers that increasing antioxidant consumption prior to exercise sessions might help reduce free radical damage and the associated fatigue/performance/recovery effects. A number of the antioxidants tested by researchers have shown promising results in the different study populations mentioned above. In fact, we wrote about several in our free report, Top 7 Supplements for Lifetime Vitality.

The Power of Antioxidant NAC

One of these antioxidants is N-acetyl cysteine (otherwise known as NAC). NAC is an amino acid that our bodies break down and process into glutathione. Glutathione is the most prevalent antioxidant found in human tissue and is heavily used to counter free radicals in muscle cells. Previous research studies have confirmed that bouts of exercise reduce the amount of glutathione found in muscle cells post-exercise, and so a number of attempts have been made to examine the benefit of supplementing NAC prior to exercise to see if it boosts glutathione in muscle cells during and after exercise, and if this increased amount of glutathione has a measurable effect on fatigue, performance and/or recovery.

For example, a new study published this month online ahead of print in the journal Respiratory Physiology and Neurobiology evaluated the effect of oral supplementation of NAC to seven young adults (average age of 20) who engaged in bouts of exercise at different intensity levels. Among other measures, the researchers sought to determine whether NAC was effective in extending the length of time the study subjects could exercise before reaching exhaustion (known as “time-to-fatigue”), and the maximum power subjects could maintain during exercise without fatiguing (known as “critical power”). Further, they desired to know whether NAC was effective at all exercise intensity levels.[1]

To assess these two measures, the study subjects first were tested for their maximum exercise capacity utilizing a stationary exercise bike known as a cycle ergometer. Then, on eight subsequent visits, the study subjects were asked to cycle at 80%, 90%, 100% and 110% of their initially established maximum exercise capacity for as long as they could before reaching exhaustion. Each visit was spaced 72 hours apart to allow for muscle recovery and for the tested supplements to fully wash out in between visits.

For half of the eight trials, the seven subjects received a placebo pill one hour prior to exercise. For the other half of the trials, subjects received 70mg of NAC per kilogram of body weight one hour prior to exercise. Given that the average body weight of the study participants was 89 kilograms (196 lbs), the study subjects on average received a total pre-exercise session dosage of 6,200 milligrams. The placebo and NAC were provided to study subjects each session without the primary researchers’ knowledge of which subjects received which pill. This type of study is referred to as a placebo-controlled, double-blind crossover trial. It is viewed as the most reliable type of study in that it removes researcher bias (by blinding researchers as to which subjects receive which supplements) and study subject bias (by testing all subjects with both supplements).

Before, during and after exercise, the researchers drew blood samples to assess the amount of free-floating glutathione. They also measured time-to-fatigue and critical power for each exercise session, along with a few other measures of exercise performance.

The blood sample results showed that the NAC supplement did break down into free-floating glutathione within one hour of ingestion, and that study subjects who received NAC had 64% more glutathione in their blood samples in comparison to when the same subjects received the placebo.

The exercise tests revealed that NAC supplementation significantly increased time-to-fatigue in the “80% of maximum exercise capacity” test, but did not significantly increase time-to-fatigue in the three other higher intensity tests implying that NAC supplementation is effective at more moderate intensity levels than higher intensity levels. In the 80% test, subjects receiving NAC experienced an average 21% improvement in time-to-fatigue.

By contrast, NAC improved critical power in all four exercise intensity levels, but by a small amount (3%). However, the researchers considered this small improvement to be statistically significant.

Based on these results, the researchers concluded, “This systematic evaluation of exercise tolerance across a range of severe work intensities demonstrated a significant increase in time to fatigue at 80% Pmax and subsequent increase in critical power (CP) with oral NAC supplementation, which was accompanied by a tendency for changes in electromyographic responses, but not in VO2 kinetics. This degree of improvement in CP and thus on time to fatigue seen in this study, while deceptively small, could have dramatic effects in athletic competition as well as in patient populations.”

Other NAC studies have reported improvements in exercise capacity in different populations, including those undergoing respiratory rehabilitation at 1,200mg/day[2] and healthy, recreationally active adults at 1,800mg/day.[3] Another recent study showed NAC supplementation reduced muscle soreness associated with exercise at 750mg/day in healthy, recreationally active adults.[4]

Choosing Your NAC Supplement

When looking for a NAC dietary supplement, we recommend starting with a dosage level closer to these latter three studies (between 750mg per day and 1,800mg per day) rather than the much larger dosage used in the initial study discussed in this article. While none of the subjects in the larger dosage study reported an adverse reaction, efficacy has been seen in the other studies at much lower dosages, and from a conservative standpoint is a better place to start in our opinion.

NAC is widely available online or in retail outlets as a stand-alone dietary supplement in dosage levels ranging from 600mg/day up to 2,000mg/day. The average cost per 30-day supply bottle of NAC ranges from $10-$20 depending on the dosage and brand purchased.


[1] Corn SD, et al. Effects of oral N-acetylcysteine on fatigue, critical power, and W’ in exercising humans. Respir Physiol & Neurobiol. 2011 June 29. [Epub ahead of print.]

[2] Stav D, et al. Effect of N-acetylcysteine on air trapping in COPD: a randomized placebo-controlled study. Chest. 2009 August; 136(2): 381-386.

[3] Kelly MK, et al. Effects of N-acetylcysteine on respiratory muscle fatigue during heavy exercise. Respir Physiol Neurobiol. 2009 Jan 1; 165(1): 67-72.

[4] Silva LA, et al. N-acetylcysteine supplementation and oxidative damage and inflammatory response after eccentric exercise. Int J Sport Nutr Exerc Metab. 2008 Aug; 18(4): 379-388.