Nitric oxide (NO) is an important cellular signaller in the regulation of blood flow, mitochondrial respiration in skeletal muscle during exercise and in the immune system. It has also been demonstrated that nitric oxide regulates blood pressure and tissue oxygen consumption and it is for these reasons that NO has become of interest for bodybuilders and even athletes, as an increase in blood flow has the potential to increase oxygen and nutrient transport, helping to support performance, recovery and muscle growth.
The amino acid L-arginine is commonly found in NO boosting supplements as it is metabolised by most cells into nitric oxide via enzymes known as nitric oxide synthases. Citrulline is also often included in NO boosting supplements as it is turned into arginine in the kidneys but is a more stable and efficient way of boosting NO. A normal, healthy individual creates nitric oxide as a product of normal metabolism but can also be created from dietary sources containing nitrates and nitrites. Nitrates and nitrates are compounds that contain large amounts of nitrogen and oxygen, hence their capacity to increase nitric oxide!
Food sources such as Beetroot juice, celery, lettuce, rocket and spinach contain high amounts of nitric oxide (approximately 2500 mg/kg). Of these, beetroot is often used and is concentrated into juice for this reason, and often nitrate salts are used as a source of nitrate to enhance the amount of nitric oxide available in the blood.
Several studies to date have shown that L-arginine supplements can improve exercise performance. However, it seems that sodium or potassium nitrate supplements have no overall effect on exercise performance in healthy athletes. On the other hand, beetroot juice supplements appear to be more efficient. This lack of effect of sodium or potassium nitrate supplements may be because healthy athletes tend to be habituated to a life of extensive exercise training regimes and may already be consuming sufficient nitrate through their diet.
The adequate daily intake of nitrate is approximately 3.7 mg/kg body weight. Care must be taken when increasing the doses of nitrate consumption due to its possible side effects. Nitrate itself is non-toxic, however, its metabolites and reaction products have been implicated in kidney cancer development. It has been stressed by various studies that athletes must refrain from uncontrolled ingestion of nitrate salts, especially those on a high-nitrate diet, until more evidence on their safety is demonstrated. Nevertheless, short term usage of nitrate salts is unlikely to be harmful.
L-arginine, beetroot juice and nitrate salts have been used by athletes to improve their performance. The mechanism of action of these supplements is through the production of nitric oxide which leads to increased mitochondrial activity in skeletal muscles during exercise. It is important to note, that we get all of the necessary nitrate through endogenous and dietary sources. Excess nitrate or nitric oxide has not been shown to be harmful if used in the short term. Furthermore, to date, studies have failed to show a strong link between the use of nitrate salts and enhanced exercise performance in highly trained athletes. However, beetroot juice supplements and citrulline have been demonstrated to have positive effects on NO production which may improve performance and support recovery and muscle growth.
Paul Rimmer (BSc, MSc, PhD).
