Sunshine and Beetroot. Performance and health benefits of nitric oxide.
In 1998 the Nobel Prize for medicine was given to three American pharmacologists for their discovery, that inside the body the gas nitric oxide (NO) is an important mediator of a wide variety of vital bodily functions. At the time this was an amazing discovery and their initial work, done in the 1980s resulted in a huge boom in research looking at related areas. Nitric oxide is now known to be a vital internal transmitter and is particularly associated with the regulation of blood pressure and blood vessel dilation, improving the ‘fuel efficiency’ of muscles, assisting the immune system fighting infection, preventing blood clots and reducing inflammation. It is a key signalling molecule in the nervous system has role in memory and behaviour and also increases sense recognition (e.g. smell) and quality of sleep. Good reviews of the health and physical performance effects of nitrites have been written by Clements, Lee & Bloomer 2014 and Andrew Jones 2014.
Once scientists knew the benefits of nitric oxide, work began on how blood nitrite levels could be increased. Initial research with direct supplementation of nitrite salts found that even tiny amounts of these increase the blood levels far too high and were toxic to humans. Nitric oxide is naturally produced from the amino acid L-arginine but supplementation of L-arginine was also unsuccessful as it does not increase blood nitric oxide (Alvares et al 2014). Scientists had to start looking for indirect ways to increase nitrite levels enough to produce more nitric oxide, but within a safe range. What they found was that supplementing with dietary nitrate (NO3-) could achieve this.
This is when the sport nutritionists got involved and beetroot juice entered the picture. Of the naturally occurring food sources of nitrate, beetroot is one of the best and most palatable in the quantities needed to significantly increase blood nitrate (and therefore nitrite) levels. About 300-500mL of beetroot juice provides the amount of nitrate needed for the average person to increase their blood nitrite, though the nitrate levels in all vegetables vary considerably depending on the ground they are grown in and the care they are given. This is why beetroot juice has become so popular – it may or may not be the best source of nitrate but it has been developed to have a specific amount in it, taking away the uncertainty.
Drinking beetroot juice has been shown to significantly improve exercise endurance and intensity but the results are not consistent in everyone, there are ‘responders’ and ‘non-responders’ and the results appear to be better in non-elite or amateur athletes. Despite this, when a recent study in elite kayakers showed only a small improvement in performance it was still thought to be highly significant as kayaking is a sport where the margin between gold and silver medals can be as small as 0.3 %. Elite teams in many different sports now use beetroot juice as an important part of their training routine. In untrained athletes drinking beetroot juice has been shown to significantly increase endurance allowing individuals to exercise up to 16% longer (Bailey et al 2009) and to lower the oxygen cost of exercise allowing individuals to work at substantially higher intensity (Ferreira & Behnke 2011).
Beetroot outside sport
Nitric oxide is a key signaling molecule throughout the body. Produced by the endothelial cells lining the arteries, it penetrates the underlying smooth muscles and acts as a potent vasodilator that relaxes the arteries. It protects the endothelium, lowering inflammation and oxidative stress. Drinking beetroot juice can help to lower blood pressure in the few hours after ingestion. One study showed that drinking just one glass lowered systolic blood pressure by an average of 4-5 points. A systematic review of this research was done in 2013 (Servo et al 2013).
The strongest research has been done on cardiovascular disease and angina but nitrites have other benefits as well. In the gastrointestinal tract, nitric oxide is known to relax smooth muscle cells and regulate intestinal peristalsis, secretion of mucus and gastric acid release. It is also known to have a role in memory, insulin signaling, bone remodeling, respiration, ATP (energy) utilization, mitochondrial biogenesis and erectile function.
Research in a completely different area has shown that sunlight may also be an important factor in nitric oxide production, possibly explaining the differences in cardiovascular disease risk seen at different latitudes. Research done by dermatologist Richard Weller and his team in Edinburgh shows that there are reserves of nitrates and nitrites in the skin and that nitric oxide can be released by the action of UV A light. Their research has shown that sunshine can directly lead to lower blood pressure and is linked to lower cardiovascular disease risk.
Richard Weller gave this 15 minute Ted talk in 2012, outlining his work. The talk is called “Why are Scots so sick?” and looks at the benefits of UV A rays on nitric oxide stores and health. His group have more recently published an new article on how UV A impacts blood pressure (Liu et al 2014).
Responders and non-responders
One continuing area of interest is why some individuals are more sensitive to both UV A and to dietary nitrates. This question has not yet been answered conclusively but it is known that dietary nitrates are not easily converted to nitrites in the body and that this is actually done by bacteria that live in your mouth and gut. It has been shown that using antibacterial mouth washes kills these bacteria. Individuals using mouth washes get no benefit from taking beetroot juice or any other dietary nitrate source. Another possibility particularly for the lower effect in elite athletes is due to the fact that exercise has been found to enhances nitric oxide production in vascular endothelium (Allen, Cobb & Gow 2005) . There is also evidence that dietary intake of phytonutrients can enhance nitric oxide action.
As Dr Richard Weller concludes in his talk, there are risks to getting sunburn but there are also increasing reasons why you should get outside and get some sun. As well as releasing nitric oxide (UV A), sunlight (UV B) produces vitamin D, which also has a growing number of health benefits.
Beetroot is a good source of dietary nitrates. Other good sources include root vegetables, carrots, spinach and rocket. Strawberries, gooseberries and currants are also relatively high. Having a healthy gut and mouth micro-flora may also help production of nitric oxide. There is good evidence that eating the right foods can enhance nitric oxide production and have real health benefits.
High intake of nitrates and nitrites is carcinogenic in animal models. In humans, the association between cancer and nitrites is uncertain. Some studies have observed a link, especially with animal-based nitrates, whereas others have not. Nitrates and nitrites themselves are not carcinogenic, but there is a small chance that produced nitrites might react with dietary amines to form carcinogenic nitrosamines (like N-nitrosodmethylamine, NDMA). This could possibly happen when meat-containing nitrites or nitrates are cooked or grilled and is the reason why there is some speculation that nitrates and nitrites in processed meats may cause cancer. I believe the benefits way out weigh the risks and vitamin C naturally inhibits the conversion to nitrosamines, which is why fresh fruits and vegetables high in nitrates are likely to cause far fewer problems than meats that are artificially high.