Sports nutrition: is dietary nitrate the key to enhanced endurance performance?
Dietary nitrate can aid mental and physical performance for athletes.
Over the past 30 years, fears over health and the environment have galvanised governments across Europe and the US to spend billions trying to eradicate nitrate from our drinking water. But, as Andrew Hamilton explains, recent research suggests that not only is dietary nitrate far from harmful, it could actually significantly aid mental and physical performance for athletes.
It can be a topsy-turvy world sometimes. Just when you think you’ve got something sussed, along comes new research that turns everything upside down. Take nitrate in food and water for example. For the past three decades, there’s been a concerted effort to reduce levels of dietary nitrates on a number of health grounds. These fears have centred around a number of particular health conditions, as well as a more general concern about long-term exposure to dietary nitrates and an increased risk of cancer. However, new findings on nitrate metabolism in the body look set completely reverse our previous thinking!
What is nitrate?
Nitrate (chemical symbol NO3-) is an inorganic compound composed of nitrogen and oxygen found naturally in soil and water. It also forms a large constituent of agricultural fertilisers added to soils by farmers. The nitrogen in nitrate is utilised by plants to produce amino acids, which are the building blocks of protein; adding nitrate to the soil therefore helps plants to grow more rapidly.
Nitrate is extremely water soluble, and this property has strong implications for the way it behaves in the environment. One consequence is that when nitrate is added to soil, it dissolves rapidly and can very easily be ‘washed out’ into lakes and rivers, and also into underground water tables. This inevitably leads to higher concentrations of nitrate in drinking water, which has long been a cause for concern among health professionals. Another consequence of this solubility is that nitrate is readily drawn up from soil water into plants. In some vegetables containing relatively high levels of water, this nitrate content can become very significant indeed, greatly increasing nitrate intake when these vegetables are consumed.
Nitrate in the diet
As mentioned above, the nitrate in our diet comes from both food and drinking water, with the highest levels of nitrate found in vegetables. However, the actual amount of nitrate consumed is highly variable, mainly being a function of the amount of water drunk, vegetables consumed and the actual levels of nitrate they contain (1). Tables 1-3 show the range of typical nitrate and nitrite (NO2-, a closely related compound) concentrations in a range of vegetables, fruits and meat.
What’s interesting about these figures is that the current recommendations of eating ‘five portions of fruits and vegetables per day’ will typically result in quite high levels of nitrate intake, which conflicts with the current health recommendations on limiting nitrate/nitrite intake. For example, some simple calculations have shown that someone consuming a diet rich in fruits and vegetables (such as that recommended in the DASH [Dietary Advice to Stop Hypertension] diet ) will exceed the World Health Organisation’s advised upper limit of nitrate intake of 3.7mg per kilo of bodyweight per day by a whopping 550%(2)!
Nitrate health fears
In years gone by, there have been two main health concerns about excessive nitrate consumption. The first of these is a condition known as methemoglobinemia or ‘blue baby syndrome’, from which infants in particular are considered at risk. In this condition, nitrate is converted to its closely related chemical cousin nitrite, which then blocks the ability of haemoglobin in the blood to carry oxygen, resulting in dangerously low tissue oxygen. In the early 1950s, methemoglobinemia was seen in infants fed formula made with contaminated well water. This was ascribed to the high nitrate content of these wells (3) and resulted in the US Environmental Protection Agency (EPA) setting a maximum contaminant level for nitrate of 44 milligrams per litre (4).
However, it is now more commonly thought that methemoglobinemia was not caused by nitrate per se but by faecal bacteria that infected the infants and produced nitrite, then nitric oxide, in their gut (nitric oxide can convert haemoglobin to methemoglobin). Indeed, studies have shown that infants exposed to far higher levels of nitrate (up to 700mg per day) did not develop methemoglobinemia (5). Moreover, recent studies on both nitrate and nitrite in healthy adult and adolescent populations have failed to find any negative health effects and a number of experts have now begun to question whether nitrates (and nitrites) in food and water really do pose a risk for this condition (6,7).
Nitrate health benefits
Over the past three decades, nitrate has been something of a dirty word among nutritionists and health professionals. However, recent research suggests that, far from being harmful, nitrate could actually be beneficial for health. This is because once consumed, nitrate can be readily converted to nitrite in the body, which can then be further converted to nitric oxide (NO).
One of the great breakthroughs in recent years is the discovery that NO is an incredibly important signalling molecule in the body and vitally important for the health of the cardiovascular system (12). For example, one key function is to facilitate vasodilation (widening) in blood vessels, promoting increased blood flow and regulating blood pressure (13). It now appears that dietary nitrate provides an important alternative route for NO production in the body, particularly when the normal pathway is compromised (14).
A diet rich in vegetable and fruits, supplying high levels of nitrate, appears then to offer a double-whammy of health benefits; first, numerous studies have confirmed that the antioxidant content of this kind of diet offers significant protection against a wide range of degenerative diseases and second, the high levels of nitrate supplied may also be beneficial for cardiovascular health.
The other concern about nitrate and nitrite consumption relates to a theoretically increased risk of cancer. The reasoning goes like this: nitrate in drinking water and food is readily converted to nitrite in the body (nitrites can also be ingested directly, as they’re often added to processed meat products). This nitrite can then go on to react with amino acids (formed from the breakdown of dietary protein) to produce carcinogenic compounds known as ‘nitrosamines’.
The problem is that while this effect has been shown in the lab and in certain species of animals at very high nitrite intakes, when you look for evidence of a link between increased nitrate/nitrite intakes and cancer in the population at large, there doesn’t really seem to be any (3,8). The one exception might be when nitrite is consumed directly in processed and cured meats. A recent study has shown that nitrite, when consumed with vitamin C (also added to processed meats) and fat (almost always present in these meats), can increase the production of certain types of carcinogens by 140-fold (9).
Contrast this, however, with research showing that increasing intake of vegetables and fruits (naturally containing high levels of nitrate/nitrite) actually decreases the risk of these harmful compounds being produced in the body, probably due to the high levels of protective antioxidants they contain (10,11). If nitrates and nitrites really were harmful, we’d expect that cancer risks would rise with increasing intakes, whereas we know that the reverse is true. Consider this too; we produce a substance called nitric oxide (see PP 271) naturally in the body, which leads to an accumulation of natural nitrite in saliva, accounting for more than half of all nitrite in the body. If nitrite really were that harmful, we should all be advised to stop swallowing, which is clearly daft!
Nitrate and athletic performance
So far, so good but it seems that athletes may have special reason to celebrate nitrate’s transformation from villain to hero. Given that nitrate can be converted to nitric oxide (NO) in the body (see box, right) and NO is involved in modulating blood flow to muscles, muscular contraction and cellular respiration, Swedish scientists decided to investigate the effect on sub-maximal exercise of giving nitrate as a supplement (sodium nitrate) (15).
In a double-blind, placebo-controlled crossover study, nine healthy young well trained men performed sub-maximal and maximal work tests on a cycle ergometer after two separate three-day periods of dietary supplementation. For one of these three-day periods, they received sodium nitrate at a dose of 0.1 millimoles per kilo of bodyweight per day (equating to about 500mgs of actual nitrate – an amount that can be consumed when consuming a high-vegetable diet). For the other period, they received an equal amount of sodium chloride (table salt – placebo).
What the scientists found amazed them. The oxygen cost at sub-maximal levels of exercise was significantly reduced after nitrate supplementation compared with placebo – in other words, they became more efficient at using oxygen. Over the four lowest work rates, their oxygen efficiency jumped from 19.7 to 21.1%. Or to put it another way, the same work output required less oxygen after nitrate supplementation compared to when nitrate wasn’t taken. Moreover, this oxygen-saving effect occurred without any increase in lactate production, indicating that energy production had become more efficient.
A gain of 1.5% in oxygen efficiency might not sound earth shattering, but it actually amounts to a potentially big advantage in competition, especially when the top performers are often separated by a fraction of one percent. As any endurance athlete knows, what limits your maximum sustainable power output before fatigue sets in is how much oxygen can be transported to and used by the working muscles. Although maximum oxygen uptake is limited, if you can use your oxygen more efficiently, you will be able to sustain higher levels of power before fatigue sets in.
Given the results achieved with supplementing nitrate, an obvious question to ask is whether the same effects would be observed with a diet high in naturally occurring nitrates in the form of vegetables? This is exactly what British scientists from the University of Exeter have been investigating in recent months. Professor Andy Jones (a contributor to PP) and his team of scientists have been looking at the effects of giving beetroot juice on moderate and high-intensity exercise during an incremental cycling test (16).
Why beetroot juice, you may ask? Well, quite simply because it provides a very rich source of nitrate indeed. In the study, the subjects consumed 500mls of beetroot juice per day (containing nearly 700mgs of naturally occurring nitrate) for the high nitrate test, whereas for the control condition (placebo) they drank the same amount of blackcurrant cordial containing negligible nitrate.
For the study, eight subjects consumed either beetroot juice or blackcurrant cordial for six consecutive days, and completed a series of ‘stepped’ moderate-intensity and severe-intensity exercise tests on a cycling ergometer during the last three days of this period. On the fourth day of supplementation, subjects completed two bouts of moderate cycling, while on days five and six the subjects completed one bout of moderate cycling and one bout of severe cycling. In the severe cycling test, the loading was increased by 30 watts per minute until the subject could no long continue.
After this six-day period, subjects underwent a 10-day ‘washout’ period during which neither beetroot juice nor cordial were consumed, then the supplement protocol was reversed (those who had taken beetroot took cordial and vice-versa). Throughout this time, all subjects refrained from eating any high-nitrate foods to ensure that any differences were due to the beetroot juice.
The results were as follows:
- As the exercise intensity increased, the extra oxygen demand to provide this increased power output rose more slowly in those taking beetroot juice (the beetroot condition required an extra 8.6mL of oxygen per minute per additional watt of power output whereas the placebo required an extra 10.8mL per minute per additional watt);
- The time-to-exhaustion in the ‘severe test’ was significantly extended when beetroot juice was taken compared to placebo (675 seconds vs. 583 seconds, an increase of 16%!);
- The ‘slow component’ of oxygen uptake (you can think of this as the ‘lag’ in the aerobic system while oxygen supply catches up with demand) was reduced by about 0.2L per minute in the severe test when beetroot juice was taken, indicating that the aerobic system was more efficient at getting up to speed during intense exercise;
Although none of these (young) subjects had high blood pressure, taking beetroot juice further reduced systolic blood pressure (a good thing).
Andy and his team summed up their findings thus: ‘It should be stressed that the remarkable reduction in the oxygen cost of sub-maximal exercise following dietary supplementation with inorganic nitrate in the form of a natural food product cannot be achieved by any other known means, including long-term endurance exercise training. And although not directly tested in this study, our results suggest that increased dietary nitrate intake has the potential to enhance exercise tolerance during longer-term endurance exercise.’
Why could nitrate improve oxygen efficiency?
It’s too early to say how nitrate supplementation might be producing these favourable effects during exercise. One theory is that once in the body, the nitrate is converted to nitrite then NO, and this NO helps promote vasodilation in muscles. With a better blood supply, more oxygen is able to diffuse deep into muscle tissue, resulting in a better supply for a larger number of contracting muscle cells.
There’s also some speculation that in this process, the NO produced might help prevent the muscle cells right next to the capillary supplying the blood from taking a lion’s share of the oxygen, allowing more distant muscle cells to achieve better oxygenation and therefore a more ‘even’ oxygen distribution in the working muscles.
Another theory is that the extra NO produced in the body when nitrate is consumed stimulates the production of muscle mitochondria. The mitochondria can be considered as the ‘energy factories’ in muscle cells and greater numbers of mitochondria would enable more efficient oxygen metabolism.
In the world of sports science, it’s all too easy to become caught up in the marketing hype of sports performance drinks and supplements and forget the dietary basics. Should these findings on nitrate be confirmed by further studies (and there’s no reason to doubt they will), the implications and benefits for endurance athletes are potentially very significant.
It seems remarkable that eating humble vegetables such as beetroot and spinach could be a far better route to enhanced performance than quite a few so-called ‘energy-boosting’ supplements. Even more remarkable is the fact that the active ingredient (nitrate) has long been considered a health villain. It just goes to show the wisdom of our elders. How right Grandma was when she told us to ‘eat up your greens’!
Andrew Hamilton BSc Hons, MRSC, ACSM is a member of the Royal Society of Chemistry, the American College of Sports Medicine and a consultant to the fitness industry, specialising in sport and performance nutrition
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