This article aims to review the latest finding and innovation in sport nutrition over the past few years – the humble beetroot, and the nutrient it is rich in – nitrate. There has been a surge in evidence over the past two to three years supporting the use of dietary nitrates, especially from beetroot juice, to improve endurance performance.
- Highlight sources of dietary nitrate
- Review how nitrates may help exercise and performance
- Examine the how, what, and when of taking beetroot juice / dietary nitrate supplementation to improve performance
Background on Dietary Nitrates
Sources of Dietary Nitrate
Nitrate (NO3) is commonly ingested as part of a healthy diet (Bryan & Hord, 2010a), and according to previous work (Ysart et al., 1999), vegetables account for approximately 60-80% of the daily NO3 intake in a western diet, with green leafy vegetables such as lettuce, spinach and beetroot being particularly rich in NO3 ̅ (Bryan & Hord, 2010b).
It is worth noting that there is a large variability in the nitrate levels in vegetables – the freshness and farming practices that the vegetable was harvested from will play a part in the nitrate content of the vegetable. Fresh vegetables grown with nitrogen containing fertilizer will yield greater amounts of nitrate. Therefore, organically grown vegetables are likely to have lower nitrate levels.
Common sources of nitrate and their typical content:
|Nitrate||Content per kg fresh veg||Common products|
|Very high||2500mg (40mmol)||Beetroot and juice, celery, lettuce, spinach|
|High||1000-2500mg (18-40mmol)||Chinese cabbage, celeriac, endive, leek, parsley|
|Moderate||500-1000mg (9-18mmol)||Cabbage, dill, turnip, carrot juice|
|Low||200-500mg (3-9 mmol)||Broccoli, carrot, cauliflower, pumpkin|
|Very low||<200mg (<3mmol)||Asparagus, artichoke, broad beans, peas, tomato, watermelon, sweet potato, potato, garlic, onion, mushroom|
*Adapted from Byran, N.S. & Hord, N.G. (2010b). Dietary Nitrates and nitrates in: Bryan N(ed), Food Nutrition and the Nitric Oxide pathway. Destech Pub Inc: Lancaster, PA, pp 59-77.
A vegetable-rich diet is greatly substantiated to have cardiovascular benefits and can be associated with a longer lifespan (Appel et al., 1997). Dietary nitrate is thought to be cardio-protective as it can reduce blood pressure (Gilchrist et al., 2010). It has been suggested that these benefits may be attributed to the high NO3 content of vegetables, especially leafy greens and beetroot (Hord et al., 2009).
Nitrate (NO3) to Nitric Oxide (NO)
In the mouth, bacteria on the surface of the tongue reduce nitrate to nitrite (NO2) (Duncan et al., 1995).This nitrate is swallowed and reduced to nitric oxide (NO) within the stomach (Benjamin et al., 1994; Lungberg et al., 2004). However, it is clear that some Nitrite (NO2) is absorbed to increase circulating plasma (Lungber et al., 2004; Dejam et al., 2004).
Nitric oxide (NO) serves as an important molecule to support dilation of blood vessels and reduce vascular resistance (Joannides et al., 1995). Stimuli from diverse signal chemicals (including neurotransmitters) and shearing stretch and vessel stretch from increased blood flow through the vessel lumen provoke NO synthesis and release by the vascular endothelium to serve its role as vascular gatekeeper. It was formerly termed endothelium-derived relaxing factor by 1998 Physiology or Medicine Nobel Prize recipient, Robert F Furchgott (McArdle et al., 2001).
NO spreads through underlying cell membranes to muscle cells within the arterial wall. Here it induces arterial smooth muscle relaxation to increase blood flow in neighboring blood vessels (Jaynor et al., 1997; Balon, 1997). Vascular wall receptors for NO contribute to blood pressure regulation in response to cardiovascular stimulation during emotionally stressful situations and exercise.
A study by Govoni et al. (2008) reviewed oral bacterias’ role in the biological activation of nitrate, by its reduction to nitrite. In their study, Govoni and colleagues found that participants who used an antibacterial mouthwash had lower levels of salivary and plasma levels of sodium nitrate, despite following the same level of nitrate supplementation. Therefore, if using dietary nitrate supplementation it is perhaps wise to avoid antibacterial mouthwashes to gain full benefit.
Natural versus Man-made Nitrates
Looking at the training supplement market, especially that aimed at the bodybuilding community, there are many products available containing Nitrates (Nitric Oxide when broken down in the body), which are aimed at giving a ‘pumped’ look and improving oxygen delivery. There appears to be no added advantage of buying these commercially available man-made products when compared to natural products like beetroot (and beetroot juice), which is much cheaper and cost-effective. A note of caution that using uncontrolled doses of commercially available nitrates (such as those on the supplement market), may cause severe hypotension in some people. The use of nitrates from natural vegetable products has not shown any adverse effects in the sport and exercise science literature, beyond potentially pink urine and discoloured stools.
Nitric Oxide and Exercise
Recently is has been shown that dietary nitrate supplementation in the form of commercially available beetroot juice has a beneficial effect in endurance performance. Langsley and colleagues (2011) found that dietary nitrate supplementation, in the form of a commercially available beetroot juice, reduced the oxygen cost of walking and running. Previous related research (Bailey at al., 2009) provided evidence that dietary nitrate supplementation reduced the oxygen cost of low intensity exercise, and enhanced tolerance to high intensity exercise. In addition, Larson et al. (2010) found that dietary nitrate supplementation (in the form of sodium nitrate [0.1 mmol/kg/day]) reduced maximal VO2 while maintaining work performance.
Endurance Exercise & Peripheral Artery Disease
Nitrate supplementation may be beneficial to those training clients who are looking for ‘an edge’ to help enhance their performance, particularly those training for endurance based events. Those findings relating to reduced oxygen cost of exercise would translate to being able to improve the quality of exercise, as the client may be able to exercise for longer, or for slightly increased intensity.
Further endurance research showed improvements in the 50 km cycling time trail (Wilkerson et al., 2012), and 5 km run performance (Murphy et al., 2012). Further research from the health field showed that beetroot juice supplementation can enhance exercise performance in patients with peripheral artery disease, which results in the inability to sufficiently supply blood and oxygen (O2) to the working musculature (Kenjale et al., 2011).
High Intensity Exercise
Much of the previous research on dietary nitrate supplementation has focused on endurance performance, while very few studies have concentrated on short duration exercise, and no studies appear to have focused on intermittent exercise. One study focusing on high intensity exercise (Bailey et al., 2010), examined the effect of dietary nitrate supplementation on reducing the ATP cost of muscle force production during high-intensity exercise. The study showed that this reduced muscle phosphocreatine (PCr) utilization and it might also have reduced muscle glycogen utilization. Since PCr and glycogen are important substrates for anaerobic metabolism within the muscle, then a reduced utilization of these substrates during exercise may increase the number of high-intensity bouts an individual can complete before these reserves attain low levels and compromise exercise performance. Therefore, the potential for sparing the limited anaerobic energy reserves and for lowering the accumulation of metabolites linked to the process of muscle fatigue after beetroot juice supplementation would be hypothesized to improve intermittent exercise performance – including activities of alternating intensities, such as interval training or soccer.
More Studies Confirm Benefits of Dietary Nitrate Supplementation
Bailey and colleagues (2011), in a double blind placebo controlled study of 8 healthy recreationally active volunteers, found that chronic dietary nitrate supplementation (11.2 ± 0.6 mH nitrate per day for 6 days), reduced the oxygen cost of low intensity exercise (ramping cycle performance), and improved tolerance to high intensity exercise (increased time to exhaustion). This supports previous work, (Larsen et al., 2009), where volunteers were exposed to two days of pharmaceutical nitrate supplementation (0.1 mml.kg per day), the equivalent of 100-300 g of natural dietary nitrate-rich food such as beetroot or spinach. Using combined arm and leg ergometry, the study found that time to exhaustion increased after nitrate supplementation compared to placebo, and a decrease in VO2.
Chronic dietary nitrate supplementation has also been found to improve the economy (O2 cost) of walking and running in a group of 9 healthy, physically active volunteers (Lansley et al., 2009). More recently, Vanhatalo and colleagues (2010), found that beetroot juice (BRJ), again reduced the O2 cost of treadmill exercise following chronic nitrate supplementation over 15 days, followed by a 15 day washout. The study also showed that an acute dose of BRJ is beneficial, as improvements were seen in a reduction of end VO2, along with peak power production when compared to placebo.
Recently examining acute nitrate supplementation, Bescós and colleagues (Bescos et al., 2012) in a sample of 11 cyclists showed that reduced O2 cost of cycling without negatively influencing time to exhaustion (nitrate 416 ± 32 seconds; placebo 409 ± 27 seconds), when ingested 3 hours prior to exercise.
Effects on the Highly Trained Athlete
The training status of the athlete may influence the effect of dietary nitrate supplementation. One such study (Christensen et al., 2012) showed that in a sample of 13 well trained cyclists, nitrate supplementation did not enhance endurance performance following a three day supplementation period. Despite a reduction in the VO2, no significant differences were reported in changes in the time to exhaustion (nitrate = 416 ± 32 s, placebo = 409 ± 27 s) or in the maximal power (nitrate = 416 ± 29 W, placebo = 410 ± 28 W). This finding suggests that highly trained athletes are perhaps not as sensitive to the effects of dietary nitrate supplementation when compared to less trained individuals.
This finding is again supported by Cermak et al. (2012) with a group of 10 highly trained cyclists (VO2max 72 ± 4 ml.kg.min-1), following a 6 day dietary nitrate supplementation period. This study showed no difference between supplementation versus control in terms of VO2 and exercise economy. While nitrate levels were greater following nitrate supplementation, the time trial was similar (18:20 and 18:37 min:s in beetroot juice and placebo respectively), as was peak and mean power outputs (290 ± 43 W in beetroot juice and 285 ± 44 W in placebo). Acute supplementation (2.5 h pre exercise) in high trained cyclists (VO2peak 60 ± 1 ml.kg.min-1) has also been shown to be ineffective for time trial events (Cermak et al., 2012), as there was no difference versus placebo.
While not intermittent in nature, rowing performance after dietary nitrate supplementation has been examined using an intermittent protocol (Bond et al., 2012). Here 14 well trained junior male rowers were exposed to 6 days of dietary nitrate supplementation (beetroot juice), which resulted in improved maximal rowing ergometer sets. The rowers were randomly assigned to a double blind crossover study ingesting either homemade beetroot juice (5.5 mmol.d nitrate), or placebo (a commercially available blackcurrant juice containing negligible nitrate). After 6 days, supplementation volunteers underwent a 6 x 500m rowing ergometer intersped with 90 s rest in between each set. Mean performance improvements across all sets were 0.4%, with the largest improvements seen around sets 4 – 6 (1.7% improvement v placebo). This indicates that beetroot juice supplementation may prove beneficial during the latter stages of competition.
The effect of dietary nitrate supplementation on exercising muscle has been examined in an animal model (Ferguson et al., 2012). Here eight rats were administered beetroot juice (1 mmol.kg.day) for five days before being tested during sub-maximal exercise (treadmill running). Improvements, shown by skeletal muscle blood flow of hind limbs, were positively associated with their percentage type IIb muscle fibers. This supports the notion that chronic dietary nitrate supplementation improves vascular control by elevating skeletal muscle oxygen delivery during exercise where type IIb muscle fibers are predominant.
Research suggests that beetroot juice may have positive effects on exercise performance in a variety of individuals including training clients, patients, as well as athletes. Studies have shown that the nitrate in beetroot juice aids exercise performance by dilating the vasculature to exercising muscles, meaning the oxygen delivery is improved and the oxygen costs of exercise is reduced. Commercially available beetroot juices are advantageous because they contain a certain quantity of nitrates and the dose is known. However, there is no evidence to suggest that homemade juices are any less effective.
Acute nitrate supplementation (up to 3 hours prior to exercise) has been shown to be effective in improving endurance performance such as 5 km run times, and 40 km cycle time trials. Although there is mixed evidence for the effectiveness of acute supplementation, it is worth noting that the least effective studies were based on the elite level athletes, where the window of improvement is much less. As mentioned above, moderately trained athletes have shown improvements in 5 km run times (21), and cycling time trials (20). Research has shown loading to be effective up to 15 days, but no studies have examined long term loading (15 days+).
Keep in mind when using dietary nitrate supplementation, especially in the form of beetroot juice, to avoid using antibacterial mouthwashes as part of your oral hygiene routine, as this can reduce the breakdown of nitrates and therefore reducing the effectiveness of the supplementation.
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