Beetroot juice (BRJ) is often ingested by athletes for its purported benefits on improving exercise capacity and exercise performance through conversion into nitric oxide (NO). NO is active in physiological mechanisms such as vasodilation, immune function, neurotransmission, mitochondrial respiration and skeletal muscle contraction. Dietary nitrate supplementation has been associated with reduced blood pressure, improved muscular contractile function, improved exercise economy, exercise tolerance and cognitive function (Jones et al. 2021). The reduction pathway of nitrate-nitrite-NO is particularly active under conditions when oxygen is limiting, suggesting it may benefit high intensity exercise and possibly performance in high altitude environments.
When dietary nitrate is ingested, the bacteria present in the mouth begins the process of reducing nitrate to nitrite which then increases circulating levels of nitrite which can ultimately be reduced to NO in the circulation and other tissues. Athletes who maintain a diet rich in leafy green vegetables and beets would ingest some amount of dietary nitrate through these food sources. However, many athletes also choose to supplement with BRJ as it can deliver a higher, measurable concentration of nitrate in an easy-to-ingest supplement that can be timed appropriately prior to training or competition. An individual’s ability to benefit from dietary nitrate is dependent on the health of the oral microbiota; and, use of antibacterial mouthwash can also inhibit the reduction of nitrate to nitrite. The composition of the oral microbiome appears to differ between women and men suggesting women may benefit less from nitrate supplementation (Wickham et al. 2019). Interestingly, compared to a high carbohydrate diet, a low-carbohydrate high fat diet also alters the composition of the oral microbiome (Murtaza et al. 2019), suggesting dietary strategies of athletes can also influence the efficacy of nitrate supplementation.
While it has been suggested that dietary nitrate supplementation may have an ergogenic effect, the mechanisms that may improve markers related to endurance exercise appear to be more prevalent in recreational athletes versus in highly trained (Senefeld et al. 2020) but it is possible that highly trained athletes may observe a benefit of supplementation on high intensity performance (Jones et al. 2018). It is not clear why elite athletes may not benefit from supplementation compared to their less trained counterparts, but possible reasons may include elite athletes already having high baseline concentrations of nitrate and nitrite and diets that are habitually high in dietary nitrate. Likewise, women do not appear to benefit from supplementation compared to males, possibly due to higher baseline nitrite levels and the oral microbiome; however, studies in women are extremely limited in the literature (Wickham and Spriet 2019). However, the ergogenic effects of dietary nitrate appear to be highly individual and complex. Recommended dose of BRJ supplementation is suggested to be >300 mg (5 mmol) or higher up to 25 mmol (Senefeld et al. 2020) for trained athletes, ingested at least 90 minutes up to 3.5 h (Jones et al. 2021, Senefeld et al. 2020) prior to exercise. Both acute and multi-day supplementation protocols may be effective (Senefeld et al. 2020).
Though NO has a very short half life in the body, it remains possible that NO can be oxidized back into nitrate and nitrite. If a high level of nitrate supplementation is regularly ingested, there may be associated health risk factors including the formation of carcinogenic nitrosamides and nitrosamines (Zamani et al. 2020). The risks of BRJ are not well studied in the literature but warrant investigation to ensure the safety as an ergogenic aid. Interestingly, Gallardo and Coggan (2019) investigated the nitrate and nitrite content of 45 commercially available BRJ supplements and found that only five of the supplements contained at least 5 mmol nitrate per serving with a moderate-large variability between samples of the same product and large variability between products. Not only are the supplements unregulated, most products do not indicate nitrate content on the label and very rarely are the supplements independently tested to ensure content accuracy.
