You are currently viewing Supplement Review: Dietary Nitrate and Beetroot Juice

Supplement Review: Dietary Nitrate and Beetroot Juice

In the human body, Nitric Oxide (NO) promotes vasodilation, or widening of the blood vessels, which increases blood flow and reduces blood pressure [1-3]. NO is heavily involved with many physiological processes that affect exercise performance, including the regulation of blood flow, skeletal muscle contraction, and mitochondrial respiration and biogenesis [3-5].

Note: You may have noticed that a few words are highlighted in blue; abbreviations for these words are used extensively in this article.

Through these mechanisms, NO can enhance exercise performance via improved function of type II (fast-twitch) muscle fibers [6], reduced ATP (energy) cost of muscle force production [7], increased efficiency of mitochondrial respiration [3], and increased blood flow (and thus delivery of oxygen and nutrients) to the muscles active during exercise [1, 7].

Nitrate

Dietary Nitrate and Endurance Exercise Performance

There’s a substantial body of evidence that shows dietary nitrate supplementation can improve endurance exercise performance. For example, oxygen efficiency during submaximal exercise [8-10], time-to-exhaustion [10-12] and time trial performances [13-16] have all been improved with dietary nitrate supplementation. A recent meta-analysis and systematic review of the literature supports these performance enhancements [17].

Endurance performances lasting ~12-40 minutes are generally enhanced by dietary nitrate supplementation [8-19, 43]. Research supporting ergogenicity for exercise tasks lasting 40 minutes is not supported [26-29]. In World-Class, short-track speed skaters, 1000-m time trial performance lasting ~90 seconds was not enhanced with chronic dietary nitrate supplementation [20]. Similarly, 4-km and 168-m performances by elite cyclists and trained swimmers, respectively, were not enhanced [21, 22]. On the other hand, dietary nitrate supplementation improved 1,500-m and 4-km time trial performances in trained runners [18] and competitive cyclists [19], respectively. Differences in supplementation timing and dosing are likely contributors to these mixed outcomes [30, 31].

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Dietary Nitrate and Intermittent High-Intensity Performance

More recently, research has focused on the potential for dietary nitrate to enhance performance in repeated high-intensity tasks, which is a physiological requirement for many team sports. Many different intermittent high-intensity protocols in various cohorts, ranging from untrained to elite team-sport athletes, have been investigated [32-39]. When dietary nitrate was consumed throughout 3 weeks of sprint interval training, maximal work rate during an incremental graded cycling test was enhanced, compared with placebo, in untrained males [34]. In high-level soccer players, 6 days of beetroot juice (i.e. dietary nitrate) supplementation improved performance on a Yo-Yo IR1 test (i.e. intermittent high-intensity running), compared with placebo [38]. Similarly, 5 days of beetroot juice intake was associated with sprint speed and Yo-Yo IR1 test performance improvements in football, rugby, and hockey athletes [36]. In recreational cyclists, national talent speed-skaters, and Olympic-level track cyclists, 6 days of beetroot juice intake reduced time to reach peak power in a 30-second all-out cycling sprint (Wingate), regardless of sport and level [32].

Nitrate 2

A recent review of the literature concluded that supplementation with beetroot juice, a common source of dietary nitrate, improves intermittent high-intensity performance, likely due to accelerated phosphocreatine (PCr) resynthesis and faster muscle shortening velocity [40]. The former would delay the PCr depletion during repeated high-intensity exercise efforts and the latter would improve muscle power output [40].

The Impact of Training Status

Training status may impact ergogenicity of nitrate supplementation [30, 41, 42], with highly trained athletes requiring higher intakes to reap performance benefits [30, 41], but this is not always true [15, 18, 34]. Overall, more research is needed to determine if and how training status impacts dietary nitrate supplementation ergogenicity.

The IOC Recommendation

Recently, beetroot juice has become a suggested, evidence-based approach to enhance athletic performance [43]. In fact, it’s one of the few supplements that the International Olympic Committee (IOC) recommends for the high-performance athlete [44].

Dietary Sources, Dosing, and Safety

Leafy green and root vegetables, including spinach, rocket salad, celery, and beetroot, are foods that contain high levels of nitrate [44]. Beetroot is perhaps the most popular for supplementation, likely due to its 100% bioavailability [49].

Acute performance benefits are generally observed 2-3 hours following ingestion [14, 45, 46]. Hoon et al. (2014) reported such benefits following 5-9 mmol of dietary nitrate intake [14]. However, it must be noted that substantial inter-individual differences in these pharmacokinetics can exist, resulting in varying optimal doses and timings for supplementation [47]. In other words, the dosage and timing that’s advised in the research may not be optimal for you or me. With that being said, 5-9 mmol seems to be the most effective dose for performance benefit, in most people [17]. However, high-level athletes may require greater intakes [7, 30, 48], perhaps between 9-11 mmol [48].

These quantities can easily be met through consumption of vegetables rich in dietary nitrate (i.e. beetroot, spinach, and rocket/rogula), but there doesn’t appear to be anything wrong with supplementary products. In general, there appears to be few, if any, side effects with nitrate supplementation in doses suggested in literature [43]. Some athletes may experience gastrointestinal (GI) upset, and thus, high dietary nitrate intakes and/or supplementation should be trialed during training, as opposed to competition [43].

Other Nitric Oxide (NO) Promoters: L-Arginine and L-Citrulline

L-arginine and L-citrulline supplementation can increase nitric oxide (NO) metabolites, but this don’t necessarily translate to increased NO in the body, or exercise performance benefits [50, 51]. Although one small study showed improved moderate-intensity exercise performance after acute 6g L-Arginine supplementation consumed pre-exercise [52], the majority of other L-arginine supplementation protocols resulted in no cardiovascular exercise performance benefit [53-58].

There’s less research on L-citrulline than L-arginine. Although there are multiple reports that indicate L-citrulline is more likely to benefit exercise performance than L-arginine [59-64], the research on L-citrulline ergogenicity is inconsistent and inconclusive, at this time [50, 64-66]. Like L-arginine, L-citrulline should not be recommended as an ergogenic aid because there is not enough research, currently, to support its efficacy.

Summary

Overall, dietary nitrate appears to be beneficial for endurance and intermittent high-intensity performance lasting 12-40 minutes, but not for performances of shorter [20-23, 43], or longer duration [26-29]. Foods high in dietary nitrate include leafy green and root vegetables, such as beets, radishes, turnips, lettuce, and spinach. Although 5-9 mmol seems like an appropriate dosage for most people [4, 17], it appears that athletes need higher intakes to experience exercise benefits. Overall, more research is needed to determine optimal dosing strategies.

The evidence on exercise performance ergogenicity when using other Nitric Oxide (NO) enhancers, such as L-arginine and L-citrulline, is less clear. L-arginine appears to offer no cardiovascular exercise performance benefit [53-58]. While L-citrulline is more likely to offer ergogenicity, reports have been inconsistent to-date [50, 59-66]. Research does not currently support supplementation with L-arginine or L-citrulline for improved cardiovascular exercise performance.

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Reference

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