Abstract
Beetroot juice contains high levels of inorganic nitrate (NO3 −) and its intake has proved effective at increasing blood nitric oxide (NO) concentrations. Given the effects of NO in promoting vasodilation and blood flow with beneficial impacts on muscle contraction, several studies have detected an ergogenic effect of beetroot juice supplementation on exercise efforts with high oxidative energy metabolism demands. However, only a scarce yet growing number of investigations have sought to assess the effects of this supplement on performance at high-intensity exercise. Here we review the few studies that have addressed this issue. The databases Dialnet, Elsevier, Medline, Pubmed and Web of Science were searched for articles in English, Portuguese and Spanish published from 2010 to March 31 to 2017 using the keywords: beet or beetroot or nitrate or nitrite and supplement or supplementation or nutrition or “sport nutrition” and exercise or sport or “physical activity” or effort or athlete. Nine articles fulfilling the inclusion criteria were identified. Results indicate that beetroot juice given as a single dose or over a few days may improve performance at intermittent, high-intensity efforts with short rest periods. The improvements observed were attributed to faster phosphocreatine resynthesis which could delay its depletion during repetitive exercise efforts. In addition, beetroot juice supplementation could improve muscle power output via a mechanism involving a faster muscle shortening velocity. The findings of some studies also suggested improved indicators of muscular fatigue, though the mechanism involved in this effect remains unclear.
Background
Because of the increase in competitive equality in high level sport, a 0.6% performance improvement is today considered sufficient to make a difference [Citation1]. In this setting of high competition, athletes often look to nutritional supplements to boost their performance [Citation2]. However, most statements about the potential effects on sport performance or health that appear on the labels of many products are not backed by clear scientific evidence [Citation2]. Because of this, institutions such as the Australian Institute of Sport (AIS) have created a system to classify supplements according to their effects on performance based on confirmed scientific evidence [Citation3]. Thus, dietary supplements assigned to class A have been proven with a high level of evidence to improve exercise performance in certain modalities when taken in appropriate amounts. The only substances in this class are β-alanine, sodium bicarbonate, caffeine, creatine and beetroot juice [Citation4]. However, it is thought that the effect of a given supplement on performance besides the recommended dose may be specific to each sport’s modality [Citation5]. This, in turn, will depend on the energy and/or mechanical requirements of each form of exercise such that some supplements will have an ergogenic effect on some types of exercise efforts and have no effects on other types.
The relationship between exercise intensity and time to exhaustion is hyperbolic [Citation6] as it is directly linked to the prevailing energy producing systems during exercise [Citation7]. Thus, depending on their bioenergetics, the different exercise efforts can be classified according to exercise duration. This means we can differentiate between explosive efforts, high-intensity efforts and endurance-intensive efforts [Citation8]. Explosive efforts are those lasting under 6 s in which the main energy metabolism pathway is the high-energy phosphagen system and there is some participation also of glycolysis [Citation9, Citation10], which gradually contributes more energy until 50% at 6 s [Citation9]. High-intensity efforts are those of duration longer than 6 s and shorter than 1 min [Citation11]. These efforts are characterized by a major contribution of glycolytic metabolism and smaller contribution of high-energy phosphagens and oxidative phosphorylation [Citation8]. Finally, intensive endurance efforts are those lasting longer than 60 s and whose main energy producing system is oxidative phosphorylation [Citation8].
Beetroot juice is used as a supplement because it may serve as a precursor of nitric oxide (NO) [Citation12]. The mechanism of NO synthesis is thought to be via the catabolism of arginine by the enzyme NO synthase [Citation13]. Effectively, arginine supplementation has been shown to increase NO levels [Citation14]. An alternative mechanism of NO genesis is mediated by inorganic nitrate (NO3 −). This means that the high amounts of NO3 − present in beetroot juice are able to increase NO levels in the organism.
In the mouth, some 25% of dietary NO3 − is reduced by NO3 − reductase produced by microorganisms [Citation15] to nitrite (NO2 −) [Citation16]. This NO2 − is then partially reduced to NO through the actions of stomach acids which is later absorbed in the gut [Citation17]. Some of this NO2 − enters the bloodstream, and, in conditions of low oxygen levels, will be converted into NO [Citation18] (Fig. ).
Fig. 1 Conversion of NO3 − in beetroot juice to NO. The diagram shows how ingested NO3 − is transformed by bacteria in the mouth containing nitrite reductase to NO2 −. Once in the gut, NO2 −enters the bloodstream and, under conditions of hypoxia, is used to generate NO
Nitrous oxide has numerous physiological functions including haemodynamic and metabolic actions [Citation19, Citation20]. Mediated by guanylyl cyclase [Citation21], NO has an effect on smooth muscle fibres causing blood vessel dilation [Citation22]. This vasodilation effect increases blood flow to muscle fibres [Citation23] promoting gas exchange [Citation24]. NO also induces gene expression [Citation25], enhancing biogenesis [Citation26] and mitochondrial efficiency [Citation27]. All these effects can favour an oxidative energy metabolism. In effect, though not all [Citation28–Citation31], numerous investigations have noted that beetroot juice supplementation boosts performance in exercise modalities involving intensive endurance efforts in which the dominant type of energy metabolism is oxidative [Citation24, Citation27, Citation32–Citation45].
To date, several reviews of the literature have assessed the effects of beetroot juice supplements on physical exercise [Citation12, Citation46–Citation49]. In addition, given that NO can potentiate the factors that limit performance when executing actions in which the predominant metabolism is oxidative, two recent reviews have explored the positive effects of this form of supplementation on endurance exercise [Citation50, Citation51]. Thus, the different studies showed that beetroot juice supplementation was effective at: lowering VO2 by −6% during a swimming test conducted at an intensity equivalent to the ventilatory threshold (VT) [Citation27]; lowering VO2 by −3% during a kayaking test conducted at 60% VO2max [Citation38] and during a cycle ergometry test conducted by recreation sport athletes [Citation45] and cyclists [Citation34] at 45–70% VO2max; increasing performance by 12–17% in cycle ergometry tests until exhaustion conducted at intensities of 60 to 90% VO2max by recreation sport athletes [Citation37, Citation42], and by 22% when conducted at a 70% intensity between VT and VO2max [Citation36]; and finally, improving times by 2.8% in trained cyclists conducting cycle ergometery tests of 4 km [Citation33], 10 km (1.2%) [Citation34], 16 km (2.7%) [Citation33] and 50 miles (0.8%) [Citation35]. However, besides the effects of NO mentioned above, other impacts need to be considered. Accordingly, it has been described that the effect of increased blood flow induced by NO is specific to type II muscle fibres [Citation20]. Moreover, in type II muscle fibres, beetroot juice intake has been found to improve the release and later reuptake of calcium from the sarcoplasmic reticulum [Citation52]. This could translate to an increased capacity for muscle strength production of these type II muscle fibres. Such effects of NO could mean a physiological advantage for efforts involving the recruitment of type II muscle fibres, such as intermittent, high-intensity efforts. Hence, given the scarce yet growing number of studies that have addressed the effects of beetroot juice supplementation on this type of intermittent, high-intensity effort [Citation38, Citation53–Citation60], here we review the results of experimental studies that have specifically examined in adults (whether athletes or not) the effects of beetroot juice supplementation on intermittent, high-intensity efforts.
References
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