Beetroot juice has been studied extensively as a supplement and ergogenic aid due to its natural high content of nitrate(Khatri, Mills, Maskell, Odongerel, & Webb, 2016). Nitrate (NO3) is reduced to nitrite (NO2) which is further reduced to nitric oxide (NO)(Khatri et al., 2016). NO acts a chemical mediator for many different processes within the body, some of which have a beneficial effect on exercise performance. Some of these effects include reduced blood pressure, decreased VO2 at submaximal exercise, increased resistance to fatigue, and increased energy production (Bailey et al., 2010a; Fulford et al., 2013; Khatri et al., 2016; K.E. Lansley et al., 2011; Mills, Khatri, Maskell, Odongerel, & Webb, 2016). The purpose of this review is to explore the literature regarding beetroot juice and its effects on exercise performance. Specifically, we will be looking at the acute and chronic effects of beetroot juice on plasma nitrate/nitrite levels, aerobic exercise, anaerobic exercise, and general health.
Articles were searched through the Boise State Library online database. The following terms were used to find published peer reviewed articles: beetroot, beetroot juice, nitrate, nitrite, nitric oxide, performance, exercise, aerobic, anaerobic. Articles were chosen based upon study design with emphasis being placed on double-blind crossover studies. Further emphasis was given to studies using nitrate depleted beetroot juice as the control beverage. This decision was to ensure minimal confounding variables would be present between the placebo/control and beetroot groups. Subsequent studies were taken from the references from the initial studies when appropriate.
Conversion of Nitrate to Nitrite to Nitric Oxide:
Digested nitrate does not have the same physiological effects that nitrite has as a chemical mediator(Khatri et al., 2016). Nitrate, commonly consumed as beetroot juice, must be reduced from NO3 to NO2 in the mouth (Fulford et al., 2013). Digested nitrate is returned to the mouth through the saliva where it is reduced to nitrite through interaction with oral bacteria (Bailey et al., 2009, 2010b; Cermak et al., 2012; Fulford et al., 2013). Once ingested as nitrite it is absorbed into the blood stream. When blood samples are measured 45 min post consumption of nitrate rich beetroot juice, minimal differences are found in plasma nitrate levels however large increases in plasma nitrite levels will be found (F. J. Larsen, Weitzberg, Lundberg, & Ekblom, 2007). This is consistent with both acute and chronic supplementation. No time effect is noted with the consumption of beetroot juice over longer durations (K.E. Lansley et al., 2011). Levels peak approximately 2.5 hours post consumption and will remain elevated as long as daily supplementation continues. Nitrite levels will drop when it is further reduced to NO (F. J. Larsen et al., 2007). This reduction occurs in the presence of hypoxia, which lowers pH creating an acid reducing environment. This environment, along with activation of various nitrite reductases, leads to NO production (Fulford et al., 2013; Khatri et al., 2016; Mills et al., 2016; Petrie et al., 2016). Nitrite plasma levels have been found to be elevated between 150-300% following both acute and chronic supplementation (Cermak et al., 2012; Fulford et al., 2013; K.E. Lansley et al., 2011; F. J. Larsen et al., 2007) with no significant time effects being noted.
Nitric oxide is also a naturally occurring chemical compound found within the body. NO is created endogenously through the interaction of L-arginine or L-Lysine along with O2 and Nitric Oxide Synthase (NOS). These amino acids can be oxidized to NO with L-arginine being the most common (Ignarro, 1989). This process is increased 100-fold with the presence of additional plasma nitrite (Khatri et al., 2016).
Effects of beetroot juice on blood pressure:
NO has been found to decrease resting and exercise blood pressure through vasodilation. This is commonly seen with decreases in diastolic BP, systolic BP as well as mean arterial pressure(MAP)(Bailey et al., 2010b; Khatri et al., 2016). These changes occur within 2.5-3 hours post consumption with no time effect being noted with chronic usage. This blood pressure response is amplified with hypertensive individuals (Khatri et al., 2016) making beetroot juice viable as a dietary aid for hypertensive populations. Decreasing MAP allows for greater cardiac output leading to better O2 delivery to the working muscles. This increased cardiac output is demonstrated with an elevated VO2 in cardiac patients at similar workloads (Mills et al., 2016). Long term studies have also shown potential benefit of reducing the reoccurrence of myocardial infarction after 1 year (Khatri et al., 2016).
Effects of beetroot juice on aerobic exercise:
The most positive effects of beetroot juice supplementation regarding exercise performance has been found in submaximal aerobic exercise. Numerous studies have demonstrated a lower VO2peak at corresponding exercise loads (Bailey et al., 2009, 2010b; Ernest G. Rimer, Linda R. Peterson, Andrew R. Coggan, 2016; Khatri et al., 2016; K.E. Lansley et al., 2011; Katherine E. Lansley et al., 2011; Mills et al., 2016) along with increased time to exhaustion (Bailey et al., 2009, 2010a; Katherine E. Lansley et al., 2011). The exact mechanism of this improvement is not well known however several concepts have been examined and discussed.
Mitochondrial biogenesis is thought to be one cause of this increased performance. NO interacts with cytochrome c oxidase within the mitochondria playing a key role in oxygen sensing and adjusting energy output based on demand. This chronic interaction has been shown to lead toward functional mitochondrial biogenesis (Clementi & Nisoli, 2005; Nisoli et al., 2004). Although these authors did find mitochondrial biogenesis in their studies, other researchers have reported improved aerobic performance with the absence of this change.
Larsen et al. (2007) observed dramatic decreases in nitrite plasma levels comparing pre and post exercise in a sodium nitrate supplemented group of cyclists after 3 days of supplementation. They noted a decline from 226uM to 137uM in plasma nitrite levels following 5 bouts of submaximal aerobic exercise followed by a ride to exhaustion. This acute supplementation period yielded improved performance through a decrease in required VO2 in the 4 lowest intensities without mitochondrial biogenesis occurring. This decrease in VO2 also occurred independent of an increase in lactate indicating greater efficiency occurring within the muscle and likely the mitochondria (F. J. Larsen et al., 2007). Since mitochondrial biogenesis is a chronic adaptation, these VO2 improvements could not be attributed to this response.
One area of study regarding this VO2 improvement involves the phosphate oxygen ratio (P/O ratio). The P/O ratio refers to the amount of oxygen consumed as it relates to the number of ATP generated (F. J. Larsen et al., 2007; Mills et al., 2016). It is postulated that with elevated plasma nitrite, and the accompanying increase in NO, increases in the P/O ratio occur. This would account for the decrease in VO2 as reduced O2 would be needed to generate the required ATP.
Increases in the P/O ratio are thought the be the result of reduced proton leakage into the mitochondria (Mills et al., 2016) and reduced expression of ADP/ATP translocase (Filip J. Larsen et al., 2011). Decreased proton leakage into the mitochondria results in less ATP required to activate ADP/ATP translocase to remove them. This ATP sparing will increase the overall yield of ATP per O2 consumption leading to higher efficiency within the mitochondria.
Whatever mechanism is at play, consumption of beetroot juice as little as 2.5 hours prior to submaximal aerobic activity has demonstrated not just decreased VO2 requirements and time to exhaustion, but also improvements in time trial performance (Katherine E. Lansley et al., 2011), power output (F. J. Larsen et al., 2007), and decreased RPE (F. J. Larsen et al., 2007; Petrie et al., 2016).
The positive effects of beetroot juice on submaximal aerobic performance begin to attenuate around the lactate threshold or roughly 75-80% VO2peak. At this point steady state VO2 is not achieved and accessory muscles begin to activate to maintain cadence along with increased glycolytic energy yield (F. J. Larsen et al., 2007).
Although the majority of research is in favor of beetroot juice as an ergogenic aid in submaximal aerobic exercise, there are studies that have failed to find these effects. Cermak (2012) found that an acute bolus of beetroot juice (8.7 mmol nitrate) did not have an effect on 1hr time trial performance on highly trained cyclists (VO2 60±1) despite the dramatic increase in blood plasma nitrite. These negative findings are in the minority and as such the general consensus is positive for the use of beetroot juice.
Effects of beetroot juice on anaerobic performance:
Less positive results have been found with performance gains in the realm of anaerobic performance. Some studies have suggested that the presence of NO or NOS has a depressing effect on force generation (Kobzik, Reid, Bredt, & Stamler, 1994). This reduction in muscle strength is thought to be the result of decreased calcium sensitivity and slower filament shortening velocity (Perkins, Han, & Sieck, 1997).
In a more recent study Fulford et al. (2013) explored the capacity of beetroot juice effects on maximal voluntary contractions. In addition to looking at PCr recovery and force generation, they were also curious to see if they could recreate the NO depressing effects previously documented. They found no decrease in power output however found only marginal, if any, improvement. The improvements that were found were in the final repetitions of a 50-repetition bout which brings into question the nature of the test remaining anaerobic. They did note previous studies hadn’t looked at maximal contractions and were more focused around submaximal. This could account for the difference found regarding depressed power output. In addition to power this study examined beetroot juices effect on PCr mechanics looking both at PCr recovery as well as PCr usage. No differences were found in PCr recovery however miner improvements in PCr efficiency were noted in the beetroot juice group. This increased PCr efficiency at a given work intensity may also play some role in the aerobic VO2 reductions noted in the previous section.
Wylie et al. (2016) found that 10 trained cyclists (VO2 58±8 ml/kg/min) did have an increase in power post beetroot consumption (8.2mmol daily) with short duration sprints (6 seconds) interspersed with rest but not with longer sprints (30 and 60 seconds). They concluded beetroot juice may have some effect on maximal efforts however the type and duration of these efforts would be impactful to the results. More research on these effects is required.
Effects of beetroot juice on other exercise markers:
Despite the decreases in submaximal VO2 and blood pressure, beetroot juice has shown minimal effects on a wide variety of other biological exercise markers. No corresponding changes have been noted in HR, VO2peak, RER, lactate response, PCr recovery, hemoglobin levels, or hematocrit in aerobic or anaerobic studies (Bailey et al., 2010a; Fulford et al., 2013; Khatri et al., 2016; K.E. Lansley et al., 2011; Katherine E. Lansley et al., 2011; F. J. Larsen et al., 2007; Mills et al., 2016). The absence of any meaningful change to these metrics further illuminates the improvements of beetroot juice being deriving from increased P/O ratio efficiency within the muscle.
Beetroot juice as a dietary aid:
Due to the improved O2 efficiency and decrease in blood pressure, beetroot juice is used to supplement the diets of the ill and elderly. Exercise is well known to improve mental cognition. Through the addition of beetroot juice, a group of elderly adults where better able to exercise compared with their control group, and as such had corresponding improvements in their mental conditions compared with control groups. Additionally, cerebral blood perfusion was increased as a result of the vasodilation effect leading to a decrease in MAP and a greater cardiac output. (Petrie et al., 2016; Presley et al., 2011).
Negative effects of Beetroot Juice:
Reviewed studies all reported no negative effects of the consumption of beetroot juice in regard to GI distress or other performance related metrics. The only negative effect is the red tinge given to both urine and feces post consumption (Bailey et al., 2010a).
Summary of results:
Consumption of beetroot juice elevates levels of blood nitrite through the reduction of its high concentration of nitrate. Nitrate is reduced to nitrite via interaction with oral bacteria in the mouth with peak plasma levels around 2.5 hours post consumption. Elevated levels of nitrite are reduced post exercise indicating the physiological improvements observed are related to the conversion of nitrite to nitric oxide. This conversion to NO from NO2 in influenced by hypoxia and the lowering of pH due to exercise demands.
Elevated levels of plasma nitrite have no significant time effect and will remain elevated as long as daily supplementation occurs. Most studies consumed a range from 6-10 mmol of nitrate which is a level that can be consumed in a daily diet consisting of nitrate rich foods. Most studies use a nitrate depleted beet root juice as a control. This removes confounding variables of other trace nutrients being the cause of physiological effects. Therefore, the only difference between beverages is the presence, or lack thereof, of nitrate.
Beetroot juice is an effective ergogenic aid in submaximal aerobic exercise. Results begin to attenuate around 80% of VO2peak which corresponds with the lactate or aerobic thresholds. Improvements below this level are reflected in a decreased VO2 at a given work rate. Although some studies state this is a result of mitochondrial biogenesis, this increase in performance is more likely due to improved mitochondrial efficiency in the form of a greater P/O ratio. This improved P/O ratio is the result of decreased proton leakage across the mitochondrial membrane requiring less activation of ADP/ATP translocase leading to ATP sparing. Thus, a greater amount of ATP is resynthesized at the same O2 consumption.
The effects of beetroot juice on anaerobic exercise is equivocal at best, however most agree that there is no improvement to performance. Disagreements remain regarding the inhibiting presence of NO or NOS on muscular contraction. This inhibition is attributed to decreased calcium sensitivity and a decrease in shortening velocity. Other studies have not found this depressed effect in response to beetroot juice supplementation.
No negative effects are reported with the consumption of beetroot juice outside of discoloration to the urine and feces of the subject.
Due to its safety and consistent results, beetroot juice should be a recommended supplement for athletes engaging in submaximal aerobic activity. For those engaging in anaerobic activities there may be negative consequences to force generation and, at best, no or minimal improvements to strength. Beetroot juice should not be recommended for athletes engaging in these types of activities or sports. Beetroot juice can also safely be used as a dietary aid to help with blood pressure, blood flow, cognition and exercise performance in cardiac patients and the elderly.
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