Beetroot

Beetroot (of the family Chenopodiaceae), usually as juice, is one of the more common supplemental sources of dietary nitrate conferring at least 1,300mg/kg fresh weight^[1] with one review noting an average value of 1,459mg/kg fresh weight (range of 644–1,800mg/kg);^[2] up to 2500mg/kg has been reported.^[3]

It does not appear to be the largest source on a weight basis (being outperformed by leafy green vegetables such as spinach, rocket, and lettuce) although the juice may be more palatable.

Beetroot tends to contain:

• Inorganic Nitrate (NO^3-), as the main bioactive for cardiovascular and endurance exercise interactions
• The pigment class of betalains, predominately betanin and vulgaxanthin^[4]

The pomace of beetroot also contains phenolics (45.68mg Gallic acid equivalents) and flavonoids (25.89mg Rutin equivalents) with both betanin (4.09mg/g) and vulgaxanthin (7.32mg/g).^[4]

The main bioactive in beetroot is nitrate, insofar that many studies use beetroot without nitrate as a placebo intervention and note significant difference in microcirculation and exercise performance. Betalains may also contribute to beetroot bioactivity

In regards to plasma nitrate (NO^3-), 0.1mmol/kg daily for 3 days via sodium nitrate can raise plasma nitrate from 27+/-6.9uM to 182+/-55uM (585% increase)^[5]

Plasma nitrite (NO^2-) levels are also increased following nitrate consumption, with up to a 400% increased level at rest^[6] or more moderate estimates of an increase to 373+/-211nM (103% higher than placebo) following consumption of 6.2mmol nitrate for 6 days.^[7] Nitrite levels appear to be 'used up' during exercise, although following nitrate consumption a 20% decrease in nitrite seem in placebo is still 54% above baseline placebo values;^[6] higher serum nitrite levels, as seen with nitrate supplementation, has an accelerated rate of decline during exercise relative to basal nitrate concentrations (which are fairly static).^[5]

Both plasma nitrate and nitrite are increased following consumption of nitrate sources including beets. Nitrate appears to be somewhat of a chronic increase and forms a 'pool' of nitrate substrate, and nitrite also forms something similar to a pool of substrate that is used up during exercise

Nitric oxide is thought to modulate respiration due to in vitro studies suggesting it is a reversible inhibitor of cytochrome C^[8][9] and some animal researching showing that administration of nitric oxide synthase (NOS) inhibitors increases oxygen consumption.^[10][11]

In healthy trained adults, ingestion of 0.1mmol/kg nitrate is associated with a reduced oxygen cost of exercise by 5.4% (assessed via VO₂) during submaximal performance and increased energy efficiency by 7.1%; these were independent of changes in heart rate, respiratory exchange ratio, and blood lactate.^[5] There was no influence of nitrate supplementation at maximal work.^[5] Another study confirms the decrease in oxygen usage but found benefit for low (20% reduction), medium (7.1%), and high (7.2%) intensity running exercises where time to fatigue was increased 15% in high intensity running following 6.2mmol nitrate for 6 days^[7] and this is noted elsewhere where nitrate at 0.1mmol/kg (via sodium nitrate) reduced VO₂ max yet did not adversely influence performance (trended nonsignificantly to increase time to exhaustion).^[12] One study has noted that muscle extraction of oxygen has been reduced 19% following supplementation of 500mL of beetroot juice (11.2+/-0.6mM nitrate).^[13]

These effects appear to occur after a single acute dose of dietary nitrate at 10mg/kg (0.16mmol/kg) reducing VO₂ max by 3.8% during endurance exercise and without adversely affecting time to exhaustion (again noting an insignificant trend to improve).^[14]

Supplementation or dietary ingestion of nitrate appears to reduce oxygen cost of exercise and reduce VO2 max without affecting exercise performance; this is thought to be secondary to increasing efficiency of substrate utilization

Beetroot supplementation has been associated with a decrease in both systolic and diastolic blood pressure. ^[15][16] Nitroglycerin, an antianginal agent used to prevent and treat acute chest pain, helps produce nitric oxide, which causes vasodilation of blood vessels and a drop in blood pressure. ^[17] Beetroot supplementation in conjunction with other blood pressure lowering agents, such as nitroglycerin, may be cautioned, to prevent a potential unsafe drop in blood pressure. There has also been mention that beetroot possibly increases blood clotting,^[18] but this has been disputed by other professionals.^[19]

A small study (n=8) of persons consuming 500mL beetroot juice for 15 days (relative to placebo, a nitrate depleted beetroot juice) has failed to significantly influence muscular power output either acutely or after 15 days; this study noted a slightly lesser usage of phosphocreatine (bioactive form of creatine) associated with maximal voluntary contractions.^[20]

There do not appear to be any significant influences of beetroot juice on acute power output

During an exercise period, in a study where plasma nitrite declined 20% during exercise in trained athletes, consumption of 490mL nitrate-rich beetroot juice prior to exercise resulted in a 52% increase in plasma nitrate (relative to their own baseline values).^[6] An increase in plasma nitrate and nitrite in those consuming beetroot juice relative to placebo during exercise trials with the most commonly ingested dosage of beetroot juice being 500mL.^{[21][13][22][14]}

Consumption of beetroot juice is associated with increased plasma nitrate and nitrate during physical exercise

In athletes subject to exercise trials, performance in a Yo-Yo intermittent recovery level 1 test (20m intermittent sprinting) increased by 4.2% relative to placebo following the consumption of 490mL beetroot juice;^[6] This increase in performance was not accompanied by any changes in plasma lactate, but accompanied by a reduction in glucose relative to placebo (9.6%) and a trend (P=0.08) to reduce the rise in serum potassium.^[6]

In a longer trial, 11 recreationally active persons consuming a single dose of beetroot juice (89kcal and over 500mg nitrate; prepared via 90 minutes of oven baking followed by food processing) relative to the placebo juice (cranberry) noted a trend (P=0.06) to improve time to complete a 5 kilometer run and this was independent of any significant changes in heart rate, blood pressure, or the rate of perceived exertion overall; the only statistical significance arose for the rate of perceived exertion in the first third of the run, and running velocity for the final stretch of the run.^[21] Overall, the 5 kilometer trial was completed 41 seconds quicker in the beetroot group (although the very limited sample size should be taken with caution^[21]) and another study using a time trial design (but for cycling) has found that acute ingestion of 500mL beetroot juice (6.2mmol nitrate) 150 minutes prior to the time trial was associated with a 2.8% improvement in the first 4km of the trial and 2.7% after all 16.1km (which was associated with improved power output but no alterations in VO2 max).^[23] 140mL beetroot juice (8.7mmol nitrate) has failed to significantly improve cycling performance in trained athletes despite increased serum nitrate.^[24]

A slight improvement in time trial performance on both running and bicycling. The studies in support of this are unanimous, but individually underpowered

For trials measuring a time to exhaustion (in which a longer time to become exhausted is indicative of enhanced physical endurance), 500mL of beetroot juice (11.2+/-0.6mM nitrate) has been shown to reduce muscle fractional oxygen extraction was reduced 19% and the time to exhaustion increased by 15.7% relative to placebo as assessed by cycle ergometer.^[13] This reduction in oxygen consumption relative to placebo has been replicated elsewhere with 0.1mmol/kg sodium nitrate (food equivalent of 100-300g beetroot) on an ergometer test^[12] and has been noted in walking and running, where the time to exhaustion in high intensity running was prolonged 15% relative to placebo.^[7]

At least one study assessing an acute dosage versus 15 days of chronic dosing has failed to find a significant difference, with both time periods of 500mL beetroot juice (5.2mmol nitrate) reducing the oxygen cost of exercise and improving performance to similar degrees.^[22] Two other studies have been conducted confirming the increase in energy efficiency (via reduction in oxygen requirement of exericse) although these two studies did not assess time to exhaustion.^[5][14]

In tests that measure time to exhaustion (rather than those measuring time to complete X distance), beetroot juice appears to be associated with an anti-fatigue effect and prolongation of exercise performance

A few studies using beetroot extract have noted anti-cancer effects in cells despite a nitrate concentration, attributed to the betalain content (due to their antioxidant properties).^[28] The apoptotic effects have been noted in breast,^[29] prostatic,^[29] liver,^[30] skin,^[31][30] and lung cancer cells.^[31] The anticancer effects have been noted with a drinking water containing the beetroot pigmentation,^[32] and have been said to possibly be a safer option of dietary nitrate for cancer patients pending more research.^[1]

Beetroot extract appears to have anti-cancer properties in vitro despite a nitrate content, thought to be related to the betalain content; no studies currently assessing living systems

There has been evidence, in rats, that increased beetroot consumption results in an accumulation of copper, iron, magnesium, manganese, phosphorus, and zinc.^[33]

Ingestion of beetroot, which is abundant with nitrates, can cause a decrease in blood pressure, resulting in hypotension.^[34]

Additionally, ingestion of beetroot can cause red urine.^[35]

One study has reported a link between beetroot supplementation and postural hypotension.^[36]

In Finland, raw beetroot consumption was linked with several cases of sudden gastrointestinal illness. No common foodborne pathogens or toxins were detected in the beetroot samples, but the beetroot samples were all of poor quality according to total bacteria counts.^[37]