Nitrate

Last Updated: September 28, 2022

Inorganic nitrate (NO3-) is an endogenously produced food product that appears to have a critical role in blood pressure and cardiovascular health management; the main ingredient in beetroot, nitrate converts to nitric oxide by various means independent of the NOS enzyme and may aid exercise.

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1.

Sources and Composition

1.1

Sources

Inorganic nitrate (NO3-) is a small molecule that is produced in the body and found in the diet. It is important for regulating circulation and blood pressure via the metabolite Nitric Oxide, which is produced in a parallel but independent pathway to L-Arginine,- nitric oxide synthase (NOS) signaling.[1]

A number of vegetables contain high amounts of dietary nitrate, with beetroot, celery, cress, chervil, lettuce, spinach, swiss chard, radishes, and rocket tending to have the highest concentrations.[2][3]

  • Beetroot at 1,459mg/kg (range of 644–1,800mg/kg)[4]
  • Radish at 1,868mg/kg (range of 1,060–2,600mg/kg)[5]
  • Rocket at 2,597mg/kg[6]
  • Spinach at 2,137mg/kg (range of 65–4,259mg/kg)[7][4][6][5][8]
  • Lettuce at 1,893mg/kg (range of 970–2,782mg/kg)[7][4]
  • Lamb's lettuce at 2,572mg/kg[9]
  • Rucola at 4,474mg/kg[9]
  • Chinese cabbage 1,388mg/kg (range of 1,040–1,859mg/kg)[5][8]
  • Swiss chard at 1,597mg/kg[9]
  • Crown daisy at 5,150mg/kg[5]
  • Dill at 2,936mg/kg[4]

With vegetables not being in the upper ranks of nitrate intake but otherwise quantified including:

  • Parsley[6]
  • Turnip at 624mg/kg (307–908mg/kg)[10]
  • Cabbage at 513mg/kg (333–725mg/kg)[10]
  • Green beans at 496mg/kg (449–585mg/kg)[10]
  • Leek at 398mg/kg (56–841mg/kg)[8]
  • Spring onion at 353mg/kg (145–477mg/kg)[10]
  • Cucumber at 240mg/kg (151–384mg/kg)[5]
  • Carrot at 222mg/kg (121–316mg/kg)[9]
  • Potato at 220mg/kg (81–713mg/kg)[9][8]
  • Dandelion averaging 195-202mg/kg[11][12] (range of 47-487mg/kg[11])
  • Garlic at 183mg/kg (34–455mg/kg)[10]
  • Sweet pepper at 117mg/kg (93–140mg/kg)[10]
  • Green pepper at 111mg/kg (76–159mg/kg)[5]
  • Onions at 87mg/kg (23–235mg/kg)[5]
  • Tomatoes at 69mg/kg (27–170mg/kg)[10]
  • Artichoke at 96mg/kg[9]
  • Soybean sprouts at 56mg/kg[5]

With tap water being noted to have a concentration of 26mg/L (range of 22.8–30.3mg/L) and mineral water (bottled) of 2.6mg/L (range of undetectable to 6.3mg/L); there are regulations in place to ensure tap water does not exceed 45-50mg/L in the US and UK;[10] nitrate may be higher in unregulated well water if said well has nitrate producing bacteria within the reservoir.[13] The variation in nitrate concentrations in vegetables (which is quite unreliable) is due to variations in growing conditions, such as light and moisture exposure of usage of nitrogen containing fertilizers.[14][15] The presence of a nitrate reductase enzyme also plays a critical role, as it is low active in lettuce (high nitrate content) and higher in activity in peas (low nitrate level).[16][17][18]

The greatest sources of dietary nitrates are the leafy green vegetable class, followed by low-calorie tuber vegetables (including turnips and beetroot). A dietary nitrate content is found in most vegetables, although to lower levels. Beetroot may merely be the most popular vessel due to the palatability of blending beetroot into shakes (relative to spinach or rocket) and low financial cost of beetroot

Although not a source of naturally occurring nitrates, cured meats may also contribute to dietary nitrate intake. A typical western diet appears to contain approximately 81–106mg of inorganic nitrate daily of which 80% appear to come from vegetable sources.[19][20] Intakes of nitrates appear to be higher in european countries (relative to mideastern and eastern countries as well as Africa and South America) with notable intakes including Belgium (148mg per person with 1992-1993 data[21]), France (121mg per person), and Italy (149mg per person).[22] The usage of nitrate as a preservative in cured meats may also contribute to dietary nitrate intake in lower quantities,[23] and some cheeses may use nitrate as a preservative and confer dietary nitrate levels.[24]

Washing, peeling, and practical cooking of vegetables may lower (but not abolish) the dietary nitrate content.[21]

In general, nitrate consumption appears to be highly correlated with vegetable consumption and further correlated with consumption of leafy green vegetables. A standard western diet is lower in nitrates relative to mediterranean diets due to this

1.2

Properties

Nitrate has historical usage as a gunpowder reagent.[25]

image

There have been anecdotal reports of free inorganic nitrate, in blenders, causing volatile results

1.3

Biological Significance

NO is a key signaling molecule that regulates a number of functions in the body. It is produced by the L-Arginine-nitric oxide synthase (NOS) pathway and also from directly from nitrate. Under normal circumstances, most NO is produced from L-arginine via the NOS enzyme. Since the L-arginine-NOS pathway is dependent on oxygen, NO can’t be produced via NOS signaling under hypoxic conditions that can occur during strenuous exercise or certain disease states. Under these conditions NO is produced directly from nitrate, making dietary nitrate of great importance for NO production under stressful conditions where oxygen levels are low.[1]

1.4

Daily Intakes and References

The acceptable daily intake (ADI) of nitrate according to the European Food Safety Authority (EFSA) has been set at 0.06mmol/kg, being 4.2mmol (260mg) for a 70kg human.[26] This value was derived initially from animal studies in 1962 noting that 500mg/kg sodium nitrate was the highest tested dose without any adverse effects in research animals and then a 100-fold safety buffer was added to result in 5mg/kg sodium nitrate (3.7mg nitrate per kilogram) and appears to have stood since then.[10][27]

It has been noted[10] that the average omnivorous diet contains around 1.2mmol of dietary nitrate daily while a vegetarian diet consumes approximately 4.3mmol, similar to the ADI.

2.

Pharmacology

2.1

Absorption

Dietary nitrate from food products has approximately 100% intestinal bioavailability[28] while dietary nitrite (possible reduction occurring in the stomach) in solution has a bioavailability in the 95-98% range.[29]

2.2

Metabolism

Dietary nitrate, upon intestinal absorption, circulates in serum until approximately 20-28% gets ejected into saliva[30][31][32] where some (approximately 20%) is metabolized into nitrite by commensal bacteria on the tongue; the nitrite is subsequently swallowed, resulting in approximately 5-8% of total dietary nitrate intake becoming dietary nitrite.[33] The salivary step appears to be critial, as not swallowing following ingestion of nitrate (via spitting saliva) prevents serum increases in nitrite,[34] and consumption of mouthwash that destroys bacteria markedly reduces the increases in serum nitrite;[35] the oral bacteria that are causative of this conversion are Veillonella, Actinomyces, Rothia, and Staphylococcus epidermidi.[36][37]

Following oral consumption of nitrate and its absorption in the intestines, it is ejected into the saliva and oral bacteria metabolize nitrate into nitrite. This step appears to be mandatory, and is inhibited by spitting and attenuated by mouthwash that destroys bacteria

Nitrite is reduced into nitric oxide by various agents that have reducing properties. Nitrite has been noted to be reduced initially by deoxyhaemoglobin[38] and other endogenous agents such as deoxymyoglobin,[39] cytoglobin,[40] neuroglobin,[41] xanthine oxidoreductase,[42][43] aldehyde oxidase,[43] carbonic anhydrase,[44] and cytochrome P450 enzymes.[45][46] Although nitrite is clearly not reliant on the nitric oxide synthase enzyme like L-Arginine is, endothelial NOS may also reduse nitrite into nitric oxide when oxygen concentrations in the blood are reduced (anoxia)[47][48] and appears to account for a large deal of nonsupplemental nitrite production in the body[49] especially in the fasted state (with no dietary nitrate intake).[50]

Nitrite may also be reduced to nitric oxide directly via antioxidants (reducing agents, with Vitamin C being noted to be capable of this[51] and other polyphenolics in wine[51]), and reduction via stomach acid has also been noted.[34][52][33]

The conversion of nitrite into nitric oxide is very general and not isolated to a single enzyme, and has been noted to occur systemically (in tissues) and in the stomach

2.3

Serum and Distribution

The plasma Tmax of nitrate from food products appears to be in the range of 1.5-1.8 hours[28] while nitrate from isolated nitrate supplementation has been noted to peak at 3 hours post ingestion.[53][10] Oral nitrite solution has a quicker AUC at 15-45 minutes.[29]

The half-life of nitrate following oral ingestion appears to be 5-8 hours[54][55][28][56][34] and nitrite appears to be rapidly taken up by tissues[57] where it has a biological half-life of 20-45 minutes,[29][58] which is greater than the 1-5 minutes seen in vitro.[55][34]

Consumption of 490mL nitrate-rich beetroot juice acutely has been associated with an increase in plasma nitrate by approximately 400%.[59]

In a randomized placebo controlled trial of amaranth (red spinach) extract in 16 healthy subjects, nitrate (NO3-) and nitrite (NO2-) levels increased by 2-fold relative to a placebo-control and remained elevated in the body for 8 hrs after a single 2g oral dose.[60]

2.4

Excretion

Ultimately, 65-75% of dietary nitrate is excreted via urination[61] with nitrite also being renally excreted, although with the potential for nitrite resorption from the kidneys to the blood.[62]

3.

Neurology

3.1

Oxygenation

Reduced oxygenation (usually secondary to reduce cerebral blood flow) in cerebral tissue of older adults is known to be related to cognitive decline and dementia risk[63][64] possibly through producing leukoaraiosis, a term used to refer to changes in cerebral white matter that are a result of chronic ischemia[65] and increased tortuosity of arterioles[66][67][68] and seem to be ubiquitous to aging-related cognitive decline.[69]

Poor cerebral oxygenation appears to be highly involved in cognitive aging (in general) and possibly causative of some observed changes

When assessing cerebral blood flow in older adults, the consumption of nitrate via beetroot has failed to significantly influence cerebral blood flow overal (globally) while blood flow increased within the subcortical and deep white matter of the frontal lobe after 4 days of supplementation of 8.5mmol nitrate.[69]

An acute study in older adults assessing nitrate supplementation noted that benefits to cardiovascular health were independent of any alteration in cognition after 3 days.[70]

A study during physical exercise during periods of hypoxia (to mimic high elevations) noted that, while nitrate supplementation (0.07mmol/kg) was able to increase muscle oxygenation (has been noted elsewhere[71]) and negated a bit of the ergolytic effect of hypoxia that intervention failed to influence cerebral oxygenation.[72]

There may be a role in dietary nitrate in preserving cerebral oxygenation rates, although the evidence of this delaying progression of neural diseases is not fully established yet

4.

Cardiovascular Health

4.1

Cardiac Tissue

Dietary nitrate in the rat (0.7mM sodium nitrate), sufficient to raise plasma nitrate concentrations by 80%, appears to have a protective effect on cardiac tissue during instances of low oxygen exposure.[73] The reduction in mitochondrial respiration seen during the low oxygen period (by 33%) and subsequent reduction in available ATP in heart cells (62%) appeared to be fully ablated in the nitrate group.[73]

4.2

Mechanisms (Nitric Oxide)

Nitric oxide (NO) is known to be produced in the body following metabolism of the amino acid L-Arginine via the nitric oxide synthase (NOS) enzyme[74] where subsequently NO exerts vasodilatory actions.[75] This pathway is the canonical nitric oxide production pathway known as the L-Arginine-NOS-Nitric oxide pathway.

Nitric oxide production secondary to nitrate appears to be independent of this pathway, where nitrate converts to nitrite (via salivary enzymes[34][33]) and then nitrite converts directly to nitric oxide; this is an alternate pathway known as the Nitrate-Nitrite-Nitric Oxide pathway.[10][25]

Nitric oxide production from dietary nitrate sources are independent of nitric oxide synthase enzymes (target of L-Arginine and Norvaline)

4.3

Microcirculation and Blood Flow

It was initially found that increasing nitrite levels in serum from 180nM to 2,600nM (nitrite injections) was able to greatly improve forearm blood flow[76] and that a significant but lesser increase was noted with the serum concentration of 350nM[58] which is achievable via food products.[25]

Dietary nitrate (via the intermediate nitrite) appears to be a bioavailable source of NO with comparably greater importance in areas of the body with low oxygen concentration[77][78] such as microcirculation[76] due to being metabolized into nitric oxide via deoxyhemoglobin and deoxymyoglobin,[76] which are increased in areas of the body suffering from hypoxia and establishes nitrite as a localized regulator of oxygen concentrations.[79][80]

Nitrite converts into nitric oxide on an as needed basis, and appears to have preference for areas with low oxygen concentrations (as the deoxygenation products of hemoglobin and myoglobin can facilitate conversion of nitrite into nitric oxide while hemoglobin and myoglobin themselves are inactive in this conversion).

Plasma nitrite levels appear to be inversely correlated with blood flow as assessed by flow mediated vasodilation, with persons experiencing endothelial dysfunction having measurably lower nitrite concentrations in serum,[81] and in animals blood flow and vascular control to peripheral limbs is increased during exercise following beetroot ingestion (1mmol/kg nitrate) relative to controls.[82]

In healthy volunteers nitrate has failed to affect flow mediated vasodilation despite a reduction in systolic blood pressure by 5mmHg[83] with another study in healty persons fed spinach (200g) noting a small but significant improvement in flow mediated vasodilation.[84]

4.4

Blood Pressure

In animals, it is noted that a reduction in blood pressure observed following nitrate consumption (1mmol/kg, human equivalent of 0.16mmol/kg) occurs during exercise (about 10mmHg), with no significant alteration in the mean arterial pressure of healthy rats at rest.[82]

Daily consumption of 250mL beetroot juice daily in older individuals with diabetes over 2 weeks was able to increase plasma nitrate and nitrite but failed to reduce ambulatory blood pressure.[85]

One study using 500g of juice (mixture of beetroot and apple at 72% and 28%, respectively, with 15mmol/L nitrate) in persons with resting systolic blood pressure over 120mmHg noted a trend to reduce blood pressure when measured 6 hours after consumption relative to placebo juice (Apple juice); this trend (P=0.064) reached significance when only analyzing the subset of men, and reached a 4-5mmHG reduction in systolic blood pressure as assessed by an ambulatory blood pressure measurement (Meditech ABM-04).[86] This study failed to detect a reduction in diastolic blood pressure or pulse rate.[86]

Another study has also found no effect on blood pressure reduction in people with hypertension who were being treated with 1-3 antihypertensive medications.[87] Their systolic blood pressure was between 120-160mmHg coming into the study, and they also had a diastolic blood pressure of less than 100mmHg.[87] They were also overweight on average (with those with a BMI >35 being excluded). The double-blind crossover trial found that 70mL of nitrate-rich beetroot juice twice a day for a week did not lower blood pressure relative to a low-nitrate beetroot juice placebo even though the high-nitrate juice increased blood and salivary nitrate/nitrite concentrations severalfold.[87]

4.5

Platelets

Nitric oxide is known to inhibit platelet adhesion to the endothelium[88] and aggregation of platelets.[89] It is thought that nitrite is inactive in this regard using studies in vitro[90][91] although it appears to be active but dependent on the addition of red blood cells and induction of hypoxia.[92]

Although one animal study delivering nitrite failed to find an influence on platelet aggregation,[57] oral potassium nitrate (2mmol nitrate) has been noted to inhibit platelet aggregation in otherwise healthy individuals[93] and consumption of beetroot juice (22.5mmol nitrate) has been noted to induce platelet inhibitory effects which are inhibited by spitting saliva (preventing enterosalivary conversion of nitrate to nitrite).[56]

May have platelet inhibiting properties, which would be cardioprotective at those at high risk for cardiovascular disease; currently has been demonstrated in humans, but no comparative studies against reference drugs exist

5.

Physical Exercise and Skeletal Muscle

5.1

Oxygen Consumption

Nitric oxide is thought to modulate respiration due to in vitro studies suggesting it is a reversible inhibitor of cytochrome C[94][95] and some animal researching showing that administration of nitric oxide synthase (NOS) inhibitors increases oxygen consumption.[96][97]

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.[98] There was no influence of nitrate supplementation at maximal work.[98] 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[99] 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).[100] 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).[101]

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).[102]

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

5.2

Power Output

Mitochondrial efficiency in skeletal muscle has been noted to be improved in humans consuming nitrate following muscle biopsy, thought to be related to reduced expression of ATP/ADP translocase.[103]

It has been noted that nitrate, in mice, increased contractile force of skeletal muscle (ex vivo) occurred when the rats were fed nitrate at 1mmol/L in their drinking water (said to be similar to doses used in human studies) and was thought to be related to increase tetanic calcium production.[104] When tested in humans, consumption of 500mL beetroot juice daily for 15 days, with muscle performance tested on day 1, 5, and 15 (via 50 maximum voluntary contractions) has failed to significantly influence power output relative to placebo.[105]

In humans fed beetroot there was slightly less usage of phosphocreatine (bioactive form of creatine) associated with beetroot.[105] This reduction in phosphocreatine usage rate has been noted elsewhere[106] without influencing phosphocreatine recovery rate[99] and may be related to less required ATP production per muscle contraction (noted in humans subject to knee flexion) following 500mL beetroot juice giving 5.1mmol (326.4mg) nitrate.[106]

Although it is theoretically plausible that nitrate can increase power output in muscle tissue, this has not yet been shown in humans and preliminary evidence suggests no significant effect with common supplemental dosages. There might be more metabolic efficiency during muscle contractions (less creatine and ATP usage per muscle contraction) although these appear to be independent of maximum power changes

Due to decreased energy usage secondary to increased metabolic efficiency, nitrate may have a role in preserving submaximal power output over time. This has been demonstrated in humans given 5.1mmol nitrate following a series of maximum voluntary contractions.[106]

May have a role in attenuating the usage of high-energy substrate (creatine and ATP) and enhancing performance in a series of muscle contractions

5.3

Cardiovascular Exercise (Anaerobic)

At times the difference between anaerobic and aerobic exercise can be a bit grey as the transition from anaerobic into aerobic is a spectrum rather than a dichotomy; this section comprises exercise which is endurance in nature but shorter than the following section

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;[59] 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.[59]

One trial using 'maximal effort running' has noted improvements in time to exhaustion where placebo was able to run for 7.6+/-1.5 minutes noted that consumption of beetroot juice enhanced this time to 8.7+/-1.8 minutes (15% increase).[99]

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 increased time to exhaustion increased by 15.7% relative to placebo as assessed by cycle ergometer.[101]

One study in elite ranking cyclists using 500mL beetroot juice prior to exercise has failed to find a significant difference in an approximately 20 minute time trial race (18:37 in placebo, 18:20 in beetroot; difference not statistically significant).[107] Power output on average was also similar, in trending towards being better in beetroot (290+/-43 Watts) rather than placebo (285+/-44 W) condition but failing to significantly outperform.[107] In well trained rowers doing a time trial, overall the benefit was not statistically significant (0.4+/-1.0% improvement in time) although there appeared to be significant improvement in the later portions of the test (1.7+/-1.0%).[108]

Studies using sprint of high intensity cycling procedures and dietary nitrate tend to note increased performance and time to exhaustion; limited studies in elite athletes are less promising and tend to border being statistically significant or not

One study assessing comparative effects of a single dose of nitrate (via beetroot) versus 15 days of loading failed to find a significant difference, with both being effective in incremental loading exercises.[109] For acute benefits, it appears that oral ingestion of nitrate containing products 2-3 hours prior to exercise is recommended.[110]

A single dose may be as efficacious as a loading protocol

5.4

Cardiovascular Exercise (Aerobic)

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.[111] 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[111]) 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).[112] 140mL beetroot juice (8.7mmol nitrate) has failed to significantly improve cycling performance in trained athletes despite increased serum nitrate.[113]

In a 10 kilometer time trial in trained cyclists, 6 days of dietary nitrate (8mmol) via beetroot juice, the time trial performance in placebo (965+/-18 seconds) was improved upon with beetroot (953+/-18 seconds) by 1.3% and met with increased average power output (2%) and reduced submaximal oxygen expenditure in beetroot.[114]

There appears to be some benefits associated with prolonged cardiovascular exercise and nitrate consumption, but the magntiude of benefit is comparatively less than that seen in anaerobic exercise and sometimes does not appear to have a large practical significance

6.

Interaction with Organ Systems

6.1

Oral Cavity

Nitrate is known to have its conversion into nitrite dependent on oral bacterial metabolism, and ingested nitrate can convert into nitrite in the mouth.[34][35] Human saliva tends to contain 9mg/L of nitrite[115] and due to secreting 500−1500mL of saliva daily there is a constant exposure of 4.5-13.5mg nitrite daily.

6.2

Thyroid

It is thought this may be related to nitrate being able to mechanistically prevent iodine uptake into the thyroid by binding to the sodium-iodide symporter, which reduces circulating T3 and T4 while increasing TSH (changes in accordance with hypothyroidism)[116][117][118] and similar to other anionic inhibitors the potency of this effect is dependent on iodine concentrations (with more iodine concentrations being able to preserve its own activity via competitive interaction with nitrate, and lower circulating iodine concentrations causing more potent inhibition of nitrate).[119][118]

Dietary nitrates has been weakly (but significantly) associated with hypothyroidism in epidemiological research (Odds Ratio 1.2; 95% CI of 1.1-1.4) and a highly variable but significant association with thyroid cancer (Relative Risk of 2.9; 95% CI of 1.0-8.1).[120] It is thought that chronic stimulation of the sodium-iodide symporter by nitrate is due to this, as chronic stimulation has been previously linked to thyroid hypertrophy, hyperplasia, and neoplasia[121][122] which has been further linked to nitrate in drinking water when the nitrate exceeds legal limits.[123][124]

Nitrate may competitively inhibit iodine uptake into the thyroid and exert hypothyroidic effects. There is a significant but weak relationship between dietary nitrate and hypothyroidism, and no intervention research exists currently and thus overall risk and potency cannot be assessed (although it doesn not seem highly potent)

6.3

Kidneys

Nitrate reductases are present in renal tissue[125] which can reduce nitrate to nitrite[126] and appears to have xanthine oxidoreductase as a critical factor, as it is inhibited by allopurinol.[125] The presence of these enzymes, and the role of nitric oxide in renal health, has led to the hypothesis that dietary nitrate may reduce vascular complications via kidney dependent mechanisms.[25][127]

A study in rats giving 1mmol/kg nitrate for 5 days (reaching serum levels seen in humans) noted significantly higher renal vascular control.[82]

Supplementation of nitrate exerts protective effects against L-NAME (NOS inhibitor) induced renal injury[128] and has shown renoprotective effects in rat model of salt-induced hypertension, which showed fairly strong protective effects as renal inflammation, protein and dimethylarginines in urine, and tubular/glomerular changes were abolished at 0.1-1mmol/kg nitrate while arterio-arteriolar sclerosis and fibrosis were attenuated.[129] As protective effects are noted with L-Arginine supplementation (to a lesser degree) in rats[130][131] it is thought these effects are mediated via nitric oxide.

Interstingly, the molecule known as assymetric dimethylarginine (ADMA) that is produced to higher levels in renal diseases and inhibits NOS (thus furthering pathology of renal disease)[132] appears to be reduced to baseline levels following supplementation of nitrate to rats;[129] possible explaining the greater relative efficacy of nitrate to L-Arginine (as nitrate is not NOS-dependent to form nitric oxide).

Research is preliminary and currently validated only in rodents, but supplemental nitrate appears to be highly protective of the kidneys possibly through nitric oxide production

7.

Interactions with Cancer

7.1

Nitrosamines

Dietary nitrate from beetroot converts to nitrite in the body, which is structurally the same as nitrite as a preservative found in cured meats due to its efficacy in preventing botulism infection of the food.[133] It has been noted that nitrite may form molecules in the class called nitrosamines including one carcinogen known as dimethylnitrosamine.[134][135][136] This class of N-nitroso compounds tends to have carcinogenic potential[137] there is currently still not a solid epidemiological link between dietary nitrate consumption and cancer rates in general.[138][12][139] Some evidence exists to suggest a link between dietary nitrate intake (CI of 1.05-1.69)[140] and thyroid cancer (Relative risk of 2.6; 95% CI of 1.1-6.2)[120] although no significant link has been reported for pancreatic,[141] stomach and esophageus carcinoma,[142] lymphoma,[143] or kidneys.[144]

It has been noted[10] that the lack of evidence to connect nitrates in vegetables to cancer may be confounded by other molecules in vegetables with anti-cancer properties. Another study[145] has concluded the weak evidence to support a link between cancer and nitrate consumption is outweighed by cardiovascular benefits.[145]

Nitrates appear to form a group of metabolites known as nitrosamines, some of which are carcinogenic in vitro; there is no clear link between nitrate consumption in a mixed diet or in the drinking supply and cancer risks, although some evidence supports a small but statistically significant link. Some sources note that the cardiovascular benefits of nitrate consumption outweight the cancer risk in practical settings

Some interactions with plant based molecules and sodium nitrate are more complex, with Vitamin C being traditionally known to reduce nitrosation rates due to being a direct antioxidant[146] yet a study assessing nitrosation rates in the presence of lipids (to mimic a mixed meal) noted that while ferulic acid and caffeic acid were inhibitory on this reaction that Vitamin C, gallic acid, and chlorogenic acid increased nitrosation[147] which is dependent on fatty acids.[148][147] In instances where cancer is experimentally induced, Vitamin E has shown an adverse interaction with nitrosamines in the stomach[149] which is similarly seen with green tea catechins.[150]

The interactions between nitrates (and subsequent nitrosamine production) and plant polyphenolics and antioxidants appear to be complex and context dependent, with no clear result of what the risk is for an intervention using nitrate supplementation

8.

Longevity and Life Extension

8.1

Mitochondrial Interactions

Nitric oxide is thought to positively modulate mitrochondrial respiration due to in vitro studies suggesting it is a reversible inhibitor of cytochrome C[94][95] in the nanomolar concentration range (half inhibition at 250nM when oxygen is at near arterial levels of 150nM, with 60nM NO increasing the KM of oxygen respiration from below 1μM to 30μM)[151][152] by competitive binding to the oxygen binding site;[94] the biological significance of nitric oxide in this role is supported by administration of NOS inhibitors (to reduce nitric oxide levels) increasing oxygen consumption in animals[96][97] and in vitro studies noting a tonic inhibition of cell respiration at cytochrome oxidase under the influence of nitric oxide.[153][154][155]

Other aspects of mitochondrial respiration are less selectively inhibited by nitric oxide such as complex I[156] via nitrosylation and nitration and these general effects appear to be mediated via the nitric oxide metabolite known as peroxynitrate[157][158][159][160] in a slower, nonselective, and irreversible manner (whereas nitric oxide is rapid and reversible while being selective).[161] These oxidative and seemingly negative mechanisms may sensitize cells to hypoxic conditions via hormetic mechanisms.[161]

This apparent increase in mitochondrial efficiency has been noted in human interventions using supplemental nitrate[103] and may underlie the reduced oxygen cost of exercise (the reduced oxygen cost being well supported, its link to mitochondrial efficiency being hypothesized).[110] The mechanisms are more linked to nitric oxide rather than nitrate per se, and at least the reduced oxygen cost of exercise has been noted with supplemental L-Arginine as well (also capable of increasing nitric oxide).[162]

Nitric oxide and its metabolite, peroxynitrate (both increased via nitrate supplementation) appear to negatively influence mitochondrial respiration directly at physiologically relevant concentrations, which may cause a hormetic protective effect and subsequent improvement in mitochondrial respiration efficiency (which is the effect noted in human interventions)

8.2

Correlation and Interventions

In mice, it has been found that serum nitrate levels are lower than a more youthful control rats, and the normalization of serum nitrate levels (50mg/L sodium nitrate in drinking water) was associated with an improvement in parameters of aging associated with the endothelium.[163]

One study using nitrate supplementation in older adults noted standard cardiovascular benefits (attenuation of oxygen consumption in exercise assessed via VO2 and reduction in blood pressure) but failed to influence functional capacity or cognitive function associated with aging.[70]

9.

Nutrient-Nutrient Interactions

9.1

Betaine

Betaine is the bioactive form of supplemental choline, is found in beetroots, and works as a methyl donating agent[164] and can be vasoprotective[165] when orally ingested. Its interactions with nitrate are investigated as betaine itself has ergogenic properties[166][167] and is a component of beetroot and thus may be related to the observed effects.

A study using an acute dose of betaine at either 1,250mg or 5,000mg or chronic loading at 2,500mg (14 days) or 5,000mg (7 days) in healthy exercise-trained men failed to find any significant influence on plasma nitrate or nitrite.[168] Elsewhere, the erogenic effects of betaine has been found independent of an increase in plasma nitrate or nitrate.[169]

Betaine, being a component of beetroot alongside nitrate and also being ergogenic, does not appear to have any proven synergistic interactions with nitrate at this moment in time

9.2

Vinpocetine

Chronic usage of nitrates or nitrate donors is known to induce a state of tolerance called 'nitrate tolerance'[170][171] which appear to be associated with an increase in the activity of PDE1A1.[172] Nitric oxide normally acts upon its receptor to increase cGMP concentrations,[173] and PDE1 enzymes (all isoforms) degrade cGMP[174][175][176] so an increase of the enzyme acts to normalize cGMP concentrations and thus the effects of nitric oxide. PDE1A1 does not fully explain nitrate tolernace, but plays a role.[172]

Vinpocetine is a PDE1 inhibitor,[177][178] and appears to partially restore tolerance to nitrates in vitro and augmented nitrate-induced vasodilation in already sensitive rings.[172]

Although there is no evidence in living systems right now, vinpocetine appears to be promising for reducing nitrate tolerance and also augmenting the ability of nitrate to induce relaxation in endothelial cells

9.3

Arginine

The chronic use of nitrate donors which produces a state of nitrate tolerance[179][180] could also be theoretically attenuated with arginine. This is because the enzyme known as arginase II has been found to regulate endothelial nitric oxide synthase (eNOS) activity by affecting intracellular L-arginine levels, which is its substrate.[181]

An in vitro experiment using rat aorta supported this hypothesis. The aorta was incubated with 0.5 mM L-arginine for two hours and showed a reduction in nitrate tolerance as measured by an attenuated shift in the concentration-relaxation curve 4.4-fold that of control values.[182]

One human study may also support the hypothesis that arginine may attenuate nitrate tolerance. The study was done in patients with stable angina using a transdermal nitroglycerin patch releasing 0.4 mg/h. 700 mg of L-arginine administered 4 times a day significantly increased the time the patients could walk on a treadmill without experiencing angina 4 hours and 24 hours after the patch was applied over placebo, suggesting attenuation of nitrate tolerance.[183]

Whether arginine would affect tolerance to supplemental nitrate in healthy individuals has not yet been studied.

In vitro and limited human evidence suggests that L-arginine may attenuate nitrate tolerance.

10.

Safety and Toxicology

10.1

Blue Baby Syndrome

There are limits on nitrate concentration in the water supply (45-50mg/L limit) due to associations with infantile methaemoglobinaemia, a condition known as 'blue baby syndrome'[184] which rarely occured below 44mg/L.[185]

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