Citrulline

L-Citrulline is one of the three amino acids in the Urea cycle alongside L-Arginine and L-Ornithine; it turns into L-Arginine after absorption and being sent to the kidneys, so it is actually a better method of getting supplemental L-Arginine into the blood rather than L-Arginine itself.

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L-Citrulline is one of the three dietary amino acids in the urea cycle (alongside L-Arginine and L-Ornithine; L-arginosuccinate is also an intermediate) and is a standalone dietary supplement for sports performance and cardiovascular health.

Citrulline is unique relative to the other two aforementioned amino acids since it uses a different set of intestinal transporters, and is not subject to reduced absorption at around 10g which may result in diarrhea with excessive oral dosing (this is seen with both arginine and ornithine). Additionally, while arginine and ornithine raise plasma arginine and ornithine respectively supplemental Citrulline raises plasma levels of all three intermediates (about a doubling of arginine and ornithine, both of which are a tad lower than supplementing those amino acids straight) and dose-dependent increases in plasma citrulline.

Due to the ability of citrulline to raise plasma ornithine and arginine, it confers both the benefits of ammonia recycling (a property of ornithine) and nitric oxide metabolism (a property of arginine).

There is not enough evidence to establish the efficiency of citrulline in increasing plasma ornithine relative to ornithine itself, but it appears that citrulline causes a doubling of ornithine and is somewhat capped at that while supplemental ornithine can cause dose-dependent increases up to 300-500%; in this sense, ornithine is a better supplement to increase plasma ornithine.

As for arginine, supplemental citrulline seems to only be able to double plasma arginine or so due to limited conversion in the kidneys. Citrulline then forms a pool of citrulline in plasma avaiting conversion into arginine. When comparing citrulline to arginine supplementation, arginine can increase plasma arginine higher for a short period but citrulline provides a more prolonged boost in plasma arginine. Due to this, supplemental arginine for purposes that are not acutely time dependent (cardiovascular health, erectile dysfunction) would be better gained through citrulline.

The functions of citrulline as it pertains to supplementation included a reduction in fatigue and promotion of endurance in prolonged exercise, and it appears to apply to both aerobic and anaerobic. There is not enough evidence to support an acute increase in power output with citrulline supplementation, only a preservation of power output throughout exercise.

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Also Known As

L-Citrulline, Stimol (Brand Name), Watermelon extract


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The lower end of dosing appears to be one gram (1,000mg) of citrulline taken thrice a day with meals, totalling 3,000mg at the end of the day. This appears to be somewhat effective for promoting circulatory health and alleviating erectile dysfunction (possible ED associated with high blood pressure).

If following the above dosing, a conversion may be needed if you are using a product containing citrulline malate rather than citrulline; for every gram of citrulline taken you would be required to take 1.76 grams of citrulline malate to account for the weight of malic acid.

For ergogenic (performance enhancing) purposes, a dose in the range of 6,000-8,000mg is taken acutely about an hour before exercise, these studies tend to use citrulline malate anyways so the above conversion is not necessary.


My thoughts on citrulline right now are:

  • We still don't know if ornithine's benefits are dose-dependent or whether a doubling is sufficient. It is wholly plausible that ornithine supplementation is not needed with citrulline

  • Regardless of whether or not arginine supplementation would confer additional benefits alongside citrulline, they should additively increase serum levels (different transporters)

It appears to be the best of the three main amino acids of the urea cycle, probably just because it is an efficient way of getting the other two increased.


Kurtis Frank

The Human Effect Matrix looks at human studies (excluding animal/petri-dish studies) to tell you what effect Citrulline has in your body, and how strong these effects are.
GradeLevel of Evidence
ARobust research conducted with repeated double blind clinical trials
BMultiple studies where at least two are double-blind and placebo controlled
CSingle double blind study or multiple cohort studies
DUncontrolled or observational studies only
Level of Evidence
EffectChange
Magnitude of Effect Size
Scientific ConsensusComments
BBlood Glucose

There do not appear to be any significant influences of supplemental citrulline on blood glucose concentrations

BInsulin

Most studies note that there is no significant change in insulin concentrations, although a lone study (not replicated) suggested that the exercise-induced increase in... show

CUrea

Minor

An increase in urea is seen with citrulline supplementation (possibly due to increased serum Ornithine sequestering ammonia)

CNitric Oxide

Notable

Nitric oxide derivatives (nitrate and urinary cGMP, since nitric oxide itself is hard to measure these biomarkers are indicative of nitric oxide production) appear to be... show

CGrowth Hormone

Minor

An increase in growth hormone has been noted with exercise, but not at rest. Practical significance of this information is unknown, since Arginine supplementation has unreliable... show

CCreatinine

Minor

A slight increase in creatinine has been noted with citrulline supplementation, practical relevance unknown.

CPlasma Glutamine

Minor

Although acute usage does not appear to influence plasma glutamine, a week of high dose supplementation (0.18g/kg) has been noted to reduce glutamine concentrations slightly.

CMuscle Protein Synthesis

Currently no human evidence to support a practically significant increase in muscle protein synthesis at rest with citrulline supplementation

CIGF-1

No demonstrated effects on IGF-1 concentrations following the usage of citrulline supplementation

CPlasma Arginine

Notable

Citrulline can increase plasma arginine concentrations, and due to acting as a resevoir of arginine it is actually more effective overall at increasing plasma arginine... show

CFatigue

Notable

The decrease in fatigue during exercise is thought to underlie most of the benefit seen with training capacity (work volume), although in men who self-report fatigue issues... show

CTraining Volume

Notable

The increase in work capacity seen with citrulline supplementation appears to be time dependent. While there are no inherent and immediate effect, the reduction of fatigue... show

CMuscle Soreness

Notable

The lone study using citrulline acutely pre-workout noted a 40% reduction in muscle soreness the following two days after the workout.

CBlood Pressure

Minor

A decrease in blood pressure is noted secondary to increasing plasma arginine (and thus increasing nitric oxide), but this blood pressure reduction may only occur in those... show

CHeart Rate

No significant influence on heart rate seems likely, although at least one study noted a random but significant reduction post-exercise

CLactate Production

No significant influence on lactate production seen with citrulline supplementation before exercise

CAerobic Exercise

No significant influence on a treadmill test to exhaustion nor oxygen kinetics during this test

CImmunity

Minor

Neutrophil oxidative burst post-workout appears to be enhanced when the exercise is preloaded with citrulline supplementation

CDNA Damage

No significant influence of citrulline on DNA damage

CLipid Peroxidation

No significant influence of citrulline on lipid peroxidation has been noted

CLiver Enzymes

Plasma concentrations of LDH are unaffected following citrulline supplementation

CBlood Flow

Minor

An increase in blood flow is noted with citrulline supplementation in persons with impaired blood flow

CPlasma Nitrate

Minor

An increase in plasma nitrate has been noted with citrulline supplementation, but not to the degree of supplemental Nitrate itself

CLDL-C

No significant influence on LDL-C concentrations in serum following citrulline supplementation

CHDL-C

No significant influence on HDL-C concentrations

CTotal Cholesterol

No significant influence on total cholesterol concentrations

CC-Reactive Protein

No significant influence on C-reactive protein concentrations

DMuscle Oxygenation

Minor

An increase in muscle ATP production via aerobic means was noted in men given 6g citrulline daily, but this appeared to be attenuated with time.

DErections

Minor

Hardness of erections in persons with mild erectile dysfunction appears to be increased following supplementation of citrulline supplementation.

DExercise Capacity (with Heart Conditions)

Minor

Citrulline has been noted to increase physical exercise capacity in persons with heart failure.

DRight Ventricular Ejection Fraction

Minor

In persons with heart failure with normal ejection fractions, right ventricular ejection fraction during exercise appears to be increased relative to control.


Studies Excluded from Consideration

  • Confounded with the inclusion of L-arginine[1]

Disagree? Join the Citrulline Discussion

Table of Contents:


Edit1. Sources and Structure

1.1. Sources

L-Citrulline is a non-protein amino acid compound (does not get used to form structural proteins such as enzymes), and unlike L-Arginine it is not widely present in all proteins. It is highest in watermelon (where it derives its name, as watermelon's are known as Citrullus vulgaris[2]) where it averages 2.1mg/g wet weight (although absolute numbers vary)[3] and consuming watermelon has been noted to acutely increase both plasma arginine and citrulline (3.3kg watermelon being equivalent to 10g supplemental L-arginine)[4][5] and to increase fasting arginine and ornithine in the range of 12-22% following consumption of 780-1560g daily.[6]

Other food sources of L-citrulline include muskmelons, bitter melons, squashes, gourds, cucumbers, and pumpkins.[2]

Citrulline is an amino acid that is not one of the essential amino acids nor a common dietary amino acid (named after its only good natural source, watermelons), but is interconverted in the body and serves roles similar to L-arginine and L-ornithine supplementation

1.2. Biosynthesis

Citrulline is created in the body by one of two pathways, either recycled from arginine (the conversion of arginine into Nitric Oxide leaves citrulline as a byproduct)[7][8] or produced from the nitrogen (and some carbon) contained in L-glutamine,[9] where the enzyme Ornithine transcarbamylase uses both ornithine and carbamoyl phosphate (which requires glutamine) to produce citrulline in enterocytes.[10][11]

It appears that the arginine pathway accounts for about 10% of circulating citrulline while the glutamine pathway accounts for 90%;[7] reducing plasma glutamine levels can reduce plasma citrulline.[12]

Two pathways mediate production of L-Citrulline, with the citrulline that is a byproduct of nitric oxide production being a minor contributor but plasma glutamine being a major contributor

As for the urea cycle (one of the mechanisms behind the 10%), L-Arginine is converted into L-Ornithine via the arginase enzyme (giving off urea as a cofactor)[13][14] and from there ornithine (using carbamoyl phosphate as a cofactor) is subject to the Ornithine carbamoyltransferase enzyme to produce L-Citrulline. In this sense, the metabolic pathway from arginine towards citrulline (via ornithine) causes an increase in urea and a concomitant decrease in ammonia, which was used by the Carbamoyl phosphate synthase enzyme to create carbamoyl phosphate.[15] If need be, arginine can directly be converted into L-citrulline via a Arginine deiminase enzyme to produce, rather than require, ammonia.[16]

The cycle is formed as citrulline then binds with L-aspartate (related to D-Aspartic Acid as its isomer) to form arginosuccinate via the arginosuccinate synthase enzyme, and then the arginosuccinate lysase enzyme degrades arginosuccinate into free arginine and fumarate; arginine then reenters the urea cycle anew.[15] Fumarate can simply enter the TCA (Krebs) cycle as an energy intermediate,[16] and citrulline negatively regulates the arginase enzyme.[17]

Citrulline's convertion into L-arginosuccinate and subsequent conversion into L-arginine also is involved in the nitric oxide cycle rather than the urea cycle, with the only difference being that Arginine directly converts into citrulling (by giving off a nitric oxide molecule) rather than being indirectly converted via ornithine.[18][19]

Citrulline can be produced by arginine by one of two mechanism, either directly via arginine giving off a nitric oxide molecule (which is the path involved in the nitric oxide cycle) or indirectly via arginine's conversion into ornithine (which is involved in the urea cycle, sequestering ammonia)

1.3. Metabolism

Citrulline per se is more of a byproduct of arginine metabolism (nitric oxide cycle) and of ornithine metabolism (urea cycle), and merely gets converted back to arginine via arginosuccinate. That being said, supplemental citrulline also beneficially affects arginine and ornithine concentrations so their bioactivity is also relevant

1.4. Regulation

The main determinent of citrulline being converted into arginine appears to merely be serum concentrations of citrulline[20] which in a rested state are in the range of 22.4-27μmol/L.[21][22] Due to this, increasing plasma citrulline appears to cause linear increases in arginine up until around a 5-10g dose (A Cmax of 1314-2756μmol/L) while higher oral doses appear to cause less increases in arginine than expected.[23]


Edit2. Pharmacology

2.1. Intestinal Absorption

L-Citrulline is taken up to a much greater degree in the gut than its counterpart L-Arginine, and results in a higher plasma level of L-Arginine via the Arginine/Ornithine/Citrulline cycle.[24] It is absorbed through numerous sodium-dependent transporters.[25]

Relative to arginine, citrulline is better absorbed in the intestines and may be a better source of bodily arginine than arginine itself

2.2. Serum

Oral supplementation of citrulline in humans at 0.18g/kg has been noted to double plasma arginine[12] which has been replicated elsewhere[22] alongside an equivalent increase in ornithine concentrations[22] but these doublings in arginine and ornithine are alongside an 6-11 fold increase in plasma citrulline.[12][22]

A single dose of 6g citrulline malate (0.08g/kg) in athletes prior to exercise has noted increases in plasma citrulline (173% increase), ornithine (152% increase), and arginine (123% increase) when measured after exercise, and these values were normalized with 3 hours rest.[21] This same dose has been noted elsewhere to raise plasma citrulline and arginine to similar degrees.[26]

Interestingly, the aforementioned studies using 0.18g/kg citrulline noted a 6-11 fold increase in citrulline alongside a mere doubling of arginine and ornithine[12][22] while the later study using 6g (calculated to be 0.08g/kg) had a much smaller increase in citrulline but still more than doubled both arginine and ornithine.[21] This was also noted in a dose-response study using 2g up to 15g citrulline where citrulline in plasma followed linear dose-dependence and both arginine and ornithine having less dose-dependence.[23] The authors hypothesized that, due to the increase in arginine being less than predicted and serum citrulline being the main predictor of arginine synthesis[20] that this indicates a rate-limiting step being reached in the kidneys.

Oral Citrulline supplementation can increase plasma urea cycle amino acids which include citrulline itself (most drastic increase), Ornithine, and Arginine. Ornithine and Arginine can reach up to a doubling or tripling with 5-10g citrulline supplementation and then seems to be restricted from increasing further, while citrulline simply follows linear dose-response patterns (more oral supplementation causing more serum citrulline)

Citrulline has been noted to not influence serum levels of any of the Branched Chain Amino Acids at rest,[22] but can accelerate depletion of BCAAs induced by prolonged exercise (via increasing their utilization as fuel).[21]

A reduction of Glutamine (13% following 0.18g/kg citrulline for 7 days) has been noted with citrulline supplementation[22] although another study noted acute usage of 6g Citrulline (0.08g/kg) failed to alter glutamine concentrations.[21]

Other amino acids tested (glutamic acid, aspartic acid, asparagine, alanine, lysine, tryptophan, phenylalanine, L-Tyrosine, histidine) are mostly unaffected.[21]

Citrulline may deplete BCAA serum concentrations during exercise, but does not appear to do this inherently at rest. There are also some interactions with glutamine, but beyond that other amino acids do not appear to be affected

2.3. Metabolism

Approximately 83% of orally ingested citrulline appears to be taken up by the kidneys[27][28][29] where it is converted into L-Arginine in the proximal tubules (via the enzymes arginosuccinate synthase and arginosuccinate lyase[30]); this conversion of citrulline into arginine (either from supplemental citrulline or that produced as a byproduct of arginine creating Nitric Oxide) accounts for 5-15% of circulating arginine but 60% of total arginine (the difference being due to the ability of arginine to be incorporated into proteins).[12][31]

The majority of L-citrulline either floats in the blood passively or gets transported to the kidneys for conversion into arginine. Due to this, most bioactivities of citrulline are secondary to arginine ingestion


Edit3. Neurology

3.1. Memory and Learning

Arginine has a few interactions with memory and learning, and as it is the molecule that citrulline acts vicariously through then reading that page will give a more complete view of citrulline's actions

Citrulline (and Beta-Alanine) have been found to be normally expressed at higher concentrations in the hippocampus of trained rats relative to untrained rats in a cognitive test.[32]


Edit4. Cardiovascular Health

4.1. Cardiac Tissue

In heart failure patients with preserved ejection fraction given 3g citrulline, pulmonary artery pressure appeared to be reduced (56.67+/-7.96mmHg to 47.67+/-8.59mmHg; 16%) alongside an improvement in right ventricular ejection fraction. Arginine was equally effective, but required a higher dose (8g),[33] and this study is duplicated in Medline.[34]

A study in healthy individuals given citrulline (3,200mg 60-90 minutes before testing) was able to decrease the ECG QT interval, indicative of a shortening of the time required to de/repolarize the myocardium.[35]

4.2. Blood Flow and Pressure

5,600mg L-citrulline daily for a week in otherwise healthy middle aged men was able to significantly reduce brachial–ankle pulse wave velocity associated with increaes in serum nitric oxide metabolites but without changes in blood pressure overall,[36] while in prehypertensive persons given 'watermelon extract' (1,350mg citrulline with 650mg arginine taken twice a day) for 6 weeks there was a reduction in aortic and brachial pulse pressure (6+/-2mmHg and 8+/-3mmHg, respectively) and a reduction in aortic systolic blood pressure by 7+/-2mmHg (brachial blood pressure and aortic diastolic unaffected).[1]

In youth given a cold-pressor test (known to increase blood pressure without affecting heart rate[37][38]), and supplemental arginine has been previously shown to attenuate the rise seen in persons with high cholesterol.[39] In otherwise healthy young men subject to the cold-pressor test after 4 weeks of 6g citrulline supplementation (two doses of 3g) there was a lesser spike in systolic blood pressure seen with the test relative to baseline in the citrulline group only.[40]

In youth with congenital heart disease having supplemental citrulline around an operation (5 doses of 1.9g/m2, which is 3.2g and 3.6g for average adult females and males), subjects with elevated citrulline concentrations in the blood appeared to be protected from developing pulmonary hypertension and there was less overall development in the citrulline treated group (due to higher serum citrulline concentrations).[41]

Citrulline supplementation appears to reduce blood pressure and improve blood flow in situations where blood flow is otherwise hindered or blood pressure higher than normal, but citrulline does not have unidirectional reducing effects; it may be ineffective in resting normotensive persons


Edit5. Interactions with Glucose Metabolism

5.1. Insulin

Citrulline at 0.1mM (physiologically relevant concentration) has been noted to increase insulin secretion from pancreatic β-cells and to augment glucose-stimulated insulin release, and these events were inhibited with a NOS inhibitor and thus secondary to nitric oxide.[42] 10-fold the concentration (1mM) does not appear to be significantly different than 0.1mM,[43] and it appears that both arginine as well as the urea cycle intermediate L-arginosuccinate both also have activating effects on pancreatic beta-cells.[43]

The exercise-induced increase in insulin has been noted to be decreased with supplemental citrulline (3g taken three hours before exercise or 9g over the course of 24 hours).[44]


Edit6. Skeletal Muscle and Physical Performance

6.1. Mechanisms

Supplementation of citrulline has been noted to modulate enzymes in the glycogenolysis (glycogen breakdown) and glycolysis (converting glucose into ATP) pathways in rat skeletal muscle, which was said to shift muscle from aerobic energy metabolism (fatty acid based) to anaerobic.[45] Elsewhere, ATP production from aerobic pathways has been noted in both rats[46] and humans given 6,000mg citrulline for 3 days[47] and phosphocreatine (bioactive form of Creatine) has been noted to have its depletion attenuated by 28%[46] and its replenishment rate during rest accelerated by 20%[47] (although this resynthesis does not appear to be reliable[46]).

Elsewhere, citrulline (as malate) has been found to increase muscular ATP efficiency[46] which is a similar phenomena as is seen with Nitrate supplementation[48] and arginine supplementation[49] due to Nitric Oxide.[50]

At least one study in rats has noted increased force output by 23% following intake of 3g/kg daily (human equivalent of 480mg/kg).[46]

Citrulline does appear to interact with ATP production (in a beneficial manner) via increasing the efficiency of energy production, but it is unclear whether citrulline increases aerobic or anaerobic energy metabolism (or both, depending on context)

Citrulline (as malate) is sometimes used as an acid and ammonia buffer in bacterial models[51] and in mammals including humans.[52] Citrulline administration does appear to stimulate hepatic ureagenesis (via Ornithine sequestering ammonia)[52] and elsewhere has been noted to simply increase ornithine concentrations itself (by approximately double, a similar increase in ornithine and arginine).[12][22] 6g of citrulline appears to increase plasma ornithine (200% or so) to a lesser degree than supplemental ornithine itself at a similar dosage (300-500%).[53][54]

Ammonia normally signals for muscle fatigue when it accumulates due to inhibiting the conversion of Pyruvate to Acetyl-CoA (via activating phosphofructokinase)[55][56] which hinders contractility of muscles.[57] Secondary to preventing ammonia accumulation (which has been confirmed in mice[58]), an increase in physical performance may occur.

Citrulline may also possess the properties of ornithine supplementation and buffer ammonia, but on a gram per gram basis appears to be less efficient at raising plasma ornithine concentrations than ornithine itself (which is opposite that seen with arginine, where citrulline is actually more effective)

6.2. Muscle Protein Synthesis

Although citrulline does not appear to be an activator of mTOR like leucine is (mTOR activation, via S6K, induces muscle protein synthesis), citrulline does appear to restore muscle protein synthesis rates[59][60] and muscular function following food restriction[61] and this positive effect on muscle protein synthesis is abolished by rapamycin and thus mediated by the mTORc1 pathway.[62][63]

Citrulline has failed to activate both the p44/42 MAPK pathway and the ERK1/2 pathway in skeletal muscle,[59] while leucine itself can activate ERK1/2 and induce 4E-BP1 (downstream of mTOR) to a much larger degree than citrulline.[59]

Citrulline positively influences the mTOR pathway, but this does not appear to be through direct activation like leucine does. If anything, citrulline normalizes suppressed mTOR signalling

For human studies, supplementation of 0.18g/kg citrulline for a week has failed to significantly modify leucine oxidation rates or whole body protein synthesis[22] but elsewhere at the same dose has been noted to improve nitrogen balance in humans in the fed state.[12] Urinary nitrogen in the fasted state with oral doses of citrulline between 2-15g does not appear to be acutely affected.[23]

Mixed evidence as to whether Citrulline supplementation can help with muscle protein synthesis

6.3. Interventions

Citrulline, when supplemented at dosages around 250mg/kg bodyweight, has been implicated in increasing anaerobic performance in rats[46] and mice.[58]

Rat studies have noted benefits with 250mg/kg citrulling (human equivalent of 40mg/kg)

In weightlifters given a training protocol (do as many reps as possible until fatigue, for 8 sets), ingestion of 8g citrulline malate alongside 10g sucrose (sucrose also supplied to placebo condition) failed to improve performance on sets 1-2 but progressively increased the amount of reps conducted relative to placebo from 10.48% to 52.92% more reps.[64]

This study also noted that while 73.17% of the subjects reported nonresponse on set 1, all subjects reported response on set 8.[64]

Muscle soreness has been noted to be reduced following weight training with 8g citrulline malate by 39.74% (24 hours after) and 41.79% (48 hours after) relative to placebo.[64]

Citrulline appears to benefit weightlifting secondary to reducing fatigue from a prolonged workout; it does not appear to be too effective on the first few reps conducted

A trial in incline treadmill walking to fatigue with either 3g citrulline acutely or 9g over 24 hours (three doses of 3g) actually noted a reduction in time to exhaustion (895.4s to 888.2s; 0.8%) and increase in the rate of percieved exertion.[44] The authors also noted that the exercise-induced increase in insulin that occured in placebo failed to occur with citrulline.[44]


Edit7. Inflammation and Immunology

7.1. Neutrophils

6g of citrulline malate has been found to augment the increases in neutrophil oxidative burst following prolonged cycling exercise, although there did not appear to be oxidative burst damage (MDA or DNA damage) associated with citrulline.[26]


Edit8. Interactions with Hormones

8.1. Growth Hormone and IGF-1

In trained athletes given 6g citrulline malate prior to a prolonged (137km) cycling test, the exercise-induced increase in growth hormone appeared to be increased; when measured immedaitely after exercise, the citrulline group had 66.8% higher GH concentrations which (after 3 hours of rest) was attenuated to 28%.[21] Elsewhere, doses of 2-15g citrulline have failed to influence growth hormone when at rest when measured over 8 hours.[23]

IGF-1 concentrations after 0.18g/kg citrulline for 7 days are not significantly affected.[22]

Has been noted to increase growth hormone concentrations, but this may require physical exercise as a prerequisite. As such, it may be more correct to say citrulline augments growth hormone release rather than directly induces it

8.2. Insulin

Insulin has been found to be unaltered by 0.18g/kg citrulline during 7 days of supplementation[22] and during prolonged exercise preloaded with either placebo or citrulline.[21]


Edit9. Interactions with Organ Systems

9.1. Penis

Citrulline is thought to be a pro-erectile agent as it is a precursor for arginine, and arginine is the substrate from which nitric oxide is produced and then can induce cGMP (via the NO/cGMP/VEGF pathway);[65] an increase in cGMP is also the ultimate effect of PDE5 inhibitors such as viagra or icariin from Horny Goat Weed.[66]

In men with erectile dysfunction as assessed by weak erections (assessed b yerectile hardness score[67]) given 1,500mg citrulline daily (two doses of 750mg) for one month was able to benefit half of the 24 patients assessed (as assessed by 'very satisfied' with treatment) while there was merely a 8.3% improvement in placebo.[68]

Citrulline appears to have pro-erectile properties vicariously through arginine. It is likely more potent than arginine at doing this due to having a greater bodily exposure to arginine, but the preliminary evidence right now does not suggest that it is more potent than the reference drug Viagra


Edit10. Nutrient-Nutrient Interactions

10.1. Branched Chain Amino Acids (special reference to Leucine)

Branched Chain Amino Acids (BCAAs) are a group of three amino acids (Leucine, Isoleucine, and Valine) which are involved with skeletal muscle synthesis and degradation. Of these three, leucine is commonly said to be the most 'important' since it is the prime mediator of muscle protein synthesis (and that appears to be the most common reason people supplement BCAAs).

When looking at citrulline ingestion at rest, 0.18g/kg has been found to improve nitrogen balance in one study when ingested in the fed state[12] while another noted a failure with the same dose, with both whole body protein synthesis and leucine oxidation rates being unaffected.[22]

During exercise, 6g citrulline in trained cyclists prior to a 137km cycling test noted that plasma BCAAs were reduced relative to control and attributed to increase utilization of BCAAs for fuel.[21]

Citrulline does appear to interact with BCAA metabolism in the body, although human studies seem to have differing results based on the context of the study

Similar to how citrulline is able to restore muscle protein synthesis rates[59][60] and muscular function[61] during aging and malnourishment in rats via mTORc1 dependent means,[62][63] citrulline itself is a weak agonist (enough that it could be false positive from sensitizing mTORc1[59]). Conversely, Leucine itself is a potent activator of mTORc1 activity.

Citrulline may positively mediate leucine's signalling through mTOR, which theoretically suggests that they are synergistic. The application of the combination towards weight lifters has not yet been investigated, so the synergism is currently just a hypothesis rather than a demonstrated fact

10.2. Nitrate

Nitrate is a small Nitric Oxide donor that is the main bioactive of beetroot juice.

Serum nitrite (reduced form of nitrate) appears to be increased during exercise following consumption of 6g citrulline malate, which is thought to be an indicator of increase nitric oxide production.[21]

10.3. Sodium Bicarbonate

Sodium Bicarbonate (baking soda) is an ergogenic supplement that is reported to increase physical performance, and although overall it is somewhat unreliable in doing so it appears to be effective for short duration activities characterized by metabolic acidosis ('the burn').

Citrulline administration appears to promote ureogenesis (production of urea) and secondary to this promoted the renal resorption of bicarbonate;[52] it is thought that citrulline may promote a slight buffering effect via preserving bicarbonate.[52]

A study assessing synergism between citrulline and bicarbonate supplementation is currently not available.

Theoretically synergistic, but has not been directly investigated at this moment in time

10.4. Statins

One study in endothelial cells (in vitro) noted that the statin tested, simvastatin, was able to increase mRNA concentrations of eNOS (the enzyme in endothelial cells that produces nitric oxide from arginine) and thus was said to act synergistically with arginine and/or citrulline for improving blood pressure in persons with high cholesterol.[69]

As simvastatin,[69] atorvastatin,[70][71] and lovastatin[72] (bioactive in Red Yeast Rice) have all been noted to increase eNOS,[70] iNOS,[70][72] and nNOS[71] it is thought that this is a common mechanisms to statin drugs in general.

Statin drugs may be able to increase the expression of the enzyme that mediates conversion of arginine into nitric oxide, and due to this there is possible synergism for anything pertaining to nitric oxide. This has not yet been tested in a living system


Edit11. Safety and Toxicity

11.1. General

Citrulline appears to be well-tolerated by rats in doses up to 3g/kg bodyweight.[58][46]

In humans, 15g citrulline taken acutely does not appear to cause diarrhea or intestinal upset[23] which is notably different than both Ornithine and Arginine that may cause diarrhea at 10g dosages when taken in boluses[73][74] due to limited absorption of these amino acids which then proceed to the colon to cause osmotic diarrhea.[73]

References

  1. Figueroa A, et al. Effects of watermelon supplementation on aortic blood pressure and wave reflection in individuals with prehypertension: a pilot study. Am J Hypertens. (2011)
  2. Kaore SN, Amane HS, Kaore NM. Citrulline: pharmacological perspectives and its role as an emerging biomarker in future. Fundam Clin Pharmacol. (2013)
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(Common misspellings for Citrulline include citruline, citrulllin, citrulin)

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