On 2015 May 04, Peter Good commented:

I asked Dr. Cynober whether oral arginine, citrulline, or glutamine would be the best source of arginine for brain nitric oxide and creatine in ASD children. He replied it was controversial whether oral arginine or glutamine produces more citrulline in the intestines. Citrulline enters the brain, he said, but whether it generates creatine there, and the balance between production of nitric oxide (NO) and creatine from citrulline-derived arginine, is unclear.[personal communication 2015]

Their 2010 paper [Cynober L, 2010] presented much evidence critical to autistic disorders (ASD). Citrulline (CIT) is not normally present in protein; its usual sources are arginine (ARG) and glutamine in dietary proteins, which produce CIT in the intestines: “CIT is almost absent from natural foods, watermelon being a notable exception.” CIT bypasses the liver and forms ARG in the kidneys, which limits wasting of nitrogen as urea, and provides ARG to many other tissues, including the brain.

“[D]irect supplementation of CIT should be more useful than ARG supplementation, leaving the kidney to convert CIT into ARG, so avoiding heavy first-pass splanchnic extraction of the ARG and the possible harmful effects of an excessive ARG [therefore NO] supply. . . . CIT is able to sustain NO production through eNOS but not iNOS. . . . CIT could also be a safe way to deliver ARG to endothelial and immune cells, and can certainly prevent excessive uncontrolled nitric oxide production. . . . [A]ntioxidant properties, together with the ability to generate NO, make CIT an excellent candidate for the treatment of pathological situations characterized by oxidative stress and decreased arginine availability . . . .”

Because CIT stimulates protein synthesis when dietary proteins (i.e. ARG and glutamine) are low, it should be given in the postabsorptive state (3–5 hrs after meals) or fasted state (before breakfast). 10–15g/day of oral citrulline in healthy adults showed high bioavailability and no adverse effects. Because CIT is synthesized almost exclusively in the intestines, it may also be a useful biomarker of functional gut tissue.[Cynober L, 2010]

In their 2005 paper [Curis E, 2005] Cynober and colleagues discussed other aspects of citrulline metabolism: “Citrulline presents the common reactivity of the α-amino acid family. In particular, it can form peptide bonds; hence it can therefore be present in proteins. However, since there is no known codon in the genetic table for this amino acid, its presence in a protein must always result from a post-translational modification of the protein. . . .

“The main reason for this citrulline metabolism split between two organs [gut and kidney] is related to the efficacy of the capture of arginine by the liver. In fact, without metabolic adaptation, almost all the arginine coming from food supply would be withdrawn from the portal blood by the liver, leaving only very low amounts of available arginine for other organs. . . .

“[M]any cell types which are able to metabolize arginine into NO are able to uptake circulating citrulline, which explains why citrulline induces certain of the NO effects . . . . The figure seems to be even more complex in the brain, since the recycling of citrulline into arginine is split between various cell types, defining a unique inter-cell-type cycle. Indeed, the brain neurones producing NO are not able to reconvert citrulline into arginine since they do not express the [necessary] enzymes. Hence, citrulline is released from the neurons and taken up by surrounding neural cells where return-conversion to arginine is performed.”[Curis E, 2005]

Romero and colleagues presented other valuable information about citrulline [Romero MJ, 2006]: “With development, intestinal synthesis of L-arginine from glutamine decreases and the small intestine gradually becomes the major site of net L-citrulline production. . . . L-citrulline is largely taken up and metabolized by the kidney, which in turn releases arginine equivalent to ~75% of the L-citrulline taken up. Thus, much of the L-citrulline produced by enterocytes reaches the systemic circulation as L-arginine. This L-arginine/L-citrulline homeostasis allows a proper supply of L-arginine for the whole body. About 60% of dietary L-arginine makes it into the hepatic portal circulation, while the rest is metabolized in the intestine. . . . L-citrulline synthesis in many tissues also occurs as a byproduct of NOS activity. . . . Although NOS is widely distributed throughout the body, its activity does not contribute substantially to whole body L-citrulline flux under normal conditions. . . .

“[A]cute oral administration of L-citrulline appears to be considerably more efficient raising plasma levels of L-arginine than L-arginine itself. Additionally, a recent study in children and young adults showed that five oral doses of L-citrulline every 12 hours (1.9 g/m2/dose) for a total dose of 9.5 g/m2 resulted in 57 and 85% increases in mean plasma levels of L-arginine and L-citrulline, respectively. . . . L-citrulline is generally recognized as safe for oral consumption. In fact, L-citrulline can prevent some of the untoward effects of L-arginine supplementation. . . . L-citrulline is a natural and apparently safe means of providing L-arginine for constitutive NOS production of NO.”[my emphasis] [Romero MJ, 2006]

Deutz also presented important observations about plasma concentrations and interorgan transport of glutamine, citrulline, and arginine [Deutz NE, 2008]: “In daily practice, the plasma concentration is usually viewed as a parameter of production. This is not always correct as the plasma concentration can be high due to an increased production of the substrate and/or a reduced capacity of the body to dispose [of] this substrate. This means that the level of plasma concentration can be misleading and does not always give reliable information whether there is actually an intracellular deficiency of a certain substrate. . . .

“The quantitative main production site [of glutamine] in the body is muscle and the main consumption sites are the gut, liver and the kidney. Liver plays a dual role as it can both produce and consume glutamine, depending on the metabolic state (fasting/fed). . . . [A]bout 80–90% of the citrulline is derived from the gut glutamine to citrulline conversion. Therefore, whole body citrulline production is related to the quantity of gut glutamine conversion to citrulline, and is most likely influenced by the amount of active gut tissue.”

Most important conclusions for ASD: (1) oral citrulline bypasses the liver and becomes arginine in the kidneys, making arginine more available to other tissues, including the brain; (2) oral citrulline supports production of constitutive nitric oxide but not inducible nitric oxide; (3) in light of the benefit of casein-free/gluten-free diets in ASD children, citrulline’s stimulation of protein synthesis when dietary protein is low may be invaluable; (4) citrulline synthesis may be a useful marker of functional gut tissue in these children; (5) oral citrulline is safer than oral arginine. The evidence speaks for itself.

Peter Good Autism Studies www.autismstudies.net autismstudies1@gmail.com

Curis E, Nicolis I, Moinard C, et al. Almost all about citrulline in mammals. Amino Acids 2005;29:177–205.

Cynober L, Moinard C, De Bandt J-P. The 2009 ESPEN Sir David Cuthbertson. Citrulline: A new major signaling molecule or just another player in the pharmaconutrition game? Clin Nutrit 2010;29:545–551.

Deutz NEP. The 2007 ESPEN Sir David Cuthbertson Lecture: Amino acids between and within organs. The glutamate-glutamine-citrulline-arginine pathway. Clin Nutrit 2008;27:321–327.

Romero MJ, Platt DH, Caldwell RB, Caldwell RW. Therapeutic use of citrulline in cardiovascular disease. Cardiovasc Drug Rev 2006;24:275–290.

On 2015 Apr 30, Peter Good commented:

None

On 2015 Apr 06, Peter Good commented:

To PubMed Commons: comment to Frye RE, 2013.<br> Frye et al. [2010] previously reported that sapropterin, a synthetic form of tetrahydrobiopterin (BH4), improved behavior in children with autism spectrum disorders (ASD) [Frye RE, 2010]. Their 2013 study [Frye RE, 2013] was intended to test whether sapropterin’s benefit was due to BH4’s role as cofactor for synthesis of the monoamine neurotransmitters dopamine and serotonin (as previous investigators suspected), or BH4’s role as cofactor for nitric oxide synthase (NOS), which produces the critical gaseous molecule nitric oxide (NO). Frye et al. concluded that improvements in communicative language in these children from sapropterin were due to restoration of NOS “coupling” disrupted by lack of BH4, which dysregulated nitric oxide metabolism. In support of their conclusion they cited evidence by Sweeten et al. [Sweeten TL, 2004] and others of high levels of nitric oxide metabolites nitrite and nitrate in blood of ASD children. In their previous study Frye et al. concluded: “[I]t is possible that BH4 in ASD could be depleted by the overactivation of the immune system and inflammatory processes during an excessive production of nitric oxide.” [Frye RE, 2010]

There may, however, be more to this story. A few months after publication of Frye et al. 2013, Stanhewicz et al. reported sapropterin increased reflex vasodilation in aging human skin by increasing release of nitric oxide by endothelial and neuronal nitric oxide synthases [Stanhewicz AE, 2013]. Nitric oxide, the primary dilator of blood vessels in the body, is produced by three different forms of nitric oxide synthase – two constitutive forms present in blood vessel endothelial cells (eNOS) and neurons (nNOS), and a third form (iNOS) induced in brain microglia and other cells of the immune system in response to infections and other agencies. Endothelial nitric oxide maintains the vasodilator tone of blood vessels. Neuronal nitric oxide may be largely responsible for neurovascular coupling – dilation of nearby blood vessels when brain neurons fire. Faraci & Brian: “. . . NO appears to mediate cerebral vasodilatation in response to local neuronal activation.” [Faraci FM, 1994]. Koehler et al.: “. . . NO is required as a mediator of neurovascular coupling in the cerebellum, whereas NO acts as a modulator in the cerebral cortex.” [Koehler RC, 2009]. Inducible nitric oxide is released in large quantities to flush infective agents and toxins, and kill damaged cells.

If nitric oxide is too high in autistic disorders, inducible nitric oxide is the form likely responsible, Frye et al. concluded. Sweeten et al. concluded likewise: “[I]t is reasonable to hypothesize that iNOS is involved in the elevated NO production in autism.” [Sweeten TL, 2004]. Yet inducible nitric oxide is often released to compensate deficiencies of constitutive nitric oxide [Hecker M, 1999;Kubes P, 2000]. One indication neuronal nitric oxide is deficient in children with autistic disorders is their failure of neurovascular coupling – their brains are often hyperexcitable, yet brain blood flow is consistently low [e.g. Ohnishi T, 2000]. Nitrite and nitrate also serve as reservoir forms to deliver nitric oxide elsewhere [Dejam A, 2005]. Lundberg & Weitzberg: “[N]itrate and nitrite should probably be viewed as storage pools for NO rather than inert waste products.” [Lundberg JO, 2005].

Did sapropterin increase endothelial and neuronal nitric oxide in the brains of ASD children in Frye et al. 2013? Why would endothelial and neuronal nitric oxide be deficient in these children? One explanation is deficiency of BH4. Another is deficiency of the amino acid arginine – only substrate for nitric oxide [Wiesinger H, 2001]. Frye et al. found higher baseline levels of blood arginine in these children, and higher ratios of arginine to citrulline, were associated with greater improvements in language from sapropterin. They noted blood arginine and the arginine/citrulline ratio did not change significantly during sapropterin treatment – but also stated improvements in language were greater in children with “an attenuated increase in arginine.” [Frye RE, 2013]

Considerable evidence argues that arginine is deficient in ASD children: (a) high levels of inducible nitric oxide; (b) consistently low brain creatine (arginine + glycine) [Friedman SD, 2003]; (c) frequent high blood ammonia [Filipek PA, 2004] which requires arginine to detoxify to urea; and (d) high levels of arginine vasopressin in autistic boys [Carter CS, 2007; Momeni N, 2005]. Furthermore, NOS produces harmful oxidants superoxide and peroxynitrite when NOS “uncouples” from lack of BH4 – or when arginine is deficient [Xia Y, 1996]. Because most supplemental arginine is taken up by the liver (thus unavailable to other tissues), citrulline (arginine’s precursor) or glutamine (citrulline’s precursor) may be better sources of arginine for NOS [Cynober L, 2010]. The evidence speaks for itself.

Peter Good Autism Studies La Pine, OR www.autismstudies.net autismstudies1@gmail.com

Carter CS. Sex differences in oxytocin and vasopressin: implications for autism spectrum disorders? Behav Brain Res 2007;176:170–186.

Cynober L, Moinard C, De Bandt J. The 2009 ESPEN Sir David Cuthbertson. Citrulline: A new major signaling molecule or just another player in the pharmaconutrition game? Clinical Nutrition 2010;29:545–551.

Dejam A, Hunter CJ, Pelletier MM, et al. Erythrocytes are the major intravascular storage sites of nitrite in human blood. Blood 2005;106:734–739.

Faraci FM, Brian Jr. JE. Nitric oxide and the cerebral circulation. Stroke 1994;25:692–703.

Filipek PA, Juranek J, Nguyen MT, et al. Relative carnitine deficiency in autism. J Autism Dev Disord 2004;34:615–623.

Friedman SD, Shaw DW, Artru AA, et al. Regional brain chemical alterations in young children with autism spectrum disorder. Neurology 2003;60:100–107.

Frye RE, Huffman LC, Elliott GR. Tetrahydrobiopterin as a novel therapeutic intervention for autism. Neurotherapeutics. 2010;7(3):241–249.

Hecker M, Cattaruzza M, Wagner AH. Regulation of inducible nitric oxide synthase gene expression in vascular smooth muscle cells. Gen Pharmacol 1999;32:9–16.

Koehler RC, Roman RJ, Harder DR. Astrocytes and the regulation of cerebral blood flow. TINS 2009;32(3):160–169.

Kubes P. Inducible nitric oxide synthase – a little bit of good in all of us. Glia 2000;47:6–9.

Lundberg JO, Weitzberg E. NO generation from nitrite and its role in vascular control. Arterioscler Thromb Vasc Biol 2005;25:915–922.

Momeni N, Nordström BM, Horstmann V, et al. Alterations of prolyl endopeptidase activity in the plasma of children with autistic spectrum disorders. BMC Psychiatry 2005;5:27–32.

Ohnishi T, Matsuda H, Hashimoto T, et al. Abnormal regional cerebral blood flow in childhood autism. Brain 2000;123(Pt. 9):1838–1844.

Stanhewicz AE, Alexander LM, Kenney WL. Oral sapropterin acutely augments reflex vasodilation in aged human skin through nitric oxide-dependent mechanisms. J Appl Physiol 2013:115:972–978.

Sweeten TL, Posey DJ, Shankar S, McDougle CJ. High nitric oxide production in autistic disorder: a possible role for interferon-gamma. Biol Psychiatry 2004;55(4):434–437.

Wiesinger H. Arginine metabolism and the synthesis of nitric oxide in the nervous system. Prog Neurobiol 2001;64(4):365–391.

Xia Y, Dawson VL, Dawson TM, et al. Nitric oxide synthase generates superoxide and nitric oxide in arginine-depleted cells leading to peroxynitrite-mediated cellular injury. Proc Natl Acad Sci USA 1996;93:6770–6774.

On 2015 Apr 06, Peter Good commented:

To PubMed Commons: comment to Frye RE, 2013.<br> Frye et al. [2010] previously reported that sapropterin, a synthetic form of tetrahydrobiopterin (BH4), improved behavior in children with autism spectrum disorders (ASD) [Frye RE, 2010]. Their 2013 study [Frye RE, 2013] was intended to test whether sapropterin’s benefit was due to BH4’s role as cofactor for synthesis of the monoamine neurotransmitters dopamine and serotonin (as previous investigators suspected), or BH4’s role as cofactor for nitric oxide synthase (NOS), which produces the critical gaseous molecule nitric oxide (NO). Frye et al. concluded that improvements in communicative language in these children from sapropterin were due to restoration of NOS “coupling” disrupted by lack of BH4, which dysregulated nitric oxide metabolism. In support of their conclusion they cited evidence by Sweeten et al. [Sweeten TL, 2004] and others of high levels of nitric oxide metabolites nitrite and nitrate in blood of ASD children. In their previous study Frye et al. concluded: “[I]t is possible that BH4 in ASD could be depleted by the overactivation of the immune system and inflammatory processes during an excessive production of nitric oxide.” [Frye RE, 2010]

There may, however, be more to this story. A few months after publication of Frye et al. 2013, Stanhewicz et al. reported sapropterin increased reflex vasodilation in aging human skin by increasing release of nitric oxide by endothelial and neuronal nitric oxide synthases [Stanhewicz AE, 2013]. Nitric oxide, the primary dilator of blood vessels in the body, is produced by three different forms of nitric oxide synthase – two constitutive forms present in blood vessel endothelial cells (eNOS) and neurons (nNOS), and a third form (iNOS) induced in brain microglia and other cells of the immune system in response to infections and other agencies. Endothelial nitric oxide maintains the vasodilator tone of blood vessels. Neuronal nitric oxide may be largely responsible for neurovascular coupling – dilation of nearby blood vessels when brain neurons fire. Faraci & Brian: “. . . NO appears to mediate cerebral vasodilatation in response to local neuronal activation.” [Faraci FM, 1994]. Koehler et al.: “. . . NO is required as a mediator of neurovascular coupling in the cerebellum, whereas NO acts as a modulator in the cerebral cortex.” [Koehler RC, 2009]. Inducible nitric oxide is released in large quantities to flush infective agents and toxins, and kill damaged cells.

If nitric oxide is too high in autistic disorders, inducible nitric oxide is the form likely responsible, Frye et al. concluded. Sweeten et al. concluded likewise: “[I]t is reasonable to hypothesize that iNOS is involved in the elevated NO production in autism.” [Sweeten TL, 2004]. Yet inducible nitric oxide is often released to compensate deficiencies of constitutive nitric oxide [Hecker M, 1999;Kubes P, 2000]. One indication neuronal nitric oxide is deficient in children with autistic disorders is their failure of neurovascular coupling – their brains are often hyperexcitable, yet brain blood flow is consistently low [e.g. Ohnishi T, 2000]. Nitrite and nitrate also serve as reservoir forms to deliver nitric oxide elsewhere [Dejam A, 2005]. Lundberg & Weitzberg: “[N]itrate and nitrite should probably be viewed as storage pools for NO rather than inert waste products.” [Lundberg JO, 2005].

Did sapropterin increase endothelial and neuronal nitric oxide in the brains of ASD children in Frye et al. 2013? Why would endothelial and neuronal nitric oxide be deficient in these children? One explanation is deficiency of BH4. Another is deficiency of the amino acid arginine – only substrate for nitric oxide [Wiesinger H, 2001]. Frye et al. found higher baseline levels of blood arginine in these children, and higher ratios of arginine to citrulline, were associated with greater improvements in language from sapropterin. They noted blood arginine and the arginine/citrulline ratio did not change significantly during sapropterin treatment – but also stated improvements in language were greater in children with “an attenuated increase in arginine.” [Frye RE, 2013]

Considerable evidence argues that arginine is deficient in ASD children: (a) high levels of inducible nitric oxide; (b) consistently low brain creatine (arginine + glycine) [Friedman SD, 2003]; (c) frequent high blood ammonia [Filipek PA, 2004] which requires arginine to detoxify to urea; and (d) high levels of arginine vasopressin in autistic boys [Carter CS, 2007; Momeni N, 2005]. Furthermore, NOS produces harmful oxidants superoxide and peroxynitrite when NOS “uncouples” from lack of BH4 – or when arginine is deficient [Xia Y, 1996]. Because most supplemental arginine is taken up by the liver (thus unavailable to other tissues), citrulline (arginine’s precursor) or glutamine (citrulline’s precursor) may be better sources of arginine for NOS [Cynober L, 2010]. The evidence speaks for itself.

Peter Good Autism Studies La Pine, OR www.autismstudies.net autismstudies1@gmail.com

Carter CS. Sex differences in oxytocin and vasopressin: implications for autism spectrum disorders? Behav Brain Res 2007;176:170–186.

Cynober L, Moinard C, De Bandt J. The 2009 ESPEN Sir David Cuthbertson. Citrulline: A new major signaling molecule or just another player in the pharmaconutrition game? Clinical Nutrition 2010;29:545–551.

Dejam A, Hunter CJ, Pelletier MM, et al. Erythrocytes are the major intravascular storage sites of nitrite in human blood. Blood 2005;106:734–739.

Faraci FM, Brian Jr. JE. Nitric oxide and the cerebral circulation. Stroke 1994;25:692–703.

Filipek PA, Juranek J, Nguyen MT, et al. Relative carnitine deficiency in autism. J Autism Dev Disord 2004;34:615–623.

Friedman SD, Shaw DW, Artru AA, et al. Regional brain chemical alterations in young children with autism spectrum disorder. Neurology 2003;60:100–107.

Frye RE, Huffman LC, Elliott GR. Tetrahydrobiopterin as a novel therapeutic intervention for autism. Neurotherapeutics. 2010;7(3):241–249.

Hecker M, Cattaruzza M, Wagner AH. Regulation of inducible nitric oxide synthase gene expression in vascular smooth muscle cells. Gen Pharmacol 1999;32:9–16.

Koehler RC, Roman RJ, Harder DR. Astrocytes and the regulation of cerebral blood flow. TINS 2009;32(3):160–169.

Kubes P. Inducible nitric oxide synthase – a little bit of good in all of us. Glia 2000;47:6–9.

Lundberg JO, Weitzberg E. NO generation from nitrite and its role in vascular control. Arterioscler Thromb Vasc Biol 2005;25:915–922.

Momeni N, Nordström BM, Horstmann V, et al. Alterations of prolyl endopeptidase activity in the plasma of children with autistic spectrum disorders. BMC Psychiatry 2005;5:27–32.

Ohnishi T, Matsuda H, Hashimoto T, et al. Abnormal regional cerebral blood flow in childhood autism. Brain 2000;123(Pt. 9):1838–1844.

Stanhewicz AE, Alexander LM, Kenney WL. Oral sapropterin acutely augments reflex vasodilation in aged human skin through nitric oxide-dependent mechanisms. J Appl Physiol 2013:115:972–978.

Sweeten TL, Posey DJ, Shankar S, McDougle CJ. High nitric oxide production in autistic disorder: a possible role for interferon-gamma. Biol Psychiatry 2004;55(4):434–437.

Wiesinger H. Arginine metabolism and the synthesis of nitric oxide in the nervous system. Prog Neurobiol 2001;64(4):365–391.

Xia Y, Dawson VL, Dawson TM, et al. Nitric oxide synthase generates superoxide and nitric oxide in arginine-depleted cells leading to peroxynitrite-mediated cellular injury. Proc Natl Acad Sci USA 1996;93:6770–6774.

On 2015 Apr 30, Peter Good commented:

None

On 2015 May 04, Peter Good commented:

I asked Dr. Cynober whether oral arginine, citrulline, or glutamine would be the best source of arginine for brain nitric oxide and creatine in ASD children. He replied it was controversial whether oral arginine or glutamine produces more citrulline in the intestines. Citrulline enters the brain, he said, but whether it generates creatine there, and the balance between production of nitric oxide (NO) and creatine from citrulline-derived arginine, is unclear.[personal communication 2015]

Their 2010 paper [Cynober L, 2010] presented much evidence critical to autistic disorders (ASD). Citrulline (CIT) is not normally present in protein; its usual sources are arginine (ARG) and glutamine in dietary proteins, which produce CIT in the intestines: “CIT is almost absent from natural foods, watermelon being a notable exception.” CIT bypasses the liver and forms ARG in the kidneys, which limits wasting of nitrogen as urea, and provides ARG to many other tissues, including the brain.

“[D]irect supplementation of CIT should be more useful than ARG supplementation, leaving the kidney to convert CIT into ARG, so avoiding heavy first-pass splanchnic extraction of the ARG and the possible harmful effects of an excessive ARG [therefore NO] supply. . . . CIT is able to sustain NO production through eNOS but not iNOS. . . . CIT could also be a safe way to deliver ARG to endothelial and immune cells, and can certainly prevent excessive uncontrolled nitric oxide production. . . . [A]ntioxidant properties, together with the ability to generate NO, make CIT an excellent candidate for the treatment of pathological situations characterized by oxidative stress and decreased arginine availability . . . .”

Because CIT stimulates protein synthesis when dietary proteins (i.e. ARG and glutamine) are low, it should be given in the postabsorptive state (3–5 hrs after meals) or fasted state (before breakfast). 10–15g/day of oral citrulline in healthy adults showed high bioavailability and no adverse effects. Because CIT is synthesized almost exclusively in the intestines, it may also be a useful biomarker of functional gut tissue.[Cynober L, 2010]

In their 2005 paper [Curis E, 2005] Cynober and colleagues discussed other aspects of citrulline metabolism: “Citrulline presents the common reactivity of the α-amino acid family. In particular, it can form peptide bonds; hence it can therefore be present in proteins. However, since there is no known codon in the genetic table for this amino acid, its presence in a protein must always result from a post-translational modification of the protein. . . .

“The main reason for this citrulline metabolism split between two organs [gut and kidney] is related to the efficacy of the capture of arginine by the liver. In fact, without metabolic adaptation, almost all the arginine coming from food supply would be withdrawn from the portal blood by the liver, leaving only very low amounts of available arginine for other organs. . . .

“[M]any cell types which are able to metabolize arginine into NO are able to uptake circulating citrulline, which explains why citrulline induces certain of the NO effects . . . . The figure seems to be even more complex in the brain, since the recycling of citrulline into arginine is split between various cell types, defining a unique inter-cell-type cycle. Indeed, the brain neurones producing NO are not able to reconvert citrulline into arginine since they do not express the [necessary] enzymes. Hence, citrulline is released from the neurons and taken up by surrounding neural cells where return-conversion to arginine is performed.”[Curis E, 2005]

Romero and colleagues presented other valuable information about citrulline [Romero MJ, 2006]: “With development, intestinal synthesis of L-arginine from glutamine decreases and the small intestine gradually becomes the major site of net L-citrulline production. . . . L-citrulline is largely taken up and metabolized by the kidney, which in turn releases arginine equivalent to ~75% of the L-citrulline taken up. Thus, much of the L-citrulline produced by enterocytes reaches the systemic circulation as L-arginine. This L-arginine/L-citrulline homeostasis allows a proper supply of L-arginine for the whole body. About 60% of dietary L-arginine makes it into the hepatic portal circulation, while the rest is metabolized in the intestine. . . . L-citrulline synthesis in many tissues also occurs as a byproduct of NOS activity. . . . Although NOS is widely distributed throughout the body, its activity does not contribute substantially to whole body L-citrulline flux under normal conditions. . . .

“[A]cute oral administration of L-citrulline appears to be considerably more efficient raising plasma levels of L-arginine than L-arginine itself. Additionally, a recent study in children and young adults showed that five oral doses of L-citrulline every 12 hours (1.9 g/m2/dose) for a total dose of 9.5 g/m2 resulted in 57 and 85% increases in mean plasma levels of L-arginine and L-citrulline, respectively. . . . L-citrulline is generally recognized as safe for oral consumption. In fact, L-citrulline can prevent some of the untoward effects of L-arginine supplementation. . . . L-citrulline is a natural and apparently safe means of providing L-arginine for constitutive NOS production of NO.”[my emphasis] [Romero MJ, 2006]

Deutz also presented important observations about plasma concentrations and interorgan transport of glutamine, citrulline, and arginine [Deutz NE, 2008]: “In daily practice, the plasma concentration is usually viewed as a parameter of production. This is not always correct as the plasma concentration can be high due to an increased production of the substrate and/or a reduced capacity of the body to dispose [of] this substrate. This means that the level of plasma concentration can be misleading and does not always give reliable information whether there is actually an intracellular deficiency of a certain substrate. . . .

“The quantitative main production site [of glutamine] in the body is muscle and the main consumption sites are the gut, liver and the kidney. Liver plays a dual role as it can both produce and consume glutamine, depending on the metabolic state (fasting/fed). . . . [A]bout 80–90% of the citrulline is derived from the gut glutamine to citrulline conversion. Therefore, whole body citrulline production is related to the quantity of gut glutamine conversion to citrulline, and is most likely influenced by the amount of active gut tissue.”

Most important conclusions for ASD: (1) oral citrulline bypasses the liver and becomes arginine in the kidneys, making arginine more available to other tissues, including the brain; (2) oral citrulline supports production of constitutive nitric oxide but not inducible nitric oxide; (3) in light of the benefit of casein-free/gluten-free diets in ASD children, citrulline’s stimulation of protein synthesis when dietary protein is low may be invaluable; (4) citrulline synthesis may be a useful marker of functional gut tissue in these children; (5) oral citrulline is safer than oral arginine. The evidence speaks for itself.

Peter Good Autism Studies www.autismstudies.net autismstudies1@gmail.com

Curis E, Nicolis I, Moinard C, et al. Almost all about citrulline in mammals. Amino Acids 2005;29:177–205.

Cynober L, Moinard C, De Bandt J-P. The 2009 ESPEN Sir David Cuthbertson. Citrulline: A new major signaling molecule or just another player in the pharmaconutrition game? Clin Nutrit 2010;29:545–551.

Deutz NEP. The 2007 ESPEN Sir David Cuthbertson Lecture: Amino acids between and within organs. The glutamate-glutamine-citrulline-arginine pathway. Clin Nutrit 2008;27:321–327.

Romero MJ, Platt DH, Caldwell RB, Caldwell RW. Therapeutic use of citrulline in cardiovascular disease. Cardiovasc Drug Rev 2006;24:275–290.