Does NO metabolism play a role in the effects of vegetables in health?

Nitric oxide formation via the reduction of nitrites and nitrates.
Ralt, D.
The Gertner Institute, Chaim Sheba Medical Center, Tel Hashomer, Israel
Medical Hypotheses, 2009, 73(5), 794-796

Abstract
A recent attribution of vegetables to human health stems from their effect on the gasotransmitter nitric oxide (NO). This commentary proposes that a major reason for this attribution is that vegetables are rich in nitrates. Recent research has shown that nitrites and nitrates are not only inert end-products of NO oxidation. In addition, they can be recycled back to bioactive NO and this pathway is an important alternative to the classical L-arginine-NO-synthase pathway.
Various chronic ailments, e.g. diabetes, obesity, high blood pressure, are symptomatic of NO bioavailability. Because NO deficiency is metabolically complex, natural nitrate-rich nutrients, such as green leafy vegetables, can improve these chronic ailments via the alternative nitrate-NO pathway.
This commentary implies an added value for vegetables in enhancing health such as cardiac health and in lowering the risks of maladies such as diabetes.

Introduction
Fruit and vegetables rich diets has been associated with lower risk of chronic diseases including cancer and cardiovascular disease and provide beneficial effects that can be attributed to a variety of protective agents [1, 2, 3]. In this commentary, the therapeutic attributes of vegetables will be discussed mainly in terms of their nitrates and nitrites content as a source for nitric oxide (NO) [4, 5, 6, 7].
The past decade has witnessed a significant increase in the interest of biologists in NO. This highly reactive free radical, first considered only a noxious air pollutant, plays a vital role in many biological events including regulation of blood flow, immunity and neurotransmission [8].
Recent data demonstrate that NO is not only a paracrine substance. It has been shown that enzymatic generation of NO in the heart is capable of modulating remote physiological actions and cell signaling [9]. NO was also shown to affect the computational ability of the brain; it regulates information transmission across neurons [10]. Extensive data from studies on NO signals, on relaxation/stress processes and on health, supports a view of an NO net, serving as a body coordinator [11, 12].

The necessity of NO for body functions is suggested by the fact that its deficiency is correlated with various chronic ailments such as, obesity [13], diabetes [14], hypertension [15], pulmonary hypertension [16], osteoporosis [17] and old age [18]. Supplementing NO levels is helpful and thus 5-phosphodiesterase inhibitors (e.g. Viagra), have a potential protective role in chronic ailments as coronary artery disease [19, 20]. It is also not surprising that though helpful, the response rate to NO related drugs as Viagra is lower in diabetics [21].

The problematic nature of chronic ailments is the occurrence of an extended, not necessarily hazardous, condition (see the following elaboration on obesity). Thus, while temporary deficiency of NO may not pose a dramatic risk, a long term NO deficiency becomes hazardous.
NO bioavailability in the body depends on the rate of its synthesis and metabolism. Though arginine is a main precursor for the NO synthesis [22], it has lately been established that mammalian nitrate and nitrite reduction can also contribute to NO biogenesis [23, 24]. This commentary recommends an increase in consumption of nitrate-rich vegetables to prevent NO shortage and its consequential ailments. These nitrate-rich nutrients support health via reduction of nitrites and nitrates to NO [25]. Though beyond the scope of this commentary, it is worth mentioning that vegetable consumption has many nutritional benefits, these include supplementing precursors not only to NO but also to other gasotransmitters such as H2S [26, 27].

Discussion
This section is composed of 3 parts:
- Description of a chronic ailment (obesity) and its NO deficiency.
- Curing NO shortage via nitrates and nitrites supplementation.
- Nitrate rich vegetables indeed improve chronic ailments.

Obesity and NO deficiency
Obesity, which begins as simple overweight, over time becomes a malady correlated with ailments such as heart disease and diabetes [28]. NO deficiency may explain this pattern [29]. There are a number of studies that support this idea.
First, NO and citrulline are decreased in obese juveniles when compared to normal weight juveniles, and are negatively correlated with body weight. Arginine (the NO precursor) however, is increased in obese juveniles and is positively correlated with body weight [13]. Therefore, whereas low levels of NO are correlated with obesity,
it is not the lack of arginine that limits NO bioavailability.
Second, another study demonstrated that excess of fat in the body results in excess of the leptin hormone, which in turn, can lower the levels of bioavailable NO [30].
In addition, it is interesting to note that caloric restriction, without malnutrition, extends life span in a range of organisms [31] and that there is an NO link between caloric restriction and mitochondria [32].

Until the last century, obesity was a rare and impermanent condition and a transient lowered NO communication capacity was not necessarily unhealthy. Today, when obesity is mostly irreversible, prolonged inhibited NO bioavailability is hazardous and can account for the numerous maladies associated with obesity [28].

The reduction of nitrites and nitrates
Increasing the sources of NO, may therefore, contribute to the health of the obese even in the absence of losing weight. When arginine oxidation to NO is prohibited, alternative routes for NO production are effective - the reduction of nitrites and nitrates [33].
Nitrate and nitrite are important alternative sources of NO in especially, but not exclusively, hypoxic states [34]. Following are some examples; The intermittent hypoxia improved glucose tolerance [35]. Nitrite therapy augments ischemia-induced angiogenesis [36], nitrate reduction is emerging as a regulator of physiological functions and tissue responses to myocardial infarction or stroke [23] and brief elevations in plasma nitrite trigger a concerted cardioprotective response [37]. In addition to supplementation of NO compounds in acute situations, it is also effective in improving the urogenital system and skeletal health [38].
As suggested by Tamaki [39], dietary nitrite-derived NO generation may serve as a backup system when the NO synthase/L-arginine-dependent NO generation system is compromised.

Best Nitrite and Nitrate Food Sources
Some of the best nitrate-rich foods are lettuce, spinach, beetroot [6] and fruits such as pomegranates [40, 41]. Accumulating data shows that these foods improve various chronic ailments. Following are some examples; Beetroot reduces high blood pressure [42], green leafy vegetable consumption is linked to lower risk for diabetes in women [3], nitrate rich vegetables were shown to decrease the oxygen demand during exercise [43], pomegranate juice consumption by patients with carotid artery stenosis decreases carotid IMT and systolic blood pressure [44] and indeed NO produced from nitrite plays important roles in limiting post-ischemic tissue injury [45].
Because increasing substantially the daily consumption of vegetables is not simple [46, 47, 48], it seems worthwhile to spread the trend of drinking vegetable juices [49] when dealing with chronic ailments.

It is interesting to note that the Dunhuang scrolls dating to approximately 800 AD, suggested that nitrite and nitrate were used by the Chinese to relieve chest pain and reduce cold in the hands [50].

Conclusions
Regulation of NO function is showing itself to be a complex event that maladies are associated with its disruption.
Here, it is proposed that vegetables can at least partly, reverse these effects by increasing NO generation via nitrate and nitrite. This explains how encouragement of daily consumption of extra vegetables supports good health.
Vegetables can be considered a natural “drug” with sustained release of low-dose nitrite into the circulatory system, alternative source to the L-arginine pathway to NO.

List of abbreviations
NO - nitric oxide
IMT - intima-media thickness


References
1. Kawashima A, Madarame T, Koike H, Komatsu Y, Wise JA: Four week supplementation with mixed fruit and vegetable juice concentrates increased protective serum antioxidants and folate and decreased plasma homocysteine in Japanese subjects. Asia Pac J Clin Nutr 2007, 16:411-421.
2. Iqbal R, Anand S, Ounpuu S, Islam S, Zhang X, Rangarajan S, Chifamba J, Al-Hinai A, Keltai M, Yusuf S; INTERHEART Study Investigators: Dietary patterns and the risk of acute myocardial infarction in 52 countries: results of the INTERHEART study. Circulation 2008, 118:1929-1937.
3. Bazzano LA, Li TY, Joshipura KJ, Hu FB: Intake of fruit, vegetables, and fruit juices and risk of diabetes in women. Diabetes Care 2008, 31:1311-1317.
4. Himeno M, Ishibashi T, Nakano S, Furuya K, Kigoshi T, Uchida K, Nishio M: A practical procedure for achieving a steady state of NOx concentration in plasma: with special reference to the NOx content of Japanese daily food. Tohoku J Exp Med 2003, 199:95-110.
5. Nam PH, Alejandra B, Frédéric H, Didier B, Olivier S, Pauss A: A new quantitative and low-cost determination method of nitrate in vegetables, based on deconvolution of UV spectra. Talanta 2008, 76:936-940.
6. van Velzen AG, Sips AJ, Schothorst RC, Lambers AC, Meulenbelt J: The oral bioavailability of nitrate from nitrate-rich vegetables in humans. Toxicol Lett 2008, 181:177-181.
7. Wink DA, Paolocci N: Mother was right: eat your vegetables and do not spit! When oral nitrate helps with high blood pressure. Hypertension 2008, 51:617-619.
8. Murad F: Nitric oxide signaling: would you believe that a simple free radical could be a second messenger, autacoid, paracrine substance, neurotransmitter, and hormone? Recent Prog Horm Res 1998, 53:43-59, discussion 59-60.
9. Elrod JW, Calvert JW, Gundewar S, Bryan NS, Lefer DJ: Nitric oxide promotes distant organ protection: evidence for an endocrine role of nitric oxide. Proc Natl Acad Sci U S A 2008, 105:11430-11435.
10. Steinert JR, Kopp-Scheinpflug C, Baker C, Challiss RAJ, Mistry R, Haustein MD, Griffin SJ, Tong H, Graham BP, Forsythe ID: Nitric Oxide Is a Volume Transmitter Regulating Postsynaptic Excitability at a Glutamatergic Synapse. Neuron 2008, 60:642-656.
11. Ralt D: NO netting, health and stress - studying wellness from a net perspective. Med Hypotheses 2008, 70:85-91.
12. Olson KR, Donald JA.: Nervous control of circulation - The role of gasotransmitters, NO, CO, and H(2)S. Acta Histochem. 2009 Jan 5. [Epub ahead of print]
13. Gruber HJ, Mayer C, Mangge H, Fauler G, Grandits N, Wilders-Truschnig M: Obesity reduces the bioavailability of nitric oxide in juveniles. Int J Obes (Lond) 2008, 32:826-831.
14. Grijalva J, Hicks S, Zhao X, Medikayala S, Kaminski PM, Wolin MS, Edwards JG. Exercise training enhanced myocardial endothelial nitric oxide synthase (eNOS) function in diabetic Goto-Kakizaki (GK) rats. Cardiovasc Diabetol 2008, 7:34.
15. Wang D, Strandgaard S, Iversen J, Wilcox CS: Asymmetric dimethylarginine, oxidative stress, and vascular nitric oxide synthase in essential hypertension. Am J Physiol Regul Integr Comp Physiol. 2009, 296:R195-200.
16. Morris CR, Gladwin MT, Kato GJ: Nitric oxide and arginine dysregulation: a novel pathway to pulmonary hypertension in hemolytic disorders. Curr Mol Med 2008, 8:620-632.
17. Wimalawansa SJ: Nitric oxide: novel therapy for osteoporosis. Expert Opin Pharmacother 2008, 9:3025-3044.
18. Lauer T, Heiss C, Balzer J, Kehmeier E, Mangold S, Leyendecker T, Rottler J, Meyer C, Merx MW, Kelm M, Rassaf T: Age-dependent endothelial dysfunction is associated with failure to increase plasma nitrite in response to exercise. Basic Res Cardiol 2008, 103:291-297.
19. Gazzaruso C, Solerte SB, Pujia A, Coppola A, Vezzoli M, Salvucci F, Valenti C, Giustina A, Garzaniti A: Erectile dysfunction as a predictor of cardiovascular events and death in diabetic patients with angiographically proven asymptomatic coronary artery disease: a potential protective role for statins and 5-phosphodiesterase inhibitors. J Am Coll Cardiol 2008, 51:2040-2044.
20. Takimoto E, Koitabashi N, Hsu S, Ketner EA, Zhang M, Nagayama T, Bedja D, Gabrielson KL, Blanton R, Siderovski DP, Mendelsohn ME, Kass DA: Regulator of G protein signaling 2 mediates cardiac compensation to pressure overload and antihypertrophic effects of PDE5 inhibition in mice. J Clin Invest. 2009 Jan 5. [Epub ahead of print].
21. Aversa A, Bruzziches R, Vitale C, Marazzi G, Francomano D, Barbaro G, Spera G, Rosano GM: Chronic sildenafil in men with diabetes and erectile dysfunction. Expert Opin Drug Metab Toxicol 2007, 3:451-464.
22. Villalpando S, Gopal J, Balasubramanyam A, Bandi VP, Guntupalli K, Jahoor F: In vivo arginine production and intravascular nitric oxide synthesis in hypotensive sepsis.
Am J Clin Nutr 2006, 84:197-203.
23. Lundberg JO, Weitzberg E, Gladwin MT: The nitrate-nitrite-nitric oxide pathway in physiology and therapeutics. Nat Rev Drug Discov 2008, 7:156-167.
24. Shiva S, Gladwin MT: Shining a light on tissue NO stores: near infrared release of NO from nitrite and nitrosylated hemes. J Mol Cell Cardiol 2009, 46:1-3.
25. Jansson EA, Huang L, Malkey R, Govoni M, Nihlén C, Olsson A, Stensdotter M, Petersson J, Holm L, Weitzberg E, Lundberg JO: A mammalian functional nitrate reductase that regulates nitrite and nitric oxide homeostasis. Nat Chem Biol 2008, 4:411-417.
26. Lefer DJ: Potential importance of alterations in hydrogen sulphide (H2S) bioavailability in diabetes. Br J Pharmacol 2008, 155:617-619.
27. Jacob C, Anwar A, Burkholz T: Perspective on recent developments on sulfur-containing agents and hydrogen sulfide signaling. Planta Med 2008, 74:1580-1592.
28. Bray GA, Wilson JF: In the clinic. Obesity. Ann Intern Med 2008, 149:ITC4-1-15.
http://www.annals.org/cgi/reprint/149/7/ITC4-1.pdf
29. Joost HG, Tschöp MH: NO to obesity: does nitric oxide regulate fat oxidation and insulin sensitivity? Endocrinology 2007, 148:4545-4547.
30. Korda M, Kubant R, Patton S, Malinski T: Leptin-induced endothelial dysfunction in obesity. Am J Physiol Heart Circ Physiol 2008, 295:H1514-H1521.
31. Civitarese AE, Carling S, Heilbronn LK, Hulver MH, Ukropcova B, Deutsch WA, Smith SR, Ravussin E; CALERIE Pennington Team: Calorie restriction increases muscle mitochondrial biogenesis in healthy humans. PLoS Med. 2007, 4:e76.
32. Guarente L: NO link between calorie restriction and mitochondria. Nat Chem Biol 2005, 1:355-356.
33. Butler AR, Feelisch M. Therapeutic uses of inorganic nitrite and nitrate: from the past to the future. Circulation. 2008, 117:2151-2159.
34. Feelisch M, Fernandez BO, Bryan NS, Garcia-Saura MF, Bauer S, Whitlock DR, Ford PC, Janero DR, Rodriguez J, Ashrafian H: Tissue processing of nitrite in hypoxia: An intricate interplay of nitric oxide-generating and -scavenging systems. J Biol Chem 2008, Oct 3 [Epub ahead of print].
35. Chiu LL, Chou SW, Cho YM, Ho HY, Ivy JL, Hunt D, Wang PS, Kuo CH: Effect of prolonged intermittent hypoxia and exercise training on glucose tolerance and muscle GLUT4 protein expression in rats. J Biomed Sci. 2004, 11:838-846.
36. Kumar D, Branch BG, Pattillo CB, Hood J, Thoma S, Simpson S, Illum S, Arora N, Chidlow JH Jr, Langston W, Teng X, Lefer DJ, Patel RP, Kevil CG: Chronic sodium nitrite therapy augments ischemia-induced angiogenesis and arteriogenesis. Proc Natl Acad Sci U S A 2008, 105:7540-7545.
37. Perlman DH, Bauer SM, Ashrafian H, Bryan NS, Garcia-Saura MF, Lim CC, Fernandez BO, Infusini G, McComb ME, Costello CE, Feelisch M: Mechanistic Insights Into Nitrite-Induced Cardioprotection Using an Integrated Metabonomic/Proteomic Approach. Circulation Research. 2009, Published online, February 19, 2009
38. Wimalawansa SJ: Nitric oxide: new evidence for novel therapeutic indications. Expert Opin Pharmacother 2008, 9:1935-1954.
39. Kanematsu Y, Yamaguchi K, Ohnishi H, Motobayashi Y, Ishizawa K, Izawa Y, Kawazoe K, Kondo S, Kagami S, Tomita S, Tsuchiya K, Tamaki T: Dietary doses of nitrite restore the circulating nitric oxide level and improve renal injury in L-NAME-induced hypertensive rats. Am J Physiol Renal Physiol 2008, 295:F1457-F1462.
40. de Nigris F, Balestrieri ML, Williams-Ignarro S, D'Armiento FP, Fiorito C, Ignarro LJ, Napoli C: The influence of pomegranate fruit extract in comparison to regular pomegranate juice and seed oil on nitric oxide and arterial function in obese Zucker rats. Nitric Oxide 2007, 17:50-54.
41. Rajfer J: Pomegranate Juice: Is It the New, All-Natural Phosphodiesterase Type 5 Inhibitor? Rev Urol 2008, 10:168–169.
42. Webb AJ, Patel N, Loukogeorgakis S, Okorie M, Aboud Z, Misra S, Rashid R, Miall P, Deanfield J, Benjamin N, MacAllister R, Hobbs AJ, Ahluwalia A: Acute blood pressure lowering, vasoprotective, and antiplatelet properties of dietary nitrate via bioconversion to nitrite. Hypertension 2008, 51:784-790.
43. Larsen FJ, Weitzberg E, Lundberg JO, Ekblom B: Effects of dietary nitrate on oxygen cost during exercise. Acta Physiol (Oxf) 2007, 191:59-66.
44. Aviram M, Rosenblat M, Gaitini D, Nitecki S, Hoffman A, Dornfeld L, Volkova N, Presser D, Attias J, Liker H, Hayek T: Pomegranate juice consumption for 3 years by patients with carotid artery stenosis reduces common carotid intima-media thickness, blood pressure and LDL oxidation. Clin Nutr 2004, 23:423-433.
45. Abe Y, Hines I, Zibari G, Grisham MB: Hepatocellular protection by nitric oxide or nitrite in ischemia and reperfusion injury. Arch Biochem Biophys 2008 Oct 12. [Epub ahead of print]
46. Dumbrell S, Mathai D: Getting young men to eat more fruit and vegetables: a qualitative investigation. Health Promot J Austr 2008, 19:216-221.
47. van Sluijs EM, Skidmore PM, Mwanza K, Jones AP, Callaghan AM, Ekelund U, Harrison F, Harvey I, Panter J, Wareham NJ, Cassidy A, Griffin SJ: Physical activity and dietary behaviour in a population-based sample of British 10-year old children: the SPEEDY study (Sport, Physical activity and Eating behaviour: Environmental Determinants in Young people).
BMC Public Health 2008, 8:388.
48. Casagrande SS, Wang Y, Anderson C, Gary TL: Have Americans increased their fruit and vegetable intake? The trends between 1988 and 2002. Am J Prev Med. 2007, 32:257-263.
49. Keen C: Vegetable juice is an effective and acceptable way to meet DASH vegetable recommendations. In American Dietetic Association Food & Nutrition conference, 25-28 October 2008, Chicago.
http://www.eurekalert.org/pub_releases/2008-10/wsw-nss102308.php
50. Gladwin MT, Schechter AN, Kim-Shapiro DB, Patel RP, Hogg N, Shiva S, Cannon RO 3rd, Kelm M, Wink DA, Espey MG, Oldfield EH, Pluta RM, Freeman BA, Lancaster JR Jr, Feelisch M, Lundberg JO: The emerging biology of the nitrite anion. Nat Chem Biol 2005, 1:308-314.

- after publication relevant articles:
* Food sources of nitrates and nitrites: the physiologic context for potential health benefits
Am J Clin Nutr. 2009, 90(1), 1-10
* The University of Leicester team believe nitric oxide may change the way we think and hear.
http://news.bbc.co.uk/2/hi/health/8116194.stm
Nitroglycerin Bioactivation
http://www.jbc.org/cgi/content/abstract/284/30/19878
http://www.sciencedaily.com/releases/2009/08/090819125042.htm