Arginine

Arginine is a conditionally essential amino acid and one of the most metabolically connected in the body. It’s the substrate the body’s nitric oxide synthase enzymes use to make nitric oxide — the molecule that widens blood vessels, helps regulate blood pressure, takes part in immune defense, and enables erections (the pathway that drugs such as sildenafil enhance). Arginine is also the molecule from which urea is split off in the urea cycle, the first building block in creatine production, and — through ornithine — a source of polyamines and proline for collagen. For anyone interested in nitric oxide and exercise, two practical points stand out: much of a supplemental arginine dose is broken down before it reaches the bloodstream, so citrulline raises blood arginine more effectively than arginine itself; and the body has a second, arginine-independent route to nitric oxide — from dietary nitrate, found in beetroot and leafy greens — which has the strongest performance evidence of all.

Classification

• Type: Conditionally essential amino acid. In adults, the body makes arginine mainly through cooperation between the gut, which produces citrulline, and the kidneys, which convert citrulline into arginine. The liver’s urea cycle also makes arginine, but it splits it straight back into urea, so it adds little to the body’s supply. Arginine becomes essential in premature infants and may fall short during severe illness, burns, and some inherited disorders
• Side chain: A guanidino group — positively charged and among the most strongly basic of all amino acid side chains. This group is transferred to glycine at the start of creatine production, and it’s the part of arginine that nitric oxide synthase converts into nitric oxide
• Metabolism: Glucogenic — arginine is broken down via ornithine and glutamate into alpha-ketoglutarate, which enters the Krebs cycle
• Urea cycle role: Both made and broken down within the cycle — arginine is formed in the cycle’s second-to-last step, and urea is split from it in the last

Key Mechanisms

Nitric oxide production:

All three forms of nitric oxide synthase (NOS) make nitric oxide from arginine:

L-arginine + O₂ + NADPH → L-citrulline + nitric oxide (NO)

The enzyme needs several cofactors: tetrahydrobiopterin (BH4), FAD and FMN (made from riboflavin, vitamin B2), a heme group containing iron, and calmodulin, which is activated by calcium. A zinc atom also helps hold the enzyme together. The three forms have distinct roles:

• eNOS (endothelial): made continuously by the cells lining blood vessels, especially in response to blood flow. The NO it produces activates an enzyme in the vessel’s muscle layer that makes cGMP, which relaxes the muscle and widens the vessel — the main way NO regulates blood pressure and blood flow
• nNOS (neuronal): found in nerve cells; involved in learning and memory, pain signalling, and erections
• iNOS (inducible): switched on by inflammatory signals, mainly in immune cells, where it produces large amounts of NO to kill microbes

What NO does:

• Widens blood vessels and lowers blood pressure
• Makes platelets and white blood cells less likely to stick to vessel walls
• Helps protect blood vessels against atherosclerosis
• Acts as a signalling molecule in the brain
• Enables erections — NO relaxes the smooth muscle of the penis through cGMP. Drugs such as sildenafil (Viagra) and tadalafil (Cialis) block cGMP breakdown, amplifying the NO signal
• Helps immune cells kill microbes

The nitrate route — NO without arginine: the body can also make NO from nitrate in food, especially beetroot, rocket (arugula), spinach, and other leafy greens. Bacteria in the mouth convert nitrate into nitrite, which the body then converts into NO — a route that works particularly well when oxygen is low, as in hard-working muscle. This pathway doesn’t depend on arginine or NOS, and dietary nitrate (usually as beetroot juice) has the best evidence of any NO-related supplement for improving endurance performance.

ADMA — a natural NOS blocker: asymmetric dimethylarginine (ADMA) is a modified form of arginine released when proteins are broken down. It competes with arginine at NOS and reduces NO production. High blood ADMA is linked with poor blood vessel function and higher risk of heart disease, high blood pressure, and kidney disease. Extra arginine may partly offset ADMA’s effect — one proposed explanation for why supplemental arginine can sometimes increase NO even though cells normally contain plenty.

The urea cycle — disposing of nitrogen:

Arginine sits near the end of the urea cycle in the liver:

1. Ornithine + carbamoyl phosphate → citrulline
2. Citrulline + aspartate → argininosuccinate
3. Argininosuccinate → arginine + fumarate
4. Arginine + water → ornithine + urea (via arginase) — the step that releases urea
5. Ornithine starts the cycle again

Urea, excreted in urine, carries most of the nitrogen from broken-down protein out of the body; smaller amounts leave as ammonium and other compounds. If the urea cycle fails, ammonia builds up to toxic levels — which is why inherited urea cycle disorders are so dangerous. Arginine or citrulline supplements are part of treatment for several of them, but in one — arginase deficiency, where arginine itself builds up — arginine must instead be restricted.

Creatine production:

1. In the kidneys: arginine + glycine → guanidinoacetate + ornithine (via the enzyme AGAT)
2. In the liver: guanidinoacetate + SAM → creatine (via the enzyme GAMT)

Arginine donates its guanidino group to glycine in the first, committed step, and that group becomes the part of creatine that carries phosphate in phosphocreatine — the muscle’s fastest way of regenerating ATP during explosive effort. Creatine production uses arginine, glycine, and methionine. Taking creatine supplements reduces the body’s own production, which in theory frees up some arginine for other uses.

NOS vs arginase — a switch in immune cells:

In immune cells called macrophages, iNOS and arginase compete for the same arginine. Studies, mainly in mice, show that which enzyme dominates helps set the cell’s role:

• Inflammatory (“M1”) macrophages favor iNOS, producing NO to kill microbes
• Repair-oriented (“M2”) macrophages favor arginase, producing ornithine for polyamines and proline that support tissue repair and collagen formation

Human macrophages handle arginine somewhat differently, so how much this applies to people is still being studied.

Polyamines:

Arginine → ornithine (via arginase) → putrescine (via ornithine decarboxylase) → spermidine → spermine

Polyamines are needed for cell growth and to stabilize DNA and RNA. Ornithine decarboxylase, which starts the pathway, is one of the shortest-lived and most tightly controlled enzymes in the body. Spermidine has attracted interest in ageing research because it promotes autophagy, the cell’s recycling process. Polyamines also come directly from food.

Wound healing and proline:

Arginine → ornithine → pyrroline-5-carboxylate → proline

Ornithine can be converted into proline, which makes up roughly 12–13% of collagen. Together with NO’s effect on blood flow and polyamines’ role in cell growth, this is why arginine has been studied for wound healing. In malnourished patients with pressure ulcers, a nutrition formula enriched with arginine, zinc, and antioxidants improved healing in a randomized trial, and arginine-containing “immunonutrition” formulas have reduced infections after major planned surgery for cancer. These benefits come from combination formulas, so the specific contribution of arginine isn’t clear.

Growth hormone release:

Large doses of arginine — especially intravenously — stimulate growth hormone release by suppressing somatostatin, the hormone that normally holds it back. Intravenous arginine is used in hospital tests of growth hormone production. Oral doses have only modest, short-lived effects that don’t translate into meaningful muscle gains in healthy adults, and some studies suggest that taking arginine before exercise may even blunt the natural growth hormone rise that exercise produces.

Recommended Intake

Found In (per 100g)

Seeds and nuts are by far the richest sources: pumpkin seeds provide over 5g per 100g, and arginine makes up 12–18% of the protein in seeds and nuts, compared with about 6–7% in meat and fish. They also have a high arginine-to-lysine ratio — about 3–4, compared with less than 1 in meat, fish, eggs, and dairy — which is why they’re the foods usually mentioned in discussions of arginine, lysine, and cold sores (see our lysine page).

Watermelon: often mentioned for arginine, watermelon actually contains very little — about 0.06g per 100g in USDA data. Its interest lies in citrulline, which the body converts into arginine, although watermelon citrulline levels vary widely between varieties, and you’d need to eat a lot of it to match a supplement dose.

Arginine vs Citrulline — The Most Important Practical Distinction

Why oral arginine underperforms:

• The gut lining and the liver contain high levels of arginase, so a large share of an oral arginine dose is broken down into ornithine and urea before it reaches the rest of the body
• Estimates of how much reaches the bloodstream vary widely between studies, but it’s well below 100% and falls further at higher doses

Why citrulline raises arginine more effectively:

• Citrulline isn’t broken down by arginase, so it largely passes through the gut and liver intact
• The kidneys convert it into arginine, which goes straight into the bloodstream
• Studies show that oral citrulline raises blood arginine more, and for longer, than the same amount of oral arginine

What that means for performance: raising blood arginine doesn’t automatically improve performance, especially in trained athletes, whose blood vessels already make NO efficiently. Meta-analyses suggest citrulline malate may slightly improve strength and power performance, and some report small benefits from arginine as well, but trials are small and results inconsistent. For endurance, dietary nitrate is the better-supported option. Citrulline malate products vary in their citrulline-to-malate ratio, so check how much citrulline a dose actually provides.

Helps With

• Making nitric oxide, for blood vessel function, blood pressure regulation, and blood flow
• Disposing of nitrogen through urea production
• Making creatine, together with glycine and methionine
• Making polyamines, for cell growth
• Making proline for collagen, through ornithine
• Wound healing, as part of specialized nutrition formulas
• Immune function, through NO and repair pathways
• Possibly erectile function — small trials suggest modest benefits for mild erectile dysfunction, though prescription medicines are far more effective

Arginine for Athletes and Active People

The “pump” and blood flow: NO widens blood vessels in working muscle, increasing blood flow — the basis of the “pump” during resistance training and the reason arginine and citrulline appear in pre-workout products. Whether supplements meaningfully increase this effect, or improve performance, in trained athletes is uncertain.

The supplement reality check: evidence for oral arginine improving exercise performance is mixed. Trained athletes already produce NO efficiently, and much of an oral dose is broken down before it reaches the muscles. If NO support is the goal, dietary nitrate (such as beetroot juice) has the strongest evidence for endurance, and citrulline is a better way than arginine to raise blood arginine — though its performance effects are small.

Creatine and arginine: creatine supplements reduce the body’s own creatine production, which in theory frees up some arginine and methionine. This is a possible minor bonus rather than a reason to take creatine.

Injury and surgery: arginine-containing nutrition formulas have shown benefits for pressure ulcer healing and after major planned surgery, but these are combination products studied in hospital patients. For athletes recovering from injury, adequate protein and energy are the priorities, and specialized formulas should be used only on medical advice.

High-protein diets: athletes eating 1.6–2.2g of protein per kg produce more nitrogen to dispose of, increasing urea cycle activity. The cycle makes its own arginine as it runs, so no extra arginine is needed.

Arginine Deficiency

In healthy adults eating enough protein: deficiency is rare. The gut and kidneys make arginine, and food provides more.

Situations where low arginine matters:

• Premature infants: their ability to make arginine is immature, so arginine is essential for them
• Urea cycle disorders: in several of these, the blocked cycle can’t make enough arginine, and arginine or citrulline supplements help keep it running — except in arginase deficiency, where arginine must be restricted
• Lysinuric protein intolerance: arginine, lysine, and ornithine are poorly absorbed and lost in the urine
• Severe illness, burns, and major surgery: blood arginine levels can fall. Supplementation isn’t straightforward, though: in critically ill patients with severe infection (sepsis), arginine-enriched formulas have raised safety concerns, so they’re used selectively and only under medical direction

Effects of low arginine depend on the cause, and can include ammonia build-up (in urea cycle disorders), poor wound healing, impaired immune function, and poor growth in children.

Excessive Arginine

At moderate supplement doses: generally well tolerated in healthy people. The most common side effects at higher doses (above about 10g a day) are nausea, stomach cramps, and diarrhea.

Important cautions:

• After a heart attack: in the VINTAGE MI trial, published in 2006, heart attack survivors given 9g of L-arginine a day had no improvement in blood vessel function, and more deaths occurred in the arginine group, so the trial was stopped early. People who’ve had a heart attack shouldn’t take arginine supplements.
• Low blood pressure: arginine can add to the effects of blood pressure medicines, nitrate medicines for angina (such as nitroglycerin), and erectile dysfunction drugs such as sildenafil and tadalafil.
• Cold sores: herpes simplex viruses need arginine to replicate in laboratory studies, so high-dose arginine supplements may trigger outbreaks in people prone to them. The evidence in people is limited, but if you get frequent cold sores, it’s sensible to avoid high-dose arginine.
• Intravenous arginine: used in hospital growth hormone tests, it can cause dangerously high blood potassium, especially in people with kidney or liver disease, and dosing errors have caused fatal overdoses in children. This isn’t a concern with oral supplements at normal doses.

Types and Forms

• L-Arginine (free form): the standard supplement form, limited by breakdown in the gut and liver.
• L-Arginine HCl: the most common salt form in supplements; behaves the same as free arginine once absorbed.
• Arginine alpha-ketoglutarate (AAKG): arginine combined with alpha-ketoglutarate, marketed as superior, but without good evidence of an advantage over standard arginine.
• L-Citrulline: raises blood arginine more effectively than L-arginine, because it avoids breakdown in the gut and liver.
• Citrulline malate: citrulline combined with malic acid — the form used in most performance studies. Check the ratio (often 2:1 or 1:1) to know how much citrulline you’re getting.
• Dietary nitrate (beetroot juice): not an arginine product, but a separate route to NO, and the best-supported NO-related supplement for endurance performance.
• Agmatine sulfate: a compound the body makes from arginine. It acts on several receptors in the brain and can inhibit NOS, so it’s a separate agent rather than a way to boost arginine.

Which form for your goal:

How to Take Arginine

If you take L-arginine: split larger doses through the day to reduce digestive upset, and check with your doctor if you take any heart, blood pressure, or erectile dysfunction medicines — and don’t take it at all if you’ve had a heart attack.

For performance: consider dietary nitrate first — beetroot juice providing roughly 6–13mmol of nitrate, taken about 2–3 hours before exercise, is the dose range used in most studies. If you prefer citrulline, studies typically used L-citrulline or citrulline malate about an hour before training.

Support the enzyme: NOS needs several cofactors, including riboflavin, iron, and BH4. A varied diet with plenty of vegetables, adequate protein, and good overall nutrition matters more than any single supplement.

If you get frequent cold sores: avoid high-dose arginine supplements.

Interactions

Lysine competes with arginine for transport into cells and across the gut, so high intakes of one reduce uptake of the other. Citrulline is both a product of NOS and a precursor of arginine, made in the kidneys, and is the most effective way to raise blood arginine. BH4 is essential for NOS — when it’s lacking, NOS becomes “uncoupled” and produces damaging superoxide instead of NO. Riboflavin provides the FAD and FMN that NOS needs. Erectile dysfunction drugs such as sildenafil, nitrate medicines for angina, and blood pressure medicines can all add to arginine’s blood-pressure-lowering effect.

Synergy

• Works with citrulline in the arginine–citrulline cycle — NOS turns arginine into citrulline and NO, and the kidneys turn citrulline back into arginine.
• Works with glycine and methionine in creatine production.
• Works with ornithine in the urea cycle and polyamine production.
• Works with proline in collagen formation, through ornithine.
• Works with glutamine in the gut — glutamine fuels gut cells, while arginine-derived NO helps regulate blood flow to the gut lining.
• Works alongside dietary nitrate, which provides a second, arginine-independent route to NO.

Antagonistic Interactions

Lysine and arginine compete for transport — the basis of using lysine for cold sores, and a minor consideration for NO production at very high lysine doses.

Arginase in the gut and liver breaks down much of an oral arginine dose before it reaches the bloodstream — the main reason citrulline is more effective.

Erectile dysfunction drugs, nitrate medicines, and blood pressure medicines combined with high-dose arginine may lower blood pressure too much.

During inflammation, increased arginase activity can deplete local arginine, limiting NO production even when dietary arginine is adequate.

Food values: USDA FoodData Central, SR Legacy — pumpkin and squash seed kernels, dried (FDC ID 170556)*; peanuts, raw (172430)*; almonds (170567)*; turkey breast, roasted (174516); chicken breast, roasted (171477); pork top loin chop, broiled (167841); ground beef, 80% lean, cooked (171797); yellowfin tuna, cooked (172006); sardines, canned in oil (175139); tofu, firm, calcium-set (172475); Atlantic salmon, farmed, cooked (175168); parmesan, hard (170848); oats, dry (173904); chickpeas, cooked (173757); egg, hard-boiled (173424); lentils, cooked (172421); black beans, cooked (173735); cottage cheese, low-fat, 2% (172182)*; white rice, cooked (168878). * SR Legacy values used as fill-ins for the Foundation Foods entries for pumpkin seeds (2515380), peanuts (2515376), almonds (2346393), and cottage cheese (328841), whose amino acid data aren’t included in our tracker. Percentages of protein and arginine-to-lysine ratios are calculated from the same USDA records; the watermelon figure is from USDA SR Legacy data for raw watermelon.