N-Acetyl L-Tyrosine vs L-Tyrosine: What the Data Show

N-acetyl L-tyrosine, usually shortened to NALT, is L-tyrosine with a small acetyl group attached to it. That one change makes it dissolve hundreds of times better in water, and it is the reason hospitals use NALT to put tyrosine into intravenous feeding. Supplement makers borrowed it from there, and NALT is often sold as the more absorbable form of tyrosine.

The human data point the other way. When researchers infused NALT into healthy volunteers, blood tyrosine rose by a quarter in one study and did not rise at all in another, and more than half of the dose left in the urine with the acetyl group still attached. The enzyme that removes the acetyl group sits in the kidney, and the kidney passes much of the NALT straight through. Meanwhile every trial that found tyrosine helps working memory or attention under pressure gave plain L-tyrosine.

One point of disclosure before the detail: Vyvamind contains L-tyrosine, so we have a position. What we can do is show the studies behind both forms and link every one of them, so you can check the numbers yourself.

What is N-acetyl L-tyrosine?

NALT is the amino acid L-tyrosine with an acetyl group (a two-carbon fragment) bonded to its nitrogen. The formula goes from C9H11NO3 for L-tyrosine to C11H13NO4 for NALT, and the molecule gets heavier: 223 versus 181 g/mol. Put another way, 81 percent of NALT by weight is tyrosine, and the rest is the acetyl group.

The body has to strip that acetyl group off before the tyrosine can do anything. Tyrosine is the starting material for dopamine and norepinephrine, the transmitters that set drive and alertness, and the rate of that synthesis responds to how much tyrosine is on hand when neurons are firing hard. NALT itself does not enter that pathway. Only the tyrosine released from it does, which is why the whole comparison turns on one question: how much of a NALT dose actually becomes free tyrosine in the blood? The chemistry of how tyrosine feeds dopamine is covered in our guide to how to increase dopamine.

Why do supplement makers use NALT instead of L-tyrosine?

Because NALT dissolves and L-tyrosine barely does. PubChem lists the water solubility of L-tyrosine at 0.45 grams per litre at 25 °C, which is less than half a gram in a litre of water. NALT's listed solubility is 297 milligrams per millilitre, about 650 times higher. Anyone who has stirred L-tyrosine powder into a glass of water and watched it settle has seen the problem.

That property matters enormously in one setting. A 1985 paper on intravenous feeding put it plainly: "The tyrosine content of parenteral solutions is limited by poor tyrosine solubility. N-acetyl-L-tyrosine has excellent solubility and is a potential source of intravenous tyrosine." A feeding bag has to carry a full day's amino acids in a limited volume of liquid, and tyrosine will not fit unless it is acetylated. NALT solved a pharmacy problem, and it is still the standard tyrosine source in adult parenteral nutrition.

Solubility in a bag is a different thing from absorption in a person. You swallow a capsule with water, the capsule opens in the stomach, and the amino acid is taken up across the gut wall by transporters that carry free amino acids. L-tyrosine's poor solubility has never stopped it from being absorbed that way: a single oral dose more than doubles blood tyrosine within two hours. The "more bioavailable" claim for NALT rests on the solubility figure, and the studies that measured what actually reaches the blood tell a different story.

Does NALT raise tyrosine in the blood?

Barely, on the two occasions it has been measured in healthy people. Both studies used an intravenous infusion, which skips the gut entirely and gives NALT the best possible chance.

In the first, at the Karolinska Institute, eleven healthy volunteers received 5 grams of NALT over four hours. Plasma NALT climbed quickly. Plasma tyrosine rose by 25 percent. Over the same four hours, 56 percent of the infused NALT was recovered from the urine, unchanged. The authors traced where the small amount of conversion happened by sampling blood across the liver and gut on one side and the kidneys on the other. They found "no net production of tyrosine" from the liver and gut, and only "a small release" of tyrosine from the kidneys, 10 micromoles a minute. Their conclusion: "under these conditions the usefulness of NAT and NAC as precursors for the corresponding amino acids in humans is not apparent."

A second group in Vienna compared NALT with two tyrosine-containing dipeptides in seven healthy controls and eight dialysis patients. The dipeptides were cleared from the blood at 1,781 and 3,169 millilitres a minute, meaning they were broken down and used fast. NALT was cleared at 284, about a sixth of the slower dipeptide. In the healthy controls, "plasma Tyr did not increase with NAc-Tyr", and 60 percent of the NALT was excreted. The dipeptides lost almost nothing to the urine.

A third study looked at the question from the hospital side. Thirteen adults on continuous intravenous feeding had their NALT retention measured, and approximately 35 percent of the administered NALT was excreted unchanged, regardless of infusion rate or kidney function. That loss is tolerable in a feeding bag, because the bag supplies NALT all day and the patient retains enough to cover the need. It is a poor result for a capsule that a person takes once, hoping for a rise in blood tyrosine over the next hour.

No study we could find has measured blood tyrosine after NALT taken by mouth in a healthy person. What does exist is a 2004 clinical chemistry report describing NALT appearing in the urine after NALT given by mouth as well as by vein, written to help laboratories tell that finding apart from a genuine metabolic disorder. The oral route, in other words, produces the same signature as the intravenous one: NALT going in, NALT coming out.

Where does the acetyl group come off, and why does that matter?

In the kidney, by an enzyme called aminoacylase-1, and that location is the whole problem. Aminoacylase-1 is the enzyme the body uses to recover amino acids from N-acetylated proteins as they are broken down, and it is most abundantly expressed in the kidney tubular epithelium, the lining of the tubes that filter blood into urine. Children born without it excrete N-acetylated amino acids in their urine, which is how the deficiency was discovered.

So NALT arrives at the very organ whose job is to remove things from the blood. Some of it meets the enzyme, loses its acetyl group and returns to the circulation as tyrosine. The rest is filtered and leaves. That is exactly the pattern the Karolinska study saw: no conversion in the liver or gut, a trickle of tyrosine from the kidney, and more than half the dose in the urine. The Vienna study adds a detail that fits. Dialysis patients, whose kidneys work poorly, cleared NALT at 129 millilitres a minute against 284 in healthy people, and the authors judged that the dipeptides, "but not NAc-Tyr, may serve as a nutritional substrate in renal failure patients."

There is one honest complication. Rats handle NALT much better than people do. In the rat feeding study, only 8 to 17 percent of the NALT was lost in the urine and the tyrosine from it turned up in tissue protein within hours. That is why NALT looked promising in the laboratory before it was tested in humans, and why any NALT claim that leans on animal data should be read with the human numbers beside it.

Which form did the cognition trials use?

Plain L-tyrosine, in every trial with a positive result that we know of. The reason people buy either form is the research showing that tyrosine protects working memory and attention when something is draining the brain's dopamine and norepinephrine supply. That research is summarised in our L-tyrosine benefits guide. The table below lists the form and dose each trial used.

The 2015 systematic review of this literature judged that tyrosine "is an effective enhancer of cognition, but only when neurotransmitter function is intact and DA and/or NE is temporarily depleted." Every study behind that sentence used L-tyrosine.

What about NALT? A PubMed search for NALT combined with cognition, memory, attention or randomised trials returns six records, and none tests NALT alone in people. The two that come closest are a pair of pre-workout trials in which 300 mg of NALT was one ingredient in a blend that also carried 284 mg of caffeine, 3 g of beta-alanine and several other compounds. The blend improved a colour-word test after one to two hours, which the 284 mg of caffeine could explain by itself. In the eight-week follow-up, the cognitive differences seen at four weeks were "similar to PLA responses after 8-weeks", meaning the placebo group had caught up. Neither trial can say anything about NALT by itself, and neither claims to.

How do NALT and L-tyrosine doses compare?

The L-tyrosine trials used 2 g, or 100 to 150 mg per kg of body weight, and NALT capsules typically hold 350 mg. Those two numbers are not on the same scale even before conversion losses are counted.

Start with the weight. Because the acetyl group is part of the molecule, 350 mg of NALT contains at most 284 mg of tyrosine. Then apply the human retention figures. If a third to 60 percent of the dose is lost unchanged, as the three human studies found, the tyrosine actually released is somewhere between 110 and 190 mg. That is the arithmetic on the best available human data. It could be worse for an oral dose, since nobody has measured one, and the gut adds a step that the infusion studies skipped.

Compare that with the plain form. Twelve fasting volunteers who took 100 mg per kg of L-tyrosine, about 7 g for a 70 kg adult, saw plasma tyrosine go from 69 to 154 nanomoles per millilitre at two hours, and the ratio of tyrosine to the amino acids that compete with it for entry into the brain nearly tripled. A 2 g dose is smaller, and it is the amount the working memory and task-switching studies used to get their results.

Commercial focus products use less tyrosine than the single-ingredient trials, because they combine it with other compounds. The point is that whatever amount a formula contains, plain L-tyrosine delivers all of it as tyrosine, and NALT delivers a fraction that no human study has pinned down.

Is NALT safe?

Nothing in the human record suggests NALT is harmful at supplement doses, and it has a long history in intravenous feeding. The two pre-workout trials that included 300 mg of NALT found no rise in reported side effects over eight weeks, although that is a blend, not NALT alone. The acetyl group itself is a normal fragment of metabolism.

L-tyrosine has the longer safety record in healthy volunteers. The 1979 dosing study noted "no side effects of orally-administered L-tyrosine" at 100 and 150 mg per kg, and the stress trials in the table above used similar doses under supervision without reported harm. The one signal worth knowing is from the dose-response study in older adults, where higher doses made working memory worse, so more is not better for either form.

Two groups should ask a doctor before taking either. People with kidney disease, because the kidney is where NALT is converted and dialysis patients cleared it at less than half the healthy rate. And anyone on regular medication, because the trials were run in healthy adults and say nothing about interactions.

Which form should you take?

L-tyrosine, if the goal is the effect the trials describe. It is the form that raised blood tyrosine in people, the form that every positive cognition trial used, and the form whose dose you can read straight off the label without guessing how much survives the kidney. NALT has a real advantage, which is solubility, and a real use, which is intravenous nutrition. Neither of those helps a person swallowing a capsule.

That reasoning is why Vyvamind uses plain L-tyrosine, 300 mg per serving, alongside citicoline and caffeine with L-theanine. The full formula and the thinking behind each dose are on our research and formulation page. If you would rather start from a finished formula than assemble the pieces yourself, our guide to the best nootropic supplement explains how we weigh ingredient forms alongside the rest of the evidence.

If you already have NALT, it is unlikely to do harm. Just do not expect a 350 mg capsule to reproduce what a 2 g dose of L-tyrosine did in the laboratory, and be skeptical of any label that calls it the better-absorbed form without a human study to show it.