A new study published in Toxicology in Vitro has been causing quite the stir in supplement circles as it appears that a common pre workout nitric oxide booster may be cytotoxic to brain cells.

The ingredient in question?

The arginase inhibitor, and nitric oxide extender, L-Norvaline.

Let’s take a look beyond the attention-grabbing headlines, dive deep into the study and see what’s really going on here.

First off….

What is L-Norvaline?

L-Norvaline is a derivative of the branched-chain amino acid (BCAA) valine that has been noted in research to be a mixed arginase inhibitor. Arginase is the enzyme in the body that degrades arginine — the primary substrate used to generate nitric oxide, increase blood flow, boost muscle pumps.

The theory goes that by limiting the actions of arginase, you’re removing the “governor” plate in your body allowing for unrestricted nitric oxide production, yielding stronger, longer-lasting pumps.

Norvaline isn’t the only arginase inhibitor commonly used in pre workout supplements either. ATP Perform for instance uses 1g Agmatine Sulfate as it’s NO-extender.

Now, as we mentioned L-Norvaline is a mixed arginase inhibitor, which means that it both degrades arginase in the liver as well as in the lumen of the GI tract and endothelium of blood vessels.

In regards to enhancing blood flow and pumps, arginase inhibition in the endothelium and lumen would be beneficial, in theory as it would lead to greater arginine availability. However, you would only benefit from the increased arginine if there was a subsequent enhancement in eNOS expression — the enzyme that actually generates nitric oxide. Furthermore, all studies using L-Norvaline are in unhealthy individuals, there is no substantial body of evidence in otherwise healthy individuals that L-norvaline exerts the same anti-arginase / anti-inflammatory activity.

Continuing on, arginase inhibition in the liver is a bit of a concern, as arginsase is a critical component of nitrogenous waste removal where it catalyzes the final step in the production of urea.

By inhibiting arginase in the liver to a large enough extent, there is the potential for an excess of ammonia in the body (hyperammonemia), which is often seen in individuals with congenital arginase deficiency, and may lead to brain injury and death.

And this brings us to the current study.

L-Norvaline and Cytotoxicity

Researchers took SH-SY5Y brain cells, placed them in petri dishes, and exposed them to 500-2000uM l-norvaline.

FYI, this verges on injecting yourself with upwards of 1g of L-Norvaline. The typical NO-boosting pre workout contains between 100-300mg of L-Norvaline.

Researchers noted that brain cells died when exposed to Norvaline, and the longer they were exposed to the common pre workout ingredient, the more toxic it was.

The method in which Norvaline caused neuronal cell death may be due to its starving the brain cells of energy, as tests showed it did not reduce the amount of mitochondria, but decreased their size.

In a follow up press release to the publication of the study, Kate Samardzic, lead author of the study said:

“Protein requirements are higher in very active individuals and proteins are considered to improve and increase performance. The demand for amino acids in supplements has expanded but in addition to the normal protein-building amino acids other ‘non-protein’ amino acids are being taken.

Some non-protein amino acids are toxic because they can mimic protein amino acids and deceive the body into making faulty proteins; a property used by some plants to kill predators.

Some plants can even release non-protein amino acids into the soil to kill other plants so that they can have access to all the nutrients. Chemical warfare among plants is a well known phenomenon. Since there was evidence that L-norvaline has antimicrobial and herbicidal activity we examined its toxicity in human cells.”

The Takeaway on L-Norvaline and Nitric Oxide

Now, it should be noted that these studies were conducted using cell cultures. They were not carried out with individuals ingesting l-norvaline as they would in a pre workout supplement. Still, this does bring a bit of concern to the ingredient especially in regards to the amount and frequency of using it pre workout.

In the end, low doses of L-Norvaline likely aren’t effective in healthy populations, high doses may be dangerous (and potentially toxic), and moderate doses taken may increase the bioavailability of co-ingested l-arginine.


  1. Arginase inhibitor in the pharmacological correction of endothelial dysfunction. Int J Hypertens. 2011;2011:515047.
  2. Ming XF, Rajapakse AG, Carvas JM, Ruffieux J, Yang Z. Inhibition of S6K1 accounts partially for the anti-inflammatory effects of the arginase inhibitor L-norvaline. BMC Cardiovasc Disord. 2009;9:12. Published 2009 Mar 13. doi:10.1186/1471-2261-9-12
  3. Samardzic, K., & Rodgers, K. J. (2019). Cytotoxicity and mitochondrial dysfunction caused by the dietary supplement l-norvaline. Toxicology in Vitro, #pagerange#. https://doi.org/10.1016/j.tiv.2019.01.020
  4. University of Technology Sydney. “Body building supplement could be bad for the brain: People taking the protein supplement L-norvaline should be aware of its potential for harm, scientists say.” ScienceDaily. ScienceDaily, 7 February 2019. www.sciencedaily.com/releases/2019/02/190207102627.htm