ZHICHENG BIO
Citrulline malate 2:1: what the ratio means, and what NMR found in the drums
Two ambiguities and one published measurement, worked through in citrulline kilograms.
The short answer
"Citrulline malate 2:1" is ambiguous before you even open the drum — a 2:1 molar ratio is 72.3% citrulline by mass and a 2:1 weight ratio is 66.7%, and labels rarely say which. The second problem is that the stated ratio may not be the delivered ratio: when Chappell and colleagues put five commercially available 2:1 citrulline malate supplements through NMR, the measured citrulline-to-malate ratios were 1.92:1, 1.62:1, 1.57:1, 1.51:1 and 1.11:1, and only one came close to the claim. Both problems are solved the same way — stop buying a ratio and start buying two separately assayed components.
- The molecular masses
- L-citrulline C6H13N3O3, Mr 175.19, CAS 372-75-8; DL-malic acid C4H6O5, Mr 134.09, CAS 617-48-1
- 2:1 as a molar ratio
- (2 × 175.19) ÷ (2 × 175.19 + 134.09) = 350.38 ÷ 484.47 = 72.32% citrulline
- 2:1 as a weight ratio
- 2 ÷ 3 = 66.67% citrulline
- What that ambiguity is worth
- 5.65 percentage points — 56.5 kg of citrulline per tonne
- What was actually measured
- NMR on five commercial "2:1" products: 1.92:1, 1.62:1, 1.57:1, 1.51:1, 1.11:1. "Only one of these products had a value close to the ratio stated by the manufacturers" (Chappell et al., J Int Soc Sports Nutr 2018;15:42, PMC6086018)
- The malate is racemic
- The standard counter-acid is DL-malic acid, not L-malic acid (CAS 97-67-6). If your specification says "malic acid" without a prefix, you have not specified it
Two readings of the same label
L-citrulline has a relative molecular mass of 175.19; malic acid has 134.09. If "2:1" means two moles of citrulline to one of malic acid — which is the chemically natural reading for a salt or co-crystal — then a mole of the pair weighs 2 × 175.19 + 134.09 = 484.47, of which 350.38 is citrulline. That is 72.32%.
If "2:1" means two parts citrulline to one part malic acid by weight — which is how a blender thinks, and how much of the sports-nutrition trade in fact uses the term — then citrulline is two thirds, 66.67%.
Five and a half percentage points does not sound like much until you put a tonne behind it: 723 kg of citrulline versus 667 kg, a gap of 56.5 kg. And because the label convention is genuinely unsettled, both suppliers can be describing their material accurately and still be selling you different things at the same price. This is not a case where one reading is right and the other is a trick; it is a case where the word "ratio" has two meanings and the purchase order has to pick one.
What NMR found when someone checked
In 2018 Chappell and colleagues, publishing in the Journal of the International Society of Sports Nutrition, ran nuclear magnetic resonance spectroscopy on five commercially available citrulline malate supplements sold as 2:1, alongside the product used in their own trial. The measured ratios were 1.92:1, 1.62:1, 1.57:1, 1.51:1 and 1.11:1. Their summary was that only one of the products had a value close to the ratio stated by the manufacturers, and that their own study supplement contained just over half the manufacturer's stated dose of citrulline.
Convert those to citrulline mass fractions, reading the NMR result as a molar ratio — which is what proton NMR integration naturally gives you. At r moles of citrulline per mole of malic acid, the citrulline mass fraction is (r × 175.19) ÷ (r × 175.19 + 134.09). That gives 71.5% at 1.92:1, 67.9% at 1.62:1, 67.2% at 1.57:1, 66.4% at 1.51:1, and 59.2% at 1.11:1 — against 72.3% for a true 2:1 molar product.
So the worst of the five delivered 59.2 kg of citrulline per 100 kg of powder where the label implied 72.3 kg: an 18% relative shortfall in the component you were buying it for. The best delivered 71.5%, which is essentially the claim. The spread between two products bearing the same 2:1 designation was 12.3 percentage points.
The published work is on finished supplements rather than bulk raw material, and that distinction should be stated rather than glossed. But the raw material is where those finished products came from, and nothing in the analysis points to blending as the source of the discrepancy.
Why the ratio is a bad thing to buy
A ratio is a derived quantity. It is the result of two measurements, and it can be hit by getting both wrong in a compensating direction. It also carries no information about anything else in the drum — a 2:1 ratio is perfectly compatible with a material that is 10% water, or that contains unreacted starting materials, because ratios normalise those away.
Assay both components separately and the ratio becomes an output rather than an input. "L-citrulline ≥ 68.0%" and "malic acid ≥ 30.0%", each by its own method, tells you the ratio, tells you the mass balance, and leaves nowhere for water or process residue to hide, because the two figures plus loss on drying have to approach 100%.
This also removes the argument. If a supplier's certificate shows 68.5% citrulline and 30.2% malic acid, nobody has to agree on what "2:1" means — the numbers are the specification. And it is the only form in which two quotations from different suppliers are directly comparable.
How to verify, and what to write down
Quantitative NMR is the method the published analysis used and it is a good one here, because a single proton spectrum shows both components simultaneously and the integration is a direct molar comparison — no separate calibration curve per analyte, no chromatographic co-elution to worry about. If you are qualifying a new supplier of this material, one qNMR run answers the ratio question outright.
For routine release, HPLC for citrulline and either HPLC or ion chromatography for malate works, provided both are reported. Add loss on drying and check that citrulline plus malate plus water approaches 100%; a large unexplained remainder is the interesting finding.
In the specification, name the acid form. "DL-malic acid" and "L-malic acid" are different substances with different CAS numbers (617-48-1 and 97-67-6) and different prices; the industry standard counter-acid for this material is the racemate, and a specification saying only "malic acid" has not said which. Chirality that goes unpriced is a recurring theme in this trade — the same failure in a different molecule is set out in the theanine enantiomer guide.
Finally, if you have a target citrulline dose in mind, write the purchase order in kilograms of citrulline rather than kilograms of citrulline malate. It converts a specification argument into an arithmetic one, which is the kind you can win.
Frequently asked
Is citrulline malate 2:1 a molar ratio or a weight ratio?
Labels frequently do not say, and both conventions are in use. A 2:1 molar ratio gives 72.32% citrulline by mass (350.38 of 484.47); a 2:1 weight ratio gives 66.67%. The difference is 5.65 percentage points, or 56.5 kg of citrulline per tonne of material. The only reliable fix is to specify the citrulline content directly as a percentage rather than specifying a ratio.
Do commercial citrulline malate products actually meet the 2:1 claim?
Not reliably, on the published evidence. Chappell and colleagues (J Int Soc Sports Nutr 2018;15:42) measured five commercially available 2:1 citrulline malate supplements by NMR and found ratios of 1.92:1, 1.62:1, 1.57:1, 1.51:1 and 1.11:1, reporting that only one was close to the stated ratio. Read as molar ratios, those correspond to citrulline mass fractions from 71.5% down to 59.2%.
How much L-citrulline is in citrulline malate?
It depends entirely on the ratio basis and the actual ratio delivered. For a true 2:1 molar product, 72.32%. For a 2:1 weight-ratio product, 66.67%. For the lowest-ratio product in the published NMR survey, about 59.2%. This is why the useful specification is a direct citrulline assay, not a ratio.
Should I specify L-malic acid or DL-malic acid?
Decide and write it down. The standard counter-acid in commercial citrulline malate is DL-malic acid (CAS 617-48-1), the racemate; L-malic acid (CAS 97-67-6) is a different, more expensive substance. A specification that says only 'malic acid' leaves the choice to the supplier.
This page describes raw-material sourcing and specification. It is not a health claim and not medical advice. Last updated 2026-08-07.