Ingredient file // Other
L-Cystine
A useful amino acid adjustment, not a coat cure
L-Cystine is a purified amino acid added to balance a recipe's sulfur amino acids. It supplies part of what a dog needs to make proteins, while methionine must still come from the rest of the food. Its presence does not promise a shinier coat.
- What it is
- A purified amino acid made of 2 linked cysteine units, added to balance a recipe's sulfur amino acids.
- Made from
- Poultry feathers or microbial fermentation
- Main job
- Sulfur amino acid balance
- Typical amount
- Not published
- Watch for
- Dogs already diagnosed with cystinuria
- Not the same as
- L-Cysteine or L-Methionine
Our read // In our catalog
In 1 of every 865 foods we track
Share of foods that list it
0%
3 of the 2,595 foods with an ingredient list on file, from 2 brands. All dry dog food.
Each square is 1% of the catalog
Where it sits on the label
Median position 24. In the top five of 0 foods, so it is usually a minor ingredient.
Above the salt
Usually listed after the salt, which caps it at roughly 1% of the recipe in 100% of foods.
Price per lb
Median of the cheapest bag price across 2 priced foods, 92% above the catalog. The price is the food's, not the ingredient's.
Who leans on it
67%
of the foods that list it are from Royal Canin. 2 brands use it in total.
What it is
How it is made
Poultry processing leaves feathers rich in keratin, which some producers break down to recover L-Cystine. Other producers grow bacteria that release cysteine into a fermentation broth. Both routes end with purified crystals for feed manufacturers.3,4,5
- 01 // Source Feathers or broth Production starts with poultry feather keratin or a bacterial culture grown to release cysteine.
- 02 // Process Release amino acids Acid breaks down feather keratin; in fermentation broth, released cysteine oxidizes into cystine.
- 03 // Process Separate crystals In the documented fermentation route, workers pasteurize the broth and separate cystine crystals by decantation.
- 04 // Process Purify and dry The crystals are dissolved, filtered, precipitated again, centrifuged and dried.
- 05 // Output L-Cystine powder A concentrated crystalline powder reaches the feed plant for measured addition to the recipe.
What is in it
Composition, dry matter
99+
% L-cystine
A fermentation product contained more than 99% cystine on a dry matter basis. This powder adjusts amino acids rather than supplying whole protein.
Why it is in the bag
What the manufacturer gets
-
01
Balances sulfur amino acids
Cystine supplies part of the sulfur amino acids needed to make body proteins. The recipe still needs enough methionine from other ingredients.1,3
-
02
Makes a measured adjustment
A small addition changes the amino acid balance without adding a larger amount of whole protein. Formulators must account for the methionine already in the food.1,3
-
03
Uses industrial supply
The feather route gives leftover poultry keratin a feed use, while fermentation provides another production route. The ingredient name does not reveal which supplier made it.4,5
How much A 32-week dog trial added cystine to keep total sulfur amino acids at 2.68 g per 1000 kcal; it did not set a standard commercial dose.1
Our read // The claims
What the evidence supports
Strong: controlled studies in dogs agree. Some: small or mixed dog data. Thin: mechanism or lab work only. None: nothing, or the evidence points the other way.
-
Brands say
Supplies part of a dog's sulfur amino acid needs
Evidence
- Strong
- Some
- Thin
- None
In a 52-dog trial, added cystine helped hold total sulfur amino acids steady while diets contained less methionine; plasma methionine did not fall over 32 weeks. The study tested carefully controlled diets, not ordinary kibble formulas.1,3
-
Brands say
Makes a dog's coat shinier
Outruns the evidence
Evidence
- Strong
- Some
- Thin
- None
A pet-industry-funded experiment exposed canine skin cells to cysteine, a related amino acid, and measured cell growth. It did not feed L-Cystine to dogs or measure coat appearance. A shinier coat remains an unsupported promise.2
-
Brands say
Boosts taurine and protects the heart
Outruns the evidence
Evidence
- Strong
- Some
- Thin
- None
In the 52-dog diet study, one test group had 19% higher plasma taurine than controls at week 16. The cardiac stress marker showed no significant diet effect. Neither result demonstrates that L-Cystine protects a dog's heart.1
-
Owners say
Causes cystine stones in healthy dogs
Outruns the evidence
Evidence
- Strong
- Some
- Thin
- None
Cornell describes cystinuria as an inherited kidney transport problem that lets cystine accumulate in urine. That diagnosis is different from seeing L-Cystine on a food label; the cited evidence does not show the ingredient causing cystinuria.6
Straight answers
What owners ask
-
Q01
Is it just filler?
NoIt has a specific nutritional job. In the adult dog trial, cystine helped keep total sulfur amino acids steady as methionine changed. This small addition says little about the quality of the food's main protein ingredients.1,3
-
Q02
Can it replace methionine?
Only partlyMethionine is essential, and L-Cystine meets only part of the sulfur amino acid requirement. The 32-week trial retained methionine in every diet. Check the food's nutritional adequacy statement rather than judging its balance from this line.1,3,7
-
Q03
Does it come from feathers?
SometimesOne documented route breaks down poultry feather keratin. Another produces cystine through fermentation and purifies the resulting crystals. A bag's L-Cystine line does not identify its production route.4,5
Who uses it
Brands, best sellers and the company it keeps
Top brands, by foods that list it
Usually listed with, share of the foods that list it
- Chicken Fat 100%
- Chicken By-Product Meal 67%
- Corn 67%
- Brewers Rice 67%
- Wheat Gluten 67%
- Natural Flavor 67%
- Corn Gluten Meal 67%
- Vegetable Oil 67%
- Dehydrated Chicken 33%
- Rice 33%
- Dehydrated Egg Product 33%
- Whole Oats 33%
Our read // On the bag
Treat it as a small recipe adjustment
L-Cystine near the end of a list is a measured amino acid addition. Read the main protein ingredients and the nutritional adequacy statement before drawing conclusions about the food. Ignore coat and heart promises tied to this line. If your dog has cystinuria, follow its prescribed diet plan.
Sources
What we read
Checked October 1, 2026.
- 1 Short-term determination and long-term evaluation of the dietary methionine requirement in adult dogs
- 2 Nutrients regulation of skin cells from canines and cats via Wnt/β-catenin signaling pathway
- 3 Safety and efficacy of l-cystine produced using Pantoea ananatis strain NITE BP-02525 for all animal species
- 4 Assessment of the feed additive consisting of l-cystine for all animal species for the renewal of its authorisation (Bretagne Chimie Fine [BCF Life Sciences])
- 5 Safety and efficacy of a feed additive consisting of l-cystine produced with Escherichia coli DSM 34232 for all animal species (Wacker Chemie AG)
- 6 Cystinuria
- 7 What's in the Ingredients List?