L-Tyrosine

The feed additive assessed by EFSA comes from feathers left by poultry processing. Acid breaks down the feather protein, and further separation isolates tyrosine. The resulting powder is an amino acid ingredient, not ground feathers.^4,5,6

1. 01 // Source Poultry feathers Feathers from poultry processing supply keratin, the protein used to make this assessed additive.
2. 02 // Process Dissolved in acid The assessed process dissolves feathers in 20% hydrochloric acid at 70°C.
3. 03 // Process Hydrolyzed and separated The mixture is heated to 106°C, then tyrosine is separated from cystine by crystallization in an alkaline solution.
4. 04 // Output L-Tyrosine powder A fine, purified powder reaches the food plant; the assessed additive must contain at least 95% L-tyrosine.

1. 01

   Adjusts amino acid balance

   Tyrosine counts toward a dog's combined phenylalanine and tyrosine intake. Adding the isolated amino acid lets a formulator raise that amount without adding another whole protein ingredient.^1,5

2. 02

   Targets black coat pigment

   Tyrosine supplies the pathway that makes melanin. Controlled diets with more tyrosine produced darker black coats in puppies and adult Labradors, although the small trials came from one company's researchers.^1,2

3. 03

   Uses a large byproduct stream

   The assessed production route extracts tyrosine from poultry feathers. Feathers are generated at large scale during poultry processing; the published assessments do not show whether this route costs less than alternatives.^5,6

4. 04

   Survives dry food storage

   EFSA reviewed tests of supplemented dry dog and cat foods and reported no loss of added tyrosine during storage. That makes a measured addition practical for shelf-stable kibble.⁵

How much EFSA reviewed dry dog and cat foods supplemented with 0.34 to 0.43% L-tyrosine; a label does not disclose the amount in its recipe.^5,9

1. Brands say

   It intensifies a black dog's coat color

   Evidence

   1. Strong
   2. Some
   3. Thin
   4. None

   A 24-dog adult trial found darker black coats after 24 weeks on the higher tyrosine diet; a 12-puppy pilot also found more intense black. Both came from Royal Canin researchers, and neither tested every breed or recipe.^1,2,10

2. Brands say

   It makes any coat shinier and healthier

   Outruns the evidence

   Evidence

   1. Strong
   2. Some
   3. Thin
   4. None

   Royal Canin reports shinier coats for a complete Coat Care diet and separately credits tyrosine with color. Its controlled tyrosine feeding work measured pigment, not shine or skin health. The whole-food result cannot be assigned to this amino acid.^1,10

3. Owners say

   It supports a dog with an underactive thyroid

   Outruns the evidence

   Evidence

   1. Strong
   2. Some
   3. Thin
   4. None

   Tyrosine residues are used to make thyroid hormones, but that pathway is not evidence of treatment. No cited dog feeding trial tested thyroid disease; the veterinary standard for canine hypothyroidism is levothyroxine replacement.^7,8

4. Critics say

   Feather-derived tyrosine is just ground feather filler

   Outruns the evidence

   Evidence

   1. Strong
   2. Some
   3. Thin
   4. None

   The assessed manufacturing process breaks feather keratin apart and crystallizes an individual amino acid. Recent tested batches contained 96.4 to 98.5% tyrosine as supplied. That is a purified supplement, not feather meal or a bulk filler.^4,5,6