Glycine

Glycine is not a leftover from another industry. Chemical plants make it on purpose, either by synthesis or by breaking down protein such as gelatin, then purify it into a powder.^2,3,4,6

1. 01 // Source Chloroacetic acid, ammonia The standard synthetic route starts from these two chemicals. Older methods broke down gelatin, cattle ligaments or silk instead.
2. 02 // Process Reacted Ammonia reacts with chloroacetic acid to give glycine, a small molecule with the formula C2H5NO2.
3. 03 // Process Purified The product is cleaned up to feed grade. The EU authorization specifies at least 98% glycine, made by synthesis or protein hydrolysis.
4. 04 // Output Glycine powder A sweet-tasting powder, weighed into the recipe in small amounts as a flavoring and amino acid.

1. 01

   Sweet taste and browning

   Glycine tastes sweet. When wet food is heated, amino acids react with sugars to form brown Maillard flavors, which a palatability review links to higher acceptance.^4,9

2. 02

   Cleared as a flavoring

   The EU authorizes it as a flavoring for all species, with a recommended maximum of 20 g per kg in dog and cat food. FDA lists it as safe in feed.^1,2

3. 03

   Slows fat rancidity

   Chemical references list glycine as retarding rancidity in animal and vegetable fats. No study has measured that effect inside a dog food.⁵

4. 04

   A glutathione building block

   Glycine is one of the parts the body uses to make glutathione, an antioxidant. Mars researchers added it to a cat diet on that basis.⁷

How much The EU recommends at most 20 g per kg, about 2%, of a 12% moisture dog food; a 34-cat trial fed 1.5%.^2,7

1. Critics say

   Free amino acids pad the protein number

   Evidence

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

   Crude protein is nitrogen times 6.25. Glycine is about 18.7% nitrogen, so the test reads it as about 117% protein. At 2% of a food that adds about 2 points, none of it from essential amino acids.^6,10,11

2. Brands say

   Glycine makes food taste better to dogs

   Evidence

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

   Glycine tastes sweet to people, and a nerve-recording study in dogs used it as a taste stimulus. No published feeding trial has tested whether dogs prefer food with added glycine.^4,8

3. Brands say

   Glycine restores antioxidant defenses in older pets

   Outruns the evidence

   Evidence

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

   A Mars-funded trial fed 34 senior cats 1.5% glycine. Red cell glutathione rose at 4 weeks but not at 8 or 12. No study has tested this in dogs.⁷

4. Brands say

   Glycine helps prevent diabetes, liver and heart disease

   Outruns the evidence

   Evidence

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

   Pet ingredient sites list diabetes, fatty liver and heart failure among glycine's benefits. No dog study supports any of them, and glycine is not among the 10 amino acids dogs must eat.^10,12