Negative nitrogen balance a word commonly heard in the PSSM community, but what exactly is it and why do PSSM horses suffer from it?

What Is Negative Nitrogen Balance?

Negative nitrogen balance is a state that can occur in any mammal where protein breakdown (known as catabolism) exceeds protein creation (known as anabolism or synthesis) resulting in tissues (including muscle) losing protein faster than it can be replaced.

What Causes Negative Nitrogen Balance?

Mammals including horses create thousands of different proteins which are encoded by different genes, including complex amino acids (structural units that make up proteins and in turn muscle etc…) but due to eating protein containing food mammals have now lost the ability to create these complex amino acids which creates a bit of a problem.

During the process of protein creation the body draws on a pool of amino acids. If there is not enough complex amino acids to respond to an increased need for protein creation, as a state of emergency the body will begin to tear down tissue (muscle) so it can get the amino acids that it needs thus causing the state of negative nitrogen balance.

What causes negative nitrogen balance?
Due to eating protein containing food mammals have now lost the ability to create complex amino acids

What Can Trigger Negative Nitrogen Balance?

Surgery, injury, illness, myopathy, or tying up

What Are The Symptoms Of Negative Nitrogen Balance?

Severe muscle wasting, depression, loss of appetite

Negative Nitrogen Balance
A Thoroughbred gelding with PSSM2 showing severe muscle wasting which also means the horse will be in a state of negative nitrogen balance

How Is Negative Nitrogen Balance Diagnosed?

When a horse is suffering an episode of negative nitrogen balance they will excrete excess nitrogen in their urine.

Negative Nitrogen Balance
Negative Nitrogen Balance

Why Do PSSM Horses Suffer With Negative Nitrogen Balance?

About 50% of the mass of a horse is muscle. Muscle is an emergency storage mechanism for amino acids including the complex ones mammals are unable to create. Any horse or mammal can suffer with negative nitrogen balance including healthy ones, but PSSM horses are at risk because a large episode of tying up with high CK & AST levels, or severe muscle wasting can cause a horse to enter the state of negative nitrogen balance.

Can The State Of Negative Nitrogen Balance Be Reversed?

Yes, the state of negative nitrogen balance can be reversed through feeding the 3 essential amino acids (Lysine, Threonine, & Methionine) which are limited in a horses diet, and/or a protein source. It is also important to liaise with your vet so they can monitor nitrogen levels.