Here is a reply I made to a horse owner on the FB group I thought you might be interested in. If so please let me know – maybe I’ll dig in deeper on the next AMA.

“Question:

I have been feeding a no grain diet for over a year and my endurance PSSM2 positive Arab mare and I have taken the time of distance riding to do some other things. So far the diet has worked fantastic.

We are beginning to condition again for distance. What might I need to do for her at competition, after long training rides, etc ?

Anyone have experience with this?”

My reply –

Energy production within the cell is a fascinating subject that is getting a lot of attention in human studies. Luckily, the use of glucose, lactate, fructose, fatty acids and amino acids for fuel is about the same for all animals at the cell level. What is different between species (obviously) is what raw materials they come from. For example we humans don’t eat like horses (though I have been accused often as someone who eats like a horse).

However through epigenetics, the expression of genes of individuals within the same species are different. Add to this the individual differences of the gut microbe population and we get a diet that works for some horses and not for others. This makes feeding them (and us) more of an individual guessing game. But there are some rules and principles…

The gut has been developed over millions of years. We need to honor this by avoiding ingredients the horse has never seen before.

Horses living here may have developed there. For example an Arab developed in the Middle East is now living in Tennessee where there is no desert. We need to asses each individual for their needs in all aspects such as their ability to use water. Remember as fat is used as a fuel source water is created. Think of the hump on the camel which allows them to survive in the desert with little water.

There are environmental factors (stress, confinement, shipping) that will express certain genes in some horses that lead to an illness or an inability to perform. PSSM (polysaccharide storage myopathy) is one of these with a possible heritability. Or is it brought out in certain environments such as a high glucose diet?

Glucose is converted into pyruvate which is then converted into one of two things. With oxygen it becomes Acetyl CoA which then proceeds into the Krebs cycle creating precursors for lots of energy. Without oxygen glucose becomes lactate which in the short term is helpful (sprinting from the enemy) but otherwise is an emergency dump of glucose metabolism beyond the limits of the amount of oxygen in hemoglobin.

Luckily the mitochondria within the cell can also use fat for fuel which is more efficient (produces more energy without the polluting free radicals. By restricting sugar intake and training, the mitochondria learn to switch between these 2 fuels: glucose (stored in the muscle as glycogen) and fat (stored as body fat or produced by the gut bacteria from cellulose). With PSSM the ability to get glucose from glycogen is impaired and the conversion of glucose into pyruvate is also impaired. This leads to lactate formation and cramping. However a horse on a limited carb diet (no grain, soak and/or limit the hay) is forced to convert body fat into energy. This is called mitochondrial flexibility. This is why the horse here is doing well in distance riding on a low carb diet.

Historically horses with PSSM are given a “high fat diet” but this is relative. A low carb diet works the same way – by forcing the energy production through fat and specifically ketones. Unfortunately people see their horse lose body fat and worry that they are “underweight” and unhealthy – that a fat horse is a healthy horse. To some degree it really is but if too fat (obese), the mitochondria are stuck and inflexible to the choice of fat as a fuel. This forces them to use glucose either in the feed or stored as glycogen and they run out of energy quickly.