I finally talked to my vet today. I wasn't expecting the test for food sensitivities/allergies to come up with anything, but it did. Tonka can't have corn, rice, or beet. I don't purposefully feed him anything with corn in it, but I find it is in his vitamins. There is rice in his probiotic. I have been feeding rice bran for the last few weeks, trying to make his food palatable for him. And he's been eating beet pulp (which he's only marginally sensitive to) for years.
So I have hope! If I cut all of these things out of his diet maybe he'll feel better. I guess it could take a couple months before it makes a difference though. It's comparable to a person with a gluten intolerance or celiac's disease, which causes cramping, diarrhea, and nausea. (I wonder if horses can't get nausea since they can't throw up?)
I also started a new treatment for the EPSM today. Instead of feeding huge amounts of fat to try to make his body burn fat, while cutting way back on starches and carbs, he will be given no extra fat (just some flax seed meal) and I have added acetyl L-carnitine. It's supposed to keep the starch burning "switch" turned on, so that he doesn't store the starches as glycogen, which is what causes his muscle trouble. I have high hopes. This particular regimen has been known to cause marked improvement in just 3 days. I also need to add a couple other things, one of which is in the mail and the other I've had to email to find out if there are any fillers that Tonka can't have. I need change his vitamin/mineral supplement too, because of the corn in HorseGuard. I don't know if I'll be able to find something without corn or rice. I wonder how much a custom supplement would cost. (cringe)
The weather is quite nice today - no moisture falling from the sky - so I'm going to go saddle Scout up and ride.