Question: I have a draft cross who is HYPP and PSSM.  It has been quite a challenge finding the right diet for him. He is doing very well, but I still think there is room for improvement in both his muscle development and his comfort level. He is a good weight, but needs more muscle, especially behind. And, although he is the most sound he has ever been, he is still hesitant to move out (but that could be his nature, not exactly a forward horse, lol). Sometimes feed labels are deceiving and don't list the potassium, and I also get confused by some of the conflicting issues of the HYPP and PSSM mechanisms. his current feeding pram is moderate in all the red flag areas (low starch, high fat, and low potassium) but not extreme in any of those, as I understand that HYPP horses need some starch to help metabolize potassium, and finding super low potassium feeds that are high in fat, but low in starch is really challenging. Any advice is welcome, but what I am specifically wondering today, is are the following three uckele supplements safe for him?  Cocosoya, E-5000, and Acetyl L-Carnitine HCL. Dr. Kellon: As you already know, with HYPP you want to keep potassium (K) in the diet at a low/slow and steady level.  Grazing is ideal for this because grass is about 80% water so the horse will not get any big blasts of K.  With PSSM (aka EPSM), low sugar and starch is the goal if the base diet.  It's true that when glucose enters a cell it takes K along with it, but you do not have to feed starch or sugar for this effect. The horse's body can manufacture all the glucose it needs.  The only time you would want to add more is if the horse is having an HYPP attack. This can be treated with Karo syrup. 
Beet pulp is the ideal feed for both conditions as it is low in K, sugar and starch - made even lower when soaked.
Symptoms of poor impulsion and poor muscular development are far more likely to be from PSSM.  The acetyl-L-carnitine can be an effective way to control PSSM without feeding high fat.  Use 1 gram per 100 lbs of body weight.  CocoSoya is fine up to 6 oz/day and you can use this to enhancer absorption of the E-5000.  Feed 10,000 IU/day of vitamin E.  You may also need to supplement selenium.  A whole blood selenium level is best to determine status.  Many PSSM horses also benefit from extra magnesium in their diet.
Give the acetyl-l-carnitine 2 weeks to begin to show you results, although many see changes beginning sooner.  If no good response after a month, you can always try the high fat feeding at 1 lb of fat/1000 lbs of body weight but good response to that takes several months.