Question: Have Friesian QH CROSS 12 Y/O. She was laid up this winter, very sound now. Very very easy keeper on Thyro L. Low energy level Is there a non fattening supplement that could help her. Schooling Third Level dressage.  My mare is weighs about 1100 – 1200 lbs.  She is getting Nutrena Balance ¾ lb twice daily, and 12 pounds of hay.  The hay analysis is attached.  She gets SmartPak’s Smart Gut Ultra  for their Coli Care program. Dr. Kellon: The diet you are feeding would be appropriate for an easy keeper while on layup but is not adequate now that she is back in work.  Underfeeding won't necessarily lead to weight loss because the horse's body can compensate by becoming more insulin resistant and shifting key energy sources to the most important organs, like heart, and away from the skeletal muscles.  The end result as you have seen is poor energy.
 To start, increase hay to 17 to 18 pounds/day on days when she is actually worked.  Also add a meal of 1 lb each (dry weight) beet pulp and plain oats, plus 8 oz of Uckele Amino Fac, to be fed within an hour after stopping work.  In the few hours after exercise the horse's muscle is "hungry" and will very efficiently take up glucose, acetate from fermentation of beet pulp, and amino acids from Amino Fac to replenish stores used during work and rebuild better levels for future work. 
Also make sure the horse is eating at least 2 oz/day of plain salt.  As little as 2% dehydration, which is not detectable by skin turgor, has a significant impact on muscle strength and endurance.