You could also consider using an equine rice bran. It is 22% fat and provides cool energy
. Equi-Jewel main
Once the feed change would be made, don't feed any more than the recommended amount on the bag, unless you call the manufacturer.
I started out feeding rice bran to my insulin resistant horse because the I-R made him lose weight. I put my other three on it to keep my life simple and discovered they are all much more laid back and peaceful.
My I-R horse is has been stabile for three years and maintains his weight on 3/4 cup of rice bran daily, along with high quality grass hay and other supplements
If you want to try rice bran and can't get Equi-Jewel in your area, just be sure it is for horses and is stabilized & calcium fortified.
Stabilized so it won't turn rancid; calcium fortified to keep the calcium:phosphorous ratio where it needs to be in the horse
As has already been stated, you would need a good quality vit/min supplement.
Hope this helps