Two of my four horses have insulin issues. All my horses have been grain-free since 2007.
Three get a ration balancer that only requires 16 ounces daily to meet their vit/min needs.
The fourth gets a condensed vitamin/mineral supplement that only requires one ounce daily.
I took this pic on July 19, 2013. The 4th horse was lagging far enough behind, that I couldn't include him but you can see these guys are healthy.
The front horse turned 27 in April. I rescued him 20 years ago, starving. He has mild gastric ulcer issues and four missing molars.
The middle horse is 25 -1/2, has metabolic issues, hind gut ulcers and colciked NINE times between March and October, 2012. The vet couldn't believe how he's snapped back and commented I "---really got lucky with this horse." That's partly because I went way outside the box to feed and care for this horse, whose been my BFF for 22-1/2 years. The other part, he is tough as nails
The third horse turned 19 in April and has some digestive issues the vet can't figure out, even with several blood tests. I've sorta got him on track with a good pre-probiotic and Remission of all things.
Meaning, I don't think anyone can say these horses aren't being kept as healthy as possible, given their ages and health issues, and they haven't had anything with grain in it since 2007