Welcome to the group. Very soon the mods will send you a plethora of info to get you started. You'll be asked to fill out a detailed case history including bloodwork so start gathering everything to scan! I wanted to quickly share my mare's story to give you hope:
I showed my 20yo mare in USEA eventing last year. She was known to be insulin resistant but I was able to keep it well-controlled with precise forage-only diet (down to the ounce!) and a very rigorous training regime. She won several events in spring and summer and easily qualified for Regionals at the end of July. So off we went on the 6-hr haul to the showgrounds with all of my expensive chopped hays in measured bags, etc. The first day in dressage my mare was a superstar, almost too aggressive! She was stabled in temp stalls on a mowed hay field. Second day, I find my mare had dug up all of her bedding and nibbled all night on the grass floor of the stall. When I got on for cross-country phase later that day, my mare would hardly move, a complete 180 from the day before. Somehow we finished cross country w/o time faults but my poor mare was really struggling. We withdrew from the rest of the show. I knew enough from her history to know that her insulin must've spiked either from the stress of the trip (altho she's a great shipper) or the few tiny bites of grass she managed to consume, or both. No matter anyway-- blood drawn Monday morning after our disastrous weekend showed my mare had actually reached diabetes mellitis status-- both her insulin and glucose were WAY above normal. So that means that no matter how carefully & perfectly I managed her diet, her body's cells were being starved for the glucose they need for basic functioning. How my mare ever finished that cross country phase I'll never know. She's a tryer.
She had a terrible fall and came close to foundering because her insulin could not be controlled. It was only after Dr. Kellon examined her case and suggested she might be an early PPID horse that my mare began to recover. Pergolide (the treatment for Cushings/PPID) was the only thing that got her insulin under control again, as nothing I did at that point got her insulin to budge (soaked hay, etc; I did everything).
Thanks to the support & dietary recommendations of this group, my mare is feeling great once again and we are gearing up for another year of USEA events. She is even able to eat unsoaked hay -- something she hasn't been able to do in years! She still remains very sensitive to grass and fats in her diet but that is easy enough to manage. Each horse is different, but my mare easily maintains good body condition (the ideal 5) on just her crappy 6% ESC+starch, very low protein (6%) free choice hay, even doing heavy eventing training (sometimes ridden 2x day). So to summarize, until the underlying IR is well-controlled, a horse's metabolism will fluctuate drastically as the insulin spikes & wanes. Last summer, as my mare's insulin levels became incresingly less stable my mare would look obese one week and haggard the next depending on what her insulin was doing, regardless of what I fed her. Now that her insulin is truly under control, she can maintain perfect condition on pretty simple grass hay while in heavy training.
You are quite far along the path for helping your mare. As I said earlier, you've found a tremendous resource being here in the group, so start getting your horse's bloodwork together and we'll help you get your mare in good shape. If I can get my old mare back in show form, you certainly can too!
Kerry in NY
Pinky Sept 2014
Tofurky Nov 2014
PS. My young horse--who, I'm not ashamed to say, moves like Totilas and will one day take the dressage world by storm--was also diagnosed IR last fall. He had been under scrutiny for four years to figure out a strange transient "lameness" (SI lux? old pelvic fracture?) and periodic freaky/dangerous behavior. Turns out he just can't handle sugars! Drylots for everyone!! Now he's perfect and *finally* in serious training :)