I lost a mare who had gotten overweight and then spent the last 6 months of her life on a miserable, low cal diet. I hated that I did that to her in the end (even though I didn't know she was going to pass). She didn't get the fatal condition from being overweight, but she did founder. I had her at a barn and regularly checked by a vet. No one ever said anything about her being overweight until I switched to an old country vet, who explained the issues of the excess weight.
So now, I keep mine on the lean side. Just slightly under weight. I did have one get sick and the vet who saw him scolded me for letting him get so "skinny." After he recovered he put the weight back on in no time. He never looked emaciated, but he definitely didn't have the weight to lose. I felt really bad, but given a choice, I will continue to keep them on the lean side, regardless of the criticism I get.
Is anyone on this forum familiar with any science on slightly over versus slightly under weight? I'd love to read what research actually says. When I was growing up in Texas, my "slightly underweight" horse would have been considered at the ideal weight. I wonder if it's attitudes or science that has changed.
Please post links if you know of any research on the topic.