For more on “Refeeding Syndrome:”
From University of Minnesota, College of Veterinary Medicine
A: We feed the horse only a small amount of food at first because we’re concerned about a metabolic crisis called refeeding syndrome, which can happen when severely starved animals receive nutritional replenishment too rapidly. Refeeding syndrome was first described in Far East prisoners of war after World War II. When the prisoners started eating again after a period of prolonged starvation, they suffered heart failure. Fortunately, the pathophysiology of refeeding syndrome has now been established. As fat reserves are consumed, starvation induces metabolic changes that allow protein (muscle) to be burned for energy. Body mineral stores, particularly phosphorus, become depleted, which can lead to respiratory failure, heart failure, arrhythmia, seizures, coma, and sudden death. Slow refeeding prevents refeeding syndrome.
Q: How much food and water do you initially give a starving horse? How often? How long does it take for the horse to get back to eating normally?
A: If the horse has had no food for 24 hours or more, it is offered handfuls of good quality grass hay on at least an hourly basis. It gets all the water it wants. After the first day, restricted hay access is provided by placing hay nets outside the bars of the stall. If the horse has had some feed prior to admission, it is permitted full access to grass hay. Within three days, most horses are back to eating all the hay they want. Concentrated horse feed is very gradually introduced on day four, using a half pound twice a day for average-sized horses."
For a technical description of what may happen during “refeeding,’ see:
Metabolic responses of chronically starved horses to refeeding with three isoenergetic diets
Christine L. Witham, DVM, MPVM, and Carolyn L. Stull, MS, PhD