A feeding study just released in the Journal of Equine Science suggests there’s more involved in the insulin response to feeding than just sugar and starch levels (non-structural carbohydrate or NSC).
In the first part of this study, two groups of 5-year-old unfit Quarter Horse geldings were fed diets composed of 1% grass hay and one of two differently formulated feeds, the first 18.05% NSC and the other 9.43% NSC. These feeds otherwise provided equal amounts of calories and were fed in the same amount each day, 7.92 pounds divided into two feedings. As expected, the 18% NSC feedings caused much higher glucose and insulin spikes.
In the second part of the experiment, the same feeds were used but fed to the two groups in amounts designed to provide the same amount of sugar and starch. That is, the horses getting the lower NSC feed were fed twice as much so that their intake of sugar and starch would be the same. Glucose responses were similar this time, although the higher NSC feed still caused a somewhat higher glucose peak. Peak insulins were also virtually identical, but the surprising finding was that both glucose and insulin stayed elevated longer with the feeding of the larger meal.
One striking difference between the two diets was that the NSC calories in the low NSC feed had been replaced with fat. This resulted in the horses being fed the high NSC diet getting only 18 grams of fat in the second part of the experiment, while the low NSC feed horses got 100 grams of fat.
High-fat feeding has already been shown to decrease insulin sensitivity in ponies. The larger meal size may also have influenced the results, possibly by delayed release from the stomach. Other differences between the two diets were an extremely high iron level in the lower NSC diet (834 ppm vs. 464 for the high NSC, with current upper limit recommendation from the NRC of 500 ppm), and a much higher calcium:magnesium ratio in the low NSC diet of 5.9:1 versus 3.6:1.
The influence of all these factors requires further study, but our advice for those with insulin-resistant horses (or potentially insulin-resistant horses) is to stay with the lowest NSC feeding regimen you can, with frequent small meals rather than large ones, and avoid added fat.
Diets based on generous amounts of low (below 10%) NSC hay fed at either 2% of ideal body weight or 1.5% of current weight if fat, whichever is the larger amount, work well. The only other things the horse needs is a protein/mineral/vitamin supplement (see article on page 3) to balance the hay and a small amount of a safe feed (like molasses-free beet pulp) to carry the supplements.