Question
I keep reading that insulin-resistant (IR) horses should eat low-NSC (nonstructural carbohydrate) hay. I understand NSC = WSC (water-soluble carbohydrate) + starch. The WSC fraction is supposedly mostly fructan, which to my knowledge cannot be digested by mammals and is fermented by bacteria in the hindgut to volatile fatty acids. This fermentation should result in minimal, if any, glucose/insulin response. Looking at hays in my database, I show mean WSC of 15.3% (with some greater than 20%) for a mean of 15.8% NSC (+/- 2%). In contrast, simple sugar (ESC, ethanol soluble carbohydrate) + starch, which should significantly influence the glucose/insulin response, is 7.7% (+/- 1.7%). Can you comment on why NSC continues to be the determining factor when feeding the IR horse? Does hindgut fermentation of grass fructans result in a glycemic response? If so, by what mechanism?
Answer
The nonstructural carbohydrate (NSC) value is used because it is a simple, easy measurement for starch and water-soluble carbohydrates (WSC). Even though it is a gross measurement, it can be used as a general guide for most of the carbohydrates that influence a glucose response in the bloodstream.
WSC is not necessarily “mostly fructans.” The WSC content will depend on whether there are fructans present in the plant, like if it is a cool-season perennial (the primary grasses that accumulate fructans). Most other feedstuffs do not contain fructans, such as warm-season perennials, grains, or high-fiber feed by-products (soy hulls, beet pulp, etc).
The hay you mention is probably a cool-season perennial like ryegrass, orchardgrass or timothy, with a WSC of 15.3% and an ESC (ethanol-soluble carbohydrate) of 7.7%, which is a fructan content of 7.6% (15.3 – 7.7 = 7.6). The low starch is characteristic of cool-season hays as well.
You are absolutely correct in your understanding of the carbohydrate fractions, but what might clarify things for you is an understanding of why fructans are a problem for horses. Fructans are fermented in the hindgut by bacteria that produce lactic acid. A buildup of lactic acid will drop the pH in the hindgut, which in turn will affect the viability of the other bacteria inhabiting the hindgut. A die-off of large numbers of bacteria can result in endotoxins getting into the bloodstream which can cause laminitis. Essentially, the danger of fructans is not related to the problem associated with insulin resistance (difficulty handling a high glycemic response). However, horses with insulin resistance have a tendency to develop laminitis, and high lactic acid in the hindgut has been associated with the development of laminitis.The recommendation of avoiding fructans for an insulin resistant horse probably comes with good intentions of trying to avoid anything that might trigger an episode of laminitis.
