Original ResearchEffects of High-Sugar and High-Starch Diets on Postprandial Inflammatory Protein Concentrations in Horses
Introduction
In horses, obesity leads to an increased risk of insulin resistance, and it is possible that consumption of high-glycemic diets (those high in starch and sugar, HSS) exacerbate the onset or degree of this dysfunction [1], [2]. Recent research on 300 horses in Virginia indicated that more than half of the studied population was overweight or obese [3]. Furthermore, 70% of the studied population was offered a grain-based concentrate meal every day. Because insulin resistance increases the risk of laminitis [4], an excruciatingly painful disease of the equine hoof, it is important to determine how HSS diets could specifically influence insulin resistance.
It is possible that HSS diets induce insulin resistance by promoting increased plasma concentrations of proinflammatory cytokines. Cytokines such as interleukin 1β (IL-1β) [5], IL-6 [6], and tumor necrosis factor (TNF)-α [7] are known to reduce insulin sensitivity in a variety of species and tissue types. Additionally, the acute-phase protein, serum amyloid A (SAA), correlates with obesity and insulin concentration in horses [8] and alters insulin sensitivity in vitro in adipocytes [9]. In ruminants, there is evidence that starch fermentation, particularly after consumption of high-starch diets, promotes increased postprandial inflammation [10]. Similarly, fermentation of starch in the gastrointestinal tract could link HSS diets to increased inflammation in horse, which could be a factor relating HSS diets to insulin resistance.
Although starch is primarily digested in the small intestine, it overwhelms the digestive capacity of the small intestine when ingested in large enough concentrations and enters the cecum and large intestine (hindgut) where it is fermented by bacteria [11], [12]. Starch is one of several carbohydrates that on reaching the hindgut are rapidly fermented. Although different from starch in its structure, oligofructose is a rapidly fermented carbohydrate that may have similar effects on the hindgut when it is consumed in large amounts. As soon as 4 hours after consuming a large quantity of oligofructose, cecal concentrations of organic acids (such as lactate) are altered, cecal pH is lowered [13], and blood concentrations of lipopolysaccharide (LPS) are increased [14]. In horses, IV infusion of LPS induces insulin resistance and increases the concentrations of several proinflammatory cytokines in plasma [15], [16], [17], [18]. The ability of starch to influence plasma LPS and inflammation has only been shown after experimental overfeeding of starch, but we hypothesize that routine consumption of HSS diets generates a whole-body state of low-grade chronic inflammation that in turn facilitates and promotes insulin resistance and, further, that this inflammation occurs due to routine exposure to increased blood LPS.
Section snippets
Materials and Methods
All procedures were approved by Virginia Tech's Institutional Animal Care and Use Committee. Methods and results of a companion study were previously published [19] and are briefly described here.
Results
Horses on the CON treatment averaged 11.7 ± 0.9 years, 610.4 ± 24.3 kg, and a BCS of 6.7 ± 0.4 initially, whereas horses on STR averaged 12.5 ± 0.9 years (P > .5), 595.7 ± 24.3 kg (P > .6), and a BCS of 6.7 ± 0.4 (P = 1.0) initially. Neither body weight nor BCS was influenced by the day × diet interaction (P > .6), or the main effects of day (P > .4) or diet (P > .4; data not shown). Horses were not tested for insulin sensitivity status before the start of the study; however, fasting insulin
Discussion
It is likely that horses evolved consuming diets that were high in indigestible and slowly fermentable carbohydrates and low in starches and sugars. However, modern feeding management practices commonly include a grain-based concentrate meal [3], which would likely be much higher in starches and sugars. The higher level of starch and sugar in grains such as barley, corn, and oats increases plasma glucose and insulin levels after consumption [26], [27], [28]. For instance, as previously
Conclusions
Feeding high-starch diets to horses may promote mild increases in IL-1β and LPS. However, it is unlikely that LPS causes the rise in IL-1β, as the peak of LPS occurred later than that of IL-1β. The potential ability of glucose to stimulate IL-1β secretion should be investigated, and the role of IL-1β in etiologies of insulin resistance should be explored.
Acknowledgments
This research was supported by the Virginia Horse Industry Board.
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