HOW-TO SESSION: FEEDING FOR FITNESS


current NRC requirements nor by dietary Se source. Oxidative and inflammatory responses were also evaluated in this same group of unfit horses in re- sponse to a mild bout of exercise (4.4 km walk/ trot).20 Although there was a transient elevation of whole blood glutathione peroxidase after exercise in horses receiving 0.3 mg/kg dry matter as Se-yeast, measures of oxidative stress and pro-inflammatory cytokine expression were unaffected by Se source or feeding rate. In another study, horses unaccus- tomed to exercise were subjected to a 2-hour (26-km) submaximal exercise bout after 5 weeks of Se in- takes of either 0.1 or 0.3 mg/kg dry matter (addi- tional Se was supplied as sodium selenite).21,22 Higher Se intake resulted in increased glutathione peroxidase activity and less lipid peroxidation fol- lowing exercise but did not alter other measures of antioxidant status or reduce muscle damage.21 Diets containing 0.3 mg Se/kg dry matter also re- sulted in less post-exercise leukocytosis and greater lymphocyte viability after ex vivo exposure to hydro- gen peroxide than diets containing 0.1 mg Se/kg dry matter.22 The exercise bout was stressful enough to suppress lymphocyte proliferation and alter lym- phocyte mRNA cytokine expression; however, this was not attenuated with Se intake above the NRC requirement. Collectively, these studies show that feeding Se above established requirements is not likely to greatly enhance antioxidant status or im- prove immunocompetence when vitamin E require- ments are also concurrently being met. Vitamin C is a part of the water-soluble antiox-


idant system where it traps peroxyl radicals and regenerates other antioxidants, including vitamin E. Healthy horses can synthesize vitamin C (ascorbic acid) from glucose in the liver; thus, no requirement has been established.7 It is pre- sumed endogenous synthesis of vitamin C is suf- ficient to meet the needs of healthy horses; however, strenuous exercise and respiratory dis- ease, such as airway inflammation has been shown to reduce circulating ascorbic acid concen- trations,23,24 suggesting some groups may benefit from vitamin C supplementation. Despite the popularity of supplementing perfor-


mance horses with vitamin C, research has not dem- onstrated a clear benefit in reducing oxidative stress.25–27 Based on current research, it is diffi- cult to tease out the contribution of vitamin C to antioxidant defense because it has typically been supplemented as part of a mixture of antioxidants. Similarly, investigations on other immune-related outcomes in response to vitamin C are limited to its supplementation as part of an antioxidant mixture. Ralston28 supplemented a small group of weanling horses with a mixture of vitamins E (800 IU) and C (5 g) or placebo beginning 5 days after the animals had been transported over long distance (2400 miles, 38 hours). Adequate responses to a routine 5-way booster vaccination were noted in 4/5 vitamin-sup- plemented weanlings and only 1/5 placebo wean-


lings. Within the 5-week period following transportation, 4/5 placebo weanlings showed signs of illness (nasal discharge, cough, fever) meriting an- tibiotic treatment. The number of vitamin-supple- mented weanlings that developed illness was not reported. Supplementation with a combination of vi- tamin E (2500 to 3000 IU), an ascorbic acid derivative (5 g), and Se (1 to 2.5 mg) did not improve neutrophil count in bronchoalveolar lavage fluid nor lung function in heaves-affected horses in clinical remission.29,30 However, one of these studies30 did observe greater exercise tolerance and lower scores for airway inflam- mation assessed by endoscopy in asthma-affected horses receiving the supplement compared with pla- cebo. Nogradi et al31 observed a reduction in bron- choalveolar lavage neutrophils and significant improvements in respiratory performance in a small number of asthma-affected horses receiving an oral supplement containing small quantities of docosa- hexaenoic acid (DHA; 1.5 to 3 g), vitamin C (2 to 4 g), methylsulfonylmethane (MSM), and “mushroom com- plex.” Differences in outcomes between these studies might relate to whether the horses were in remis- sion29,30 or had active airway inflammation.31 Taken together, the findings from these studies suggest that a mixture of antioxidants may be useful for modulat- ing immune responses in stressed horses; however, the specific role of vitamin C alone on immune function in horses remains unclear. Toxicity from excess ascor- bic acid has not been reported in horses; no side effects were reported in horses supplemented with 20 mg/kg BW (10 g/day for a 500-kg horse) for 8 months.32 In other species, high oral doses can reduce the body’s own synthesis of vitamin C, so abrupt cessation of ascorbic acid supplementation should be avoided.


3. Immunomodulation with Omega-3 Fatty Acids


In addition to serving as a calorie source, some di- etary fats play an immunomodulatory role in the body. More specifically, fatty acids belonging to the omega-3 and omega-6 families are part of the in- flammatory cascade and exert direct effects on im- mune cells.33,34 Similar to all mammals, horses require a dietary


source of the omega-6 fatty acid, linoleic acid (LA; 18:2n-6) and the omega-3 fatty acid -linolenic acid (ALA; 18:3n-3) because they lack the enzymes to synthesize them. LA and ALA can be oxidized for energy or incorporated into cell membranes where they contribute to membrane fluidity, but otherwise have very few direct biological effects. Rather, the dietary essentiality of LA and ALA is based on their role as precursors for the biosynthesis of longer- chain omega-6 and omega-3 fatty acids that help regulate many biological processes in the body. ALA is the “parent” omega-3 fatty acid and can be elongated and desaturated by the horse to form ei- cosapentaenoic acid (EPA; 20:5n-3) andDHA(22:6n- 3). In the omega-6 family, LA competes for the same elongase and desaturase enzymes to form arachidonic acid (ARA; 20:4n-6). These long-chain polyunsatu-


AAEP PROCEEDINGS  Vol. 66  2020 77


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