Twenty-four horses (450 kg, 5–15 yr) were involved in an 11-wk trial to study the effect of both selenium (Se, Se-yeast) and vitamin E (E, α-tocopheryl) supplements on serum concentration of muscle damage enzymes (creatinine phosphokinase, CK; aspartate aminotransferase, AST and lactate dehydrogenase, LDH) in horses under moderate exercise. Horses were not exercised for a month before this study; they were housed in individual stables and assigned into a factorial trial arrangement (2 × 2; Se, E levels; n = 6) with repeated measures. Basal ration was deficient in Se and vitamin E (<2 μg; 14.4, IU/kg DM). The supplementation levels met NRC 2007 requirements: low (L: Se, 0.1 mg/kg DM; E, 1.6 IU/kg BW) and high (H: Se, 0.3 mg/kg DM; E, 2 IU/kg BW) for horses under moderate or intense exercise, respectively. Hence, treatments were: LSeLE, HSeLE, LSeHE and HSeHE. Exercise was done from wk 4 to wk 7: 3 consecutive days for 30 min (5–20–5), including: warm up-moderate gallop-cool down. Serum samples were taken every week (wk 4-wk 7, at the end of last exercise day). Creatine phosphokinase, AST and LDH were quantified by spectrophotometry. Data were analyzed using the SAS PROC MIXED procedure (SAS Inst. Inc., Cary, NC) for the design described above. Fixed effects were Se, E, wk, and their interactions, while horse nested within treatment was the random effect. Tukey-Kramer test was used to compare LSM. Neither Se nor vitamin E levels affected enzymes concentrations (P < 0.01). An effect of wk was observed for CK and AST at wk 6, exercise explained this fact; CK, AST and LDH increased at wk 3 and wk 11 (unsupplemented horses) Table 1. Serum CK, AST, and LDH (U/L) concentrations for weeks 1 to 11 (wk 0 = baseline)

Item1	Adaptation period				Exercise				Rest		No supplementation
0	1	2	3	4	5	6	7	8	9	10	11
CK	180.5bc	164.4c	182.5bc	224.8a	168.7bc	186.3bc	227.9a	199.5bc	194.8b	159.4c	197.1bc	217.5ab
AST	319.8ab	261.8c–f	226.6f	330.4a	284.0b–e	301.0a–c	332.3a	322.5ab	276.7c–e	250.4ef	256.9d–f	292.2a–d
LDH	469.6de	469.5de	504.5c–e	606.8a	514.5b–d	506.5cd	524.3bc	490.0c–e	424.2e	391.8f	473.7c–e	563.3ab

^a–fMean values within a line with different letters are different (P < 0.05). ¹SEM: CK, 10.4; AST, 13.3; LDH, 24.48.