Study compared serum vitamin E concentrations in horses supplemented with either natural (d-alpha-tocopherol acetate) or synthetic (dl-alpha-tocopherol acetate) vitamin E. Over a 16-week period, both forms increased serum alpha-tocopherol levels to within normal ranges. However, horses receiving synthetic vitamin E showed higher serum concentrations at certain time points. The study highlights the importance of considering both form and dosage when supplementing vitamin E.