Old horses might need additional nutritional supplementation if they become deficient as they age. | iStockQ: My horse will be turning 20 soon, and my barnmate told me I need to be more cautious about how I feed him vitamins and minerals to ensure I meet his daily requirements. Will his requirements change as he ages? Are there any specific vitamins he might become deficient in?A: Just as a humans nutritional requirements change as they age, so will your horses; however, working with your veterinarian and an equine nutritionist can help you ensure he is getting the nutrition he needs during his senior years.Vitamin C can be particularly interesting for senior horses. In a study that looked atphysiologic differences between young horses and older horses, researchers found that older horses had lower vitamin C levels overall. Of course, horses are a species, unlike humans or guinea pigs or any other animals, that can actually synthesize their own vitamin C,so we dont know if horses just cant produce enough vitamin C, or perhaps they use more of it because it is a pretty powerful antioxidant. This might mean that its necessary to supplement vitamin C if you have an older horse.Just like if a child ate too many vitamin C orange-flavored vitamins, oversupplying most nutrients, including vitamin C, is not a major concern because these can be excreted through the urine. However, there are some theories that oversupplying some vitamins, such as vitamin C, could inhibit the horses innate ability to produce them, making continued supplementation necessary.As horses get older, they might also become deficient in vitamin E and require supplementation, especially if they have little pasture available to them. When evaluating your horses vitamin E levels, it is important toanalyze your horses diet. However,you can also ask your veterinarian for bloodwork to ensure that your horses vitaminE status is good.Vitamin E is relatively safe, even in higher doses, but it can potentially prevent the absorption of other nutrients if overfed in excessive quantities. Ultimately, working with an equine nutritionist to regularly evaluate your horses diet,and with yourveterinarian totest your horses blood levels will help ensure that they receive proper nutrition as they age.