A method for identifying pre-disposition to cognitive decline in a subject, the method comprising determining levels of: (a) omega-3 fatty acids, and vitamin D or a metabolite thereof; (b) omega-3 fatty acids, and homocysteine; (c) vitamin D or a metabolite thereof, and homocysteine; or (d) omega-3 fatty acids, vitamin D or a metabolite thereof, and homocysteine, independently in one or more samples obtained from the subject.