As always, single healthful foods added to most diets would most likely bring improvements. The best way to do nutritional research would be to actually prepare and deliver the food for say, a clean ketogenic diet and a Mediterranean diet, for six months to a year, and then assess all of the measures of health. Adding in one thing to a relatively poor diet is bound to help.
When my clients begin I often suggest they eat an apple a day with a few nuts. Simple, healthy change. Most report feeling better and having more energy. One Simple Change.