The recommended daily dietary allowance for iron for 19-to-50-year-old women is 18 mg and 8 mg for men. It doesn’t sound like much, but between 10 and 15 percent of adults in the U.S. don’t have enough and thousands are hospitalized every year. Lack of the essential nutrient is the most common nutritional deficiency in the world, and women are affected more than men, according to the American Family Physician.