Mostly members of the crucifer family, Asian greens are high in fiber and contain large amounts of vitamins C and E, as well as beta carotene, key among the antioxidants that are thought to fight cancer. In addition, the darker greens, such as bok choy and mustard greens, contain significant amounts of folic acid, which helps prevent certain birth defects and may assist in preventing some cancers.
Most Asian greens are high in essential minerals—for example, bok choy is significantly higher in calcium than other cabbages, while mustard greens contain calcium, iron, and large amounts of potassium. Asian greens also contain phytochemicals, micronutrients such as glucosinolate, indoles, isothiocyanates, thiocyanates, and lutein—now considered by some mainstream nutritionists to be as important as beta carotene in cancer prevention. Since scientists have not yet proved which substances provide which preventive functions, however, the best advice is probably still what we heard from our mothers: "Eat all your leafy greens."