Chinese cabbage (Brassica rapa, subspecies pekinensis and chinensis) can refer to two distinct varieties of Chinese leaf vegetables used often in Chinese cuisine. These vegetables are both related to the Western cabbage, and are of the same species as the common turnip. Both have many variations in name, spelling and scientific classification–especially the "Bok choy" or chinensis variety.
This group is the more common of the two, especially outside Asia; names such as napa cabbage, da baicai (lit. "large white vegetable"); Baguio pechay or pechay wombok (Tagalog); Chinese white cabbage; baechu (Korean), wongbok and hakusai (Japanese) usually refer to members of this group. Pekinensis cabbages have broad green leaves with white petioles, tightly wrapped in a cylindrical formation and usually forming a compact head. As the group name indicates, this is particularly popular in northern China around Beijing (Peking).
Chinensis varieties do not form heads; instead, they have smooth, dark green leaf blades forming a cluster reminiscent of mustard or celery. Chinensis varieties are popular in southern China and Southeast Asia. Being winter-hardy, they are increasingly grown in Northern Europe. This group was originally classified as its own species under the name B. chinensis by Linnaeus.
Other than the ambiguous term "Chinese cabbage," the most widely used name in Northern America for the chinensis variety is bok choy (from Cantonese, literally "white vegetable"; also spelled pak choi, bok choi, and pak choy). Less commonly, the Mandarin term xiao baicai ("small white vegetable") as well as the descriptive English names Chinese chard, Chinese mustard, celery mustard, and spoon cabbage are also employed.
Bok choy contains a high amount of Vitamin A per 4 oz. serving - about 3500 IU. Bok choy also contains approximately 50 mg of Vitamin C per 4 oz. serving.
Bok choy contains glucosinolates. These compounds have been reported to prevent cancer in small doses, but are toxic to humans in large doses. In 2009, an elderly woman who had been consuming 1 to 1.5 kg of raw bok choy per day developed hypothyroidism, resulting in myxedema coma.