"Vitamin U" was the name given in 1952 to a substance found in cabbage juice by Dr. Garnett Cheney.
Drinking about a quart of cabbage juice per day is an excellent remedy for peptic ulcers, but the identity of the active ingredient remains unknown. A common guess is that it is allantoin, which is also found in comfrey root, and has long been used in folk medicine both externally and internally. Other guesses are that it is a relative to the amino acid L-methionine.
Vitamin U is not acknowledged by mainstream nutrition as a vitamin, so it joins about a dozen other obscure "vitamins" identified only by a letter. Some nutrients that started out as an "unknown substance" with only a letter to identify them, are now verified as an essential nutrient. For instance, the originally termed "vitamin H" is now known as the B-vitamin biotin, and "vitamin M" is folicin, a form of the B-vitamin folic acid. Sometimes, if the active ingredient is later found to be several different substances, it is easier to just keep the letter. For example, bioflavonoids as a group are still called "vitamin P". Because many of them have similar biological effects, it would be difficult to single out one and call only that one "vitamin P."
Although he did not use the term "vitamin U" to describe what he found, Dr. William Shive from the Clayton Foundation Biochemical Institute at the University of Texas, found that a high concentration of L-glutamine was part of the reason that cabbage juice is capable of healing ulcers. It appears that a specific food may have more than one component with a beneficial effect, and that all these components likely work better together via synergy to provide more benefits than from one single ingredient.
These observations confirm what Roger J. Williams termed the "teamwork concept" of nutrition. With antioxidant supplementation this concept has been repeatedly demonstrated to be valid. For example, vitamins C and E work stronger together than each would individually. We also find that three or more antioxidants used together not only perform better, but also exhibit fewer side effects than if each had been taken alone. Likewise among the B-complex vitamins, vitamin B-2 is required to change vitamin B-6 into the form actually used by our body cells, so balance is important.
Whole foods will supply at least some of virtually all essential nutrients (there are exceptions however, for example there is no vitamin B-12 in plant foods). Thus a variety of whole foods is the best assurance that you will receive the nutrients you need to maintain good health.