Vitamin U is not actually a vitamin, but is instead a term used to refer to a substance called S-Methylmethionine. It's found in several different types of food and is used as a naturopathic supplement to treat a variety of health problems. Though several studies have shown its effectiveness, the results are not conclusive. As of 2012, there was no established Recommended Daily Allowance (RDA) for this substance.
S-Methylmethionine is found in both plants and animals, and it can also be made synthetically. The most common way to consume it is in liquid or spray form, but it can also be taken in tablet form or directly from foods. The most common natural sources are green cabbage leaves, alfalfa sprouts, spinach, kale, tomatoes, celery, wheat, turnips, radishes, and parsley. Some health experts recommend getting vitamin U from raw or fermented foods, since appears to be much more effective when cold, and cooking may make it not work as well. Since there is no RDA for this substance and it does not have an identified toxicity level, those interested in adding it as a supplement to their diet should consult a healthcare professional for proper dosing instructions.
Whether taken as a supplement or from foods, vitamin U has been shown to be able to treat a variety of gastrointestinal conditions, including ulcerative colitis, acid reflux, and peptic ulcers. It may also be able to treat skin lesions, improve the symptoms of diabetes, and strengthen the immune system. Some studies show that it can also help prevent liver damage by protecting the organ from the effects of high doses of acetaminophen. Additionally, it may be able to reduce allergies and sensitivities to cigarette smoke and improve cholesterol levels. Many people also choose to use this vitamin as part of a detox, since it may be able to help the body get rid of environmental toxins.
The positive effects of S-Methylmethionine on the gastrointestinal system were discovered by Dr. Garnett Cheney. The studies Cheney, G. 1952 and Cheney et al. 1956 demonstrated that a previously unknown substance in raw cabbage juice helped soothe and heal peptic and gastric ulcers, sometimes faster than other means of treatment. They also showed that the results were greatly diminished if the juice was heated, which implies that the substance is very sensitive to high temperatures. Cheney's work prompted the nickname "Vitamin U" for this substance due to its ability to treat ulcers.
Subsequent studies also demonstrated the potential benefits of this substance. Roediger et al. 1996 demonstrated that S-Methylmethionine and similar substances may have positive effects for those with ulcerative colitis, while Peltz et al. 2007 demonstrated that it may prevent liver damage. Shaw et al. 2009 also showed that this substance may improve the function of the intestines; however, the study was done on chickens so it's not clear whether the results would be the same in humans.