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Uridine (symbol U or Urd) is a glycosylated pyrimidine analog containing uracil attached to a ribose ring (or more specifically, a ribofuranose) via a β-N1-glycosidic bond. The analog is one of the five standard nucleosides which make up nucleic acids, the others being adenosine, thymidine, cytidine and guanosine. The five nucleosides are commonly abbreviated to their symbols, U, A, dT, C, and G, respectively. However, thymidine is more commonly written as 'dT' ('d' represents 'deoxy') as it contains a 2'-deoxyribofuranose moiety rather than the ribofuranose ring found in uridine. This is because thymidine is found in deoxyribonucleic acid (DNA) and usually not in ribonucleic acid (RNA). Conversely, uridine is found in RNA and not DNA. The remaining three nucleosides may be found in both RNA and DNA. In RNA, they would be represented as A, C and G whereas in DNA they would be represented as dA, dC and dG.
Some foods that contain uridine in the form of RNA are listed below. Although claimed that virtually none of the uridine in this form is bioavailable "since – as shown by Handschumacher's Laboratory at Yale School of Medicine in 1981 – it is destroyed in the liver and gastrointestinal tract, and no food, when consumed, has ever been reliably shown to elevate blood uridine levels'. This is contradicted by Yamamoto et al., plasma uridine levels rose 1.8-fold 30 minutes after beer ingestion, suggesting, at the very least, conflicting data. On the other hand, ethanol on its own (which is present in beer) increases uridine levels, which may explain the raise of uridine levels in the study by Yamamoto et al. In infants consuming mother's milk or commercial infant formulas, uridine is present as its monophosphate, UMP, which is both bioavailable and able to enter the circulation from the digestive tract.
Consumption of RNA-rich foods may lead to high levels of purines (adenine and guanosine) in blood. High levels of purines are known to increase uric acid production and may aggravate or lead to conditions such as gout.
Harvard researchers report that omega-3 fatty acids and uridine, two substances in foods such as fish, walnuts, molasses, and sugar beets, prevented depression in rats as effectively as antidepressant drugs. "Giving rats a combination of uridine and omega-3 fatty acids produced immediate effects that were indistinguishable from those caused by giving the rats standard antidepressant medications," said lead author of the study William Carlezon, director of McLean's Behavioral Genetics Laboratory.
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