Taken from Q&A in NEW SCIENTIST magazine: (I dont know how true this is…)

Q. I have heard that it is possible to live on Guinness and milk alone. Is this true, or even partially true?

A. This is not quite true. Guinness does contain many vitamins and minerals in small quantities, but is lacking vitamin C, as well as calcium and fat. So, to fulfil all of your daily nutritional requirements you would need to drink a glass of orange juice, two glasses of milk, and 47 pints of Guinness.