Gately's 'Drink: A Cultural History of Alcohol'. said:
Christian Europe emerged from the Dark Ages as a heavy-drinking culture. Alcohol had the reputation of a saint. No medical prescription was complete without it, nor, indeed, was any meal. Mothers brewed ale for their children; alchemists used spirits in their search for the secrets of how to turn other substances into gold; priests held wine aloft in chalices and declared it to be the blood of Christ; and drunkenness, especially during the barbarian festivals that had been adopted by Mother Church, was regarded as a natural, indeed blameless, condition.
[...]
Their staple was ale, which, to them, was rather food than drink. Men, women, and children had ale for breakfast, with their afternoon meal, and before they went to bed at night. To judge by the accounts of the great houses and religious institutions to which they were bound by feudal ties, they drank a great deal of it—a gallon per head per day was the standard ration. They consumed such prodigious quantities not only for the calories, but also because ale was the only safe or commonly available drink. Water was out of the question: It had an evil and wholly justified reputation, in the crowded and unsanitary conditions that prevailed, of being a carrier of diseases; milk was used to make butter or cheese and its whey fed to that year's calves; and cider, mead, and wine were either too rare or too expensive for the average commoner to use to feed themselves or to slake their thirsts.
[italics mine]
