It is said that in the year 1087, William the Conqueror (who became King of England after his success at the Battle of Hastings) found he could no longer ride his horse because he was too fat. He reportedly refused to get out of bed, and began drinking alcohol instead of eating food in an attempt to lose weight. If this story is true, it may be the first recorded instance of someone changing food intake in order to reduce their bulk.(thus dieting)
Although it is apparently true that he had grown quite fat by the end of his life, we have no record of what success King William's alcoholic 'liquid diet' might have had. King William died that same year, but since he died from injuries he suffered when his horse fell, we may assume his regime was at least partially successful, because he was on his horse once again.