During the Age of Exploration, it wasn't uncommon for ships to lose the majority of their crews to scurvy. While there were many theories as to what caused scurvy, a lot of people suspected it was a dietary deficiency. After all, long ocean voyages required foods that could be preserved and stored a long time, which meant they were pretty plain. Ship captains tried various kinds of supplements, and several found that citrus fruits or sauerkraut could fend off scurvy, yet neither scheme was well communicated nor adopted in the shipping industry. Provisioners had to keep costs down, and vitamin C wouldn't be discovered until 1912.
Then in 1747, Scottish naval surgeon Dr. James Lind selected 12 sailors suffering from scurvy to test six different treatments. Besides the experimental treatment, they were all kept in the same conditions and fed the same food. The two men who were given oranges and lemons improved quickly, one going back on duty in just six days! The men who were given vinegar, seawater, sulphuric acid, or other treatments shows no significant improvement.
Unfortunately, since this was the first clinical trial of its kind, Lind didn't have enough confidence in his methods or results. It would take several more decades before citrus fruits were widely adopted as a scurvy preventative. Read the story of that first clinical trial at Amusing Planet.
(Cropped image credit: Robert Alan - Parke, Davis & Company)