Navy Rum Rations, The Proof of Rum's Strength, & Scurvy

The history of rum and scurvy is more or less parallel.

Scurvy is a disease caused by insufficient vitamin C in the diet.  Vitamin C is found in citrus fruits and vegetables.  From wiki:

. . .  Early symptoms include weakness, feeling tired, and sore arms and legs. Without treatment, decreased red blood cells, gum disease, changes to hair, and bleeding from the skin may occur. As scurvy worsens there can be poor wound healing, personality changes, and finally death from infection or bleeding. . . . It takes at least a month of little to no vitamin C before symptoms occur.

Although scurvy has been known since ancient times, and indeed, was a problem for Crusaders making the long sea voyage to the Holy Land, it truly became a significant problem in Europe with the long sea voyages of the Age of Discovery beginning in the late 15th century. Fresh fruits and vegetables were not available to sailors.  Between 1500 and 1800, "it has been estimated that scurvy killed at least two million sailors. Jonathan Lamb wrote: "In 1499, Vasco da Gama lost 116 of his crew of 170; In 1520, Magellan lost 208 out of 230;...all mainly to scurvy.""

From wiki:

In 1734, the Leiden-based physician Johann Bachstrom published a book on scurvy in which he stated, "scurvy is solely owing to a total abstinence from fresh vegetable food, and greens; which is alone the primary cause of the disease", and urged the use of fresh fruit and vegetables as a cure. 

However, it was not until 1747 that James Lind formally demonstrated that scurvy could be treated by supplementing the diet with citrus fruit, in one of the first ever reported controlled clinical experiments in the history of medicine. In 1753, Lind published A Treatise of the Scurvy, in which he explained the details of his clinical trial, but it occupied only a few paragraphs in a work that was long and complex and had little impact. In fact, Lind himself never actively promoted lemon juice as a single ‘cure’. He shared medical opinion at the time that scurvy had multiple causes – notably hard work, bad water, and the consumption of salt meat in a damp atmosphere which inhibited healthful perspiration and normal excretion - and therefore required multiple solutions.  He was also sidetracked by the possibilities of producing a concentrated ‘rob’ of lemon juice by boiling it. Unfortunately this process destroyed the vitamin C and was unsuccessful. 

During the 18th century, disease killed more British sailors than enemy action. It was mainly by scurvy that George Anson, in his celebrated voyage of 1740–1744, lost nearly two-thirds of his crew (1300 out of 2000) within the first 10 months of the voyage. The Royal Navy enlisted 184,899 sailors during the Seven Years' War; 133,708 of these were "missing" or died from disease, and scurvy was the leading cause. 

Although throughout this period sailors and naval surgeons were increasingly convinced that citrus fruits could cure scurvy, the classically trained physicians who ran the medical establishment dismissed this evidence as mere anecdote which did not conform to current theories of disease. Literature championing the cause of citrus juice, therefore, had no practical impact. Medical theory was based on the assumption that scurvy was a disease of internal putrefaction brought on by faulty digestion caused by the hardships of life at sea and the naval diet. Although this basic idea was given different emphases by successive theorists, the remedies they advocated (and which the navy accepted) amounted to little more than the consumption of ‘fizzy drinks’ to activate the digestive system, the most extreme of which was the regular consumption of ‘elixir of vitriol’ – sulphuric acid taken with spirits and barley water, and laced with spices. 

In 1764, a new variant appeared. Advocated by Dr David McBride and Sir John Pringle, Surgeon General of the Army and later President of the Royal Society, this idea was that scurvy was the result of a lack of ‘fixed air’ in the tissues which could be prevented by drinking infusions of malt and wort whose fermentation within the body would stimulate digestion and restore the missing gases.  These ideas received wide and influential backing, when James Cook set off to circumnavigate the world (1768–1771) in HM Bark Endeavour, malt and wort were top of the list of the remedies he was ordered to investigate. The others were beer, sour crout and Lind's ‘rob’. The list did not include lemons. 

Cook did not lose a single man to scurvy, and his report came down in favour of malt and wort, although it is now clear that the reason for the health of his crews on this and other voyages was Cook's regime of shipboard cleanliness, enforced by strict discipline, as well as frequent replenishment of fresh food and green stuffs. Another rule implemented by Cook was his prohibition of the consumption of salt fat skimmed from the ship's copper boiling pans, then a common practice in the Navy. In contact with air the copper formed compounds that prevented the absorption of vitamins by the intestines.