The British Royal Navy Considered Rations Of What Foodstuff A Military Secret?
Answer: Lime Juice
One of the perils of life at sea is a lack of fresh fruits and vegetables (and the nutritional deficiencies that result from said scarcity). In the 18th and 19th centuries, the British Royal Navy successfully struck a blow against a disease of deficiency that plagued sailors, scurvy, by rationing out lime juice.
Limes, and other citrus fruits, are high in Vitamin C (ascorbic acid) and will keep scurvy at bay. To that end, all sailors in the British Navy were given lime juice mixed in with their daily rations and, as such, were able to stay out at sea longer than the sailors of other navies (thus leading to lime juice being treated as a military secret).
Interestingly enough, the mechanism by which limes prevented scurvy was poorly understood. The British knew that citrus fruits prevented scurvy but they thought the mechanism of action was based on the acidity of the fruit. Because of this understanding, they replaced lemons (which have almost twice the amount of Vitamin C as limes) with highly acidic limes which were very high in citric acid but lower in ascorbic acid.
The use of limes in the British Navy is also where the term “limey” (referring to a British person) originated from. Initially used by sailors to refer to sailors in the British Royal Navy (and later British merchant sailors), the term eventually spread to refer, in general, to any British person.