Many people believe that carrots are good for their eyesight. There is a germ of truth in this in that carrots are indeed a good source of vitamin A, and vitamin A is required for, among other things, correct functioning of the visual system. However just because they are required for correct functioning does not necessarily mean that eating more of them will improve your vision. The origin of the myth that carrots are good for your eyesight dates back to World War II and the Battle of Britain that occurred in 1940 and marked a turning point in the war. The Battle of Britain refers to the air battle between the German and British air forces for air superiority. A German victory would have permitted a cross channel invasion of England.
Although the principles of Radar had been known for a long time it was British scientists who produced the first working system, enabling ground controllers to direct British planes to intercept German planes in poor visibility and at night. In order to maintain their advantage the British spread the rumor that their pilots were being successful because of their extraordinarily good eyesight. A feat that was achieved by the mass consumption of carrots. This rumor affected both the Germans and the British. Due to the blackout at night and the frequent requirement to move to air-raid shelters people were keen to improve their night vision and carrots were readily available from allotments and gardens.
I do not know whether the Germans fell for this story. There are a few suspicious parts to it that may not have been fully understood at the time. Or maybe this was a sneaky part of the plan, to encourage German pilots to poison themselves with large amounts of Vitamin A. As Claudia pointed out today, if you want to propose ingesting large amounts of a vitamin you might be better off picking one that is water, rather than fat, soluble. As vitamin A is fat-soluble, disposing of any excesses taken in through diet is much harder than with water-soluble vitamins B and C. As such, vitamin A toxicity can result. This can lead to nausea, jaundice, irritability, vomiting, blurry vision, headaches, muscle and abdominal pain and weakness, drowsiness and altered mental status. Too many carrots will also cause you to turn orange from the accumulating beta-carotene. Another good source, in fact a much better source, of vitamin A is liver. Too much liver is even worse and can kill you quite quickly as Arctic explorers who were reduced to eating polar bear and sled dog liver discovered. Polar bears have very high concentrations of vitamin A in their liver and the native Inuit were well aware of this fact:
After killing a bear, the Inuit ate the meat and used the fur to make warm trousers for men and kamiks for women. An average polar bear would yield three pairs of trousers and one kamik. The only part of the bear that was not used was the liver. This was immediately thrown out, as it could make even the sled dogs violently ill.
From Polar Bear International.