Answer: Vitamin A

Nyctalopia, or night blindness, is a disorder, as the name would imply, that reduces or outright removes the ability of the patient to see clearly at night due to decreased receptivity to light in darkened conditions.

While most of us are happier when we can see everything clearly in the bright light of day, at night we can still see well enough by street lights (and even moonlight on a clear night) to navigate. Those with nyctalopia perceive a darkened room or street so dimly that only the brightest points of light register. Thus while an unafflicted person might see a dimly lit road, outlines of buildings, and the alternative pools of light and shadows found beneath and in between street lamps, a person with nyctalopia might only see the bright points created by the street lamp bulbs themselves and little else.

Although there are multiple independent causes of nyctalopia such as the genetic disorder retinitis pigmentosa (wherein the rod cells in your eyes slowly lose their ability to respond to light), cataracts, and retinal detachment, the one cause of night blindness which is most within our control is nutrition-based: vitamin A deficiency.

The rods of your eye contain a pigment called rhodopsin, a critical protein that is key to the functioning of the photoreceptors found within the eye. Rhodopsin is made of the protein opsin and a reversibly covalently bound cofactor, retinal. That retinal is derived from retinol created via the breakdown of Vitamin A. If you don’t get enough Vitamin A in your diet, found in a wide variety of vegetables like carrots, broccoli, kale, spinach, and tomatoes as well as animal sources such as liver and cod liver oil, your body fails to product enough rhodopsin.

In fact you may have heard that carrots are very good for your eyesight, a tidbit often told to children reluctant to eat their vegetables. While eating enough vegetables is good for general health and, in fact, carrots are very high in Vitamin A and thus ensure you won’t suffer from night blindness, the old bit about carrots radically increasing your vision (especially your night vision) is actually an old bit of World War II propaganda.

When the British invented radar they did their best to keep a lid on the secret. In order to throw German forces off they spread propaganda that British pilots ate huge amounts of carrots and other Vitamin A rich foods in order to boost their night vision (as a way of explaining the success of British pilots during night time missions). While we’re sure the pilots derived great benefit from a healthy diet, being the first military to deploy radar for air-based combat certainly helped a wee bit more.