The basis for this "old wive's tale" is that, accurately, carrots as well as other yellow/orange vegetables and gourds (squash, pumpkin) are fantastic sources of vitamin a, an essential vitamin required for photoreceptor (cone/rod) metabolism. While severe vitamin a deficiency
may lead to serious, but generally reversible, vision problems
, such as night blindness
, most people in developed countries meet more than their daily vitamin a requirements from normal diets
and there is no need to consume exorbitant amounts of carrots.
In fact, vitamin a excess may be toxic to the liver and some clinical studies have demonstrated that excess vitamin a metabolites may actually be potentially toxic to photoreceptors.