Vitamin A

What it does

Vitamin A is best known for its role in vision. In the retina it forms retinal, the light-sensitive part of the pigment rhodopsin, which the rod cells need to detect dim light. Without enough vitamin A the eye cannot regenerate this pigment quickly, which is why one of the earliest signs of deficiency is difficulty seeing in low light (night blindness). It also keeps the surface of the eye — the cornea and conjunctiva — moist and healthy.

Beyond the eye, vitamin A is a master regulator of cell growth and differentiation. Its active form, retinoic acid, switches genes on and off that tell immature cells what type of tissue to become. This is central to maintaining the "barrier" tissues of the body — the skin and the linings of the airways, gut and urinary tract — which are the body's first line of defence against infection. It is also essential for normal growth in children and for the development of a baby during pregnancy.

Vitamin A supports the immune system in a broad way, helping to keep those barrier surfaces intact and supporting the production and function of white blood cells. It comes in two dietary forms: preformed vitamin A (retinol) from animal foods such as liver, oily fish, eggs and dairy, and provitamin A carotenoids such as beta-carotene from colourful plant foods, which the body converts into vitamin A as needed. Because it is fat-soluble and stored in the liver, the body can draw on reserves between meals — but this same storage means very high supplement doses can accumulate to harmful levels.