Vitamin A

Vitamin A is not a single molecule: broadly, it is a group of fat-soluble compounds, meaning they dissolve in fat. They include retinol and beta-carotene. The body needs them for vision, for cell differentiation, which is when each cell matures into its specific role, for keeping intact the epithelia, the tissues that line the body's surfaces, and for immune function, the body's defenses.

It arrives by two routes. One is preformed vitamin A, which comes from animal products. The other is beta-carotene, called provitamin A, which is found in colorful fruits and vegetables. Once inside, in the tissues, both are oxidized, meaning they change chemically, and are converted into retinal and retinoic acid.

When it is lacking, the rods, cells in the retina that capture light, can't properly rebuild their visual pigment, and night blindness may develop: seeing poorly in low light, like walking into a room at night without turning on the light. And in the eyes and in the respiratory, digestive and genitourinary tracts, the normal epithelium is replaced by a keratinized one, more like the tough outer layer of the skin.