Vitamin A is a fat soluble (oil based) vitamin that your body stores predominately around the liver. Like all fat soluble vitamins, it is stored in the body, so it does not need to be replenished daily, which water based vitamins (like vitamin C) do. In fact, vitamin A stores so well, you can actually poison yourself with it! If you eat the liver of a predatory animal (such as a turtle or a dog), you can actually die from too much vitamin A. Also, too much can make pregnant people have babies with birth defects, and may contribute to osteoporosis.
Vitamin A is used for vision (its true you have better night vision if you eat your carrots), bone growth (the job of osteoblasts), reproduction, cell division (making fresh stem cells) and differentiation (making stem cells into specialist cells), the immune system (making and strengthening white blood cells), and creates strong linings in the respiratory, urinary and intestinal systems. If you don't get enough vitamin A all these things are weakened, which makes it easier to get an infection, and harder to fight it.
There are two forms of vitamin A: preformed vitamin A, which comes from animal sources, and provitamin A carotinoid, which comes from plant sources.
Preformed vitamin A from animals is the easiest to absorb, as it has already converted into vitamin A pure. However, it is more dangerous as an excess is hard for the body to get rid of. It is found in milk, eggs, and liver.
Plant forms of vitamins, provitamin A carotinoids, are harder to turn into pure vitamin A, but it is also harder to consume an excess of, and can still be removed from the digestive system if necessary. Darkly coloured, orange or yellow fruit and vegetables tend to have more vitamin A in them. Carrots, cantaloupes, sweet potato, spinach, kale, and apricots are a good source of vitamin A.
Zinc is a mineral that is needed to release vitamin A stored around the liver into the blood stream so it can be used by other parts of the body. An excess of Vitamin A may inhibit vitamin D, which is necessary for building bones, amongst other things.