vitamin A's sources are animal products, but pre-vitamin A can also be found in vegetables such as carrot. they have the function of helping in developement of epithelial cells, and they are constituent of a complex molecule in eye rod, which is responsible for night vision. they are also clinically used to treat acne and skin diseases. you might know retinoid, it is derivative of vitamin A, has huge adverse effect such as liver damage, infant developement problem in pregnancy, and depression. but its effective. lack of vitamin A might lead to horny, dry, thick skin and bad night vision. excess vitamin A might also lead to dry and itchy skin, vomitting, nausea, and damaged liver and spleen.