Vitamin A

Vitamin A is a vitamin, a group of organic chemicals including retinal and several carotenoids. The human body does not make these chemicals, and must take them from food. Egg yolks and liver are both rich in vitamin A. Many parts of the body need vitamin A. For example, vitamin A helps sight and is good for the immune system. It is also important for a growing embryo.

β-carotene, a provitamin found in foods like carrots can be converted to vitamin A. The efficiency of the conversion depends on genetics and fat consumption.

Too little vitamin A is dangerous to health. People who do not get enough of the vitamin may lose the ability to see in poor light and suffer from a weakened immune system. They may also have problems with memory, because vitamin A is important for the brain. People with malnutrition often have too little vitamin A. This is common in poor countries.

 Hypervitaminosis A 
Too much vitamin A is also dangerous to one's health. It is a condition known as hypervitaminosis. Vitamin A is not soluble in water, and the human body can not get rid of the excess vitamin A easily by urination.  Too much vitamin A can make someone very sick.  People may get too much vitamin A from taking too many vitamin pills or from eating too much A-rich food like liver.

What applies to humans applies to all mammals at least. Some large predators, such as the big cats, seem not to eat their preys' livers. On the other hand, polar bears apparently do so, and they store vitamin A in their liver cells.Senoo H, Imai K, Mezaki Y, Miura M, Morii M, Fujiwara M, Blomhoff R. 2012. Accumulation of vitamin A in their liver cell of arctic top predators. Anatomical Record 295 (10): 1660–8  However, they have long, sometimes very long, periods between kills. In contrast, African big cats kill about every three or four days, and they do seem to avoid eating the preys' livers.

 References 

Category:Fat-soluble vitamins