Vitamins are defined as any group of substances required in small quantities for healthy growth and development. They cannot be manufactured in the body so they are therefore vital constituents of the diet.
There are two types of vitamins according to whether they are soluble in water or fat. Vitamin A (Retinol) is a fat soluble vitamin and this is not excreted from the body in the urine so excessive intake can have a negative effect on health. Vitamin A is present in animal origin foods eg. liver, milk and egg yolk. 
It is formed in the body from the pigment Beta-carotene which is present in certain vegetables eg. carrots, cabbage and lettuce.
Vitamin A has a number of roles in the body. It is necessary for normal cell division and growth. It is important for maintenance of mucous membranes of the respiratory, digestive and urinary tracts. Retinol is a pale yellow solid which dissolves easily in oil and fat and it is essential for good eye sight, particularly vision in dim light as well as having an important role in normal development of the embryo. It can be produced synthetically and therefore used to enrich margarines.
The best source of retinal is liver and a small amount 3g meets the body’s daily requirements. 700mcg per day is a sufficient amount per adult which can also be derived from the beta carotene foods eg. 2oz raw carrots.
Deficiency of Vitamin A causes stunted growth, night blindness and drying up of mucous membranes which increases the risk of infection. A continued lack of Vitamin A can lead to worsening of vision and lead to blindness.
image sources: https://intermountainhealthcare.org/blogs/topics/live-well/2017/10/the-abcs-of-vitamins/
https://www.femina.in/wellness/diet/sources-of-vitamin-a-218652.html
Why not check out our recommended nutrition guide at http://www.nutritional-supplement-guides.com/nut-ebook.html