Retinol is preformed vitamin A. It is the most active form of vitamin A in the human body. Retinol can only be found in animal foods.

Retinol

Yellow-orange pigments found in plants. The human body can convert certain carotenoids into vitamin A.

Carotenoids

Vitamin A is a fat-soluble vitamin involved in:
- Immune function
- Bone health
- Vision
- Cell production and communication
- Growth and development

University of Pittsburgh

These vitamins are able to be stored in the body in fatty tissue and i the liver. Vitamins A, D, E, and K are fat-soluble. 

Fat-Soluble Vitamins

Schiff, Wendy J. Nutrition for Healthy Living. 5th Ed., McGraw-Hill, 2019. 

Nutrition for Healthy Living 5th Ed.

Vitamin A

Vitamin D is a nutrient that is synthesized endogenously from 7-dehydrocholesterol by the skin after UV-B exposure from sunlight and is also found in some foods.  It is processed in the liver by the enzyme 25-hydroxylase to form 25-hydroxyvitamin D [25(OH)D].  It is then further processed by the enzyme 1-ɑ-hydroxylase (CYP27B1) to form the biologically active molecule 1,25-dihydroxyvitamin D [1,25(OH)2D].  This molecule can then go on to interact with vitamin D receptors in a variety of tissues which are involved in physiological processes such as the endocrine regulation of minerals and some cell signaling mechanisms related to immune responses.  

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