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

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.  

Vitamin D

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. 

University of Pittsburgh

A vitamin is a complex organic compound that regulates particular metabolic processes in the body. 
Vitamins must meet the following criteria:
- the body cannot synthesize enough to maintain good health
- compound is naturally occurring in commonly eaten foods
- if substance is missing from diet, signs and symptoms of deficiency disorder eventually occur
- health can be restored by supplying missing compound if deficiency is treated early

Vitamins

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

Nutrition for Healthy Living 5th Ed.

Fat-Soluble Vitamins

These vitamins are not able to be stored in the body. They are excreted from the body in the urine. 
Water-soluble vitamins include:
- Thiamin
- Riboflavin
- Niacin
- Vitamin B-6
- Pantothenic Acid
- Folate
- Biotin
- Vitamin B-12
- Vitamin C

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