What are vitamins?: 
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Sources:
https://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/
https://www.biology-online.org/dictionary/Aldehyde
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3679378/
http://lpi.oregonstate.edu/mic/dietary-factors/phytochemicals/carotenoids
https://en.wikipedia.org/wiki/Apocarotenal
https://en.wikipedia.org/wiki/Chylomicron
http://lpi.oregonstate.edu/mic/vitamins/vitamin-A#function
https://en.wikipedia.org/wiki/Cryptoxanthin
https://en.wikipedia.org/wiki/Retinoic_acid
https://en.wikipedia.org/wiki/Retinal#Vitamin_A_metabolism
https://www.healthline.com/health/hypervitaminosis-a#treatment
https://www.consumerlab.com/RDAs/#VitaminA
https://dietarysupplementdatabase.usda.nih.gov/ingredient_calculator/help.php
https://www.bluebulbprojects.com/measureofthings/results.php?amt=.0009&comp=weight&unit=gms&searchTerm=.0009+grams
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