Vegan principals sound endearing but there’s strong evidence that nutrition can’t be substituted by plants or supplements for many reasons which include the contamination of supplements, toxic excipients, missing nutrient cofactors and supplement induced imbalances, poor bioavailability of plant based nutrients, and vitamins or minerals in plants that are inferior to animal sources or cannot be properly processed without animal sources.
Take vitamin A for example...
Vitamin A in the retinoid form is naturally only found from animal sources (please read this if nothing else: Bowman B, Russell RM, editors. Present Knowledge in Nutrition, 8th ed. Washington, DC:.ILSI Press; 2001, Chapter 11). Liver has the highest quantity (Vitamin A). The assumption that an individual can reach sufficiency with plant-source beta-carotene (i.e. avoiding animal sources) is problematic (Error - Cookies Turned Off, global clinical view on vitamin A and carotenoids, Bowman et al 2001).
Getting nutrients from supplements can also be very problematic due to serious issues with quality with supplement manufacturers and mislabeling (paid reports from ConsumerLab.com - independent tests and reviews of vitamin, mineral, and herbal supplements), adulterants (Unapproved Pharmaceutical Ingredients in Dietary Supplements Associated With FDA Warnings, Principal component analysis of synthetic adulterants in herbal supplements advertised as weight loss drugs - ScienceDirect, https://www.tandfonline.com/doi/abs/10.1080/19390211.2017.1360976?journalCode=ijds20), contaminants from chemical synthesis (https://community.dur.ac.uk/i.r.baxendale/papers/Tet2016.72.1645.pdf), added excipients (https://jpharmsci.org/article/S0022-3549(16)00447-0/fulltext), and imbalance of other co-factors, and ease for which an individual can overdose in comparison with food sources of nutrients (https://www.ncbi.nlm.nih.gov/pubmed/16140692, https://www.hindawi.com/journals/crie/2011/424712/, https://pediatrics.aappublications.org/content/118/2/820?download=true).
