What are the Functions of Vitamins? – Vitamins A, C, D, E, and K

Vitamins are non-nutritive compounds that play various roles in health. This lesson covers the fat-soluble vitamins ADEK and one of the water-soluble vitamins, C.

Vitamin A

  • Retinol is most usable form, along with retinyl ester (storage form)
  • Functions of Vitamin A
    • Translation of light to visual images via vit A-dependent proteins rhodopsin and iodopsin
    • Protein synthesis and cell differentiation 
    • Fetal and childhood growth
    • Sperm production and normal menstrual cycles
  • Daily needs
    • 2310 IU for women
    • 3000 IU for men
  • Toxicity liver toxicity 
  • Deficiency: blindness, immune problems

Vitamin D

  • Two forms 
    • Vitamin D3 (cholecalciferol): found in animal foods, supplements, and produced in the skin
    • Vitamin D2 (ergocalciferol): found in plants and supplements
  • Functions
    • Vitamin D increases production of calbindin in the enterocytes to increase Ca absorption
    • Vitamin D deficiency has been associated with increased incidence of depression. Needs
  • Daily Needs 
    • 15 to 20 micrograms (600 – 800 IU) for adults
    • 15 ug for kids
  • Toxicity: hypercalcemia
  • Deficiency: rickets/osteomalacia 

Vitamin E

  • Eight forms of E but alpha-tocopherol is the most active
  • Functions
    • Antioxidant: protects phospholipids and LDL
    • Anticoagulant: reduces “stickiness” of platelets and cells lining lymph and blood vessels 
  • Daily Needs
    • 15mg per day (22.4 IU)
  • Toxicity: no toxicity found from foods; from supplements, may cause hemorrhaging, may interfere with the balance of other antioxidants in the body
  • Deficiency: nerve problems, muscle weakness, and uncontrolled movement 

Vitamin K

  • Forms
    • Phylloquinone (K1): found in plants, primary source of vitamin K in the diet
    • Menaquinone (K2): produced by gut bacteria 
    • Menadione (K3): form found in supplements – no longer used because it was found to damage the liver
  • Functions
    • Coagulation: arboxylase is dependent on Vitamin K as a coenzyme–forms clotting factors such as prothrombin 
    • Bone health: carboxylation of osteocalcin and matrix Gla protein is needed for calcium ions to bind to the matrix
  • Daily needs
    • Difficult to determine because we don’t know how much the microflora contribute
    • AI 90 ug (1000 IU) for women and 120 ug (1300 IU) for men
  • Toxicity no known effects so no UL; those on anticoagulants need to maintain consistent intake
  • Deficiency: side effects are rare in healthy individuals; those with conditions that reduce fat absorption such as gallbladder disease, are at higher risk

Vitamin C

  • C is not stored, taken up when needed by glucose transport proteins (because ascorbic acid is similar to glucose)
  • Functions:
    • Collagen synthesis
    • Antioxidant 
    • Iron absorption
    • Immune System
  • Daily Needs
    • 90mg for men, 75mg for women
  • No known toxicity (other than nausea, cramps, and diarrhea over 3000mg), upper limit set at about 2000mg (can also exacerbate gout and kidney stones)
  • Deficiency: scurvy is related to inability to make collagen (swollen and bleeding gums, rough rash, coiled/curly arm hair, wounds that won’t heal)


Note: some of the general information from this video came from Nutrition: From Science to You (3rd Edition) by Joan Salge Blake, Kathy D. Munoz, and Stella Volpe

Vitamin A Fact Sheet for Health Professionals: https://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/#h7

The Mechanism for Vitamin A Improvements in Night Vision: https://www.ebmconsult.com/articles/vitamin-a-eye-vision-mechanism 

Vitamin A and Pregnancy: A Narrative Review (2019): https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6470929/

Vitamin A in Reproduction and Development (2011): https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3257687/pdf/nutrients-03-00385.pdf

Teratogenic effects of vitamin A and its derivates (1997): https://www.ncbi.nlm.nih.gov/pubmed/9345570

Vitamin D Metabolism, Mechanism of Action, and Clinical Applications (2014): https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3968073/pdf/nihms-557928.pdf

Effect of Vitamin D Supplement on Mood Status and Inflammation (2019): https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6390422/  

The role of vitamin D in nervous system health and disease (2013): https://onlinelibrary.wiley.com/doi/full/10.1111/nan.12020

Vitamin E Fact Sheet for Health Professionals: https://ods.od.nih.gov/factsheets/VitaminE-HealthProfessional/

Effects of Random Allocation to Vitamin E Supplementation on the Occurrence of Venous Thromboembolism (2007): https://www.ahajournals.org/doi/full/10.1161/CIRCULATIONAHA.107.716407?url_ver=Z39.88-2003&rfr_id=ori%3Arid%3Acrossref.org&rfr_dat=cr_pub%3Dpubmed

Peripheral Neuropathy Due to Vitamin Deficiency, Toxins, and Medications (2014): https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4208100/pdf/20141000.0-0016.pdf

Vitamin K Fact Sheet for Health Professionals: https://ods.od.nih.gov/factsheets/Vitamink-HealthProfessional/

The Roles of Vitamin C in Skin Health (2017): https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5579659/pdf/nutrients-09-00866.pdf

Vitamin C and Immune Function (2017): https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5707683/pdf/nutrients-09-01211.pdf

Interaction of vitamin C and iron (1980): https://www.ncbi.nlm.nih.gov/pubmed/6940487