Vitamin D

Vitamin D

Vitamin D has two structures: vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Vitamin D3 can be synthesized by the skin from sunlight in adequate amounts for the human body; however, the differences amongst regions and physiological capabilities make vitamin D essential. 

Absorption: Vitamin D is packed into micelles and enters enterocytes. Within enterocytes, vitamin D is packed into chylomicrons and enters lymph circulation.² If some vitamin D pairs with vitamin D-binding protein (DBP), it is transported to tissues such as skeletal muscle and adipocytes.² Whether bound to DBP (40% of vitamin D) or in chylomicrons (60% of vitamin D), vitamin D is transported to the liver. ²

Circulation and Metabolism: Vitamin D within chylomicrons and vitamin D transported by DBP eventually enter hepatocytes.² Vitamin D is converted into 25-hydroxycholecalciferol in the liver, and 25-hydroxycholecalciferol is converted into 1,25 dihydroxycalciferol within kidney cells.² 1,25 dihydroxycalciferol is the active form of vitamin D.¹ ² 1, 25 dihydroxycalciferol is released within the blood and binds to intestinal cells to form a calcium uptake protein known as calbindin to help raise blood calcium levels. 

The amount of 1,25 dihydroxycalciferol released is controlled by an enzyme called 1a-hydroxylase.² Parathyroid hormone (PTH) and insulin-like growth factor (IGF) promote the activity of this enzyme, and calcium and phosphorus inhibit its activity.² Low blood calcium levels stimulate 1a-hydroxylase to synthesize 1,25 dihydroxycalciferol and calcium reabsorption within the kidneys.  

Storage and Excretion: As a fat-soluble vitamin, vitamin D is found within hepatocytes and adipocytes.² Excretion occurs mostly in bile but can also be excreted through urine.¹

Functions: Vitamin D has many other methods of calcium and phosphorus regulation methods, among other functions, such as:

Vitamin D Receptor (VDR)/1,25 dihydroxycalciferol complex: The VDR/1,25 dihydroxycalciferol complex is formed when vitamin D binds to a vitamin D receptor (VDR).²

  • Gene expression is controlled when the complex interacts with vitamin D-responsive elements (VDREs) found within 1,25 dihydroxycalciferol- activated genes, such as osteoblasts and osteoclasts.²

  • Other than kidney calcium and phosphate reabsorption, vitamin D via VDR aids in calcium and phosphate reabsorption in the gut and bone. 

  • The VDR complex is found within bone, pancreatic beta cells, skin, prostate, breast, liver, immune, kidneys, muscle, and fat tissues.² 

  • The VDR complex is thought to play a role in a variety of diseases. VDR complex may alter cell growth factors and proliferation prevalent among cancer while promoting cell differentiation. 

  • VDR complex may also play a role in autoimmune disorders and is prescribed for skin disorders.

Dietary Reference Intake (DRI) - The recommended dietary allowance (RDA) for adult men and women is 15 micrograms or 600 IU.³ For adult men and women older than 70 years old, the RDA increases to 20mcg or 800 IU.³

Deficiency - Vitamin D deficiency in children can present as rickets which is the softening in long bones due to failure of mineralization.² In adults, reduced calcium and phosphate absorption leads to a reduction in mineralization of bones.² This can further lead to osteomalacia and decreased bone density. Risk groups of vitamin D deficiency include older adults, darker-pigmented individuals,  gastric bypass recipients, and individuals with fat absorption disorder.³

Toxicity - Hypervitaminosis symptoms (lack of appetite, vomiting, nausea, headache, bone and muscle pain, etc) may develop. Vitamin D toxicity can cause hypercalcemia; and in extreme cases, renal failure, calcium deposits within blood vessels, cardiac arrhythmias, and death.³

Dietary Sources of Vitamin D

Vitamin D is found naturally in animal products, such as meat and dairy products.² Vitamin D has been fortified in grains and dairy/dairy alternative products such as cow milk and plant milk. Vitamin D is naturally found in mushrooms but no other vegetables and fruits. 

Fruits/Vegetables - mushrooms, fortified orange juice 

Grains - fortified cereals 

Meat/ poultry products - fish (salmon, trout, sardines), egg 

Dairy/dairy alternatives/fats - milk (cow, soy, almond, oat), cheese, butter, margarine 

Source(s)

1. Jerry Tortora and Bryan Dickerson, Principles of Anatomy & Physiology, 16th ed. (New Jersey: John Wiley & Sons, 2021).

2. Denise M Medeiros and Robert E.C. Wildman, Advanced Human Nutrition, 4th ed. (Burlington, MA: Jones & Bartlett, 2019).

3. https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/