Vit D Flashcards
What is Vitamin D? what forms does it have and where are they found? why is it important?
A fat-soluble vitamin that is essential for calcium/ phosphorus homeostasis, bone health and immune function
-Vitamin D3 (Cholecalciferol) – Found in animal products, synthesized in skin.
-Vitamin D2 (Ergocalciferol) – Found in plant sources and fortified foods.
How is Vitamin D Absorbed and Stored?
Absorption: Occurs in the small intestine with dietary fats via chylomicrons.
Storage: Primarily in adipose tissue and liver.
Transport: Bound to Vitamin D Binding Protein (DBP) in circulation.
Excess adipose tissue can store Vitamin D but may reduce availability for use.
How does sunlight contribute to Vitamin D synthesis?
1) UVB radiation (290-315 nm) converts 7-dehydrocholesterol in the skin to pre-vitamin D3.
2) Thermal isomerization converts pre-vitamin D3 into cholecalciferol (Vitamin D3).
-This process occurs in the epidermis of the skin.
What factors increase the synthesis of Vitamin D from sunlight?
-Summer months (higher UVB exposure).
-Midday sun exposure (UVB rays strongest).
-Lower latitudes (closer to the equator).
-Lighter skin tones (less melanin to block UVB).
What is a challenge with Vit D metabolism?
Vit D has the ability to isomerize into other metabolites along each rxn of the pathway
How is Vitamin D metabolized in the body?
Liver: Converts Vitamin D3 (Cholecalciferol) into 25-hydroxyvitamin D [25(OH)D] (inactive storage form).
Kidneys: Convert 25(OH)D into 1,25-dihydroxyvitamin D [1,25(OH)2D] (active form).
Final Activation: 1,25(OH)2D is the hormone-like active form, regulating calcium and phosphorus absorption.
what are the steps involved in vit D synthesis?
1) absorption of DHC on skin
2) Thermal isomerization to form Vit D3 (cholecalciferol)
3) translocation to liver via vit D binding protein where Pre vit D is activated into active D3
what is the best measure of Vit D status? why?
25-hydroxyvitamin D3
-this is vitamin D before it is re-arranged into other metabolites, measuring [ ] of this molecule is more reflective of serum Vit D levels
How does Vitamin D regulate calcium and phosphorus homeostasis? in the intestine, kidney and bones?
Intestines: Increases absorption of calcium and phosphorus.
Kidneys: Reduces calcium excretion and regulates phosphorus levels.
Bones: Stimulates osteoclast activity to release calcium when needed
How is vit D impacted by PTH?
Interaction with Parathyroid Hormone (PTH):
Low calcium levels → PTH increases → Kidney converts 25(OH)D (inactive) to 1,25(OH)2D (active).
1,25(OH)2D increases calcium absorption in the intestine to restore balance.
What are the differences between 25-hydroxyvitamin D [25(OH)D] and 1,25-dihydroxyvitamin D [1,25(OH)₂D]?
What are the best food sources of Vitamin D?
Natural sources:
Fatty fish (salmon, mackerel, sardines).
Cod liver oil.
Egg yolks.
Fortified foods:
Milk, plant-based milk (soy, almond, oat).
Orange juice.
Cereals.
What are the Recommended Dietary Allowances (RDA) for Vitamin D?
What are the differences between Vitamin D2 and Vitamin D3? what are they also called?
Vit D2= ergocalciferol
-less active, found in plants
Vit D3= cholecalciferol
- more active, found in animals
What can result from Vitamin D deficiency in children, adults and elderly people? what are common risk factors
Children: Rickets – Soft, weak bones, bow-legged appearance.
Adults: Osteomalacia – Bone softening, increased fracture risk.
Elderly: Osteoporosis – Decreased bone density, higher fracture risk.
Common risk factors:
Low sun exposure, malabsorption disorders, darker skin, kidney/liver disease.
how much exposure is needed to get vit D toxicity through skin?
toxicity of vit D in the skin is impossible
What are the causes of Vitamin D deficiency?
1) Low dietary intake (lack of fatty fish, fortified foods).
2) Limited sun exposure (northern latitudes, sunscreen, winter months).
3) Malabsorption disorders (Crohn’s disease, celiac disease).
4) Kidney or liver disease (inhibits activation of Vitamin D).
what occurs wiht excessive vit D intake?
hypervitaminosis D –> hypercalcemia (high blood calcium levels) –> soft tissue calcification –> renal / CV damage
what is the optimum level of serum 25-hydrocyvitamin D (25[OH]D)?
80-120 nmol /L
