Vitamin D

Assesment of Vitamin D

-Plasma concentration of 25-OH D3 (calcidiol) is often used as an index of vitamin D status -Concentrations less than 37 nmol/L = subclinical deficiency -Less than 28 nmol/L = deficiency -Optimal levels: 80-120 nmol/L -Toxicity is considered when 25-OH D3 concentration typically exceeds 375 nmol/L *Serum Ca and P are not good indicators of vitamin D status UL= 4000 IU per day

Hydroxylation process

Cholecalciferol or D2 ----> 25-OH D3 (calcidiol; 2 OH's) -Hydroxyl group added at position 25 -via 25-OHase in liver **1st step in the liver **Second Step of Activation (in the kidney): 25-OH D3 (calcidiol) ---> 1-25 (OH)2 vitamin D3 (Calcitriol) via vitamin d-hydroxylase -Now has 3 hyroxyl groups *Calcitriol = biologically most active form *Tightly regulated step via vit D-hydroxylase

Metabolism in the liver

In the liver vitamin D is hydroxylated by 25 hydroxylase --> 25 hydroxy D made --> exported into the blood and binds to vitamin d binding protein (binds cholecalciferol, 25 hydroxy D, 25 hydroxy D3 or D2) *25 Hydroxylase acts on D2 and D3 -Hepatic 25 hydroxylase not tightly regulated: can easily hydroxylate vitamin D in liver cells -Circulating levels of 25-hydroxy D3: reflect intake and exposure to UV light and is commonly used to asses status -->first stage of activation and this molecule tends to stay in the blood for a long time -In the sun: levels of 25-OH D3 will go up in the blood (seasonal areas) Most of the 25-hydroxy D is taken up by the kidney

Dietary sources

Naturally occurring vitamin d in food = rare -Fortified products = main dietary source -Fortified milk us suppose to be fortified at 10 ug/quart but not required by law *Fortified milk, fatty fish, fortified cereals

Deficiency in adults

Osteomalacia: soft bones -Defect in mineralization -Results from abnormal phosphorus and calcium absorption/excretion -->less calcium is absorbed, declining blood calcium triggers secretion of PTH--> PTH can remain elevated in blood for awhile and lead to bone resorption --> increase urinary phosphorus excretion *Need a specific ratio of calcium to phosphorus to make bones so bones will become soft with low P -Turnover occurs the bone matrix becomes progressively demineralized which leads to bone pain and osteomalacia

Vitamin D deficiency in Children

Rickets -Failure of bone to mineralize -Epiphyseal (growth plate) cartilage continues to grow and enlarge without bone matrix or minerals --> become soft -Bone deformities, bowed legs, racitic rosary of rib cage -Can be cause by dietary deficiency, or lack of sunlight or combo -Kid susceptible: low sun exposure, craniotabes (softening of bones and skill) -sitting/crawling/walking are delayed -bowing of arms, knees or outward bowing

Synthesis of Vitamin D2 and D3

Via diet: D2 comes from plants ---> plants dont have cholesterol so they use ergosterol to make vitamin D *Ergosterol is in plants and yeast D3: animals contain cholesterol and use it to make D3 -D3 comes from animal products or the sun **this is the form that we make in our body (D2 can still be activated/hydroxylated and function like vitamin D3) Cholesterol ---> 7 Dehydrocholesterol (takes place in the skin via sebaceous glands) ---> UV exposure converts it to Pre-Vitamin D3 (precalciferol) (ring opens up becuase of UV) --> Few options (from UV exposure) 1) Lumisterol 2) Tachysterol Body heat 3)--> Vitamin D3 (cholecalciferol) Pre Vitamin D2 (ergosterol)---> Vitamin D2 (ergocalciferol) via irradiation *UV is required for making vitamin D - > sun protects them by making vitamin D -Stay in the sun too long: doesn't become toxic -Conversion of previtamin D3 to lumisterol and tachysterol is a protective mechanism (extended exposure to UV creates lumisterol and tachysterol which are two molecules that don't become vitamin D; don't leave the skin; not harmful) D3 made in the skin -->binds vitamin D binding protein in the blood --> transports it to tissues to be hydroxylated

Genes with VDRE sequences in their promotor regions

Calbindin: in enterocytes and kidney cells; takes calcium from calcium pump and moves through the cell Osteocalcin: in bones; involved in calcium metabolism

MEmbrane VDR's

1) Calcitriol binds cell surface receptors (VDR) --> signal in cell --> triggers response (kinase cascade) --> transcaltachia (calcium movement) and opening of Ca Channels ==> Increase Ca uptake

Why the elderly may be at risk for low vitamin d

1) Inadequate sun exposure 2) poor vitamin D intake (decrease milk, fish consumption) 3) Aging reduces synthesis of cholecalciferol on the skin (we make less and decreased activation) 4) Decreased renal-1-hydroxylase synthesis in response PTH (PTH isn't as effective) 5) Disorders affecting the parathyroid, liver, kidney will impair synthesis of active form of the vitamin

Calcitriol --> Phosphorus absorption

*Calcitriol = major mineral in bone Phosphorus = second major mineral 1) Calcitriol increases activity of brush border alkaline phosphatase --> phosphatase hydrolyzes phosphate ester bonds --> allowing phosphorus to be absorbed *Basically: Calcitriol enhances phosphorus absorption by increases a phosphates on the brush border 2) Calcitriol increases the number of transporters for Na-dependent phosphorus absorption at the brush border

Signs and symptoms of toxicity

Anorexia, Vomiting, weakness -High amounts can lead to calcification of soft tissues (kidneys, lungs, blood vessels) -Vitamin K is usually missing in supplements which is important for making sure vitamin D goes where it should

Toxicity

-Sunlight not a problem -Due to dietary ingestion: most likely of all the vitamins to cause overt toxic reactions -Chylo remnants deliver vitamin D to the liver and liver hydroxylates to 25-OH (liver will hydroxylate it, even with significant amounts) -- > end up with calcidiol and our body will downregulate 25-OH to 1-25 OH but 25-OH can stimulate some of the same effects as calcitriol (can bind receptors)

Nuclear Receptors

*Main function of calcitriol is increased calcium absorption which involves nuclear receptors in enterocytes 1) Vitamin D Binding protein with calcitriol in the blood (VDB protein will bind vit. D3, calcitriol and calcidiol) --> releases calcitriol --> across plasma membrane --> D3 ---> nuclear membrane 2) D3 vinds VDR ---> causes conformational change --> binds RXR: retinoid X receptor --> binds vitamin D response element (VDRE) -->helps recruit other transcription factors/RNA polymerase to the promotor region --> affect gene expression

Calitriol and it's actions (Receptors, VDRE, etc)

-Calcitriol (D3) is the main active form of vitamin D and functions like a steroid hormone -D3 must bind to a receptor in order to elicit a response -Calcitriol binds two different types of receptors: 1) Membrane receptors 2) Nuclear receptors *Vit. D is genomic and nongenomic --> These receptors are denoted as Vitamin. D receptors (VDR) Vitamin D is involved in many other things besides calcium absorption: cell membrane receptors are present in the intestine, kidney, bone, cardiac, muscle, pancrease, brain, skin, etc.

Calcitriol and the kidney

-Involved in stimulating reabsorption of calcium and phosphorus in the kidney -Calbindin D 28K = plays a role in renal calcium reabsorption --->Synthesis of this protein = calcitriol responsive -->synthesis may be increases by nVDR mechanism (nuclear) -->Larger version of the calbindin protein of SI **Basically: More calbindin in kidney cells = less calcium that gets excreted in the urine

Calcitriol and the bone

-PTH alone or with calcitriol = leads to mobilization of Ca and phosphorus from bone to maintain blood levels -This can be mediated by calcitriol's effects on bone cell differentiation -->Calcitriol can induce bone marrow (stem) cells ==> osteoclasts (dietary calcium low, vitamin D participates in this role) -->Osteoblasts also respond to calcitriol by producing: collagen, osteocalcin, and other proteins for bone mineralization (with adequate vitamin D and Calcium in the diet then vitamin D functions in this role)

Study on fortified milk

80% of samples = didn't contain suggested amount of fortified vitamin D 50%= had 50% of the vitamin D they should of have 15%= no detectable vitamin D *Fortified/Supplements= D3 form -vitamin d supplementation can be the most toxic -If it had calcitriol you're bypassing the regulation of vitamin --> too much of the active form could cause toxicity issues *USP label on supplementation

Major functions of calcitriol

Calcitriol= been activated twice/most activated form of vitamin D3 1) Maintain serum Ca and Phosphorus levels at high concentrations for bone mineralization (main function) 2) Maintain Ca homeostasis to support cellular metabolic processes and neuromuscular functions (vitamin D will help break down bone to release calcium) Performs function by: -Increasing Ca and P in SI -Increasing Ca and P mobilization from bone -Increasing Ca and P reabsorption in the kidney **main loss of calcium = urine--> vitamin D can decrease losses by increasing blood calcium

Adequate Intakes

Controversial how much we should have in our diet and in tissues -In 2010 AI's became RDA -RDA's: Adults: 15 ug (600 IU) Elderly over 70: 20 ug (800 IU) -Conversion: 1 ug = 40 IU) -10 minutes in the sun (face and hands exposed): 10 ug of vitamin D (400 IU) -As you get older, synthesis becomes less efficient / activation becomes less efficient

Background Info

Fat Soluble Hormone -Functions: absorption of calcium (bones), immune system--> protects us, critical for growth. regulates gene expression (lots of target genes) -Helps differentiate different cells and reduces likelihood of becoming different cancers -Controversial: how much is enough --> look at vitamin D health indicators

Overview of hydroxylation and calcitriol effects (draw out)

Vitamin C ---> Liver (hydroxylated to 25-hydroxylase) ---> Kidney (tightly regulated via PTH) PTH= Parathyroid hormone; Increases kidney 1-hydroxylase activity --> Calcitriol *low blood calcium stimulates PTH release Calcitriol effects: -Calcitriol and PTH together help convert bone stem cells to osteoclasts --> erode the skeleton to increase blood calcium and phosphorus -Increases Ca absorption in small intestine (major function) -->can enhance absorption efficiency from 30%-80% **These mechanisms are tightly regulated --> PTH is activated if a person isn't consuming enough dietary calcium --> breaking down bone to maintain blood calcium --> bone can erode and increase risk of osteoporosis over a long period of time *Hard to tell if a person is consuming enough calcium --> can use PTH marker (measure blood PTH levels)

Two types of Vitamin D

Vitamin D2: Ergocalciferol -Active in the body -Functions as vitamin D -double bond on carbon chain Vitamin D3: Cholecalciferol -more important/biologically active form of vitamin D in the body -Supplement forms *Carbon chains are different Hormone function: binds proteins and changes function of proteins --> then proteins affect gene expression *Our cells can distinguish between D3 and D2

Absoorption and Transort

Vitamin D3 and D2 from the diet are absorbed from micelles (fat soluble) via going through the stomach, SI then bile --> micelle --> absorption is by passive diffusion --> enterocytes Once in enterocyte: -50% of dietary cholecalicferol is absorbed (any form of dietary vitamin D) -In intestinal cells most of the vitamin is incorporated into chylomycrons -D3 transferred to DBP can be delivered to extrahepatic tissues (muscle, adipose) or the liver -Vitamin D that remains in chylos is delivered to the liver as chylo remnants ===> 1st step of vitamin D activation -In the liver: hydroxylated by 25 hydroxylase