Shady Mahmoud

Vitamin D metabolism

Vitamin-D is a sterol which contains steroid nucleus. It is a fat soluble vitamin and functions like a hormone.

Sources of vitamin D:1. Diet.

2. Sun exposure.

Vitamin D in the dietIt occurs in two forms:Vitamin D2 (Ergocalciferol).Vitamin D3 (Cholecalciferol).

Ergocalciferol (vitamin D2) is formed from ergosterol and is present in plants.

Cholecalciferol (vitamin D3) is found in animals (fatty fish, fish liver oils, egg yolk etc).

Milk is not a good source.

Both the sterols are similar in structure except that ergocalciferol has an additional methyl group.

They are sources for vitamin D activity and are referred as provitamins.

Absorption: vitamin D2 and D3 are absorbed from upper small intestine and bile is essential.

Mechanism: vitamin D3 and D2 form mixed micelles by combining with bile salts (micelles).

During the course of cholesterol biosynthesis 7-dehydrocholesterol is formed as an intermediate.

On exposure to sunlight, 7-dehydrocholesterol is converted to cholecalciferol in the skin (dermis and epidermis).

Sun exposure

The production of vitamin D in the skin is directly proportional to the exposure to sunlight and inversely proportional to the pigmentation of skin.

Excessive exposure to sunlight does not result in vitamin D toxicity since excess provitamin D3 are destroyed by sunlight itself.

Transport:Vitamin D is transported from intestine to

the liver by binding to vitamin D binding globulin.

25 OH D3 and 1,25 OH2 D3 are also transported in the blood by binding to it.

Storage:25 OH cholecalciferol is the major

storage and circulatory form of vitamin D.

Activation of Vitamin D:Active form: the active form of vitamin D is

1,25 OH cholecalciferol and is also called as calcitriol.

Cholecalciferol is first hydroxylated at 25th position to 25 OH cholecalciferol by a specific hydroxylase present in liver.

Kidney possesses a specific enzyme, 25 OH cholecalciferol 1 α hydroxylase.

1 α hydroxylase hydroxylates 25 OH cholecalciferol at position 1 to produce 1,25 – OH cholecalciferol (calcitriol).

Both hydroxylase enzymes (of liver and kidney) require cytochrome P450, NADPH and molecular oxygen for hydroxylation process

Regulation Formation of 1,25 – DHCC is regulated by

the regulation of renal 1 α – hydroxylase.

1 α – hydroxylase activity is increased by hypocalcemia.

Hypocalcemia stimulates PTH secretion which, in turn, increases 1 α – hydroxylase.

1 α – hydroxylase activity may be feedback inhibited by 1,25 – DHCC

FunctionVitamin D regulates the plasma levels of calcium

and phosphorous.Plasma calcium levels are regulated by effects of

1,25 – DHCC on small intestine, kidney, and bone.

It maintains the plasma calcium levels by increasing absorption of calcium from small intestine, increasing reabsorption of calcium by renal distal tubules and increasing mobilization of calcium from bone

24,25 – DHCC is another metabolite of vitamin D

It is synthesized in kidney by 24 - hydroxylase

If Calcitriol concentration is adequate, 24 – hydroxylase acts leading to the synthesis of a less important compound 24,25 – DHCC

To maintain calcium homeostasis, synthesis of 24,25 – DHCC is important

Recommmended daily intakeChildren - 10 gm/day or 400

IU/dayAdults - 5 gm/day or 200

IU/dayPregnency, lactation -10 gm/day or 400

IU/dayAbove the age of 60 yrs - 600 IU /day

Vitamin D tradenames

Decal B 12 120 ml syrupEach 5 ml syrup

contains:Ca 50 mgVitamin D3 1000 IUVitamin B12 10 mcg

Dose: 1 teaspoonful

Vidrop 2000 IU/mlEach drop contains 100

IU of vitamin D3 (cholecalciferol)

1 ml =28 dropsDose: 5000 IU daily for 3

weeks.

Devarol-S 200000 IU ampoule Each 2 ml contains:

Vitamin D3 (Cholecalciferol) 5 mg (equivalent to 200000 I.U.).

Dose: 1 ampoule every 6 months.

Sterogyl 2,000,000 ul/100 ml Each drop contains 400

IU of vitamin D2 (ergocalciferol).

1 ml = 50 drops. Dose: 2 – 5 drops daily

provide 1000-2000 IU.

Alfacareno 10 ml drops It contains alphacalcidol

(1 OH cholecalciferol).Used especially in renal

rickets.Dose: < 20 kg: ½ the

weight drops daily.> 20 kg: the weight

drops daily.

THANK YOU

Shady Mahmoud

Vitamin D and Sun

The part of the sun’s rays that is important is ultraviolet B (UVB). This is the most natural way to get vitamin D.

Which rays

Duration of exposure.Type of the skin.The amount of exposed skin. Sunscreen use.Age: elderly produce less vitamin D.Altitude: higher altitude produce more

vitamin DCloud weather.Air pollution: Polluted air soaks up UVB or

reflects it back into space.Being behind glass: glass blocks all UVB.

Factors affecting vitamin production by sun exposure

Sunlight exposure typically between 10 am and 3 pm in the spring, summer, and fall.

When the sun’s rays enter the Earth’s atmosphere at too much of an angle, the atmosphere blocks the UVB part of the rays, so the skin can’t produce vitamin D. This happens during the early and later parts of the day and during most of the day during the winter season.

Time of exposure

Exposing the skin for a short time will make all the vitamin D can produce in one day.

The body can produce 10,000 to 25,000 IU of vitamin D in just the time it takes for the skin to turn pink (could be just 15 min. in fair skinned people).

How much exposure

Infants and children should be kept out of direct strong sunlight (spend time in the shade). 

As Infants have delicate skin which burns more easily, the American Academy of Pediatrics recommends that babies under six months old should stay out of the sun completely.

What about children?

Pregnants: In two recent studies from the University of South Carolina, they found that pregnant women need to take 4,000 IU of vitamin D everyday to make sure that their newborn child has enough when he is born.

Lactation :the Vitamin D Council recommends that:

a) If she takes a supplement of 6,000 IU of vitamin D each day, she shouldn’t need to give her baby any vitamin D supplement.b) If she is not taking a supplement or getting a good amount of sun exposure, or if she is taking less than 5,000 IU/day of vitamin D, the baby should be given a vitamin D supplement.

RDAs for Vitamin D:

Infants (<1 year): 400 IU / day

Children and adults: 600 IU / day

Elderly (> 70 years): 8oo IU / day

Reference: Institute of Medicine, Food and Nutrition Board. Dietary Reference Intakes for Calcium and Vitamin D. Washington, DC: National Academy Press, 2010.

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