Vitamin D

Vitamin D

Q. 1

Which among the following doesn’t have a role in regulating Vitamin D metabolism?

 A Liver
 B Lungs 
 C

Kidney

 D

Skin

Q. 1

Which among the following doesn’t have a role in regulating Vitamin D metabolism?

 A Liver
 B Lungs 
 C

Kidney

 D

Skin

Ans. B

Explanation:

Lungs REF: Guyton physiology 11th edition page 983

  • Cholecalciferol (Vitamin D3) Is Formed in the Skin.
  • Cholecalciferol Is Converted to 25–Hydroxycholecalciferol in the Liver.
  • Formation of 1, 25-Dihydroxycholecalciferol takes place in the Kidneys

Q. 2 Vitamin D is synthesized by? 
 A

Keratinocytes

 B Prickle cells
 C Melanocytes
 D Granular cells
Q. 2 Vitamin D is synthesized by? 
 A

Keratinocytes

 B Prickle cells
 C Melanocytes
 D Granular cells
Ans. A

Explanation:

Keratinocytes REF: Harrison 18th ed chapter 352

“Vitamin D is produced in keratinocytes of skin and maximum production is in stratum basal”

1, 25-dihydroxyvitamin D [1, 25(OH)2 D] is the major steroid hormone involved in mineral ion homeostasis regulation.

In response to ultraviolet radiation of the skin, a photochemical cleavage results in the formation of vitamin D

from 7-dehydrocholesterol. Cutaneous production of vitamin D is decreased by melanin and high solar protection factor sunblocks,

which effectively impair skin penetration by ultraviolet light.

Vitamin D from plant sources is in the form of vitamin D2, whereas that from animal sources is vitamin D3.

Vitamin D is subsequently 25-hydroxylated in the liver.

25-hydroxyvitamin D [25(OH)D], is the major circulating and storage form of vitamin D

The second hydroxylation, required for the formation of the mature hormone, occurs in the kidney to form

1,25-dihydroxyvitamin D [1,25(OH)2D]


Q. 3

Rate limiting step in vitamin D synthesis is?

 A 25 cholecalciferol
 B 1, 25-dihydroxycholecalciferol
 C 24, 25- dihydroxycholecalciferol
 D 7-dehydrocholesterol
Q. 3

Rate limiting step in vitamin D synthesis is?

 A 25 cholecalciferol
 B 1, 25-dihydroxycholecalciferol
 C 24, 25- dihydroxycholecalciferol
 D 7-dehydrocholesterol
Ans. B

Explanation:

1, 25-dihydroxycholecalciferol REF: Ganong 22ed chapter 21, Harrison 17TH edition chapter 21

Vitamin-D: Synthesis and Metabolism

1, 25-dihydroxyvitamin D [1, 25(OH) 2D] is the major steroid hormone involved in mineral ion homeostasis regulation

Vitamin D is subsequently 25-hydroxylated in the liver by cytochrome P450—like enzymes in the mitochondria and microsomes. The activity of this hydroxylase is not tightly regulated, and the resultant metabolite, 25-hydroxyvitamin D [25(OH) D], is the major circulating and storage form of vitamin D

The second hydroxylation, required for the formation of the mature hormone, occurs in the kidney. The 25-hydroxyvitamin D-1-hydroxylase is a tightly regulated cytochrome P450—like mixed function oxidase expressed in the proximal convoluted tubule cells of the kidney

Quiz In Between


Q. 4

Which of these is a poor source of Vitamin D?

 A

Fish fat

 B

Whole milk

 C

Cod liver oil

 D

Egg

Q. 4

Which of these is a poor source of Vitamin D?

 A

Fish fat

 B

Whole milk

 C

Cod liver oil

 D

Egg

Ans. B

Explanation:

Food   Vit D µg/ 100 g of food
  Butter   0.5-1.5
  Egg   1
  Whole milk   0.1
  Fish fat   5-30
  Cod liver oil   200-750
  Halibut liver oil   500-10,000

Ref: Park’s textbook of Preventive and Social Medicine, 21st edition, page-570


Q. 5

Which among the following is the richest source of vitamin D?

 A

Milk

 B

Egg

 C

Green leafy vegetables

 D

Fish oil

Q. 5

Which among the following is the richest source of vitamin D?

 A

Milk

 B

Egg

 C

Green leafy vegetables

 D

Fish oil

Ans. D

Explanation:

The main natural source of vitamin D in the diet is in the livers of fish, which ingest ultraviolet-irradiated sterols (in phytoplankton and zooplankton) and store the vitamin D produced in their livers.
A major source of vitamin D is its synthesis in the skin upon ultraviolet B (UV-B) (wavelength, 290–315 nm) exposure.
Except for fish, food (unless fortified) contains only limited amounts of vitamin D.
Vitamin D2 (ergocalciferol) is obtained from plant sources and is the chemical form found in some supplements.
 
Ref: Shoback D.M., Sellmeyer D.E. (2010). Chapter 17. Disorders of the Parathyroids & Calcium & Phosphorus Metabolism. In McPhee S.J., Hammer G.D. (Eds),Pathophysiology of Disease, 6e.

 


Q. 6

Vitamin D from the skin or diet is metabolized primarily in the liver to 25-hydroxyvitamine D3. What is the rate limiting enzyme in Vitamin D synthesis?

 A

25 cholecalciferol

 B

1, 25-dihydroxycholecalciferol

 C

24, 25- dihydroxycholecalciferol

 D

7-dehydrocholesterol

Q. 6

Vitamin D from the skin or diet is metabolized primarily in the liver to 25-hydroxyvitamine D3. What is the rate limiting enzyme in Vitamin D synthesis?

 A

25 cholecalciferol

 B

1, 25-dihydroxycholecalciferol

 C

24, 25- dihydroxycholecalciferol

 D

7-dehydrocholesterol

Ans. B

Explanation:

Vitamin D from the skin or diet is metabolized primarily in the liver to 25-hydroxyvitamine D3. 25D is the ‘storage’ form of vitamin D and is used to determine the vitamin D status of individuals. The last and rate limiting step in the synthesis of ‘active’ 1,25D is catalyzed by the mitochondrial enzyme 1 hydroxylase and is conventionally known to take place in kidneys.

Quiz In Between


Q. 7

Which among the following doesn’t have a role in regulating Vitamin D metabolism?

 A

Liver

 B

Lungs

 C

Kidney

 D

Skin

Q. 7

Which among the following doesn’t have a role in regulating Vitamin D metabolism?

 A

Liver

 B

Lungs

 C

Kidney

 D

Skin

Ans. B

Explanation:

In response to ultraviolet radiation of the skin, a photochemical cleavage results in the formation of vitamin D from 7-dehydrocholesterol. Vitamin D enters the circulation, whether absorbed from the intestine or synthesized cutaneously, bound to vitamin D–binding protein, an alpha-globulin synthesized in the liver. Vitamin D is subsequently 25-hydroxylated in the liver by cytochrome P450–like enzymes in the mitochondria and microsomes. The second hydroxylation, required for the formation of the mature hormone, occurs in the kidney. The 25-hydroxyvitamin D-1 alpha-hydroxylase is a tightly regulated cytochrome P450–like mixed-function oxidase expressed in the proximal convoluted tubule cells of the kidney.

 
Ref: Molina P.E. (2013). Chapter 5. Parathyroid Gland and Ca2+ and PO4- Regulation. In P.E. Molina (Ed), Endocrine Physiology, 4e.

Q. 8

Vitamin D is absent in –

 A

Egg

 B

Milk

 C

Fish fat

 D

None

Q. 8

Vitamin D is absent in –

 A

Egg

 B

Milk

 C

Fish fat

 D

None

Ans. D

Explanation:

Ans. is ‘None’ 

Sources of Vit D :

a)    Sunlight – Vit D is synthesised by body by the action of UV rays or 7-dehydro cholestrol

b)  Foods – only of animal origin

i)   Liver

ii)  Egg yolk

iii) Milk, butter, cheese

iv) Fish fat

v) Fish liver oils (richest source)


Q. 9

Richest source of vitamin D is:  

September 2003

 A

Fish

 B

Soyabean

 C

Halibut liver oil

 D

Vegetables

Q. 9

Richest source of vitamin D is:  

September 2003

 A

Fish

 B

Soyabean

 C

Halibut liver oil

 D

Vegetables

Ans. C

Explanation:

Ans. C i.e. Halibut liver

Quiz In Between


Q. 10

Which pathway describes the synthesis of metabolically active vitamin D:

March 2004

 A

Liver – skin – kidney

 B

Gut – kidney – liver

 C

Kidney – liver – skin

 D

Skin – liver – kidney

Q. 10

Which pathway describes the synthesis of metabolically active vitamin D:

March 2004

 A

Liver – skin – kidney

 B

Gut – kidney – liver

 C

Kidney – liver – skin

 D

Skin – liver – kidney

Ans. D

Explanation:

Ans. D i.e. Skin – liver – kidney


Q. 11

Vitamin D is not found in sufficient quantities in:

March 2012

 A

Fish fat

 B

Egg

 C

Halibut liver oil

 D

Milk

Q. 11

Vitamin D is not found in sufficient quantities in:

March 2012

 A

Fish fat

 B

Egg

 C

Halibut liver oil

 D

Milk

Ans. D

Explanation:

Ans: D i.e. Milk

Foodstuff and vitamin-D content

  • Fish fat contains 5-30 microgram/100 gram of vitamin D
  • Egg contains 1.25-1.5 microgram/100 gram of vitamin D
  • Halibut liver oil contains 500-10,000 microgram/100 gram of vitamin D
  • Milk has 0.1 microgram/100 gram

Q. 12

Vitamin D is maximally found in:         

March 2011

 A

Milk

 B

Egg

 C

Green leafy vegetables

 D

Halibut liver oil

Q. 12

Vitamin D is maximally found in:         

March 2011

 A

Milk

 B

Egg

 C

Green leafy vegetables

 D

Halibut liver oil

Ans. D

Explanation:

Ans. D: Halibut liver oil

Halibut liver oil is the richest source of vitamin D (and also vitamin A)

Vitamin D

  • It is a fat-soluble vitamin that is naturally present in very few foods, added to others, and available as a dietary supplement.
  • It is also produced endogenously when ultraviolet rays from sunlight strike the skin and trigger vitamin D synthesis.
  • Vitamin D obtained from sun exposure, food, and supplements is biologically inert and must undergo two hydroxylations in the body for activation.
  • The first occurs in the liver and converts vitamin D to 25-hydroxyvitamin D [25(OH)D], also known as calcidiol.
  • The second occurs primarily in the kidney and forms the physiologically active 1,25-dihydroxyvitamin D [1,25(OH)2D], also known as calcitriol.
  • Vitamin D promotes calcium absorption in the gut and maintains adequate serum calcium and phosphate concentrations to enable normal mineralization of bone and to prevent hypocalcemic tetany.
  • It is also needed for bone growth and bone remodeling by osteoblasts and osteoclasts.
  • Without sufficient vitamin D, bones can become thin, brittle, or misshapen. Vitamin D sufficiency prevents rickets in children and osteomalacia in adults.
  • Vitamin D has other roles in the body, including modulation of cell growth, neuromuscular and immune function, and reduction of inflammation.
  • Serum concentration of 25(OH)D is the best indicator of vitamin D status.
  • It reflects vitamin D produced cutaneously and that obtained from food and supplements and has a fairly long circulating half-life of 15 days.
  • Serum 25(OH)D levels do not indicate the amount of vitamin D stored in body tissues.
  • In contrast to 25(OH)D, circulating 1,25(OH)2D is generally not a good indicator of vitamin D status because it has a short half-life of 15 hours and serum concentrations are closely regulated by parathyroid hormone, calcium, and phosphate.
  • Levels of 1,25(OH)2D do not typically decrease until vitamin D deficiency is severe
  • The flesh of fatty fish (such as salmon, tuna, and mackerel) and fish liver oils (Shark-30-100 microgram/ 100 gram; Cod-200-750 microgram/ 100 gram; Halibut-500-10,000 microgram/ 100 gram) are among the best sources.
  • Small amounts of vitamin D are found in beef liver, cheese, and egg yolks.
  • Vitamin D in these foods is primarily in the form of vitamin D3 and its metabolite 25(OH)D3.

Q. 13

All are true regarding vitamin D, except ‑

 A

Sunlight is important

 B

1-hydroxylation in liver

 C

Active form is calcitriol

 D

RDA is for children is 400 IU

Q. 13

All are true regarding vitamin D, except ‑

 A

Sunlight is important

 B

1-hydroxylation in liver

 C

Active form is calcitriol

 D

RDA is for children is 400 IU

Ans. B

Explanation:

Ans. is ‘b’ i.e., 1-hydroxylation in liver

Quiz In Between



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