Vitamin D is an essential hormone needed for normal calcium and phosphate homeostasis. Unfortunately, over 100 million Americans, including children have a vitamin D deficiency.
Vitamin D is important for proper maturation and development of bone. The important of vitamin d is also discussed in immunity, cancer and other conditions. Some people claim vitamin D to be a miracle hormone.
Vitamin D is essential for strong bones because it helps the body use calcium from the diet. The main action of vitamin D is absorption of calcium and phosphate from the intestines.
Where does vitamin D come from?
Vitamin D comes from diet and exposure to the sun! Vitamin D is also naturally found in fish. Both vitamin D from diet and converted by sun light and not biologically active. They need to be activated before achieving all of its function.
What foods are rich in vitamin D?
Cod liver oil is the best source of vitamin D but it has a very bad taste. Fish such as salmon, tuna, sword fish and oysters contain some vitamin D, but to obtain the required amount a person would need to eat a lot of it. Other foods rich in vitamin D include ham, salami, sausages, fortified cereals, mushrooms, eggs, dairy products such as milk and cheese (not as rich in vitamin D), and supplements.
3.5 ounces of cold liver oil contain 8500 international units of vitamin D. In 3.5 ounces of raw herring there are 900 international units of vitamin D. Salmon (3.5 ounces) contains 600 international units. A multivitamin containing vitamin D has 400 international units. One cup of milk contains only 100 international units.
How about the sun?
Known as the sunshine vitamin, vitamin D is produced by the body in response to skin being exposed to sunlight. Cholesterol derived precursor (pro-vitamin D3) is formed in the liver and stored in the skin. Pro-vitamin D is activated by ultraviolet light, yet it still requires more steps for activation. Ten to fifteen minutes per day is enough exposure for light skinned individuals. Darker skinned people need more exposure. If you are in the sun for 15 minutes, you may get about 10,000 units of vitamin D.
How does metabolic activation occur?
Metabolic activation occurs by adding two hydroxyl groups. One is at the carbon 25 in the liver and one at the carbon 1 in the kidney. Hydroxylation to 25 vitamin D3 occurs in the liver. The big organ takes the big number – 25. 25(OH) – Vitamin D3. à Another hydroxylation occurs in the kidneys. The small organ takes the small number – 1. The result will be 1,25 (OH)2 - Vitamin D3. This is the active form of vitamin D and works mainly on the intestines and bones. Activation of vitamin D is controlled by the PTH hormone (parathyroid hormone). Any problem with this activation process of vitamin D will lead to deficiency due to an insufficient diet, lack of sun exposure, liver or kidney disease. Dilantin impairs metabolism of vitamin D in the liver.
Once the vitamin D is activated in the kidneys it is then carried by a carrier with vitamin D binding protein to its target tissue. The target cells have receptors and vitamin D is bound to these receptors which lead to conformational changes in the nucleus. The receptors are found in the intestines, the bones and maybe in the kidneys.
The primary function of vitamin D is absorption of calcium and phosphates from the intestines. Vitamin D influences activity of the cells that remodel and build bone. 1,25 dihydroxy vitamin D stimulated RANKL expression. RANKL (ligand) is secreted by osteoblasts and binds to the RANK receptor on osteoclast precursor and mature osteoclast cells.
Osteoclast activation stimulates bone resorption. In the kidneys, vitamin D helps in regulation of calcium absorption in the tubules. Elevated vitamin D decreases the formation of parathyroid hormone. Parathyroid hormone stimulates the conversation of 25 vitamin D3 to 1,25 hydroxyvitamin D in the proximal tubules of the kidneys by stimulating 1α – hydroxylase enzymes.
Receptors are also found in the cardiovascular and immune systems. So, what are the stimuli for the release of 1,25 vitamin D? These stimuli include low serum calcium concentration, low serum phosphate concentration, elevated PTH levels.
What are the factors that inhibit the production of vitamin D3?
These factors are decreased parathyroid hormones and elevated serum calcium and elevated serum phosphate.
What are the net effects of vitamin D on plasma calcium and phosphate concentration?
The net effects are increased plasma calcium and increased plasma phosphate.
Rickets
There are three types of rickets related to vitamin D; vitamin D deficient, vitamin D dependent, vitamin D resistant.
