Vitamin D Synthesis, Metabolism, and Biological Effect ☀️

Vitamin D is a fat-soluble vitamin that is essential for maintaining the health of bones and teeth, supporting the immune system, brain, and nervous system, and regulating insulin levels, among other functions. It can be obtained through certain foods, supplements, and exposure to sunlight.

𝗩𝗶𝘁𝗮𝗺𝗶𝗻 𝗗 𝗦𝗼𝘂𝗿𝗰𝗲𝘀

𝐒𝐮𝐧 𝐄𝐱𝐩𝐨𝐬𝐮𝐫𝐞

Vitamin D can be synthesized in the skin through exposure to ultraviolet B (UVB) rays from sunlight. UVB radiation triggers the conversion of 7-dehydrocholesterol, a substance found in the skin, into previtamin D3. Previtamin D3 is then converted to vitamin D3 (cholecalciferol) through body heat.  

𝐃𝐢𝐞𝐭

Vitamin D can also be obtained from dietary sources, such as fatty fish, egg yolks, and fortified foods. Vitamin D from food sources is typically vitamin D3 or vitamin D2 (ergocalciferol).  

𝗩𝗶𝘁𝗮𝗺𝗶𝗻 𝗗 𝗠𝗲𝘁𝗮𝗯𝗼𝗹𝗶𝘀𝗺

𝐓𝐫𝐚𝐧𝐬𝐩𝐨𝐫𝐭 𝐭𝐨 𝐭𝐡𝐞 𝐋𝐢𝐯𝐞𝐫

Once vitamin D is obtained from either sun exposure or diet, it is transported by vitamin D-binding protein (DBP) in the bloodstream to the liver.  

𝐌𝐞𝐭𝐚𝐛𝐨𝐥𝐢𝐬𝐦 𝐢𝐧 𝐭𝐡𝐞 𝐋𝐢𝐯𝐞𝐫

In the liver, vitamin D undergoes its first hydroxylation by the enzyme 25-hydroxylase (CYP2R1) to form 25-hydroxyvitamin D (25(OH)D). 25(OH)D is the major circulating and stored form of vitamin D in the body. 

𝐌𝐞𝐭𝐚𝐛𝐨𝐥𝐢𝐬𝐦 𝐢𝐧 𝐭𝐡𝐞 𝐊𝐢𝐝𝐧𝐞𝐲𝐬

25(OH)D can then be further hydroxylated in the kidneys by the enzyme 1α-hydroxylase (CYP27B1) to form the biologically active form of vitamin D, 1,25-dihydroxyvitamin D (1,25(OH)2D). The kidneys tightly regulate the production of 1,25(OH)2D in response to calcium and phosphate levels in the blood.  

𝗥𝗲𝗴𝘂𝗹𝗮𝘁𝗶𝗼𝗻 𝗼𝗳 𝗩𝗶𝘁𝗮𝗺𝗶𝗻 𝗗 𝗠𝗲𝘁𝗮𝗯𝗼𝗹𝗶𝘀𝗺

Several factors can regulate vitamin D metabolism, including:

1️⃣ 𝐏𝐚𝐫𝐚𝐭𝐡𝐲𝐫𝐨𝐢𝐝 𝐇𝐨𝐫𝐦𝐨𝐧𝐞 (𝐏𝐓𝐇): PTH increases the production of 1,25(OH)2D in the kidneys by stimulating the expression of 1α-hydroxylase (CYP27B1).  

2️⃣ 𝐏𝐡𝐨𝐬𝐩𝐡𝐚𝐭𝐞 𝐋𝐞𝐯𝐞𝐥𝐬: Low phosphate levels increase the production of 1,25(OH)2D in the kidneys.  

3️⃣ 𝐅𝐢𝐛𝐫𝐨𝐛𝐥𝐚𝐬𝐭 𝐆𝐫𝐨𝐰𝐭𝐡 𝐅𝐚𝐜𝐭𝐨𝐫 𝟐𝟑 (𝐅𝐆𝐅𝟐𝟑): FGF23 is a hormone produced by osteoblasts (bone cells) that inhibits the production of 1,25(OH)2D in the kidneys.  

𝗕𝗶𝗼𝗹𝗼𝗴𝗶𝗰𝗮𝗹 𝗘𝗳𝗳𝗲𝗰𝘁𝘀 𝗼𝗳 𝗩𝗶𝘁𝗮𝗺𝗶𝗻 𝗗

1️⃣ 𝐈𝐧𝐭𝐞𝐬𝐭𝐢𝐧𝐚𝐥 𝐂𝐚𝐥𝐜𝐢𝐮𝐦 𝐚𝐧𝐝 𝐏𝐡𝐨𝐬𝐩𝐡𝐚𝐭𝐞 𝐀𝐛𝐬𝐨𝐫𝐩𝐭𝐢𝐨𝐧 

1,25(OH)2D increases the absorption of calcium and phosphate from the intestine, which is essential for bone mineralization and maintenance of blood calcium levels.  

2️⃣ 𝐁𝐨𝐧𝐞 𝐇𝐞𝐚𝐥𝐭𝐡

Vitamin D promotes bone mineralization by stimulating the intestinal absorption of calcium and phosphate, and by regulating the activity of osteoblasts (bone-forming cells) and osteoclasts (bone-resorbing cells).  

3️⃣ 𝐌𝐮𝐬𝐜𝐥𝐞 𝐅𝐮𝐧𝐜𝐭𝐢𝐨𝐧

Vitamin D helps to maintain muscle strength and power.  

4️⃣ 𝐈𝐦𝐦𝐮𝐧𝐞 𝐅𝐮𝐧𝐜𝐭𝐢𝐨𝐧

Vitamin D supports the immune system and helps to protect against infections.  

5️⃣ 𝐂𝐞𝐥𝐥 𝐆𝐫𝐨𝐰𝐭𝐡 𝐚𝐧𝐝 𝐏𝐫𝐨𝐥𝐢𝐟𝐞𝐫𝐚𝐭𝐢𝐨𝐧

Vitamin D can regulate cell growth and proliferation and has been shown to have anti-cancer effects.  

6️⃣ 𝐎𝐭𝐡𝐞𝐫 𝐇𝐞𝐚𝐥𝐭𝐡 𝐄𝐟𝐟𝐞𝐜𝐭𝐬 

Vitamin D has been implicated in a variety of other health effects, including cardiovascular disease, diabetes, and autoimmune diseases. 

𝗗𝗲𝗳𝗶𝗰𝗶𝗲𝗻𝗰𝘆 𝗮𝗻𝗱 𝗧𝗼𝘅𝗶𝗰𝗶𝘁𝘆  

𝐕𝐢𝐭𝐚𝐦𝐢𝐧 𝐃 𝐃𝐞𝐟𝐢𝐜𝐢𝐞𝐧𝐜𝐲  

Vitamin D deficiency is a common problem worldwide. It can be caused by inadequate sun exposure, insufficient intake of dietary vitamin D, or impaired absorption of vitamin D from the intestine. Vitamin D deficiency can cause rickets in children and osteomalacia in adults, which are conditions that lead to soft and weak bones.  

𝐕𝐢𝐭𝐚𝐦𝐢𝐧 𝐃 𝐓𝐨𝐱𝐢𝐜𝐢𝐭𝐲  

Vitamin D toxicity is a rare condition that occurs when a person has excessive levels of vitamin D in their body, most commonly due to taking high doses of vitamin D supplements for an extended period. Vitamin D toxicity can lead to a buildup of calcium in the blood (hypercalcemia), which can cause a variety of symptoms, including:  

𝐍𝐚𝐮𝐬𝐞𝐚 𝐚𝐧𝐝 𝐕𝐨𝐦𝐢𝐭𝐢𝐧𝐠

Excessive levels of calcium can irritate the gastrointestinal tract, leading to nausea and vomiting.  

𝐋𝐨𝐬𝐬 𝐨𝐟 𝐚𝐩𝐩𝐞𝐭𝐢𝐭𝐞

High calcium levels can suppress appetite, leading to weight loss and malnutrition.  

𝐂𝐨𝐧𝐬𝐭𝐢𝐩𝐚𝐭𝐢𝐨𝐧

Hypercalcemia can slow down the digestive system, causing constipation.  

𝐖𝐞𝐚𝐤𝐧𝐞𝐬𝐬 𝐚𝐧𝐝 𝐅𝐚𝐭𝐢𝐠𝐮𝐞

Elevated calcium levels can affect muscle function, leading to weakness and fatigue.  

𝐅𝐫𝐞𝐪𝐮𝐞𝐧𝐭 𝐔𝐫𝐢𝐧𝐚𝐭𝐢𝐨𝐧 

High calcium levels can impair kidney function, causing frequent urination and dehydration.  

𝐊𝐢𝐝𝐧𝐞𝐲 𝐒𝐭𝐨𝐧𝐞𝐬

Excess calcium can lead to the formation of kidney stones, which can cause severe pain and urinary problems.  

𝐂𝐨𝐧𝐟𝐮𝐬𝐢𝐨𝐧 𝐚𝐧𝐝 𝐃𝐢𝐬𝐨𝐫𝐢𝐞𝐧𝐭𝐚𝐭𝐢𝐨𝐧

Hypercalcemia can affect the nervous system, leading to confusion and disorientation.  

In severe cases, 𝐤𝐢𝐝𝐧𝐞𝐲 𝐝𝐚𝐦𝐚𝐠𝐞 𝐚𝐧𝐝 𝐡𝐢𝐠𝐡 𝐛𝐥𝐨𝐨𝐝 𝐩𝐫𝐞𝐬𝐬𝐮𝐫𝐞.  

𝗥𝗲𝗰𝗼𝗺𝗺𝗲𝗻𝗱𝗮𝘁𝗶𝗼𝗻𝘀

The recommended daily intake (RDI) of vitamin D varies depending on age. Here are some general guidelines:

▪️𝐈𝐧𝐟𝐚𝐧𝐭𝐬 (𝐮𝐩 𝐭𝐨 𝟏 𝐲𝐞𝐚𝐫): 𝟒𝟎𝟎 𝐈𝐔 

▪️𝐂𝐡𝐢𝐥𝐝𝐫𝐞𝐧 (𝟏-𝟏𝟑 𝐲𝐞𝐚𝐫𝐬): 𝟔𝟎𝟎 𝐈𝐔  

▪️𝐀𝐝𝐨𝐥𝐞𝐬𝐜𝐞𝐧𝐭𝐬 𝐚𝐧𝐝 𝐀𝐝𝐮𝐥𝐭𝐬 (𝟏𝟒-𝟕𝟎 𝐲𝐞𝐚𝐫𝐬): 𝟔𝟎𝟎 𝐈𝐔  

▪️𝐀𝐝𝐮𝐥𝐭𝐬 (𝐨𝐯𝐞𝐫 𝟕𝟎 𝐲𝐞𝐚𝐫𝐬): 𝟖𝟎𝟎 𝐈𝐔   

This comprehensive overview of vitamin D provides valuable insights for us (nutritionists and healthcare professionals) helping us to guide our patients in achieving and maintaining adequate vitamin D levels for overall well-being.