Functions of vitamin D

FUNCTIONS OF VITAMIN D

The main role of vitamin D is to regulate the intestinal absorption of calcium and phosphorus, promoting normal bone formation and mineralization. Furthermore, vitamin D is involved in the process that guarantees normal muscle contractility and interacts with the immune system, exerting an immuno-modulating effect.
Vitamin D VOLCHEM cholecalciferol or vitamin D is the same as that which is naturally formed in the skin from 7-dehydrocholesterol. Taken orally, it is transformed first into 25-hydroxy vitamin D (25(OH)D) in the liver and then into 1,25-dihydroxy vitamin D (1,25(OH)2D), the active form of vitamin D, in kidneys. This regulates the concentration of calcium and phosphorus in the blood by keeping it around 10 mg of calcium and 5 mg of phosphorus/100 ml of blood. These minerals are essential not only for maintaining a robust bone structure but also for ensuring normal nerve transmission and cardiac and muscular contractility. Vitamin D shows immune activity by strengthening the host's defenses against pathogens and simultaneously limiting any excessive specific response. 
 

 












NORMAL BLOOD VITAMIN D 
The measurement of the serum concentration of vitamin D in the form of 25(OH)D is used in clinical practice as an expression of the body's "vitamin D reserve" because the 25(OH)D form is the most represented in circulation and is relatively stable in serum with a half-life of 2–3 weeks. Its activated form, 1,25(OH)2D, has a half-life of approximately 15 hours, and a circulating concentration (pg/mL) one thousand times lower than that of its precursor 25(OH)D.
 
Normal blood values for vitamin D are (NB - 1 mcg = 1 µg = 1000 ng):
• at 20 ng/mL optimal calcium absorption and PTH control are guaranteed
in almost all of the analyzed sample.
• levels above 20 ng/mL do not add appreciable clinical advantages.
Recommended values without adverse effects are: 20-40 ng/ml.
 
FREQUENTLY ASKED QUESTIONS ABOUT VITAMIN D
 
1. What is vitamin D?
Vitamin D is a fat-soluble prohormone produced in the skin by the action of UVB rays. Sun exposure represents the main natural source of vitamin D, however, it can also be taken in through the diet.
 
2. What does a vitamin D deficiency cause?
In many people, vitamin D deficiency is completely asymptomatic. Severe deficiency causes rickets in children and osteomalacia in adults. Furthermore, conditions of hypovitaminosis D can also lead to reduced muscle strength and widespread pain.
 
3. How can the availability of vitamin D in the human body be assessed?
Measurement of vitamin D in the serum 25(OH)D form is the most accurate method for estimating the vitamin D reserve status in the body (Binkley N et al. 2010).
 
4. When is vitamin D dosage necessary?
The dosage should be performed only in the presence of specific risk conditions, on the advice of the doctor. Vitamin D dosing [25(OH)D dosing] extended to the general population is inappropriate.
 
5. What are natural sources of vitamin D?
Effective exposure to sunlight and dietary intake of vitamin D are the main factors determining serum 25(OH)D levels.
 
- SUN EXPOSURE
The skin produces vitamin D stimulated by UV rays of wavelength between 290 and 315 nm. Exposure must be direct, not through the glass.
Sun exposure does not cause hypervitaminosis because the excess vitamin is destroyed in the skin.
Sun exposure in a swimsuit leading to a slight erythema is equivalent to the intake of 10,000 - 25,000 IU of vitamin D (NB - NRV = 5 mcg = 200 IU).
In young adults, exposure to the summer sun without sunscreen of about 25% of body surface area (face and arms) for 15 minutes 2-3 times a week is equivalent to an oral dose of 25 mcg or 1000 IU of vitamin D.
On the market there are devices that generate UV rays (tanning lamps) which emit from 2 to 6% of beta-type radiation with a wavelength useful for the production of Vitamin D.
The results of the studies are not unequivocal and non-negligible risks emerge on the development of skin neoplasms. The FDA recommends caution: it supports the oral intake of vitamin D rather than exposure to tanning lamps.

- DIETARY INTAKE
Most foods contain low amounts of vitamin D, so diet alone cannot be considered an adequate source. Vitamin D is relatively stable and is little affected by storage and cooking. The table below shows a list of foods with corresponding vitamin D content.
 

§UI: International Units *Adapted from Saggese et al. 2018

6. In which cases is the use of vitamin D expected to prevent or treat deficiency?
The use of vitamin D, regardless of the measurement of 25(OH)D, is expected:
• in elderly residents of health-care residences
• in pregnant or breastfeeding women
• in people suffering from osteoporosis from any cause or established osteopathies for which remineralizing therapy is not indicated
 
While use after measuring 25(OH)D is expected:
• in people with serum 25(OH)D levels < 20 ng/mL and symptoms attributable to hypovitaminosis (asthenia, myalgias, diffuse or localized pain, frequent unmotivated falls)
• in people diagnosed with hyperparathyroidism secondary to hypovitaminosis D
• in people suffering from osteoporosis of any cause or established osteopathies for whom the correction of hypovitaminosis should be preparatory to the start of remineralizing therapy.
• in case of long-term therapy with drugs interfering with the metabolism of vitamin D (antiepileptics, glucocorticoids, antiretrovirals, antifungals, etc.).
• in case of diseases that can cause malabsorption in adults (cystic fibrosis, celiac disease, Crohn's disease, bariatric surgery, etc.).
 
7. Why is it important to stick to the correct dosage? What adverse health effects can an excess of vitamin D cause? Why is it always important to consult your doctor before taking vitamin D medications?
Vitamin D is fat-soluble and therefore tends to accumulate in the human body. Taking high doses for long periods can cause serious health effects, for example hypercalcemia and nephrolithiasis (kidney stones), as well as increasing the risk of fractures in the months following the administration of vitamin D boluses (Sanders KM et al. 2010) and the risk of some cancers in populations with 25(OH)D levels > 40-50 ng/mL (IOM 2011). For these reasons it is always important to consult your doctor before taking vitamin D-based medications and follow the correct prescribed dosage. Furthermore, it is necessary to remember that an overdose of vitamin D during the first 6 months of pregnancy can have toxic effects on the fetus and therefore also in this case the intake of these medicines during pregnancy cannot be done without a medical prescription.
 
8. In which cases has taking vitamin D proven ineffective?
The available scientific evidence indicates that the administration of vitamin D for cardiovascular and cerebrovascular prevention and for the prevention of tumors is ineffective and, therefore, inappropriate.
 
9. Can vitamin D-based medicines or products for special medical purposes be purchased without medical supervision or prescription?
No, as your doctor must be aware of the intake of these products in consideration of the possible adverse events.
 
HOW MUCH VITAMIN D?
 
The recommended daily dose for the general population by EFSA - EU and FDA - USA (2023) is:
 
◦ 400 IU – 10 mcg for children < 12 months
 
◦ 400 - 800 IU 10-20 mcg from 1 to 70 years, for pregnancy and breastfeeding.
 
◦ 800 IU IU 20 mcg per day > 70 years
 
◦ 1600-2000 IU 40-50 mcg per day in the presence of hypovitaminosis (< 20 ng/ml) or predisposing conditions (see par. 6 above).
 
Daily oral administration of Vitamin D3 may increase the blood vitamin D level by 1 ng/mL for every 100 IU - 2.5 mcg administered over 2 months. In subjects of normal weight, it would therefore be sufficient to take 2000 IU per day to bring even significant deficiency conditions back to normal (> 20 ng/mL) in 2-4 months.
In cases where this dose is not sufficient, even higher doses could be used (e.g. 10,000-100,000 IU) to be taken under medical supervision.
Once normal blood values have been reached (20-40 ng/ml) the maintenance dose can drop to 400 - 800 IU 10-20 mcg.
 
References
Epicentro – Istituto Superiore di Sanità – 10 dicembre 2018
Nutrients 2022, 14, 4148, SIOMMMS 
AIFA - vitamin D – 2018
Vitamin D – EFSA 2019
US 2022 Recommended Dietary Allowances (RDAs) for Vitamin D