Vitamin D

1. What is Vitamin D and how does it operate in our body?

Vitamin D is a general name given to a group of liposoluble compounds that are essential for maintaining the mineral balance in the body. It is also known as calciferol. Its main forms are known as:

• Vitamin D3 (cholecalciferol: of animal origin), formed by the action of ultraviolet light in the vitamin D precursors in the skin;

• Vitamin D2 (ergocalciferol: of vegetable origin) less potent than D3.

Vitamin D is metabolized in the liver to form 25-hydroxyvitamin D or 25(OH)D; this is the most abundant metabolite in the circulation and serves as an indicator of the amount of vitamin D.

In the kidney, 25(OH)D is converted to 1,25 – dihydroxyvitamin D or 1,25(OH)2D or Calcitriol, which is the hormonal form of vitamin D. Its secretion is stimulated by a decreased amount of serum calcium or the increase of the parathyroid hormone (PTH).

The main function of 1,25(OH)2D is to maintain the calcium homeostasis, promoting bone mineralization, by increasing the calcium and phosphorus from the diet, at an intestinal level, and mobilizing calcium from bones.

Recent studies show that apart from kidney cells, many others have the enzyme that synthesizes 1,25(OH)2D.

Many of these cells also possess vitamin D receptors, which facilitates the action of the active hormone. In the extrarenal system, calcitriol controls the expression of over 200 genes involved in cellular differentiation, replication and immunity. These features of vitamin D are currently being actively researched. 

2. What are the sources of Vitamin D?

The largest source of vitamin D in the human body is endogenous production through exposure to sunlight. Exposing your arms and legs to sunlight for 5 to 30 minutes between 10 am and 3 pm, twice a week, is often enough. Sunscreen, so important for protection against skin cancer, blocks the formation of this vitamin.

Other natural sources rich in vitamin D are fish liver oils and saltwater fish such as sardines, herring, salmon and Atlantic bonito. Eggs, meat, milk and butter also contain small amounts. Plants are weak sources and fruit and nuts have no vitamin D. The amount of vitamin D in human milk is insufficient to meet the children’s needs.

3. What is the amount of Vitamin D needed?

Currently, the proposed levels of Total 25-hydroxyvitamin D measured in the serum are:

• Deficiency <10 ng/ml (0-25 nmol/L)

• Insufficiency 10-30 ng/ml (25-75 nmol/L)

• Sufficiency 30-100 ng/ml (75-250 nmol/L)

• Toxicity >100 ng/ml (>250 nmol/L)

It is estimated that one billion people worldwide do not reach the optimal minimum concentration of 30 ng/ml. However, experts believe that the population of all age groups may need more vitamin D than current guidelines recommend.

4. Who is at risk of Vitamin D deficiency?

• People with limited sun exposure;

• People with dark skin;

• Adults over 50;

• People with conditions that cause fat malabsorption, such as pancreatitis, cystic fibrosis, celiac disease, and resection of the gall bladder;

• People with liver or kidney disorders  and enzyme deficiencies;

• People living in Nordic climates during the winter;

• Obese people.

5. Is it true that maintaining sufficient levels of Vitamin D helps prevent certain diseases and maintain good general health?

The role of vitamin D in regulating circulating levels of calcium and phosphorus to ensure bone mineralization is well known. The diseases classically associated with vitamin D deficiency are rickets (in children) and osteomalacia (in adults).

Currently, we know that vitamin D is important for the normal functioning of many types of cells, but the needs of each tissue and response to vitamin D vary.

The various effects of vitamin D are mediated by receptors regulating over 200 genes. In addition to the receptors in the intestine and bone, there were also identified vitamin D receptors in the brain, prostate, breast, colon, immune system cells, smooth muscle and cardiomyocytes.

An increasingly large number of studies associate vitamin D deficiency with an increased risk of developing several diseases, namely:

• Type 1 diabetes;
• Breast cancer and other cancers (in post-menopausal women);
• Fractures in the elderly, as complications from osteoporosis;
• Cardiovascular disease, hypertension, and death from myocardial infarction and stroke;
• Multiple sclerosis and, eventually, other autoimmune diseases.

6. What are the recommendations for Vitamin D dosing?

This test is recommended for those at risk of Vitamin D deficiency who, should their levels be below 30 ng/ml (75 nmol/L), should take vitamin supplements. It is recommended that the dosage of 25(OH)D lasts at least 3 months after beginning treatment with supplements (to reach stable levels).

Experts also recommend the use of a test that determines both 25(OH)D2  and 25(OH)D3, i.e., a test that measures the Total 25-hydroxyvitamin D, especially in countries where both forms of vitamin D are available in dietary supplements.