In recent decades, interest in vitamin D has increased exponentially, mainly because its deficiency has been associated with multiple diseases and there seems to be a high deficiency of this micronutrient in the general population.
Since the chemical structure of vitamin D was identified in 1930, there have been significant advances in research into its functions in the body. Initially, studies focused on the role of this compound and its metabolites in calcium homeostasis and bone metabolism.
Later, with the discovery of 25-hydroxyvitamin D (25(OH)D) in 1968 and 1,25-hydroxyvitamin D (1,25(OH)2D) later, research expanded and focused on the role it plays in the appearance of immunological diseases, infections, cancer and chronic non-communicable diseases (cardiovascular diseases, obesity, type 2 diabetes, etc.).
There is currently no doubt that vitamin D plays a role in the regulation of the immune system. In fact, its deficiency is linked to a worse prognosis of Covid-19 infection .
A growing deficit
Current epidemiological data show that 40% of the European population, 24% of the American population, and 37% of the Canadian population are deficient in this vitamin. These are very high figures that may cause some alarm. The population groups most at risk of developing hypovitaminosis are pregnant women, children, the elderly, obese people, individuals with darker skin tones, and those with little exposure to sunlight.
Humans can cover part of their vitamin D needs through cutaneous synthesis from cholesterol, if they are sufficiently exposed to sunlight. It is difficult to specify the minimum recommended time, as it depends on factors such as the season of the year, time of day, geographic latitude, age, or skin phototype.
A panel of experts from the Spanish Society for Bone and Mineral Metabolism Research recommends that the Caucasian population have 15 minutes of daily sun exposure on the face and arms between March and October. For the elderly and patients with osteoporosis, the advice is to extend this to 30 minutes. In both cases, a protection factor of between 15 and 30 should be used, depending on the latitude and the intensity of UV (ultraviolet) radiation.
However, dietary intake is also necessary. Good dietary sources of this substance include oily fish (especially salmon and trout), non-skimmed dairy products, margarine, and fortified vegetable drinks.
So what could be the reason for the growing vitamin D deficiency? Factors such as the increasing use of sunscreen or the downward trend in the consumption of fatty foods could contribute to this situation.
When should you take vitamin D supplements?
Currently, to assess vitamin D levels, the serum concentration of 25(OH)D is determined, although the results may vary depending on the analytical method.
In general, values greater than 20 nanograms per milliliter (ng/mL) are considered optimal for the general population, and greater than 30 ng/mL for people over 65 years of age, patients with bone conditions or with chronic pharmacological treatments (corticosteroids, anticonvulsants, etc.).
Values between 12 and 20 ng/mL are considered insufficient, and below 12 ng/mL, are deficient. There is also clear concern about the risks of hypervitaminosis D, which is associated with 25(OH)D levels above 100 ng/mL.
The need or convenience of prescribing vitamin D supplements to people with adequate serum 25(OH)D levels, with the aim of improving their immune response, is a controversial issue.
In this regard, a recent meta-analysis evaluated a supplementation of 1,000 – 2,000 international units (IU) per day in healthy individuals and concluded that it did not lead to significant improvements in immune system function. It was also not useful as a prevention tool for acute respiratory diseases, influenza, Covid-19 infection, etc.
However, other authors have observed positive effects of this intervention in individuals with respiratory diseases, especially in those with vitamin deficiency. There are also contradictory results about its benefits in patients with metabolic diseases and neurodegenerative conditions.
The dangers of hypervitaminosis
The intake of vitamin D present in food is unlikely to cause problems, but indiscriminate supplementation, without a deficiency to justify it, can cause chronic toxicity. Thus, for example, the administration of vitamin D supplements in doses greater than 4,000 IU/day for prolonged periods could raise the serum concentration of 25(OH)D to values greater than 50 ng/ml, with the consequent risk of hypervitaminosis.
The most characteristic manifestation of hypervitaminosis D is hypercalcemia, characterized by the appearance of gastrointestinal symptoms (anorexia, nausea, vomiting, constipation, etc.), weakness, and fatigue. In the most severe cases, it can cause polyuria (excessive urine production), polydipsia (abnormal increase in thirst), kidney failure, ectopic calcifications (out of their proper place), depression, confusion, bone pain, fractures, and kidney stones.
In recent years, due to the increased consumption of supplements, cases of toxicity have increased significantly. Thus, the report of the National Toxicity Data System of the United States indicates that this overexposure to vitamin D has caused an increase in cases of hypervitaminosis, rising from an annual average of 196 in the period 2000-2005 to 4,535 in the following five years.
We must act with caution.
In conclusion, interest in vitamin D has increased significantly due to its association with multiple diseases and a possible, but perhaps not well-established, deficiency in the population. However, the effects of supplementation in people who do not have a deficiency are by no means conclusive.
There is evidence of its effectiveness in reducing the severity of respiratory diseases in people with a deficiency of the micronutrient in question. Hypervitaminosis D, resulting from indiscriminate supplementation, is a real risk and can be dangerous, causing hypercalcemia and other health problems.
It is therefore essential to approach this issue with caution, always acting on the basis of scientific evidence and with prudence when taking or recommending a vitamin D supplement.
With current scientific evidence, the myth of super vitamin D is debunked and the reality that more research is needed prevails.