Vitamin D Intake From Sunlight, Food And Supplementation: Myths And Realities

Exposing yourself to the sun for 15 to 20 minutes a day during the coolest hours of the day guarantees a sufficient intake of vitamin D. You can also eat foods rich in this vitamin, which is essential for the body (oily fish, fortified dairy products, etc.). However, a portion of the population is thought to suffer from a vitamin D deficiency. But beware of the risks associated with inconsiderate supplementation.

In recent decades, interest in vitamin D has grown exponentially, mainly because its deficiency has been associated with multiple diseases and because there appears to be a high deficiency of this micronutrient in the general population.

Since the identification of the chemical structure of vitamin D in 1930, research into its functions in the body has progressed considerably. 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 then 1,25-hydroxyvitamin D (1,25(OH)2D) , research expanded and focused on its role in the development of immune system diseases, infections, cancer and chronic non-communicable diseases (cardiovascular diseases, obesity, type 2 diabetes, etc.)

There is no longer any doubt that vitamin D is involved in the regulatory mechanisms of the immune system. Indeed, its deficiency is associated with a worse prognosis of Covid-19 infection.

A growing deficit

Current epidemiological data indicate vitamin D deficiency in 40% of the European population, 24% of the American population and 37% of the Canadian population. These figures are very high and may give rise to concern.

In the population, the groups most at risk of hypovitaminosis D are pregnant women, children, the elderly, obese people, individuals with darker skin and those who have little exposure to the sun.

Humans can cover part of their vitamin D needs through skin synthesis from cholesterol, if they get enough exposure to the sun. It is difficult to specify the minimum recommended duration, as it depends on factors such as season, time of day, geographical latitude, age and skin phototype .

A group of experts from the Spanish Society for Research on Bone and Mineral Metabolism recommends that the Caucasian population get 15 minutes of daily sun exposure on the face and arms between March and October. In the elderly and patients with osteoporosis, it is advisable to increase this duration to 30 minutes.

In both situations, it is advisable to use a protection factor [via the use of sunscreen, Editor's note] between 15 and 30, depending on the latitude and the intensity of UV (ultraviolet) radiation.

However, dietary intake is also necessary. Good dietary sources of vitamin D include oily fish (especially salmon and trout), non-fat dairy products, and fortified margarines and vegetable drinks.

In France, the National Agency for Food Safety (ANSES) reminds us that there are two ways to cover daily vitamin D requirements: exposing yourself to the sun "15 to 20 minutes in the late morning or in the afternoon" and consuming foods rich in vitamin D such as oily fish (herring, sardines, salmon, mackerel), dairy products enriched with vitamin D, certain mushrooms, etc., Editor's note.

So what is the reason for the increasing vitamin D deficiency? Factors such as the increasing use of sunscreens or the downward trend in the consumption of high-fat foods could contribute to this situation.

When should you take vitamin D supplements?

Currently, vitamin D levels are assessed by serum 25(OH)D concentration , although results may vary depending on the testing method.

In general, values ​​above 20 nanograms per milliliter (ng/mL) are considered optimal for the general population, and above 30 ng/mL for people over 65 years of age, patients with bone disorders or those on chronic drug treatment ( corticosteroids, anticonvulsants [prescribed for epilepsy, editor's note], etc.).

Values ​​between 12 and 20 ng/mL are considered insufficient and vitamin D deficiency is referred to if they are below 12 ng/mL. The risks of hypervitaminosis D, associated with 25(OH) D levels above 100 ng/mL, also raise serious concerns.

The need or advisability of prescribing vitamin D supplementation to individuals with adequate serum 25(OH)D levels, in order to enhance their immune response, is a controversial issue.

In this regard, a recent meta-analysis evaluated supplementation of 1,000 to 2,000 international units (IU) per day in healthy people and concluded that it did not lead to significant improvements in immune system function. Nor was it useful as a tool for preventing acute respiratory diseases, influenza, Covid-19 virus infection, etc.

However, other authors have observed positive effects of this intervention in people with respiratory diseases , especially those with a deficiency in this vitamin. The results are also contradictory regarding its benefits in patients with metabolic diseases and neurodegenerative disorders.

The dangers of hypervitaminosis

Vitamin D intake through diet is not likely to cause problems, but indiscriminate supplementation without a deficiency can lead to chronic toxicity.

For example, vitamin D supplementation at doses greater than 4,000 IU/day for prolonged periods could increase serum 25(OH)D concentrations to values ​​greater than 50 ng/mL, with a 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 more severe cases, it can lead to polyuria (excessive urine production), polydipsia (abnormally increased thirst), kidney failure, ectopic (out of place) calcifications, depression, confusion, bone pain, fractures and kidney stones.

In recent years, due to increased consumption of supplements, cases of toxicity have increased sharply. The US National Toxicity Data System report states that this overexposure to vitamin D has led to an increase in cases of hypervitaminosis, from an annual average of 196 for the period 2000-2005 to 4535 for the following five years.

Caution must be exercised

In conclusion, interest in vitamin D has increased considerably because of its association with multiple diseases and possible deficiency, which may not yet be well established in the population. However, the effects of supplementation in non-deficient individuals are far from conclusive.

There is evidence that it is effective in reducing the severity of respiratory illness in people who are deficient in this micronutrient. But hypervitaminosis D, which results from indiscriminate supplementation, is a real risk and can be dangerous because it can cause 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 caution when taking or recommending vitamin D supplementation.

In light of current scientific evidence, the supervitamin D myth is collapsing and the reality that more research is needed is taking over.

This article was first published by Carmen Vidal Carou, Maria Teresa Veciana Nogues, Natalia Toro Funes and Sonia Sanchez Perez in The Conversation.