We are almost at the end of autumn and, according to experts, this is the time when levels of 25-hydroxyvitamin D (the form of measuring vitamin D) are generally higher. If we had to get a check-up, this would be the time, although in reality, it is only recommended if we are at risk of deficiency. So why does everyone seem lately determined to check their levels and/or take supplements even on their own and without medical supervision?
"The widespread concern about vitamin D in the healthy population is exaggerated," states Dr. María Cortés Berdonces, endocrinologist and coordinator of the Mineral and Bone Metabolism Group of the Spanish Society of Endocrinology and Nutrition (SEEN). "A certain alarm has been generated above reality," confirms Dr. Miguel Ángel Acosta, family physician and member of the Spanish Society of Family and Community Medicine (semFYC). In a topic where there are many controversial points among professionals, it is even more important to highlight the aspects that are not debated, and this is one of them.
This does not mean that vitamin D does not deserve all our attention; in fact, it is a "necessary nutrient, essential in our diet, which is involved in many processes," emphasizes Acosta. Its discovery a hundred years ago made it the paradigm of rickets treatment, but in the 90s, it was found that its potential was much greater.
"Successive discoveries showed that the hormone D (calcitriol) could be synthesized from 25-hydroxyvitamin D not only in the kidney but in most cells of the body, and observational and retrospective studies in humans associate low levels of 25(OH)D with many diseases," explains Dr. José Manuel Quesada Gómez, an endocrinology specialist and recognized expert in vitamin D. This led to the idea that supplementing this vitamin could be beneficial not only to prevent skeletal conditions but also other pathologies such as cardiovascular diseases, cancer, lung infections, diabetes, autoimmune diseases, etc.
However, so far, no randomized clinical trial has demonstrated these benefits in the healthy population except, as Quesada states, in autoimmune cases and in delaying cellular aging. At the same time, symptomatic vitamin D deficiency, explains Dr. Acosta, is only justified by severe deficiency or by the presence of certain health problems. In summary, it should not be a concern or require intervention in the healthy population.
We then move on to the next question, is there a deficiency? And this is where the dance of figures begins. Quesada talks about the widespread sensation of a "pandemic" persisting and refers to the analysis conducted by Hilger on global patterns of vitamin D status, concluding that 88% of the world's population has less than 30 ng/mL, 37% less than 20 ng/mL, and "up to an impressive" 7% less than 10 ng/mL. "If we use those cutoff points, we would have to say that yes, there is a considerable deficiency in the population," affirms the expert.
And if the cutoff points were different? "The perception of an epidemic is largely due to the lack of consensus on diagnostic thresholds and the increase in analytical determinations and supplementation without clinical indication," explains Dr. Cortés Berdonces.
"Reference values are still under study," explains family physician Miguel Ángel Acosta, who regrets the application, until recently, of values that do not correspond to the Spanish population. He is in favor of personalizing them, measuring levels in relation to the patient's pathology, although "it is true that we are becoming stricter in defining deficiency, lowering the thresholds even to 10."
The extremes remain more or less clear, but the zone of uncertainty in intermediate values can lead to overdiagnosis. The Endocrine Society updated its clinical guideline in 2024 and chose to eliminate any recommendation to define insufficiency or sufficiency due to the "lack of robust clinical evidence linking specific levels of 25(OH)D with health benefits in healthy adults," as explained by SEEN.
Dr. Quesada believes that most randomized clinical trials on vitamin D had significant design limitations, implementation, and interpretation, hence the lack of consensus, and considers that the recent proposal by the Endocrine Society "has enhanced 'empirical' decision-making (based on experience)," although he affirms that it does not change the perception of widespread deficiency in the population.
The same uncertainty hovers over the recommended vitamin D doses, and even at the upper limit of tolerable intake. Quesada recalls in any case that vitamin D toxicity "is almost always due to excessive intake of this vitamin through inadequately controlled supplements."
The same applies to screening, although the number of determinations has increased exponentially in recent years, no current clinical guideline recommends routine systematic measurement in healthy adults (nor the prescription of supplements). "The benefit of supplementation seems to be limited to individuals with proven deficiency or specific risk factors," as explained by SEEN.
It is the peculiarity of vitamin D, and also what makes it not really a vitamin: it is synthesized in the skin by the action of ultraviolet rays, and not even excessive exposure is necessary to maintain normal levels, with 10-12 minutes on the face, forearms, and hands several times a week being sufficient, as stated by Manuel Fernández Lorente, a dermatologist from the Jaén Group.
This situation means that, a priori, countries with higher latitudes (less UVB radiation) are at a disadvantage, but the most recent epidemiological data reveal that northern countries paradoxically have higher levels of vitamin D than Mediterranean countries. How is that possible, if we have plenty of sun? The answer lies mainly in habits: we have hours of sun, yes, but our exposure may be ineffective or limited due to work-related issues (working indoors), cultural factors (clothing that covers the whole body), or other reasons (institutionalized individuals, excessive sun avoidance). Latitude can therefore play a relevant role, but "it is not the only or main determinant in Europe," states endocrinologist María Cortés Berdonces.
What is the role of sunscreen?
In laboratory conditions, it could "reduce vitamin D production by up to 80%", says Fernández Lorente, but this does not happen in practice. Azahara Rodríguez-Luna, a pharmacology professor and researcher in Photobiology at Loyola University, cites scientific studies showing that actual application is usually insufficient to completely block synthesis and that even with topical sun protection, there is enough exposure to maintain adequate levels with healthy outdoor habits. And if necessary, she reminds us that in case of deficiency, the solution lies in supplementation, not in avoiding sun protection.
There are other factors that are indeed determining, such as pathologies or medications that limit vitamin D absorption, dark skin pigmentation, obesity, or advanced age. "An aspect that is not taken into account and that can have a great influence on 25-hydroxyvitamin D levels," adds José Manuel Quesada, "is the wide variety of xenobiotics we coexist with, such as bisphenols, which besides other harmful actions can decrease levels." His group has shown that sulfated 25-hydroxyvitamin D (phase II metabolism to eliminate toxic products from the body) even exceeds normal levels without sulfation in a healthy young population.
A diet rich in vitamin D
While 80% of this nutrient's production is attributed to the sun, the remaining 20% depends on diet. "Vitamin D is mainly found in the fats we eat," explains family doctor Miguel Ángel Acosta, "for example, in milk cream, oily fish, egg yolks, some vegetables..."
This is another possible explanation for the difference in vitamin D levels between the Nordic countries and Spain. According to Dr. Cortés Berdonces, our country has one of the lowest levels of fortified foods in Europe, with less than 2% of pre-packaged products fortified and an average vitamin D intake below European recommendations. Nordic countries, however, have mandatory fortification policies and "have significantly reduced the prevalence of population deficiency, even in latitudes with less solar radiation."
Should we implement a similar policy in view of the results?
The answer is not simple, as despite the examples mentioned, the evidence of benefit in healthy populations is less solid than in risk groups, as the SEEN reminds us. The semFYC also considers that it is preferable to consume vitamin D in its original form, which we could achieve simply by favoring the Mediterranean diet.
A change in habits, as we have seen, could solve much of the problem, to the extent that, as José Manuel Quesada explains, withdrawing treatment from a patient once the target levels have been reached causes a new decline if the person, as is to be expected, maintains the personal habits that may have contributed to the deficiency.
Quesada also reminds us that "abuse does not eliminate use." Recent cases of vitamin D overdose poisoning (in this case due to a defective over-the-counter supplement) have raised the alarm about the proven abuse of vitamin D testing and prescription, as well as the unsupervised use of supplements, but "we must be careful with the tendency to want to ban something necessary just because there is a fundamental danger of abuse."