It is one of the best-selling drugs in France, according to Social Security databases, and links have been established between vitamin D deficiency and many diseases. Some people would like to prescribe it to everyone, especially during winter. But are we really all deficient in vitamin D? Does this supposed deficit give rise to many diseases? Should you supplement? “Le Figaro” takes stock.
Vitamin D plays an essential role in the absorption of calcium by the intestine, and by the kidneys of calcium and phosphorus. It is therefore essential for good bone mineralization, and its lack can be responsible for rickets in children (with growth delays and bone deformities), and osteomalacia (“soft” bone) or (but this point is controversial) osteoporosis (less dense bone) in adults. More recently, research has identified vitamin D receptors in most tissues of the body; it would therefore be involved in numerous physiological processes and would have anti-infectious, anti-inflammatory, anti-tumor and cardiovascular effects.
It is found in certain foods, but "the body's vitamin D comes 80-90% from skin biosynthesis under the effect of ultraviolet (UV) radiation from the sun", recalled the High Council for Public Health (HCSP ) in an opinion published in 2022. It is synthesized in our skin, from a cholesterol derivative; under the effect of UV rays, it transforms into a molecule called cholecalciferol. This is metabolized by the liver into 25-hydroxy-vitamin D (the molecule sought in blood tests of vitamin D), then by the kidney into 1-25-dihydroxy-vitamin D (the active form of vitamin D). Combined with a balanced diet, simple exposure of the forearms and face for a few minutes to a few tens of minutes two to three times a week, depending on the amount of sunlight and the color of the skin, would be enough to meet the needs of most people. 'between us.
In the general population, we are talking about a deficiency below 10 nanograms per milliliter of blood of 25-hydroxy-vitamin D (1 nanogram is a billionth of a gram), and an insufficiency between 10 and 20 ng/ml. The HCSP specifies that, “for the sake of caution, the maximum concentration (…) not to be exceeded is currently set between 50 and 60 ng/ml”. In the general population, 4% to 6% of French adults are deficient, and 40% to 50% are deficient. And 80% are below 30 ng/ml, and considered in deficit by some experts. Can such a large part of the population really be deficient in such an essential vitamin without worrying the public authorities?
In reality, these thresholds come from opinions of groups of experts on relatively fragile bases, and they are contested, in particular that of 30 ng/ml. Especially since one of the popes of vitamin D in the United States, editor of American guidelines on the subject, was singled out for his links with vitamin D manufacturers. In 2011, a few months after a report from the American Academy of Medicine who estimated that the vast majority of Americans did not need vitamin D supplementation or dosing, Boston University endocrinologist Michel Holick oversaw a report on behalf of the Endocrine Society to the findings radically different, which serves as recommendations in the United States. However, Dr. Holick, reported in 2018 by the New York Times, admitted to having been largely paid as an expert by the pharmaceutical industry and producers of food supplements, and to having received research funds from manufacturers of UV cabins. He defended himself by asserting that this had not influenced his scientific considerations on the subject, but it casts at least an insistent shadow on the sincerity of his positions.
Another subject of distrust: the reliability and usefulness of vitamin D dosages, which are not easy to carry out and are poorly standardized, which complicates the comparison of two results obtained in two different laboratories.
In France, the High Authority for Health (HAS) reserves them for 6 specific clinical situations (suspected rickets or osteomalacia, monitoring of adults with kidney transplants, before and after bariatric surgery, elderly people prone to repeated falls, taking of medications recommending the measurement of vitamin D). Outside of these situations, their cost must be borne by the patient according to Health Insurance, for whom they are very expensive: 6.8 million dosages representing a cost of 41.7 million euros were carried out in the private laboratories in 2022, compared to 3.9 million in 2015, it highlighted last July in its “Expenses and income report for 2024”. 90% of dosages carried out in 16-65 year olds would have been wrongly prescribed and did not comply with HAS recommendations.
Cancers, cardiovascular diseases, fractures, stroke, kidney diseases, multiple sclerosis, Alzheimer's disease or more recently Covid-19... Many studies have established links between vitamin D deficiency and poor health. But they only establish a statistical link between the two, without allowing us to know whether we fall ill because we lack vitamin D, or whether we lack it because of a poor state of health. Many teams have sought to supplement patients in the hope that this would improve their health, but almost all large, well-conducted trials have failed to show an effect, including in populations with a deficiency at the start of the study. Some seemed to favor a positive effect, which was later denied. Thus with asthma: in 2016, a literature review by the Cochrane collaboration judged that vitamin D seemed to reduce the risk of asthma attacks; but seven years later, in February 2023, the same authors updated their work and did not find proof of this protection.
Epidemiologists from the International Prevention Research Institute (Ipri), in Lyon, estimated, after a major literature review carried out around ten years ago, that, for all non-skeletal diseases, a low level of vitamin D was only a biological marker of deteriorated health, not its cause. In 2022, the HCSP delivered the same conclusion: “Drug supplementation trials have not yet made it possible to confirm a positive (or deleterious) effect on most pathologies potentially associated with vitamin D insufficiency in the general population. »
Among children, there is rather consensus on the need for supplementation, particularly among toddlers, who should not be exposed to the sun. French experts have published new recommendations in 2022, calling for daily supplementation until the age of 2, then ideally daily or, failing that, quarterly until the age of 18. But even this supplementation raises questions about its real effectiveness, indicates a meta-analysis by the Cochrane collaboration carried out in 2019.
In adults, vitamin D may be recommended in a few specific clinical situations (defects in mineralization or bone density, renal insufficiency, risk of falls in the elderly, pathologies or medications affecting metabolism, etc.). But “systematic drug supplementation with vitamin D in the general population is not possible given the current state of knowledge,” insists the HCSP. If you are an adult in good general health, with reasonable access to daylight, supplementation is unnecessary.
Cases of toxicity have been exceptionally reported. Thus, in 2019, Canadian nephrologists described the case of a 54-year-old man who suffered kidney failure after taking high doses of vitamin D (8,000 to 12,000 international units per day, or 2 to 3 times l tolerable maximum intake, according to health authorities and learned societies). Cases of overdose in children have also been reported with alerts from the Medicines Agency in 2021, then in 2023, linked to the taking of food supplements and which may have led to the hospitalization of previously healthy infants. After the Second World War, sporadic epidemics of vitamin D poisoning could also be observed in children whose milk was fortified.
Fortunately rare cases. “The risks of acute vitamin D poisoning, apart from exceptional pathologies, are practically zero,” notes the HCSP. On the other hand, there is a lack of data on possible negative effects of high doses administered regularly over the medium and long term. »