Vitamin D is a fat-soluble vitamin. In the absence of vitamin D, calcium cannot be incorporated into bones and teeth. It can also strengthen the immune system, reduce inflammation in the body and help regulate cell division and blood sugar metabolism. Its deficiency increases the risk of many diseases, such as cancer, cardiovascular disease, high blood pressure, etc.

Our bodies can produce it in response to sunlight. 15-30 minutes of exposure to sunlight and open air per day is sufficient to meet the vitamin D needs of a healthy adult. However, by the end of the winter, vitamin D levels in the general population are very low, as most people spend less time outdoors and the weaker sunlight is not sufficient for vitamin D production.

Among foods, the best sources of vitamin D are mushrooms, fatty marine fish, fish liver oils, liver, eggs, milk and dairy products, avocados, almonds, fortified foods. However, dietary intake might not be sufficient, and vitamin D supplementation may be necessary during the autumn-winter period (October to April) to prevent deficiency. However, as vitamin D can be stored in the body, excessive intake of vitamin D should also be avoided as it can raise blood calcium levels, which can cause calcium to build up in the body: in the vascular network or in the kidneys, where it can form kidney tissue, and can also cause calcification of soft tissues or growth disorders in children.

There are several recommendations for vitamin D intake. In 2016, the European Food Safety Authority (EFSA) issued an opinion on dietary reference values for vitamin D. The recommendations for the adequate intake (AI) are listed below:

• For infants aged 7–11 months, an AI for vitamin D is set at 10 lg/day
• For children aged 1–17 years, an AI for vitamin D is set at 15 lg/day
• For adults, including pregnant and lactating women, an AI for vitamin D is set at 15 lg/day

The Hungarian recommendation suggests the following amounts expressed in International Units (IU), where 1 IU corresponds to 0.025 μg vitamin D₃:

• for infants 400-1000 IU per day
• for children 600-1000 IU per day
• for adolescents 800-1000 IU per day
• for adults and pregnant women 1500-2000 IU per day

The proportion of people taking vitamin D has increased significantly in recent years. This is partly due to an increased awareness of the global prevalence of vitamin D deficiency. On the other hand, the spread of the word that vitamin D may also help in protecting against coronavirus infection, reducing the risk of infection and improving disease outcomes, may also have played a role over the past year and a half. There have been several publications on the subject, showing that low vitamin D levels or vitamin D deficiency increase the risk of hospitalisation and mortality due to SARS-CoV-2 infection (e.g. Chiodini et al, 2021), or the likelihood of a severe disease (e.g. Kaya et al, 2021). However, these claims have not been proven beyond reasonable doubt, as research has been published that did not confirm this effect.

Certain groups may be particularly affected by vitamin D deficiency, such as the elderly, pregnant and breastfeeding women, infants and young children. For the latter, this is mainly because babies are less likely to be exposed to direct sunlight and do not consume much vitamin D-rich food. Breast milk contains vitamin D, but not enough to meet babies' daily needs. In their case, it is of paramount importance that vitamin supplementation is safe and of good quality, using a pharmaceutically manufactured product.

In recent years, the French health authorities have reported several cases of vitamin D overdose in young children following the use of food supplements fortified with this vitamin. These cases present as hypercalcaemia (excessive calcium in the blood), which can have serious consequences on the kidneys such as lithiasis or nephrocalcinosis (calcium deposition in the kidneys).

According to a study from the UK, around 40% of prescriptions for vitamin D supplementation in primary care continued to be for supplements in 2018. In the UK, as in many other countries, there are 2 types of vitamin D products: medicinal products, which are used in the clinic to prevent and treat vitamin D deficiency and are covered by the legislation on medicines, and food supplements, which are used to supplement intake from food sources and are covered by the provisions of general food law. The regulatory requirements for the manufacture, quality control and labelling of medicines are much stricter than those for food supplements.

When using vitamin D with food supplements, it is very important to ensure that the correct dosage is taken and that the instructions for taking are followed. There is no maximum amount of vitamin D in a food supplement, so some products may contain several times the maximum tolerable daily intake (4000 IU/day). In the UK study mentioned above, the composition of 11 food supplements and 2 pharmaceutical preparations were tested. The laboratory tests showed that the vitamin D content of the supplements can differ by up to 60% from the label, with a potential risk of under- or over-dosing the patient. The products included in the study suggest that some supplements may contain several times the recommended daily dose, which may have long-term adverse health effects. For the two authorised products for medicinal use, the difference between the actual content and the amount declared on the label was much smaller.

The analysis concludes that the current regulation of vitamin D preparations should be reviewed to minimise risks and support consumers in making informed choices about the use of vitamin D preparations.

In Hungary, according to a news from 2021, the recommended daily dose for vitamin D supplementation has not changed, but the required amount of vitamin D - calculated at a healthy adult vitamin requirement of 2000 IU - can be supplemented either once a week (14,000 IU) or once a month (60,000 IU). According to the experts, a higher daily dose is not necessary, except in the case of severe COVID-19 disease, where the patient's vitamin D stores are saturated during hospitalisation.

There are also plenty of dietary supplements in the domestic market that contain vitamin D. Further information is available on the website of OGYÉI in Hungarian.

In the light of the above, it is important that consumers seeking to meet their vitamin D needs through food supplements should be careful to take the correct dosage, avoid excessive intake and avoid products from unreliable sources, such as online stores. For infants and young children, avoid the use of food supplements and use only preparations manufactured under pharmacy conditions.