Vitamin D3
Overview
Vitamin D3, or cholecalciferol, is a hormone that occurs naturally in the body. The body obtains it from food, but also through endogenous synthesis triggered by sunlight. Most of the vitamin D3 in our bodies comes from this synthesis (between 80 and 90 per cent). (1) Known as the ‘sunshine vitamin’, it is produced in the skin upon exposure to UV-B rays.
This biosynthesis (Figure 1) begins primarily in the skin, where UV-B rays react with 7-dehydrocholesterol (cutaneous provitamin D) to produce provitamin D3. This provitamin D3 is subsequently isomerised into cholecalciferol (or vitamin D3). Vitamin D3 is therefore not active and is converted in the liver and then in the kidneys into its biologically active form, 1,25-dihydroxyvitamin D3 or 1,25 (OH)₂ D₃. Cytochromes P (CYP) are responsible for this activation.
In the liver, 25-hydroxyvitamin D₃ or 25(OH)D₃ is synthesised from vitamin D₃ via the action of CYP27A1, CYP2R1, CYP2J3 or CYP3A4. In target tissues, the 1α-hydroxylase CYP27B1 synthesises the biologically active form, 1,25-dihydroxyvitamin D3 or 1,25(OH)₂D3.
Once synthesised, active vitamin D3 diffuses throughout the body and acts on its target organs, such as the intestine, bones, kidneys and parathyroid glands. Its catabolism (primarily in the kidney) is initiated by the 24-hydroxylase CYP24A1. (2)
Whilst a large proportion of vitamin D3 is endogenous, the remainder is exogenous. In other words, it comes from foods of animal origin (unlike vitamin D2, which is obtained through a plant-based diet). In general, foods rich in vitamin D3 include:
- Oily fish (salmon, herring, sardines, mackerel, etc.)
- Cod liver oil
- Offal
- Eggs
- Milk and soya drinks
- Butter and certain cheeses
According to ANSES recommendations, the daily requirement for vitamin D3 in adults is 15 µg or 600 IU. This requirement increases to 20 µg (800 IU) in people over the age of 70. (3)
Physiological roles
Vitamin D3 performs various functions in the body and has been extensively studied by the scientific community. We will analyse the following two statements regarding vitamin D3 supplementation using clinical studies and scientific articles:
- Vitamin D3 helps to maintain the immune system.
- Vitamin D3 helps to maintain normal muscle function.
Supporting the immune system
Over the last 20 years, a growing body of scientific research has revealed a link between vitamin D deficiency and an increased risk of various infectious diseases, as well as a poorer prognosis for patients with vitamin D deficiency when they contract infections. (4)
The primary function of vitamin D is to regulate and strengthen the immune system. It activates and contributes to the formation of the immune system, but its main action is to regulate it. It is therefore an essential and indispensable element for the body’s resistance. It regulates not only the innate immune system but also the adaptive immune system. In the innate immune system, vitamin D promotes the production of antibodies such as defensins and cathelicidins, enhancing their antibacterial and antiviral action. Furthermore, vitamin D3 stimulates the breakdown of pathogens by macrophages.
As for the adaptive immune system, it exerts an anti-inflammatory effect that reduces the growth of TH2 cells and reinforces the regulatory T-cell compartments. In this way, it helps prevent an excessive immune response and ensures that the body can combat pathogens through the formation of antibodies.
Vitamin D is not only essential in the face of acute conditions, but also helps prevent the onset of chronic inflammatory and autoimmune diseases. It therefore performs multiple functions, particularly in the distribution of antibodies within bodily fluids. It also regulates the immune response to prevent chronic inflammation and moderates the immune system’s reactivity. (5)
A study conducted by Adrian N. Martineau et al. in 2011 highlighted the role of vitamin D3 in immunity. (6) This was a multicentre, randomised controlled trial of vitamin D as an adjuvant treatment in adults with pulmonary tuberculosis.
A total of 126 participants were divided into two groups to receive 2.5 mg of vitamin D3 or a placebo at baseline and at 2, 4 and 6 weeks after starting standard anti-tuberculosis treatment. The primary endpoint of the trial was the time elapsed from the start of antimicrobial treatment to sputum culture conversion.
The median time to sputum culture conversion was 36.0 days in the vitamin D3 group and 43.5 days in the placebo group. Therefore, the administration of four 2.5 mg doses of vitamin D3 increased serum 25-hydroxyvitamin D concentrations in patients receiving intensive treatment for pulmonary tuberculosis and reduced the time to sputum culture conversion in the participants.
Maintenance of normal muscle function
According to various studies and clinical trials, there appears to be a link between muscle strength and vitamin D. In fact, vitamin D3 influences physical performance. A deficiency can, for example, increase the risk of falls in older people. It is also important to note that, according to epidemiological surveys and studies, a decline in muscle mass and capacity, as well as a lack of strength in older people, is consistently associated with vitamin D deficiency. (5)
In 1999, Heike A. et al. demonstrated the relationship between vitamin D and muscle strength in older people. (6) The study involved 319 patients selected at random from participants in an ongoing interdisciplinary study on ageing.
