In this article you can find:

  • What is vitamin D?
  • What is the difference between vitamin D2 and D3?
  • What are the benefits of vitamin D?
  • Vitamin D and Covid-19
  • What are the signs and symptoms of deficiency?
  • Who is at risk of deficiency?
  • Sources of vitamin D and recommended dose

What is vitamin D?

Vitamin D is a fat-soluble vitamin (vitamins that are absorbed along with fats in the diet and can be stored in the body's fatty tissue) that plays a major role in the way the body absorbs other nutrients. It is also a vital factor in ensuring that your muscles and organs work efficiently. It is a unique type of vitamin since it acts more like a hormone than a dietary aid, since the body can synthesize it from cholesterol after the skin is exposed to UVB rays from the sun and is not readily available in many foods. Often referred to as the "sunshine vitamin", vitamin D is sourced primarily from the sun (we obtain about 80-90% of it this way). It is seasonal and cannot be made during the winter, while the amount made in the summer is subject to sunshine, weather and other factors. In Ireland, it can be produced from 10-15 minutes of sun exposure from late March to late September (1).

What is the Difference Between Vitamin D2 and D3?

Vitamin D is available in two forms: vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Vitamin D2 is manufactured from plants and fungi. Vitamin D3 is created from animal products and is the type most similar to that which is naturally made by the body through sunlight exposure. Studies show that they both have equal health benefits in the recommended dosages, however their actions may vary slightly at high doses – when the impact of Vitamin D2 is less potent (2). Vitamin D3 tends to be the form in which the majority of people are deficient.

What are the Benefits of Vitamin D?

Vitamin D3 can have an impact on as many as 2000 different genes within the body (3), and has a direct role on far more aspects of health than many might realize. It had been linked to numerous health conditions, from bone and muscle health, to mental health, immune activity, cardiovascular functions and more.

Bone and Muscle Health

Our muscles and bones are constantly being restructured by mineralization. Vitamin D plays a vital role in this process by regulating the body's ability to absorb phosphorus and calcium (two compounds that provide density and strength to the skeletal system and teeth) and by enhancing the absorption of dietary calcium from the large and small intestines. A deficiency in vitamin D can result in bone deformities such as rickets in children, and bone pain caused by a condition called osteomalacia in adults (3).

Mental Health

Vitamin D plays a role in regulating neurotransmitter synthesis. Serotonin, the key hormone that stabilizes our mood, rises when exposed to bright light and falls with decreased exposure. Studies suggest that increasing levels of vitamin D overtime could effectively help to reduce the symptoms of depression, particularly for those who suffer from seasonal effective disorder (5). Other treatments may also be required, but first ensuring proper absorption of the vitamin can be critical.

Immune Function

Vitamin D has also been found to dramatically influence the immune system. It has shown to affect the key cells of the immune system such as dendritic cells' ability to activate T cells. In healthy people, T cells play a vital role in helping to fight infections. However, for those with autoimmune diseases, they can start to attack the body's own tissues. Studies suggest that vitamin D can result in dendritic cells producing more of a molecule called CD31 on their surface which can hinder the activation of T cells (4). The prevention of cell contact (an essential part of the activation process) can result in the reduction of an immune reaction. Researchers say the findings shed light on how vitamin D deficiency may regulate the immune system and influence susceptibility to autoimmune diseases. The benefit on immune function counters inflammation and may help prevent respiratory infections (in those who have low vitamin D levels).

Heart Health

Research has shown that those with high blood pressure experienced a drop in numbers when vitamin D levels were increased. Vitamin D3 actively reduces the concentration of renin, an enzyme secreted by the kidney that has an effect on blood vessels (6). In turn, improving cardiovascular endurance and keeping heart muscle cells from growing too large.

Insulin Control

Vitamin D3 stimulates the pancreas and triggers the process of insulin production. This is essential for regulating blood sugar levels more effectively and can help those with diabetes manage their condition (6).

Vitamin D and Covid-19

Co-author of the Irish Longitudinal Study on Ageing, professor of medical gerontology at Trinity College Dublin, Rose Anne Kenny stated that people have "nothing to lose" and potentially much to gain by taking vitamin D supplements as protection during the current pandemic. Evidence portraying a relationship between vitamin D deficiency and severe Covid-19 disease was described to be "circumstantial but considerable". In addition, recent research conducted in the US revealed that coronavirus patients are four times less likely to require admission to ICU if their vitamin D levels were adequate.

What are the Signs and Symptoms of Deficiency?

Almost half the Irish population is deficient in vitamin D. Due the fact that a lot of us have been spending more time indoors this year and with winter approaching, this number could increase.

Signs and symptoms of deficiency can include:

  • Bone pain
  • Back pain (particularly lower back pain)
  • Muscle aches, weakness and cramps
  • Fatigue and tiredness
  • Low mood (depression)
  • Getting sick or infected often (frequent coughs and colds)

Who is at Risk of Deficiency?

Vitamin D deficiency can occur across the population spectrum. Studies have shown it's difficult to find populations not deficient in vitamin D, especially those where sunshine is not in abundance such as Ireland.

This risk increases for those again who:

  • Eat an inadequate amount of fortified foods
  • Have a dark skin tone (greater amounts of the pigment melanin in the skin lessen the ability of the skin to absorb UV rays for vitamin D production)
  • Have a chronic disease and lung condition
  • Are elderly (as you age, your skin doesn't synthesis as much vitamin D as when you're younger)
  • Smoke
  • Are obese (those of a larger body size require more vitamin D for optimum function, in addition, since vitamin D is stored in fat it is harder to keep it in blood circulation)
  • Spend more time indoors than out (those who are housebound/confined)
  • Work night shifts
  • Live in areas that reduce direct sunlight (cities with tall buildings)
  • Live in areas with high pollution that can filter out UV rays

Sources of Vitamin D and Recommended Dose

Vitamin D3 is found in more food sources than vitamin D2.

Natural food sources of vitamin D3 include:

  • Mackerel
  • Salmon
  • Sardines
  • Herring
  • Tuna
  • Red meat
  • Liver
  • Fortified foods (breakfasts cereals, dairy products, orange juice, infant formulas)
  • Egg yolks

Natural food sources of vitamin D2 include:

  • Eggs yolks
  • Mushrooms that have been grown in ultraviolet light (shiitake mushrooms, portobello mushrooms)
  • Fortified foods (breakfasts cereals, dairy products, orange juice, infant formulas)

Supplemental sources include:

  • Vitamin D3
  • Vitamin D2
  • Cod liver oil (this has the added benefit of vitamin A and essential fatty acids)
  • Multivitamin (these tend to not contain as much vitamin D as the others so are best for those who do not need to increase their daily intake by as much)

10 UG is the minimum recommended daily intake during the wintertime. Between 15 – 20 UG is recommended for most at-risk groups (1).


  1. Laird, E. and Kenny, R., 2020. Vitamin D Deficiency In Ireland - Implications For COVID-19. Results From The Irish Longitudinal Study On Ageing (TILDA). [ebook] Dublin 2: The Irish Longitudinal Study on Ageing. Available at: [Accessed 14 October 2020].
  2. National Institutes of Health (Office of Dietary Supplements). 2020. Vitamin D: Health Professionals Fact Sheet. [online] Available at: [Accessed 14 October 2020].
  3. Wacker, M. and Holick, M., 2013. Vitamin D — Effects on Skeletal and Extraskeletal Health and the Need for Supplementation. Nutrients, 5(1), pp.111-148.
  4. Saul, L., Mair, I., Ivens, A., Brown, P., Samuel, K., Campbell, J., Soong, D., Kamenjarin, N. and Mellanby, R., 2019. 1,25-Dihydroxyvitamin D3 Restrains CD4+ T Cell Priming Ability of CD11c+ Dendritic Cells by Upregulating Expression of CD31. Frontiers in Immunology, 10.
  5. Penckofer, S., Kouba, J., Byrn, M. and Estwing Ferrans, C., 2010. Vitamin D and Depression: Where is all the Sunshine?. Issues in Mental Health Nursing, 31(6), pp.385-393.
  6. Vaidya, A. and Williams, J., 2012. The relationship between vitamin D and the renin-angiotensin system in the pathophysiology of hypertension, kidney disease, and diabetes. Metabolism, 61(4), pp.450-458.