Vitamin D is actually a group of fat-soluble vitamins responsible for the intestinal absorption of calcium, magnesium, and phosphate. The most important from the group are vitamins D3 (also known as cholecalciferol) and D2 (ergocalciferol). It is essential for the normal absorption of calcium and phosphorus, which are needed for healthy bones. Recent studies showed that many other cells have vitamin D receptors, especially the immune system cells 1)Vitamin D and the Immune System – Cynthia Aranow, MD.


The vitamin D produced by the body when exposed to UVB (with a wavelength of 290–320 nanometers) or ingested through food or supplements is biologically inert. It has to go through two hydroxylations in the body in order to become active. The first one occurs in the liver and converts vitamin D to 25-hydroxyvitamin D, also known as calcidiol. The second one occurs in the kidney and produces 1,25-dihydroxyvitamin D, also known as calcitriol2)Vitamin D.

Sunshine Vitamin


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Is also called the “sunshine vitamin” because our bodies are able to synthesize this when exposed to sunlight. In the presence of UV radiation (especially UVB), the skin produces Vitamin D3 (cholecalciferol) from a derivative of the steroid cholesterol. When we don’t expose ourselves enough to sunlight, we can develop vitamin D deficiency.

UVB radiation does not penetrate glass, so exposure to sunshine indoors is not helpful. A complete cloud cover blocks about 50% of the UVB radiation, while severe pollution blocks up to 60% 3)Rickets – Wharton B, Bishop N. Despite the importance of sun exposure for this purpose, it is recommended to limit exposure to UV radiation because it is a carcinogen. Besides that, lifetime cumulative UV damage to the skin is the cause of age-associated dryness and other cosmetic changes.

You can also get your Ds from food and supplements. When it comes to food, only small fish (sardines and herring) and fatty fish (like tuna, mackerel, and salmon) have significant amounts of it. You can also get some amounts from beef liver, cheese, and egg yolks or from fortified foods like milk, soy milk, orange juice, cereals.

Bottom line is that if you don’t get enough sun exposure, it is unlikely to get enough from your food, so you should also take some supplements. Cod liver oil is a popular supplement, containing about 448 IU per teaspoon. It is also rich in vitamin A and omega-3 fatty acids.

Vitamin D Deficiency

I will focus more on the effects of not having the needed amounts in your body, but this will also shed light on the benefits of vitamin D.

The most known effect of its deficiency is age-dependent:

  • in case of kids, it can cause rickets, a condition that results in weak bones, bowed legs, bone pain, large forehead trouble sleeping;
  • for adults, it leads to osteomalacia, causing bone pain and muscle weakness, and leads to easy fractures.

In addition to its crucial role in bone metabolism, the sunshine vitamin has been associated with multiple autoimmune diseases in several epidemiological studies. Vitamin D deficiency may contribute to the onset and progression of autoimmunity 4)The Implication of Vitamin D and Autoimmunity: a Comprehensive Review – Chen-Yen Yang, Patrick S. C. Leung, Iannis E. Adamopoulos, and M. Eric Gershwin.

Signs of Deficiency

Most people don’t realize that they’re vitamin D deficient, because symptoms are generally subtle. Even though they have a significant negative effect on your quality of life, you might not easily recognize them.

Here are some of the most important signs of deficiency:

Clinical data suggest that vitamin D3 insufficiency is associated with an increased risk of several CNS diseases, including multiple sclerosis, Alzheimer’s and Parkinson’s disease, seasonal affective disorder and schizophrenia. In line with this, recent animal and human studies suggest that vitamin D insufficiency is associated with abnormal development and functioning of the CNS 13)Vitamin D, nervous system and aging – P.Tuohimaa, T.Keisala, A.Minasyan, J.Cachat, A.Kalueff.

A blood test cand determine if you are deficient or not, and if you are, the doctor will probably order also an X-ray to check for bone density problems. The serum concentration of 25-hydroxyvitamin D is the best indicator of vitamin D status. It has a circulating half-life of 15 days, and functions as a biomarker for exposure (it is not clear to what extent it serves as a biomarker for effect). Serum 25-hydroxyvitamin D levels do not indicate the amount stored in body tissues. Circulating calcitriol has a half-life of only 15 hours, and it is not a good indicator 14)Vitamin D.

Causes of Deficiency

The main cause of its deficiency is lacking sun exposure, but it can be accentuated by using sunscreen, living in an area with high pollution or having a darker skin tone, being obese or being elderly. Your geographical position also has an important role: people who live at latitudes above 37 degrees north or below 37 degrees south of the equator don’t get enough UVB energy from the sun for proper vitamin synthesis.

How Much Do You Need?

Vitamin D

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Dosage can be expressed as micrograms per or international units (IU) per day.  An international unit (IU) is an internationally accepted amount of a substance. This type of measure is used for the fat-soluble vitamins (such as vitamins A, D, and E) and certain hormones, enzymes, and biologicals (such as vaccines) 15)Medical Definition of IU (international unit). You can convert between IU and mcg using this formula: IU/40 = 1 mcg 16)

The current intake recommendations from the Dietary Reference Intakes are:

  • Children and teens:  600 IU/day
  • Adults, up to age 70: 600 IU/day
  • Adults, over age 70:  800 IU/day
  • Pregnant and breastfeeding women: 600 IU/day

Some authorities recommend 800 or even 1000 IUs a day. Beware though that this is a fat-soluble vitamin, so it’s stored in the body’s adipose tissue (fat). This is good as the body can use the reserves when your intake and production are low, but it can also become bad when excessive doses build up to toxic levels. At toxic levels, it can drive up calcium levels in the blood, causing grogginess, constipation, and even death. One needs massive overdosing in order to reach this state – doses up to 2,000 IU are considered safe. There is no determined threshold for toxicity – some indicate 4,000 IU, while others claim that overt toxicity appears after several months of 50,000 IU intake 17)


Both an excess and a deficiency in vitamin D appear to cause abnormal functioning and premature aging. You need to make sure you help your body maintain adequate levels of this vitamin and simple sun exposure is not as efficient as it used to be decades ago. Try to eat fish (especially sardines and herring), and take supplements.


1Vitamin D and the Immune System – Cynthia Aranow, MD
2, 14Vitamin D
3Rickets – Wharton B, Bishop N
4The Implication of Vitamin D and Autoimmunity: a Comprehensive Review – Chen-Yen Yang, Patrick S. C. Leung, Iannis E. Adamopoulos, and M. Eric Gershwin
5Association of back pain with hypovitaminosis D in postmenopausal women with low bone mass – Silva AV, Lacativa PG, Russo LA, de Gregório LH, Pinheiro RA, Marinheiro LP.
6Low vitamin D, and bone mineral density with depressive symptoms burden in menopausal and postmenopausal women – Bener A, Saleh NM
7Vitamin D and the Immune System – Cynthia Aranow, MD
8A review of the critical role of vitamin D in the functioning of the immune system and the clinical implications of vitamin D deficiency – Schwalfenberg GK
9Vitamin D deficiency and lower respiratory tract infections in children: a systematic review and meta-analysis of observational studies – Jat KR
10Quality of life is impaired not only in vitamin D deficient but also in vitamin D-insufficient pre-menopausal women – Ecemis GC, Atmaca A.
11Resolution of hypersomnia following identification and treatment of vitamin d deficiency – McCarty DE
12Vitamin D status in patients with musculoskeletal pain, fatigue and headache: a cross-sectional descriptive study in a multi-ethnic general practice in Norway – Knutsen KV, Brekke M, Gjelstad S, Lagerløv P
13Vitamin D, nervous system and aging – P.Tuohimaa, T.Keisala, A.Minasyan, J.Cachat, A.Kalueff
15Medical Definition of IU (international unit)