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What is vitamin D? A guide to the sunshine vitamin

Vitamin D is a fat soluble nutrient. In fact, it’s not technically a vitamin, it’s actually a steroid hormone that regulates a vast proportion of our bodily functions. Vitamin D plays a role in modulating our immune system, and is powerful in supporting hormone regulation, the bones, teeth, and muscles. 

Vitamin D is produced in the skin when it’s exposed to the sun. The UVB rays from the sun generate pre-vitamin D, which then undergoes further reactions to make an active form of vitamin D, cholecalciferol or vitamin D3.

Vitamin D2 and D3

There are two main forms of vitamin D, vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Vitamin D2 is produced by plants via UVB exposure (sunlight), and vitamin D3 is made in the skin from the same UVB rays. Both forms are inactive and need to go through further reactions in the liver and kidneys to be converted into the active form of vitamin D before it can perform its functions within the body.  

Although both forms of vitamin D are important, Vitamin D3 is more important for our health and wellbeing. Our bodies absorb and use vitamin D3 better, and it is vitamin D3, not D2, that maintains the levels of vitamin D in our body during winter months. 

Vitamin D benefits

This fat-soluble vitamin is well known for its role in supporting bone density, but is also a vitally important nutrient for other body functions, including strengthening and balancing our immune system, reducing inflammation, and supporting the cardiovascular, nervous, and reproductive systems.

Vitamin D supports bone health

Vitamin D (in its active form) is necessary for the body’s absorption of calcium as well as regulating calcium and phosphorus levels in the blood, protecting against osteoporosis, rickets, and bone fracture. Without vitamin D, calcium absorption is affected, resulting in low levels of circulating calcium. This leads to the body taking calcium from its stores in bone which weakens existing bones and prevents new ones from forming (1) (2).

Vitamin D supports immune function

Vitamin D plays key roles in supporting the immune system and reducing our risk of infection and autoimmune diseases (3).

Vitamin D can regulate both the innate and adaptive immune systems, the two main branches of the immune system that keep us in balance. It is required for the following immune system functions: 

  • Producing, modulating and activating the immune cells that help fight infection by destroying infected or mutated cells. 

  • Inhibiting the growth and production of inflammatory proteins and potentially mutated cells. 

A review of 25 randomised controlled trials that involved about 11,300 people suggests that participants who were vitamin D deficient saw a 12 percent reduced risk for respiratory infections supplementing with vitamin D, though not every study saw a clear benefit from supplementation (4).

Vitamin D supports blood sugar levels

Vitamin D has been shown to help regulate insulin secretion and sensitivity and balances blood sugar.

Observational studies suggest that vitamin D may help increase insulin sensitivity, boost beta cell function, and reduce inflammation, which are all potential benefits for reducing the risk of and helping manage type 2 diabetes (5).

Vitamin D supports mood

Because vitamin D acts like a hormone within our bodies and affects brain function, vitamin D deficiency has been linked to an increased risk for mood disorders, including depression, seasonal affective disorder, and severe mood problems experienced during PMS, insomnia and anxiety (6).

People with seasonal affective disorder appear to produce less vitamin D, which may affect the activity of the neurotransmitter serotonin. Serotonin is our feel-good hormone, so when the serotonin levels are low, it may lead to a greater risk for mood disorders (7).

Vitamin D deficiency & depletion 

The main cause of vitamin D deficiency these days is our lack of sunlight exposure from the modern indoor lifestyle many of us lead, as well as a low vitamin D intake through diet. The level of vitamin D synthesis is influenced by a number of factors, including season of the year, skin pigmentation, latitude, use of sunscreen, clothing, age and amount of skin exposed. 

Other factors which can contribute to low vitamin D levels include:

  • Pregnancy and breastfeeding
  • Old age 
  • Obesity
  • Low magnesium status 
  • Low fat diet 
  • Genetic factors
  • High cortisol levels (caused by stress or medications like steroids)

A vitamin D deficiency can lead to a number of health complications, largely associated with bone health e.g. rickets in children and osteomalacia or osteoporosis in adults. It causes the bones to become soft and weak, often leading to deformities and a higher risk of fractures. 

Health conditions and symptoms related to vitamin D deficiency include:

  • Rickets, osteomalacia and osteoporosis
  • Muscle pain and weakness 
  • Growth problems
  • Bone and joint aches
  • Increased autoimmunity, poor immunity and greater risk of infections
  • Constipation and/or diarrhoea

Vitamin D RDA

Public Health England (PHE) and other organisations suggest that we take a dietary supplement of vitamin D3 during the winter months, between October and March to protect bone and muscle health. Their recommendation is an intake of an average of 10 micrograms per day. 

Vitamin D sources 

Vitamin D-rich foods

Very few foods are naturally rich in vitamin D. The main sources are: 

Vitamin D3:

  • Oily/fatty fish and fish liver oils (e.g. mackerel, salmon and tuna)
  • Egg yolks
  • Beef liver
  • Cheese
  • Fortified products including some dairy, cereals, juices, etc. 

Vitamin D2:

  • Some species of mushrooms

It’s important to note that dietary sources of Vitamin D3 can only be found in animal products and fortified foods, so if you are vegan, you need to pay extra attention to supplementing your diet with Vitamin D3.

Vitamin D supplements 

While some of us may be able to get vitamin D from UVB sun exposure in the summer months, and eating a diet high in oily fish, there are many factors that can inhibit our production and ability to get the adequate amounts for optimum health. Taking a vitamin D3 supplement daily will ensure you are getting your daily needs to remain healthy and prevent disease. 

Vegan sources of vitamin D include lichen algae. While this wouldn’t be pleasant to eat, it means vegan vitamin D3 supplements are accessible. 


Vitamin D is vital for your overall health. It helps to support healthy bones, immune system and even moods and mental health. 

It is important to take steps to get your recommended daily amount of this nutrient through diet, supplements and sunlight. During the winter months, it is a requirement to supplement with vitamin D to maintain optimal blood levels. Generally, 4,000 IU or less per day is considered safe, as long as your blood values are being monitored. 

Getting regular blood tests is a good way to monitor your blood levels and make sure they stay within healthy ranges to prevent unwanted health conditions. 


    1. Vitamin D deficiency
    2. Calcium and Vitamin D: Important at Every Age
    3. Vitamin D - The Nutrition Source
    4. Vitamin D supplementation to prevent acute respiratory tract infections
    5. Vitamin D and diabetes
    6. Vitamin D deficiency and depression in adults
    7. What has serotonin to do with depression?