There are 2 types of vitamin D, including vitamin D2 (ergocalciferol), which is gained through certain foods like milk and juices and vitamin D3 (cholecalciferol) that is synthesised through the skin from sun exposure, hence its nickname as the 'sunshine vitamin'. Because vitamin D3 can be gained through exposure to sunlight, it is not classed as an essential dietary vitamin.
As your skin absorbs ultraviolet B rays from the sun, it is made into a substance called pre-vitamin D3. This then travels to the liver to become vitamin D3 or 'calcidiol' and is carried by special proteins to the kidneys to be turned into vitamin D's biologically active form 'calcitriol'. The amount of vitamin D3 that your body is able to produce depends on the amount of sunlight that you are exposed to, which is obviously affected by certain factors like conditions of sunlight (season, cloud cover, altitude) as well as age and colour of skin.
What is vitamin D for?
It is considered that vitamin D3 tablets are more effective in humans than vitamin D2. No matter how much calcium you consume, you still need the right amount of vitamin D3 supplements to control the absorption of dietary calcium. With the right balance of calcium and vitamin D, the calcium will be absorbed and bones health will be maintained and strengthened. Vitamin D is also suggested to regulate the growth of cells, including the cells that kill bacteria and activate your immune response. There is also evidence to suggest vitamin D plays a role in lowering blood pressure and there is growing research to also look into a possible link between vitamin D and the prevention of cancer.
Vitamin D3 can apparently be obtained after 10-15 minutes of full sun exposure (without sunscreen), which is recommended 3-4 times per week. However, since you cannot be certain how much vitamin D3 your body is gaining from sunlight and we are advised to limit sun exposure, adding or supplementing vitamin D into your daily diet could be most effective.