Sunlight and Vitamin D Synthesis

What type of sunlight provides the best vitamin D?

Exposure to the sun’s ultraviolet (UV) rays is required for vitamin D3 (colecalciferol) to be produced. The sun emits three different types of radiation, UVA, UVB and UVC. The ozone layer is good at blocking all of the UVC radiation and most of the UVB, however, none of the UVA radiation. The UVA penetrates deep into the skin and can cause aging and wrinkles. UVB can cause sunburn and acceleration of skin aging. UVB radiation from the sun is also the best source of vitamin D.


Skin type Season Skin exposed Recommended time of day Sun exposure
Moderately fair Summer Arms or equivalent Mid morning or mid afternoon 5 – 10 minutes*
Darker skin Summer Arms or equivalent Mid morning or mid afternoon 15 – 60 minutes*

*depends on location within Australia and type of skin.

Skin type Season Skin exposed Recommended time of day Sun exposure
Moderately fair Winter Arms or equivalent Midday 7 – 30 minutes*
Darker skin Winter Arms or equivalent Midday 20 min – 3 hrs*

*depends on location within Australia and type of skin.


How does the light get converted to vitamin D in the skin?

When ultraviolet rays from sunlight penetrate the skin, they react with 7-dehydrocholesterol which is a precursor to cholesterol. This reaction forms vitamin D3 (colecalciferol) in the skin. It is then quickly converted by the liver to the pre-hormone 25(OH) D and then further converted in the kidneys to the hormone 1, 25(OH) D2 which is the active form of vitamin D.


Does sunscreen inhibit the production of Vitamin D?

Vitamin D is produced in the skin when it’s exposed to UV radiation from the sun. Sunscreens prevent the sun’s UV radiation from penetrating the skin and therefore may diminish production of vitamin D. Obtaining the balance of sufficient sunshine for vitamin D production whilst practising safe sun exposure is important.

The Sun Protection Factor (SPF) rating of sunscreens indicates the level of protection against ultraviolet radiation (UVR). The higher the rating the greater the protection. The highest SPF for sunscreens available in Australia is SPF50+. If used correctly, this can protect skin from 98 per cent of UVR.


Does protective clothing & hats inhibit the production of Vitamin D?

Like sunscreens, protective clothing and hats blocks the sun’s UV rays from penetrating the skin. Clothing that offers the best protection will cover most of the body’s surface and have a tighter weave, however as UV rays must penetrate the skin to produce colecalciferol (vitamin D3), clothing can reduce its production.


If I get sunlight through a glass window is that ok?

Glass windows are protective barriers for your skin from the sun’s UVB rays. UVB rays cannot penetrate glass. As such, to obtain any vitamin D from sunlight, the skin must come into direct contact with the sun’s UVB rays. People who drive throughout the day with the windows up are at risk of missing out on vitamin D derived from the sun. This also applies to people who spend the majority of their time indoors such as office workers, the elderly in age care facilities or hospitalised people.


How does my geographical location affect the amount of sunlight I need?

Geographical location and position to the equator affects the amount of vitamin D obtainable from UV sunlight exposure. As one moves away from the equator towards the poles, UV levels decrease. At the equator the solar zenith angle (SZA) is the smallest, resulting in higher UV levels. At higher latitude locations, the SZA increases, decreasing the UV levels.

Altitude also affects the amount of UV observed. At higher altitudes UV radiation travels through less atmosphere and therefore has less opportunity to interact with aerosols and other particles in the atmosphere. This results in higher UV radiation levels.

In addition to latitude and altitude, several factors can influence UV radiation levels: Ozone, time of day, season, surface reflectivity of the environment and cloud cover. For information on the safest times to be in the sun, visit the Australian Bureau of Meteorology website. Here you will find information on UV radiation levels for locations around Australia.


How does skin type affect vitamin D production?

People with naturally darker skin may not be obtaining sufficient vitamin D from sun exposure as dark skin contains higher amounts of melanin – melanin impedes vitamin D synthesis. Light coloured skin naturally contains less melanin hence lighter skinned people can convert the sun’s UV rays into vitamin D more readily. Melanin however also protects against UV radiation therefore lighter skinned people will burn more easily in the sunlight and require greater sun protection.




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