Question of the Week

Is making vitamin D akin to human photosynthesis?

Sun, 12th Aug 2012

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David Wood asked:

Hello I'm David from The States and i have 2 questions about the same thing.


I heard that we get vitamin D from the sun if that's true does that mean that to a point we go through a kind of photosynthesis?


And, if so, does sunblock block the Vitamin D getting to us?


Seth Raven



We put this question to Dr. Laura Tripkovic from Surrey University, and Professor Richard Cogdell from Glasgow University:

Laura -   We need vitamin D to maintain our bones and to make sure we absorb enough calcium from our diet.  One of the main sources to humans, radiation from the sun can penetrate the layers of the skin called the epidermis, while there's a chemical called 7-hydrocholesterol and this is observed through UV light and then will produce the pre-vitamin D molecule.  And then because the skin is quite warm, this pre-vitamin D3 will spontaneously convert to vitamin D3 and the vitamin D3 will move from the skin, and will be pushed out into the capillary system, and into your blood system, so it can then be activated and used.

Sunbathing in Naama Bay, Sharm el-Sheikh, Egypt.Hannah -   So, human skin does make vitamin D using a photochemical reaction.  How similar is this process to the photosynthesis are carrying in the chloroplasts of plants which uses the sunís rays to convert carbon dioxide into oxygen and carbohydrate?

Richard -   My name is Richard Cogdell and I'm the Hooker Professor in Botany at Glasgow University.  In vitamin D production, the wavelength of light which triggers that reaction or drives that reaction is the ultraviolet wavelength.  So itís the wavelength that actually would burn you if you're out in the sun for too long.  The wavelengths of light which drive photosynthesis are the wavelengths that give rise to the blue and red regions in the spectrum because itís light absorbed by chlorophyll.  And so, photosynthesis is not driven by ultraviolet light.  Itís driven by a visible radiation in the blue and the red regions of the spectrum.

Hannah -   So what happens during the winter months when the intensity of all wavelengths of sunlight hitting the UK decreases?  Plants enter dormancy phase and use remaining carbohydrates stored up during the summer months but what happens to us humans?  Back to Laura.

Laura -   We can only have vitamin D3 here in the UK between the months of April and September.  We can be out in the sun for about 10 to 15 minutes, three times a week, just around your face.  You donít need to strip off, and this will see you through into early autumn. We now know that the half-life of vitamin D can be around a month.  Now there are dietary sources of vitamin D that can support your levels during the winter time and they will be things like oily fish, like sardines and mackerel, there's a little in eggs, if you like shitake mushrooms it's in there, and there's also little bits added into breakfast cereals, milk, and soya milk and things. But you need to check the label to see if they're actually in there.


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Good Question , ummm... well plants photosynthesis in need of food. Plants obviously won't live if they can't make or have food. For humans vitmain D isnt exactly food. Even though it is a very important vitamin , especially for our bones , we humans can probably live without vitamin D . Our bones might weaken , we might not even be able to move without this very important vitamin. But we could just manage to survive without it , so ... No , making vitamin D isn't human photosynthesis.
And also if using SPF 8 blocks it literally blocks about 95% of the sun rays blocking with it the special vitamin.
:) :) :)
Well , I hope this helps!  lujainthatsme, Thu, 19th Jul 2012

In common usage, "photosynthesis" refers to plants producing sugar and oxygen from carbon dioxide and water plus light using chlorophyll.

However, in a more general sense, any chemical reaction which is activated by light (photons) could also be called "photosynthesis".

Since Vitamin D is produced by the action of sunlight, it could be called photosynthesis, in this rather unconventional way - but it's best to avoid it, as it will bring up visions of green-skinned humans!

Sunblock does reduce the level of Ultraviolet light reaching your skin - it's a tradeoff between reducing the incidence of skin cancer and increasing the incidence of osteoporosis. evan_au, Sat, 21st Jul 2012

Photosynthesis is a special biochemical process that uses solar energy to create relatively complex and energy-rich sugar molecules from CO2, a simple and low-energy source of carbon.  Essentially, solar energy is stored in the chemical bonds of sugars.  Because of this, plants are able to make their own food.

The creation of vitamin D in humans is not similar to photosynthesis for two reasons.  First, we are not converting solar energy into chemical energy, and second, we are not using these molecules for food. 

In humans, vitamin D is created from a precursor molecule that is already present in our skin.  The sun's energy is used to slightly change this pre-existing precursor molecule into a form that is more usable to our cells.  Once modified by solar rays, this early form of vitamin D will be later modified in the kidneys before becoming functionally mature. 

UV light is especially important in this process.  Using sun block reduces the amount of UV reaching your skin cells, so it would reduce vitamin D production.  However, if you spend several hours in the sun, it is likely that enough UV radiation is penetrating this protective layer and enabling your body to make sufficient quantities of vitamin D.
-Matt F, Washington State amplexity, Fri, 10th Aug 2012

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