Why Can’t I Produce Vitamin D Even if it is Sunny Outside?

26 April 2021

As we mentioned in our previous post (25 April 2021), in order for your skin to produce Vitamin D from sunlight there must be UVB radiations. The problem is that in most of the northern hemisphere there is no UVB in winter, and part of autumn and spring. The reason is that most of the UVB that comes from the sun is absorbed by the ozone layer and the atmosphere before it reaches the ground during this time of the year.

As a result, the amount of Vitamin D produced in the skin by sun exposure is lowest (and nearly zero during Winter in the northern hemisphere). The graph below clearly shows this fact, as the synthesis of previtamin D3 (the precursor to Vitamin D3) from the skin is very low from October to March in 3 northern cities (Boston, USA; Edmonton, Canada; Bergen, Norway), and then increases several folds during Spring and Summer.

Fig 1: Synthesis of previtamin D3 in the skin at various times of the year in 3 northern cities. Taken from M. Wacker and M. F. Holick, Sunlight and Vitamin D, Dermato-Endocrinology 5:1, 51–108; January/February/March 2013.

Moreover, notice how the amount of previtamin D3 decreases the farther you go north (higher latitude) from Boston to Edmonton to Bergen. For example, in July the % of previtamin D3 produced in Edmonton (latitude 52° N) is approx. 50% more than in Bergen (latitude 60° N), and in Boston (latitude 42° N) it is approx. 200% more (or 3 folds) than in Bergen.

The amount of UVB absorbed is highly dependent on your exact location with respect to the poles (e.g. latitude). This has to do with the angle at which sunrays reach the surface of the earth (which depends on the earth’s orientation with respect to the sun), and with how much UVB is absorbed by the ozone layer and the atmosphere.

Besides seasonal variations in UVB, the amount of Vitamin D you can produce from exposing your skin to sunlight depends on several other factors: your skin type, how much skin area is exposed, for how long, what time of the day, and more!

Therefore, even if it is sunny outside, you may not be able to produce any Vitamin D at certain times of the year. This is also true at certain times of the day (even in summer). This will be the topic of another post, so stay tuned if you are interested in knowing what time of the day is best to produce Vitamin D from sunlight.

Sunnily yours,

Stanbridge

References and further reading:

Matthias Wacker and Michael F. Holick, Sunlight and Vitamin D, Dermato-Endocrinology 5:1, 51–108; January/February/March 2013.

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