Seasonality of Vitamin D in the UK
Blood levels of vitamin D are lower in the winter and spring compared the summer and autumn (Figure 1).
This is both known and expected, as most of our vitamin D comes from the direct action of the sun on our skin. The blood levels of vitamin D are slightly higher in women than in men.
In Figure 2 we see that in the winter more than 80% of British adults have vitamin D levels less than the ideal, and it is more than 60% in the summer. The British summer does not give a great deal of sunlight energy, due to a combination of latitude well away from the equator and a great deal of cloud cover.
In the winter almost 50% have vitamin D levels less than 16ng/mL, and 15% in the summer and autumn.
15% have in the winter and spring serious deficiency of vitamin D, less than 10ng/mL. Some have this serious deficiency even in the summer.
Illness is more common at times when vitamin D levels are lowest.
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