Do we need vitamin D in the winter?

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Do we need vitamin D in the winter?
« on: 29/01/2010 11:30:01 »
Carolyn asked the Naked Scientists:
I have read online that Vitamin D3 supplement is needed during winter months when sunlight is a rare commodity and was wondering if this is a valid claim?

What do you think?
« Last Edit: 29/01/2010 11:30:01 by _system »


Offline Kevan Gelling

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Do we need vitamin D in the winter?
« Reply #1 on: 31/01/2010 23:39:44 »
We obtain the majority of our vitamin D3 from the sunshine (90-95%), specifically UVB and heat converts a by-product of cholesterol into vitamin D3 (cholecalciferol).  According to research from Canada1, UVB light is too weak between September and March for vitamin D3 to be created at a latitude of 52o and above - e.g. Birmingham, UK.

During this period your vitamin D3 level (calculated by measuring the blood content of 25-hydroxy-vitamin-D or 25(OH)D) may drop between 10-15 ng/ml2.

Depending on your vitamin D3 level in September and your chosen vitamin D sufficiency level (typically 20-30 ng/ml), you may need supplementation (most people in England and almost all of Scotland appear to be below 30 by March2).  As a rough guide is 100 IU/d is approximately 1 ng/ml.

That's the long answer.

The short answer is Yes and at least 1,500 IU/d.

1. Webb, A. R., Kline, L. &  Holick, M. F. Influence of season and latitude on the cutaneous synthesis of vitamin d3: Exposure to winter sunlight in boston and edmonton will not promote vitamin d3 synthesis in human skin. J Clin Endocrinol Metab  67, 373-378 (1988). URL [nofollow].
2. Hyppönen, E. & Power, C. Hypovitaminosis d in british adults at age 45 y: nationwide cohort study of dietary and lifestyle predictors. The American journal of clinical nutrition  85, 860-868 (2007). URL [nofollow].