Vitamin D Supplements — Your Winter Supplement Prep: Vitamin D and Immune Season

Vitamin D gets seasonal attention because skin synthesis drops when winter sun weakens. The immune case is only partly proven. A 2017 meta-analysis of 25 randomized trials, published in the BMJ, found fewer respiratory infections with supplements, with the strongest benefit in people who started deficient. This article sums up why the nutrient dips in winter, what the trial pool shows for infections, and where the evidence stops. Doses appear only as facts about studies or official guidance, never as advice. The gaps in the evidence are stated plainly. Why does vitamin D dip in winter? Sunlight is the main route. Skin makes vitamin D when UVB light reaches it, and living at high latitudes cuts that exposure, according to Wikipedia’s overview. Blood levels by the end of winter can run one-third to half below summer values. Older skin also makes less of the nutrient. The National Library of Medicine adds a second brake with age: the kidneys convert less vitamin D into its active form.

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