Vitamin D's role in calcium absorption and bone health is well known, but that's just the beginning. Vitamin D receptors exist in virtually every cell type — brain, heart, immune cells, muscles, and gut. It modulates immune function (both boosting pathogen defence and reducing autoimmunity), regulates mood via serotonin synthesis, and influences insulin sensitivity.
Research links vitamin D deficiency to depression, multiple sclerosis, cardiovascular disease, type 2 diabetes, and increased cancer risk. A meta-analysis in the BMJ found that daily vitamin D supplementation reduced overall mortality, particularly from cancer, by a small but significant amount.
Modern life conspires against adequate vitamin D. We spend most of our time indoors, and when we go outside, we wear sunscreen (SPF 30 blocks 97% of vitamin D synthesis). People living in northern latitudes get virtually zero vitamin D from the sun between October and April. Darker skin requires 3-5 times more sun exposure to produce the same amount.
Food sources are limited — fatty fish, egg yolks, and fortified foods provide some, but not nearly enough for optimal levels. The old recommendation of 400 IU daily was set to prevent rickets, not to optimise health. Most functional medicine practitioners now recommend maintaining blood levels of 40-60 ng/mL.
Get your 25-hydroxyvitamin D blood level tested — it's a simple, inexpensive blood test. If you're below 40 ng/mL, supplementation is warranted. Most adults need 2,000-5,000 IU of vitamin D3 daily to maintain optimal levels, though some need more depending on body weight and absorption.
Always take vitamin D with fat (it's fat-soluble) and pair it with vitamin K2 (MK-7 form). K2 directs calcium into your bones and teeth rather than allowing it to deposit in your arteries. Without K2, high-dose vitamin D can potentially contribute to arterial calcification. A good ratio is 100mcg K2 per 5,000 IU D3.
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