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Open Source Apothecary At Home / Metabolic and endocrine
Correct a deficiency. Do not mega-dose, because high intermittent dosing increased falls and fractures.
remedyVitamin D is a hormone precursor rather than a vitamin in the ordinary sense, and it is synthesised in skin from cholesterol under ultraviolet B. The two things worth knowing are that deficiency is common at high latitude in winter and is worth correcting, and that giving more than enough is not better and is sometimes worse.
Correcting a deficiency works. Vitamin D supplementation in deficient people reduces respiratory infection modestly, improves bone mineral density modestly, and in asthma is associated with fewer exacerbations — with the benefit concentrated in people who were deficient.
Exceeding sufficiency does not. In trials, high intermittent dosing of vitamin D increased falls and fractures compared with lower daily dosing. This is the U-curve pattern that recurs throughout this handbook: the relationship runs the wrong way above a point. The recommended intake for adults is around 600-800 IU daily, rising to 800 IU over 70, with higher doses reserved for documented deficiency under medical supervision.
Who is at risk of deficiency. People at high latitude (New Martinsville at 39.66 N is above the line where winter synthesis is meaningful), people with darker skin at high latitude, people who cover up or spend little time outdoors, older adults (whose skin synthesises less efficiently), exclusively breastfed infants, people with malabsorption or obesity, and anyone on long-term anticonvulsants or with kidney or liver disease.
Sun versus supplement, honestly. The amount of sun needed for synthesis is small and is not the amount that burns — a few minutes of midday exposure for fair skin. But there are two reasons not to rely on sun alone: at high latitude in winter there is essentially no UVB regardless of exposure time, and burning carries cumulative DNA damage and skin-cancer risk. The recommendation is to supplement rather than to burn. The dose-response curve for sun-derived vitamin D is capped by the skin's own regulation, which is not true of a tablet.
The testing question. A 25-hydroxyvitamin D level is the test. Testing is most useful when there is a reason to suspect deficiency, or before high-dose treatment. Routine testing of everyone is not recommended.
The one genuine toxicity. Vitamin D is fat-soluble and accumulates. Hypercalcaemia from excessive supplementation causes nausea, confusion, kidney stones and kidney injury. It takes sustained large doses, not a normal supplement.
Based on absolute risk reductions and thresholds considered clinically meaningful, this review found little to no benefits from use of calcium, vitamin D, or combined supplementation on the prevention of fractures and falls.PubMed 42161415 ↗
This updated meta-analysis yielded a similar point estimate for the overall effect of vitamin D supplementation on ARI risk to that obtained previously, but the 95% CI for this effect estimate now includes 1·00, indicating no statistically significant protection.PubMed 39993397 ↗
PURPOSE: This systematic review and meta-analysis assesses the efficacy of combined calcium and vitamin D supplementation on bone mineral density (BMD) and fracture risk among postmenopausal women with osteoporosis.PubMed 41063100 ↗
Observational and experimental evidence suggests that vitamin D plays a role in type 2 diabetes (T2D).PubMed 40199888 ↗
Vitamin D supplementation can lower the occurrence of exacerbations in children with asthma, along with the improvement of FEV1.PubMed 39421966 ↗
Vitamin D supplementation can significantly reduce serum FBG, HbA1c, HOMA-IR and fasting insulin levels in T2D patients; the effects were especially prominent when vitamin D was given in a short-term, high dosage to patients with a vitamin D deficiency, who were overweight, or had an HbA1c of 8% or higher at baseline.PubMed 39355942 ↗
Dosing
Getting it from sun (with the caveats)
Who should ask about testing
Babies
What not to do
Where it fits with other things