Vitamin D
Correcting a genuinely common deficiency, with much weaker evidence once your levels are already normal.
Not medical advice — a blood test is the only reliable way to know if you're actually deficient before deciding on a dose.
How It Works
Vitamin D is produced in the skin from sun exposure and obtained from a small number of foods (fatty fish, fortified milk, egg yolks). It's essential for calcium absorption and bone health, and is increasingly studied for roles in immune and muscle function. Because so few foods naturally contain it and sun exposure varies enormously by season, latitude, skin tone, and sunscreen use, low levels are common rather than rare.
The Research
The key nuance with vitamin D is that its supplementation research tells two different stories depending on your starting point.
Deficiency prevalence: Herrick KA, et al. "Vitamin D status in the United States, 2011–2014," American Journal of Clinical Nutrition, 2019. Using NHANES data, about 5% of the U.S. population age 1+ was at risk of deficiency (blood levels under 30 nmol/L), and another 18% at risk of inadequacy (30 to under 50 nmol/L) — meaning roughly 1 in 4 people fall short of sufficient levels.Read the study →
Supplementing when you're already sufficient: Manson JE, et al. "Vitamin D Supplements and Prevention of Cancer and Cardiovascular Disease" (VITAL), New England Journal of Medicine, 2019. In a large trial where participants were largely vitamin D-replete at baseline (average blood level around 30 ng/mL), 2,000 IU/day supplementation showed no significant reduction in cancer or cardiovascular disease incidence overall. Related musculoskeletal sub-analyses similarly found no meaningful benefit in healthy, already-sufficient older adults.
Read the trial →
Taken together: vitamin D supplementation clearly matters for people who are actually low — correcting a real deficiency supports bone health and prevents conditions like rickets and osteomalacia — but the evidence for extra benefit in people who are already at normal levels is much thinner. This is a case where testing your actual level matters more than defaulting to a high dose "just in case."
Typical Dosing
Per the NIH Office of Dietary Supplements: the RDA is 600 IU (15mcg)/day for most adults, rising to 800 IU (20mcg)/day for adults over 70. The tolerable upper intake level is 4,000 IU (100mcg)/day for adults — higher therapeutic doses for confirmed deficiency are sometimes prescribed by a doctor based on blood work, but that's a different situation from routine self-supplementation.
NIH Office of Dietary Supplements — Vitamin D Fact Sheet →
Who Should Be Cautious
Sustained high-dose supplementation without medical supervision can lead to hypercalcemia (excess blood calcium), which can cause nausea, weakness, and in severe cases kidney problems — a real risk only at doses well above the RDA, not at normal intakes. People with certain conditions (e.g., primary hyperparathyroidism, sarcoidosis, some kidney conditions) should get medical guidance before supplementing, since these can affect how the body handles vitamin D and calcium.