Vitamin D is the rare nutrient your body can make entirely on its own — if you give it enough sunlight. It's also the rare nutrient that millions of people supplement without a test, based on a list of hoped-for benefits that ranges from the proven to the wishful. Sorting the two takes about five minutes.
How Your Body Makes It
Your skin synthesizes vitamin D when UVB light hits it. The compound then travels to the liver and kidneys, where it's converted into its active form — a hormone that helps regulate calcium absorption, bone remodeling, muscle function, and immune signaling. Only a handful of foods are naturally rich sources (fatty fish, egg yolks, UV-exposed mushrooms), which is why many countries fortify dairy and cereals. If you live far from the equator, wear sunscreen diligently, work indoors, or live through long winters, your skin synthesis may be low — which is exactly when diet and supplements matter.
Who's Actually Deficient
The definition of "deficiency" (blood level of 25-hydroxyvitamin D below 30 nmol/L, per most guidelines) is not exotic. Large surveys consistently find elevated deficiency rates among:
- Older adults in care settings or with limited outdoor exposure
- People with darker skin living at high latitudes (melanin reduces UVB synthesis)
- People who cover their skin for religious or cultural reasons
- People with malabsorption conditions (celiac, Crohn's, after bariatric surgery)
- Anyone who spends the working day indoors year-round
What Vitamin D Is Proven to Do
The evidence base is unusually strong in two areas:
- Bone health. Severe deficiency causes rickets in children and osteomalacia in adults. In randomized trials, supplementation in deficient older adults improves bone density and — combined with calcium — reduces fracture risk.
- Fall prevention in older adults. The landmark 2018 Cochrane review found vitamin D supplementation reduced the rate of falls in institutionalized older adults — a meaningful, practical benefit.
What the Big Trials Show About Everything Else
Here's where the marketing and the data diverge. Over the past decade, large randomized controlled trials — most notably VITAL (25,000+ U.S. adults) and D2d (diabetes prevention) — tested vitamin D against placebo for outcomes like cardiovascular disease, cancer, and type 2 diabetes. The results were largely null: vitamin D did not reduce heart attacks, strokes, cancer, or diabetes incidence in generally healthy, non-deficient adults. Observational studies had long suggested associations; the trials showed the associations were mostly confounding — healthy people with time outdoors have many other advantages.
The nuanced bottom line: correcting real deficiency helps; mega-dosing in healthy people does not.
Supplements: Dose, Toxicity, and Testing
- Who should test? Guidelines differ, but a reasonable approach: test if you're in a high-risk group, have bone disease, or have unexplained fatigue with risk factors. Routine universal testing isn't recommended.
- Dose. For correction of deficiency, daily doses of 800–2000 IU are common and safe; higher doses are reserved for confirmed deficiency and short-term correction under supervision.
- Toxicity is real. Vitamin D is fat-soluble and accumulates. Very high doses (tens of thousands of IU daily for months) cause hypercalcemia — dangerous calcium buildup — so "more is better" is actively wrong.
- Test after months, not days. Blood levels rise slowly; if you're tracking, wait 3–4 months on a stable dose before rechecking.
The Bottom Line
Vitamin D's proven value is concentrated in bone health and fall prevention for at-risk people. Its broader health halo — heart, cancer, immunity — has not survived randomized trials in non-deficient populations. The sensible play: get sensible sunlight when you can, eat the fatty fish, and if you're in a risk group, get tested and supplement to correct, not to chase an imagined optimal level. Treat deficiency, don't worship the number.