Vitamin D

One of the cheapest tests in the supplement aisle and the most useful: a meaningful share of adults in higher latitudes runs below the threshold where bone, immune, and possibly cardiovascular function start to wobble, and a low-cost lab plus a cheap pill closes that gap. But the trial evidence for routinely taking vitamin D when you're already replete is much thinner than the marketing implies.

For decades, observational studies linked low 25-hydroxyvitamin D to almost everything — heart disease, cancer, infections, depression, autoimmune conditions. The randomized trials that followed have been more measured: they confirm the importance of correcting genuine deficiency, but do not support routine supplementation in already-replete adults for primary cardiovascular or cancer prevention. The interesting exceptions are a handful of long-tail and subgroup findings — autoimmune disease, late-window cancer mortality, and biological aging — where the signal is real but the effect size is modest.

The headline evidence

The largest randomized trial of vitamin D in adults is ^[1]. The primary endpoints — invasive cancer and major cardiovascular events — were null.

Three pre-specified or exploratory analyses pointed in a more useful direction:

Cancer mortality fell by 25% after excluding the first two years of follow-up.^[2] The pattern suggests vitamin D may affect cancer progression more than initiation — consistent with the idea that established tumors take years to manifest as death. A later pooled analysis of individual patient data sharpened the practical lesson: daily D3 cut cancer mortality by about 12%, while intermittent high-dose bolus regimens showed no benefit — steady levels seem to matter more than total dose.^[3]
Incident autoimmune disease fell by 22% over 5 years.^[4] This is the strongest randomized signal vitamin D has produced outside of bone.
Biological aging slowed on two markers vitamin D researchers watch closely — telomere length and epigenetic clocks. This is the most intriguing extra-skeletal signal in the recent literature, and it gets its own section below.

Two important negative findings round out the picture:

Fracture risk in non-deficient adults was unaffected by 2000 IU/day in the VITAL Fractures analysis.^[5] Vitamin D matters for bone, but mainly via correcting deficiency, not via "more is better" in already-replete adults.
Cognitive decline has not moved in any major randomized trial of vitamin D supplementation in initially-replete adults.

Vitamin D and biological aging

Two recent results moved vitamin D out of pure disease prevention and into the territory this site cares about most — the pace of aging itself.

Telomeres. Telomeres are the protective caps on chromosomes that shorten with each cell division; their attrition is a hallmark of aging. In a four-year sub-study of the large VITAL trial, adults taking 2000 IU/day of D3 lost significantly less telomere length than those on placebo — roughly 140 base pairs spared, which the investigators frame as around three years of normal attrition.^[6] That sits on top of a large observational literature: across about 185,000 people, higher 25-hydroxyvitamin D tracks with longer leukocyte telomeres, most steeply in those climbing out of deficiency.^[7]

Epigenetic clocks. Combining vitamin D with omega-3 and exercise nudged DNA-methylation aging clocks in adults over 70 in the DO-HEALTH trial — individual effects were modest, and the three interventions stacked.^[8] Cross-sectional data echo the direction: lower 25-hydroxyvitamin D associates with an older biological age on the PhenoAge clock. But the relationship is not "more is always younger" — in men, levels above roughly 92 nmol/L were associated with a slight acceleration of epigenetic age, a U-shape that argues against chasing the highest tolerable number.^[9]

The honest caveat: telomere length and methylation clocks are surrogate markers. They are the best biological-age readouts we have, and the VITAL telomere result is a real randomized finding — but neither is the same as a demonstrated gain in healthy years.

What "deficient" actually means

The most widely-used cutoffs:

25(OH)D level	Status	Action
<30 nmol/L (<12 ng/mL)	Deficient	Correct urgently
30–50 nmol/L (12–20 ng/mL)	Insufficient	Supplement
50–125 nmol/L (20–50 ng/mL)	Sufficient	Maintain
>125 nmol/L (>50 ng/mL)	High	Reduce or stop supplementation
>250 nmol/L (>100 ng/mL)	Toxic risk	Stop; investigate hypercalcemia

Severe deficiency (<30 nmol/L) is uncommon in healthy adults eating a mixed diet at lower latitudes. Suboptimal status — 30–75 nmol/L — is widespread at higher latitudes, especially through autumn and winter, and is the gap most supplementation actually addresses.

The "vitamin D deficiency epidemic" framing from the 2010s was somewhat overstated; large-scale severe deficiency is rare in adults who eat normally and get any sun exposure. But the trial evidence supports correcting any insufficiency more strongly than chasing the highest tolerable level — supplementing toward 75–125 nmol/L is the defensible target.

Where the guidelines split

The mortality data anchor the low end. In cohort studies, all-cause and cardiovascular mortality fall steeply as 25-hydroxyvitamin D climbs out of deficiency and then flatten — the curve is essentially level above roughly 65 nmol/L (26 ng/mL).^[10] Most of the survival benefit is in escaping the bottom, not in reaching the top. That leaves a genuine disagreement about how high to aim:

The conservative position. A 2024 clinical guideline advises healthy adults aged 19–74 against routine 25-hydroxyvitamin D testing and against empiric high-dose supplementation, backing only the standard 600 IU/day — on the grounds that randomized trials have not shown extra-skeletal benefit in already-replete people.^[11]
The optimization position. A longevity-oriented camp argues for targeting 100–150 nmol/L (40–60 ng/mL), pointing to the telomere, epigenetic, autoimmune, and cancer-progression signals that emerge above the deficiency threshold. A conference analysis presented in 2025 reported a large reduction in repeat heart attacks when doses were titrated to keep blood levels high in cardiac patients — intriguing, but not yet peer-reviewed and not a licence to megadose.^[12]

The defensible middle hasn't changed: correcting any insufficiency toward 75–125 nmol/L has the strongest evidence behind it. The case for climbing into the 100–150 range rests on surrogate markers and subgroup findings rather than hard randomized outcomes, and the male epigenetic U-shape is a standing reminder that higher is not reliably better.

Practical dosing

Test first. A single morning 25-hydroxyvitamin D draw in autumn or winter (the seasonal trough) gives the most useful read.
Replete adults at the seasonal trough: 1000–2000 IU/day usually maintains target year-round.
Insufficiency (30–50 nmol/L): 2000 IU/day for 12 weeks, then retest.
Deficiency (<30 nmol/L): 4000 IU/day for 8–12 weeks, retest, then drop to maintenance.
Always take with a fat-containing meal. Vitamin D is fat-soluble; absorption on an empty stomach is poor.
Re-test annually at the end of winter once your maintenance dose is set.
Mind your magnesium. The enzymes that convert vitamin D into its active hormone are magnesium-dependent, so low magnesium can blunt the response to supplementation. If your 25(OH)D isn't rising as expected on a sensible dose, magnesium status is one thing to check. See Magnesium.

Forms and timing

D3 (cholecalciferol) is the right form for almost everyone. Beyond raising blood levels more reliably than D2 (ergocalciferol), D2 appears to accelerate the clearance of the body's own D3 — a meta-analysis of D2 trials found circulating 25-hydroxyvitamin D3 dropping even as total vitamin D rose.^[13] Choose D3.
Daily dosing beats weekly or monthly megadosing — the body handles steady-state vitamin D more cleanly than large boluses.
A drop or softgel is fine. The vehicle (olive oil, MCT) doesn't matter much as long as you take it with a meal.

Sun is not a substitute in winter

Above about 35° latitude (most of Europe and the northern US), winter UVB is too low for the skin to make any vitamin D between roughly October and March. The half-life of 25-hydroxyvitamin D is several weeks; a summer of good sun cannot cover a high-latitude winter. Use oral D3 across the dark months. See Sun exposure for the broader picture, including the data that strict sun avoidance roughly doubles all-cause mortality in long-running cohorts.

Pair with K2 if you're targeting bone

Vitamin D raises calcium absorption from the gut. Vitamin K2 (menaquinone-7, MK-7) activates the proteins that direct that calcium into bone matrix rather than soft tissue and arterial walls. The combination has more biological coherence than either alone.

For most healthy adults, 90–180 µg/day of MK-7 alongside D3 is reasonable. Coordinate with a prescriber if you're on warfarin — vitamin K antagonises it. See Vitamin K2 for the full evidence picture and Bone density for the loading side.

Cautions

Doses above 4000 IU/day without monitoring can cause hypercalcemia (rare but real). Stop and check serum calcium and 25(OH)D if you've been megadosing.
Granuloma-forming diseases (sarcoidosis, some lymphomas, tuberculosis) can dysregulate vitamin D metabolism — work with a clinician.
High-dose calcium supplementation has its own risk profile; dietary calcium does not show the same cardiovascular signal as high-dose calcium supplements. See Supplements to avoid.
Clinically meaningful vitamin D toxicity is rare from supplementation alone but real with cumulative megadosing over years.