Vitamin D: who actually needs supplementation

Vitamin D occupies an unusual position in nutrition. The biology is real, the deficiency is widespread, and the supplement is cheap and safe. But the gap between "vitamin D moves a lab number" and "vitamin D supplementation improves clinical outcomes" turned out to be much wider than the field assumed. The big randomized trials of the past decade have humbled the case for routine supplementation in adults who aren't deficient. Both findings are real; both deserve airtime.

The biology

Vitamin D is technically a hormone, not a vitamin in the strict sense. The skin synthesizes vitamin D3 (cholecalciferol) when UVB radiation hits 7-dehydrocholesterol; the liver hydroxylates it to 25(OH)D (the blood marker); the kidneys further hydroxylate it to the active 1,25(OH)2D, which regulates calcium absorption, bone mineralization, and a large list of secondary effects on immune function, inflammation, and gene expression. The receptor is present in nearly every tissue, which is why optimistic researchers in the 2000s saw vitamin D as the master nutrient. The clinical trials haven't quite borne that out.^[1]

The deficiency case is real

Frank vitamin D deficiency, serum 25(OH)D below 20 ng/mL (50 nmol/L), has well-replicated consequences:

• Increased risk of osteomalacia and accelerated bone loss in older adults.
• Higher fall and fracture rates in the elderly, particularly with concurrent low calcium intake.
• In severe pediatric deficiency, rickets.
• Pregnancy-related concerns including preeclampsia and low birth weight in some studies.^[2] Evidence: A

For these populations, repletion to at least 20-30 ng/mL has a clear clinical case. The intervention is cheap (vitamin D3 at 1000-2000 IU daily costs roughly $20/year), well-tolerated, and the upside is real.

The U.S. Endocrine Society recommends levels above 30 ng/mL as optimal; the Institute of Medicine considers 20 ng/mL sufficient for most adults. Reasonable clinicians disagree on the optimal target. The threshold for "this person is genuinely deficient" is the less-contested call.

The big-trial humility

The 2018 VITAL trial randomized 25,871 healthy adults (≥50 years for men, ≥55 for women) to 2000 IU/day vitamin D3 or placebo for an average of 5.3 years. The primary endpoints:

• Cancer incidence: no significant reduction.
• Major cardiovascular events: no significant reduction.
• All-cause mortality: no significant reduction.^[3] Evidence: A

The trial had real strengths (large, well-randomized, ethnically diverse, primary-care population, full intention-to-treat analysis) and real limitations (most participants had already-adequate baseline vitamin D, which dilutes the effect). The honest reading: in already-replete adults, routine vitamin D supplementation does not reliably move hard outcomes.

VITAL was followed by similar negative findings in D-Health (43,000 Australians) and a long list of secondary VITAL papers. The blood-level marker moves; the clinical outcomes mostly don't.^[4]

For fall and fracture prevention specifically, the picture is more mixed. Meta-analyses show modest benefits in older adults with low baseline status; the same benefit attenuates or disappears in already-replete populations.^[5] Evidence: B

How to think about this

The trials have been clear, even if the influencer ecosystem hasn't caught up:

If your 25(OH)D is under 20 ng/mL: there's a real case for supplementation. Replete to over 30, maintain. If your 25(OH)D is 20-30 ng/mL: reasonable to supplement; the marginal benefit on bone health is plausible and small. If your 25(OH)D is over 30 ng/mL on no supplement: the case for adding one is genuinely weak. The VITAL-class evidence doesn't support routine supplementation purely to push the number higher.

The single most useful thing: get tested once. Most labs offer a 25(OH)D test for $20-50. It tells you which of the three buckets you're in.

The dose

For maintenance in an adult with low-normal levels:

• 1000-2000 IU/day of vitamin D3 (cholecalciferol). D3 is meaningfully more effective than D2 (ergocalciferol) at raising blood levels.^[6]
• Take with a meal containing some fat for absorption.
• Recheck blood level after 8-12 weeks of consistent supplementation.

For frank deficiency:

• A short-term repletion protocol (often 50,000 IU weekly for 6-8 weeks) prescribed by a clinician, then maintenance on a lower daily dose.

For routine use, don't exceed 4000-5000 IU/day without bloodwork. Vitamin D toxicity is rare but real, typically at sustained intakes above 10,000 IU/day, and presents as hypercalcemia (nausea, kidney stones, cardiac arrhythmia). The upper limit isn't a marketing claim; sustained high-dose use without monitoring is the actual risk.

The sun question

Sun exposure is the natural source. The constraints are real:

• Latitude. Above ~37° N (a line roughly through San Francisco, St. Louis, Richmond), UVB intensity is too low for vitamin D synthesis from October through March. Below that line, year-round synthesis is possible.
• Skin exposure. Hands and face alone are not enough; the relevant dose requires arms and legs (or more) at sufficient intensity.
• Time. 10-20 minutes of midday sun on bare skin for a fair-skinned person; 30-60 minutes for darker skin tones. Through window glass is essentially zero (glass blocks UVB).

For someone who lives at a low latitude, works partly outdoors, and gets bare-skin sun a few times a week, supplementation is genuinely optional. For a northern-latitude indoor worker, sun exposure is, in practice, not enough during winter months, and supplementation makes sense.

This is also the right place to note: aggressive avoidance of all sun exposure has its own costs. We covered this in the morning sun piece for the circadian effect; the vitamin D effect adds a separate case. Some daily sun is good; chronic sunburn is not. Both can be true. We don't recommend avoiding sunscreen entirely, and we don't recommend chronic sun avoidance either.

The practical case

1. Test 25(OH)D once. The single most informative move.
2. If deficient, repletion is straightforward. Daily 1000-2000 IU with a fat-containing meal; recheck at 8-12 weeks.
3. If already-replete, don't blindly supplement. The VITAL-era evidence doesn't support routine supplementation past adequacy.
4. Sun matters and isn't always enough. Get some bare-skin sun when you can, particularly at lower latitudes in summer; supplement during the months when it isn't possible.
5. Treat the supplement aisle's "vitamin D for everything" framing with skepticism. The clinical-trial humility is real.

Vitamin D is a useful tool for the right person. The right person is the one with a measured low number, not the one who heard about it on a podcast.

FAQ

Should I supplement? Test 25(OH)D first. Below 20 ng/mL: yes. 20-30: reasonable. Above 30: weak case.

Dose? 1000-2000 IU/day for maintenance; 4000-5000 IU is the upper end for routine use; don't go higher without bloodwork.

Sun enough? Sometimes, at the right latitude and exposure. For most northern-latitude indoor workers in winter, no.

References

1. 1.Holick MF (2007). Vitamin D deficiency. New England Journal of Medicine 357(3):266–281. PMID: 17634462. Link
2. 2.National Institutes of Health, Office of Dietary Supplements (2026). Vitamin D, Health Professional Fact Sheet. NIH ODS. Link
3. 3.Manson JE, et al. (VITAL Research Group) (2019). Vitamin D supplements and prevention of cancer and cardiovascular disease. New England Journal of Medicine 380(1):33–44. PMID: 30415629. Link
4. 4.Neale RE, et al. (D-Health Trial) (2022). The D-Health Trial: a randomised controlled trial of the effect of vitamin D on mortality. The Lancet Diabetes & Endocrinology 10(2):120–128. PMID: 35026164. Link
5. 5.Bischoff-Ferrari HA, et al. (2012). A pooled analysis of vitamin D dose requirements for fracture prevention. New England Journal of Medicine 367(1):40–49. PMID: 22762317. Link
6. 6.Tripkovic L, et al. (2012). Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: a systematic review and meta-analysis. American Journal of Clinical Nutrition 95(6):1357–1364. PMID: 22552031. Link