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Vitamin D Deficiency at Latitude
Live north of the line through San Francisco, Madrid, and Beijing and your skin makes no vitamin D for four months a year: winter sun hits too flat for the chemistry to start Webb 1988. The big trials that "found no benefit" were run on people already fine, but the benefit is real for the roughly one in three who are genuinely low. The whole game is knowing which group you're in, and the fix costs about $10 a year.
Do Β· Daily Evidence Moderate Chapter Supplements

The split, not the pill, is the finding. In adults who start with normal levels, supplementing does close to nothing: null on cancer, heart attack, stroke, fractures, and depression in the 25,871-person VITAL trial 1 2. In people who start genuinely low, the same interventions land hard.

Winter respiratory infection drops about 12% across all adults but around 70% in the deeply deficient 3. New autoimmune disease fell 22% over five years on 2,000 IU/day 4. And moving a deficient person from 25 to 50 nmol/L tracks with about a 20% lower all-cause mortality risk; above 50, the curve flattens 5.

You're likely low if you live above 40Β°N (New York, Madrid, Beijing) or below 40Β°S; have darker skin, which needs three to five times the sun for the same yield 6; are over 75; live mostly indoors; carry a BMI over 30, which sequesters D in fat; or are pregnant. About a third of adults worldwide sit below the 50 nmol/L sufficiency line 7. You're likely fine with lighter skin, below 35Β°N, outside most days, and none of the above.

Skip the blood test; classify yourself instead. The 2024 Endocrine Society guideline retired routine testing and the old 30 ng/mL target for healthy adults, and the USPSTF found no basis to screen people without symptoms 8 9. Decide by risk group, then dose.

If you were actually low, the wins stack over time.

  • First winter: the cold that would have flattened you for two weeks is one you shake in four days, or skip 3.
  • A year or two: if you're older, fewer falls and near-misses on the stairs 10.
  • Five years: a fifth fewer new autoimmune diagnoses across a cohort like yours 4.
  • A decade out: the all-cause mortality curve shifts down for the deficient who repleted 5.

If you weren't low to begin with, you feel nothing, and that is the expected result.

The fine print β€” when to skip it, and what people get wrong
  • "The big trials proved it doesn't work." They proved it does nothing in already-replete adults. The deficient subgroups within those trials benefited 3.
  • "A good summer banks enough for the year." Stored D has a half-life near two months; summer sun can't carry you through February at high latitude.
  • "Higher is always better." The mortality benefit flattens above 50 nmol/L, and chronic doses over 10,000 IU/day can cause hypercalcaemia 5 6.
References
  1. 1Manson JE, Cook NR, Lee IM, et al. (2019). Vitamin D Supplements and Prevention of Cancer and Cardiovascular Disease. New England Journal of Medicine. link
  2. 2LeBoff MS, Chou SH, Ratliff KA, et al. (2022). Supplemental Vitamin D and Incident Fractures in Midlife and Older Adults. New England Journal of Medicine. link
  3. 3Martineau AR, Jolliffe DA, Hooper RL, et al. (2017). Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ. link
  4. 4Hahn J, Cook NR, Alexander EK, et al. (2022). Vitamin D and marine omega 3 fatty acid supplementation and incident autoimmune disease: VITAL randomized controlled trial. BMJ. link
  5. 5Sutherland JP, Zhou A, HyppΓΆnen E (2022). Vitamin D Deficiency Increases Mortality Risk in the UK Biobank: A Nonlinear Mendelian Randomization Study. Annals of Internal Medicine. link
  6. 6Holick MF (2007). Vitamin D Deficiency. New England Journal of Medicine. link
  7. 7Cui A, Xiao P, Ma Y, et al. (2023). Global and regional prevalence of vitamin D deficiency in population-based studies from 2000 to 2022: A pooled analysis of 7.9 million participants. Frontiers in Nutrition. link
  8. 8Demay MB, Pittas AG, Bikle DD, et al. (2024). Vitamin D for the Prevention of Disease: An Endocrine Society Clinical Practice Guideline. Journal of Clinical Endocrinology & Metabolism. link
  9. 9USPSTF (2021). Screening for Vitamin D Deficiency in Adults: US Preventive Services Task Force Recommendation Statement. JAMA. link
  10. 10Bischoff-Ferrari HA, Dawson-Hughes B, Staehelin HB, et al. (2009). Fall prevention with supplemental and active forms of vitamin D: a meta-analysis of randomised controlled trials. BMJ. link
Related in the handbook (10)
Helps
  • β€” Low vitamin D is tied to recurrent inner-ear vertigo (BPPV); topping it up measurably reduces repeat attacks of the spinning.
Works well with
  • β€” Low vitamin D weakens bone; it's one of the levers behind a poor density scan.
  • β€” Repleting deficient vitamin D cuts falls and fractures and is part of the bone-protection foundation.
Related
  • β€” Untreated celiac blocks vitamin D absorption, so deficiency hangs on despite sun or pills until the gut actually heals.
  • β€” Getting outside for your circadian dose also makes vitamin D, but far from the equator the winter sun is too weak, so a supplement covers the gap.
  • β€” Magnesium is a cofactor your body uses to activate vitamin D β€” low magnesium can quietly limit the benefit of a D supplement.
  • β€” Daily sunlight makes vitamin D in summer, but past a certain latitude winter sun can't, so a pill fills the gap.
  • β€” The same dark winters that drop your vitamin D drive seasonal depression β€” but light, not the pill, is the actual treatment.
  • β€” Daily sunscreen filters the UVB your skin uses to make D. If you're already low at your latitude, get it from food or a pill, not burns.
  • β€” K2 directs the calcium your D helps absorb into bone instead of artery wall; they're a natural pair.
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