Vitamin D and Longevity: Separating Genuine Benefit from Wishful Thinking
Vitamin D is the most commonly supplemented micronutrient in the world and one of the most scientifically contested. Observational data linking low vitamin D to almost every disease known to medicine generated enormous enthusiasm. But RCTs have repeatedly failed to confirm these associations as causal. Here is the honest accounting.
- Low vitamin D levels are linked in observational studies to many disorders, from heart disease and cancer to autoimmune and infectious diseases, but a causal link for many of them has not been clearly established.1
- In the VITAL trial of 25,871 adults, 2,000 IU a day of vitamin D3 for a median of 5.3 years did not lower cancer, cardiovascular events or deaths from any cause, and the 17% lower rate of cancer death was not statistically significant.2
- One explanation for the gap between observational studies and trials is reverse causation and confounding: poor health can lower vitamin D levels, rather than low vitamin D causing poor health.
- Fracture results depend on who is treated: 700 to 800 IU a day cut hip fractures by 26% in older adults in a 2005 meta-analysis, but VITAL found no fracture reduction in generally healthy adults not selected for deficiency.4,7
- The 2024 Endocrine Society guideline suggests against routine vitamin D testing in the general population and found no clear evidence for an optimal blood level; it suggests empiric supplementation for ages 1 to 18, adults 75 and older, pregnancy and high-risk prediabetes.1
The vitamin D story is a cautionary tale in nutritional epidemiology. Studies have linked blood levels of 25-hydroxyvitamin D to musculoskeletal, metabolic, cardiovascular, malignant, autoimmune and infectious diseases, and those associations drove widespread supplementation and testing. Yet a causal link for many of these disorders has not been clearly established.1
Observational associations can be explained by reverse causation and confounding: people who are ill, inactive or obese may spend less time outdoors and make less vitamin D, and they also have higher rates of chronic disease. That is why randomized trials matter.
The VITAL Trial: What It Found
VITAL randomized 25,871 US men aged 50 or older and women aged 55 or older to 2,000 IU a day of vitamin D3 or placebo, alongside a separate comparison of omega-3 fats. Over a median of 5.3 years, vitamin D did not lower invasive cancer (hazard ratio 0.96) or major cardiovascular events (0.97). Among the secondary end points, the hazard ratio for death from cancer was 0.83 (95% CI 0.67 to 1.02), which was not statistically significant, and for death from any cause it was 0.99.2 In a separate VITAL analysis, confirmed autoimmune disease was 22% less common with vitamin D (hazard ratio 0.78, 95% CI 0.61 to 0.99, P = 0.05).6 New Zealand's ViDA trial, which gave 100,000 IU a month to 5,110 adults aged 50 to 84, found no benefit for cardiovascular disease, falls, non-vertebral fractures or cancer.3
Bones, Fractures and Infections
Vitamin D is needed for calcium absorption and bone health, but fracture trials give mixed answers. A 2005 meta-analysis of adults aged 60 and older found that 700 to 800 IU a day reduced hip fractures by 26% and non-vertebral fractures by 23%, while 400 IU a day was not enough.4 In contrast, VITAL found that 2,000 IU a day did not lower total, non-vertebral or hip fractures in generally healthy midlife and older adults who were not selected for deficiency, low bone mass or osteoporosis.7 For respiratory infections, an individual-participant meta-analysis of 25 trials found a modest protective effect overall (adjusted odds ratio 0.88), strongest with daily or weekly dosing in people who were very deficient (below 25 nmol/L) and absent with large bolus doses.5
Testing and Supplementing: What the 2024 Guideline Says
The guideline suggests against routine 25-hydroxyvitamin D testing in the general population and found no clear evidence defining an optimal target level. It suggests empiric vitamin D, meaning intake above the dietary reference intakes without testing, for four groups:1
| Group | Why the 2024 guideline suggests empiric vitamin D |
|---|---|
| Children and adolescents aged 1 to 18 | To prevent nutritional rickets and because it may lower the risk of respiratory tract infections |
| Adults aged 75 and older | Because it may lower the risk of death |
| People who are pregnant | Because it may lower the risk of preeclampsia, intra-uterine death, preterm birth, small-for-gestational-age birth and newborn death |
| People with high-risk prediabetes | Because it may reduce progression to diabetes |
Outside these groups, decisions about testing and dose are best made with a clinician, partly because the doses used in the trials varied considerably.1
Put this research into practice: Supplement Evidence Database · Biomarker Reference Tool
References
- 1Demay MB, et al. "Vitamin D for the Prevention of Disease: An Endocrine Society Clinical Practice Guideline." J Clin Endocrinol Metab. 2024;109(8):1907-1947. PubMed · DOI
- 2Manson JE, et al. "Vitamin D Supplements and Prevention of Cancer and Cardiovascular Disease." N Engl J Med. 2019;380(1):33-44. PubMed · DOI
- 3Scragg RKR. "Overview of results from the Vitamin D Assessment (ViDA) study." J Endocrinol Invest. 2019;42(12):1391-1399. PubMed · DOI
- 4Bischoff-Ferrari HA, et al. "Fracture prevention with vitamin D supplementation: a meta-analysis of randomized controlled trials." JAMA. 2005;293(18):2257-64. PubMed · DOI
- 5Martineau AR, et al. "Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data." BMJ. 2017;356:i6583. PubMed · DOI
- 6Hahn J, et al. "Vitamin D and marine omega 3 fatty acid supplementation and incident autoimmune disease: VITAL randomized controlled trial." BMJ. 2022;376:e066452. PubMed · DOI
- 7LeBoff MS, et al. "Supplemental Vitamin D and Incident Fractures in Midlife and Older Adults." N Engl J Med. 2022;387(4):299-309. PubMed · DOI
