Vitamin D in winter in Quebec: public health minimum or optimal level for longevity?
400 IU per day: enough to get through a Quebec winter, or just the minimum to avoid deficiency? Vitamin D seen through a longevity lens, evidence and limitations included.
In brief
The 400 IU recommended after age 50 aims for a public health minimum, not an optimal level. A statistical calculation published in 2014 even suggests that the recommended intake has been underestimated. From a longevity perspective, the reasonable goal is a measured blood level, often targeted between 75 and 125 nmol/L, reached with a daily dose adjusted to your weight, and not mega-doses without monitoring.
Understand in 1 minute
Three forms, three roles
- D3 (cholecalciferol): the form your skin makes; as a supplement, it comes from sheep's wool (lanolin) or lichen. D2 (ergocalciferol) comes from mushrooms or yeast.
- 25(OH)D: your liver turns it into this storage form. This is what a blood test measures.
- Calcitriol: your kidneys draw the active hormone from it as needed. Its level is very finely tuned and can remain normal during a full-blown deficiency: measuring it tells you nothing about your reserves[1].
The winter gap at 45° N
The skin only makes vitamin D if the sun is high enough. In 1988, Webb and Holick’s team measured no production in Boston (42° N) from November to February, or in Edmonton (52° N) from October to March[2]. At 45° N, Gatineau and Montreal are in between: we can estimate a “vitamin winter” of four to six months. In Canada, the percentage of people under 30 nmol/L rises from 5% to 13% in winter[6].
No need to turn red
Your skin makes vitamin D long before you turn red: small doses of UV, below the sunburn threshold and repeated often, are more effective than exposures near that threshold[3]. Beyond that, the excess is transformed into inactive compounds[1]. Burning therefore achieves nothing: enjoy the sun without burning, starting with shade, hats, and clothing.
The dosage debate
Avoiding deficiency and aiming for an optimal level are two different goals. They do not call for the same dose.
1. The public health minimum
Health Canada sets the recommended intake at 600 IU per day up to age 70 and 800 IU thereafter, from all sources combined, and the upper limit at 4,000 IU for adults[4]. Canada’s Food Guide recommends a 400 IU supplement from age 51 onwards[5]. These benchmarks aim for a level of at least 50 nmol/L for bone health: they prevent deficiency in a population, but they do not seek to optimize your long-term health.
2. The criticism: a calculation error
In 2014, two researchers from the University of Alberta, Paul Veugelers and John Paul Ekwaru, showed that the recommended intake had been calculated based on study averages rather than the variation between individuals. Corrected, the calculation indicates that about 8,895 IU per day would be needed for 97.5% of people to reach 50 nmol/L; they specify themselves that this figure exceeds studied doses and should be interpreted with caution[7]. In 2015, Robert Heaney and his colleagues arrived at a similar order of magnitude with different data: about 7,000 IU per day from all sources, including nearly 3,900 IU in supplements[8]. Recommendations have not been revised since.
3. Dosage based on weight
At the same dose, blood levels rise less in heavier individuals: according to extensive Canadian data, a dose 1.5 times higher is needed for those overweight and 2 to 3 times higher for those with obesity[9]. In Quebec, some practitioners use about 50 IU per kilo per day, a rule attributed to pharmacist Jean Dion by biologist Éric Simard (Vitoli conference, 2022). This is a practice rule, not a trial result, and Éric Simard’s company sells vitamin D drops.
4. Why large trials are not the final word
VITAL, the largest trial (25,871 adults, 2,000 IU/day, 5 years), reduced neither cancers, nor cardiovascular diseases, nor mortality[11]. But its participants started at about 77 nmol/L (31 ng/mL): most were not deficient. Robert Heaney reminded us that a trial on a nutrient must take the baseline status into account, ideally by recruiting people who are lacking it[10]. You don’t test water on people who are already hydrated. The proper summary of VITAL: no benefit when adding to an already sufficient level, which is not the same as saying “vitamin D is useless.”
5. Signals that interest longevity
Autoimmunity
In VITAL, 2,000 IU/day was associated with 22% fewer autoimmune diseases (HR 0.78, at the limit of statistical significance)[12].
Telomeres
In a VITAL sub-group (1,031 people), vitamin D reduced telomere shortening by 140 base pairs over 4 years[13]. A promising aging marker, not yet linked to clinical outcomes.
Prediabetes
Three trials combined show 15% less progression toward diabetes (HR 0.85)[14]. Among treated individuals, those who maintained at least 125 nmol/L had a 76% lower risk than those who remained between 50 and 74 nmol/L, a non-randomized comparison that suggests rather than proves.
What remains to be proven
No trial has shown that vitamin D extends life: these signals are emerging.
The ceiling and the real risks
More is not always better. In Calgary, 373 healthy adults took 400, 4,000, or 10,000 IU per day for 3 years: wrist bone density decreased more at 10,000 IU (-3.5%) and 4,000 IU (-2.4%) than at 400 IU (-1.2%)[15]. The same trial also shows more calcium in the urine (31% at 10,000 IU vs 17% at 400 IU) and some slight increases in blood calcium, especially at the highest dose[16]. Toxicity is rare, but real with high and prolonged intake[26].
What “longevity perspective” means
Aim for a measured level, not an endless dose. In 2011, the Endocrine Society considered a level of at least 75 nmol/L (30 ng/mL) to be sufficient[17], and many integrative or longevity medicine clinicians now target about 75 to 125 nmol/L. Beyond 125 nmol/L, authorities note possible adverse effects[26]: aiming for the top of this range should therefore be done with monitoring.
The Endocrine Society in 2024: a shift
The Endocrine Society is the largest international society of endocrinologists. In 2024, it published new guidelines that shift away from its 2011 position[18]:
- Supplementation suggested for children, pregnancy, high-risk prediabetes, and age 75+.
- No routine testing or target levels: no benefit threshold is established for healthy individuals.
- A daily dose rather than high, spaced-out doses.
These rely mostly on trials like VITAL, conducted on individuals who were not very deficient: this is exactly what proponents of optimal levels criticize.
In practice, this winter in Quebec
The form
D3 in oil, in drops or softgel; it raises levels better than D2[19]. Liposomal forms have little independent evidence; a benefit in cases of malabsorption remains to be demonstrated.
With your fattiest meal
With fat, the blood spike of D3 is about 32% higher than with a fat-free meal[20]. Morning or lunch is best: some people report disrupted sleep in the evening (anecdotal observation).
Every day rather than bolus
Against respiratory infections, a meta-analysis found protection with daily doses (400 to 1,000 IU)[22], an effect that became non-significant in the 2025 update[23]. And 500,000 IU once a year increased falls and fractures in elderly women[24].
The cofactors. Magnesium participates in vitamin D activation. In a randomized trial, it raised 25(OH)D in those starting around 75 nmol/L and lowered it in those starting higher (up to 125 nmol/L): a regulatory role[21]. Vitamin K2 directs calcium to the bones; solid logic, mixed clinical evidence, especially relevant at high D doses. From about 50 µg/day, K2 (MK-7) can interfere with warfarin[25].
The blood test. For longevity, measure your 25(OH)D before changing your dose, then about three months later. In Quebec, however, INESSS considers this test not indicated without symptoms or high risk (kidney or liver disease, malabsorption, fragility fracture, certain medications)[27]: talk to your doctor and inquire about possible costs.
Safety
Above 4,000 IU per day, do so with a healthcare professional and follow-up blood tests[4][16], especially in cases of kidney disease, kidney stones, or sarcoidosis.
Sunlight, much more than a source of vitamin D: light as a signal
A pill can replace vitamin D. It cannot replace light.
Established: light sets your clock
Thanks to melanopsin, your retina directly informs the brain's clock: morning light is the main signal that synchronizes your sleep[28].
Emerging: UVA and nitric oxide
In 24 volunteers, UVA lowered blood pressure temporarily via skin nitric oxide, without vitamin D[29]. In Sweden, avoiding the sun was associated with higher mortality (association, not proof)[30].
Emerging: red and near-infrared light
They act on a mitochondrial enzyme, cytochrome c oxidase[31]; plausible mechanism, few human trials.
Research lead (hypothesis)
Cells emit extremely weak light, “biophotons,” primarily from oxidation. Their role in cellular communication remains a debated hypothesis[32].
Hana’s perspective
The following is my personal perspective, not established science. I see the solar day as a series of signals. In the morning, when the sun is low, light contains proportionally more red and infrared than UV; I like to think it “wakes up” our mitochondria. Around noon, UVB allows the skin to make vitamin D. In the evening, the light fades, then darkness—not red light itself—lets melatonin rise. To me, the solar cycle is not decorative: it is our primary natural therapy. Inspired by Dr. Courtney Hunt, I see light as an interface between our environment and our DNA. And I am convinced that some of the research lag is due to the fact that sunlight cannot be patented, which attracts little funding.
To put this vision to the test: human trials comparing morning light exposure to a placebo, looking at mitochondrial markers and concrete health outcomes.
— Hana
The Ayurvedic perspective: the winter ritucharya
Ayurveda adjusts lifestyle to the seasons (ritucharya). For winter, the Charaka Samhita recommends unctuous foods, dairy products, and oil massage[33]. It is a tradition, not proof, but it aligns with an established fact: vitamin D is absorbed better with fat[20]. Taking it with the meal where you use your ghee combines both perspectives.
This will surprise you
- The recommended intake may be based on a statistical error; if corrected, it would be much higher[7][8].
- Your skin produces vitamin D long before it turns red: getting burned does not produce more[1][3].
- From November to March, 13% of Canadians are below 30 nmol/L, compared to 5% the rest of the year[6].
To place vitamin D among other longevity supplements, also read our analysis of longevity supplements and their evidence.
Supplementing with discernment
Vitamin D is one of the nutrients covered in our guide Supplementation & Functional Optimization: forms, dosage benchmarks, precautions, and interactions, to help you make informed decisions rather than acting on impulse.
Consult the full guideFrequently asked questions about vitamin D in winter
How many IUs of vitamin D should I take per day in winter?
Health Canada recommends 400 IU for those 51 and older, as a minimum. For an optimal level, the dose depends on your starting point and your weight. If exceeding 4,000 IU/day (the upper limit), do so under the guidance of a healthcare professional and with blood tests.
Should D3 be taken with K2?
Not mandatory at modest doses; more relevant at high doses, with mixed clinical evidence. If you are on warfarin, talk to your pharmacist first.
Is a daily or weekly dose better?
Daily: the 2024 guidelines prefer it over high, spaced-out doses, and a mega-dose once a year was even shown to increase falls and fractures in one trial.
Can you take too much vitamin D?
Yes, through supplements, never through the sun. At 10,000 IU/day for 3 years, a trial observed more bone loss and urinary calcium than at 400 IU. Prolonged excess raises blood calcium: nausea, intense thirst, confusion, kidney stones.
How do I know if I'm absorbing my vitamin D well?
Only a blood test (25(OH)D), taken before and then about three months after a dose change, will tell you. If your levels rise little despite taking it with a fatty meal, talk to your doctor: malabsorption or a magnesium deficiency could be the cause.
Scientific references
- Holick MF. Vitamin D deficiency. N Engl J Med. 2007;357(3):266-281. DOI.
- Webb AR, Kline L, Holick MF. Influence of season and latitude on the cutaneous synthesis of vitamin D3: exposure to winter sunlight in Boston and Edmonton will not promote vitamin D3 synthesis in human skin. J Clin Endocrinol Metab. 1988;67(2):373-378. DOI.
- Webb AR, Alghamdi R, Kift R, Rhodes LE. 100 years of vitamin D: dose–response for change in 25-hydroxyvitamin D after UV exposure: outcome of a systematic review. Endocr Connect. 2021;10(10):R248-R266. DOI.
- Health Canada. Notice of intent to amend the Prescription Drug List: vitamin D (table of reference nutritional values). canada.ca.
- Health Canada. Canada's Food Guide: advice on vitamin and mineral supplementation. canada.ca.
- Statistics Canada. Shedding light on Canada's vitamin D levels as we approach the shortest day of the year. StatCan Plus, 2023 (updated 2024). Data from the Canadian Health Measures Survey, 2016-2019. statcan.gc.ca.
- Veugelers PJ, Ekwaru JP. A statistical error in the estimation of the recommended dietary allowance for vitamin D. Nutrients. 2014;6(10):4472-4475. DOI.
- Heaney R, Garland C, Baggerly C, French C, Gorham E. Letter to Veugelers, P.J. and Ekwaru, J.P., A statistical error in the estimation of the recommended dietary allowance for vitamin D. Nutrients. 2015;7(3):1688-1690. DOI.
- Ekwaru JP, Zwicker JD, Holick MF, Giovannucci E, Veugelers PJ. The importance of body weight for the dose response relationship of oral vitamin D supplementation and serum 25-hydroxyvitamin D in healthy volunteers. PLoS One. 2014;9(11):e111265. DOI.
- Heaney RP. Guidelines for optimizing design and analysis of clinical studies of nutrient effects. Nutr Rev. 2014;72(1):48-54. DOI.
- Manson JE, Cook NR, Lee IM, et al. Vitamin D supplements and prevention of cancer and cardiovascular disease. N Engl J Med. 2019;380(1):33-44. DOI.
- Hahn J, Cook NR, Alexander EK, et al. Vitamin D and marine omega 3 fatty acid supplementation and incident autoimmune disease: VITAL randomized controlled trial. BMJ. 2022;376:e066452. DOI.
- Zhu H, Manson JE, Cook NR, et al. Vitamin D3 and marine ω-3 fatty acids supplementation and leukocyte telomere length: 4-year findings from the VITamin D and OmegA-3 TriaL (VITAL) randomized controlled trial. Am J Clin Nutr. 2025;122(1):39-47. DOI.
- Pittas AG, Kawahara T, Jorde R, et al. Vitamin D and risk for type 2 diabetes in people with prediabetes: a systematic review and meta-analysis of individual participant data from 3 randomized clinical trials. Ann Intern Med. 2023;176(3):355-363. DOI.
- Burt LA, Billington EO, Rose MS, Raymond DA, Hanley DA, Boyd SK. Effect of high-dose vitamin D supplementation on volumetric bone density and bone strength: a randomized clinical trial. JAMA. 2019;322(8):736-745. DOI.
- Billington EO, Burt LA, Rose MS, et al. Safety of high-dose vitamin D supplementation: secondary analysis of a randomized controlled trial. J Clin Endocrinol Metab. 2020;105(4):1261-1273. DOI.
- Holick MF, Binkley NC, Bischoff-Ferrari HA, et al. Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2011;96(7):1911-1930. DOI.
- Demay MB, Pittas AG, Bikle DD, et al. Vitamin D for the prevention of disease: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2024;109(8):1907-1947. DOI.
- Tripkovic L, Lambert H, Hart K, et al. Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: a systematic review and meta-analysis. Am J Clin Nutr. 2012;95(6):1357-1364. DOI.
- Dawson-Hughes B, Harris SS, Lichtenstein AH, Dolnikowski G, Palermo NJ, Rasmussen H. Dietary fat increases vitamin D-3 absorption. J Acad Nutr Diet. 2015;115(2):225-230. DOI.
- Dai Q, Zhu X, Manson JE, et al. Magnesium status and supplementation influence vitamin D status and metabolism: results from a randomized trial. Am J Clin Nutr. 2018;108(6):1249-1258. DOI.
- Jolliffe DA, Camargo CA, Sluyter JD, et al. Vitamin D supplementation to prevent acute respiratory infections: a systematic review and meta-analysis of aggregate data from randomised controlled trials. Lancet Diabetes Endocrinol. 2021;9(5):276-292. DOI.
- Jolliffe DA, Camargo CA, Sluyter JD, et al. Vitamin D supplementation to prevent acute respiratory infections: systematic review and meta-analysis of stratified aggregate data. Lancet Diabetes Endocrinol. 2025;13(4):307-320. DOI.
- Sanders KM, Stuart AL, Williamson EJ, et al. Annual high-dose oral vitamin D and falls and fractures in older women: a randomized controlled trial. JAMA. 2010;303(18):1815-1822. DOI.
- Schurgers LJ, Teunissen KJF, Hamulyák K, Knapen MHJ, Vik H, Vermeer C. Vitamin K-containing dietary supplements: comparison of synthetic vitamin K1 and natto-derived menaquinone-7. Blood. 2007;109(8):3279-3283. DOI.
- National Institutes of Health, Office of Dietary Supplements. Vitamin D: fact sheet for health professionals. ods.od.nih.gov.
- Institut national d'excellence en santé et en services sociaux (INESSS). Judicious use of biomedical analyses: 25-OH vitamin D dosage. Quebec, 2025. inesss.qc.ca.
- Blume C, Garbazza C, Spitschan M. Effects of light on human circadian rhythms, sleep and mood. Somnologie. 2019;23(3):147-156. DOI.
- Liu D, Fernandez BO, Hamilton A, et al. UVA irradiation of human skin vasodilates arterial vasculature and lowers blood pressure independently of nitric oxide synthase. J Invest Dermatol. 2014;134(7):1839-1846. DOI.
- Lindqvist PG, Epstein E, Landin-Olsson M, et al. Avoidance of sun exposure is a risk factor for all-cause mortality: results from the Melanoma in Southern Sweden cohort. J Intern Med. 2014;276(1):77-86. DOI.
- Hamblin MR. Mechanisms and applications of the anti-inflammatory effects of photobiomodulation. AIMS Biophys. 2017;4(3):337-361. DOI.
- Mould RR, Mackenzie AM, Kalampouka I, et al. Ultra weak photon emission: a brief review. Front Physiol. 2024;15:1348915. DOI.
- Charaka Samhita, Sutra Sthana 6 (seasonal regimen, ritucharya). Online edition Charak Samhita Online. carakasamhitaonline.com.
