Vitamin D is supplement #2 in the Foundation Stack, and for good reason. Approximately 42% of American adults had blood levels at or below 20 ng/mL in national survey data (Forrest and Stuhldreher, 2011); by the National Academies cutoffs that NIH uses, about 18% of Americans are at risk of inadequacy and 5% at risk of deficiency (NIH ODS). Low vitamin D is linked to more respiratory infections, poor bone health (rickets in children, osteomalacia in adults), a higher risk of depression in observational studies, and weaker muscle function, though supplement trials on muscle strength have been inconsistent. It's referenced 31 times across Health Britannica articles, the most-mentioned compound without a dedicated guide until now. This roundup ranks 5 D3+K2 supplements and explains why the K2 pairing matters.
Best premium: Thorne Vitamin D + K2 Liquid ($0.06 per 1,000 IU serving, customizable dose), precise dosing, every ingredient tested for purity and potency
Best budget: NOW Foods D3 2,000 IU ($0.04/day), cheapest effective option, no K2 (add separately)
Best vegan: Garden of Life mykind Organics D3 (about $0.16 per 1,000 IU spray at $20), lichen-derived D3, plant-based
Best for athletes: Momentous Vitamin D3 ($0.32/day), NSF Certified for Sport, 5,000 IU, which is above the 4,000 IU adult upper limit, so use it only with your doctor's say-so
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Why D3+K2 together (not D3 alone)
Vitamin D3 increases calcium absorption from your gut. That's good for bones, but without vitamin K2, that absorbed calcium may not be steered where you want it. Vitamin K activates osteocalcin, a bone protein involved in mineralization, and matrix Gla protein (MGP), a protein in blood vessel walls that might help reduce abnormal calcification (NIH ODS). NIH also says the role of the different forms of vitamin K in arterial calcification is still unclear, so treat the artery benefit as a strong hypothesis, not a settled fact. Taking D3 without K2 is like adding more cars to a highway without lane markers, the calcium goes everywhere instead of where it's needed.
Our editorial rule: pair them at D3 doses above 2,000 IU/day. At lower doses, dietary K2 from fermented foods may be sufficient: natto is the richest source, cheese contains menaquinones, and eggs carry modest amounts (NIH ODS). But since most people don't eat enough K2-rich foods, a D3+K2 combination product is the safest default.
How much vitamin D do you need?
The National Academies set the RDA at 600 IU/day for adults 19 to 70 and 800 IU/day over 70 (NIH ODS). The Endocrine Society's current guideline, published in 2024, suggests extra vitamin D for children and teens 1 to 18, adults 75 and older, pregnancy, and high-risk prediabetes, but suggests against supplementing above the RDA in healthy adults under 75 and against routine 25-hydroxyvitamin D testing (Demay et al., 2024). On blood levels, 20 ng/mL or more is sufficient for most people and levels above 50 ng/mL are linked to potential adverse effects (NIH ODS); the Society found no clear evidence for any higher "optimal" target. The tolerable upper limit is 4,000 IU/day for adults (NIH ODS), though many practitioners use higher doses under medical supervision to correct a documented deficiency. Get tested if you are in a risk group (older adults, limited sun exposure, darker skin, a condition that limits fat absorption, obesity or gastric bypass): a simple blood test from your doctor then sets your starting point and dose.
Most vitamin D sold to men comes as 5,000 IU, which sits above that 4,000 IU upper limit; our vitamin D for men comparison maps six products against NIH's lines and shows which one NSF lists.
Our top 5 vitamin D supplements
#2: Thorne Vitamin D + K2 Liquid ($34/600 servings on thorne.com in October 2026, $0.06 to $0.17/day)
Liquid drops allow precise dose customization: 2 drops = 1,000 IU D3 + 200mcg K2 (as MK-4, menatetrenone). Thorne's label says 2 drops one to three times daily, so 1,000 to 3,000 IU depending on your blood levels. Thorne tests every ingredient for identity, purity, potency and contaminants such as heavy metals. Thorne's quality standards are the same that earned our #1 picks for creatine and curcumin. Best for people who want precise dosing or have trouble swallowing pills. On warfarin? Each serving carries 200mcg of K2, so the same caution applies: vitamin K can interfere with warfarin (Coumadin) and similar blood thinners, so keep your intake steady and clear it with your doctor first (NIH ODS, vitamin K). Prop-value: Evidence 9.5, Value 9.5, Purity 9.5
#3: NOW Foods Vitamin D3 2,000 IU ($8/240 softgels, $0.03/day)
The absolute cheapest effective D3 supplement. No K2 included, add a separate K2 supplement if dosing above 2,000 IU. Third-party tested, GMP certified. At 3 cents per day, cost is never a reason to skip vitamin D. Prop-value: Evidence 9.5, Value 10/10, Purity 8.5
#4: Garden of Life mykind Organics Vegan D3 Spray (1,000 IU per spray, about 126 sprays per 58 mL bottle, $20 when we last checked in March 2026)
Vegan D3 derived from lichen (most D3 comes from lanolin/sheep wool). USDA Organic, spray delivery. Best for vegans who want a certified plant-based D3. Slightly more expensive but the only fully vegan option with organic certification.
#5: Momentous Vitamin D3 5,000 IU ($18.99/60 caps, $0.32/day; above the 4,000 IU adult upper limit)
Higher dose (5,000 IU, above the 4,000 IU adult upper limit, so take it with your doctor's say-so) for athletes and people with documented deficiency (NIH ODS). NSF Certified for Sport. For daily maintenance, Momentous also sells a 2,000 IU version in olive oil ($19.99). Best for drug-tested athletes who need certified supplements. Same trusted brand as our magnesium recommendations.
Vitamin D across the stacks
Foundation: Supplement #2, take with breakfast alongside omega-3 for fat-soluble absorption.
Immunity: The immune master switch: lab work shows human T cells need vitamin D signalling to activate fully (von Essen et al., 2010). In trials, supplementation lowered the odds of acute respiratory infection by about 8% overall (Jolliffe et al., 2021), with the strongest protection from daily or weekly dosing in people who started out very deficient (Martineau et al., 2017).
Mood: Low vitamin D is associated with depression in observational studies, but clinical trials so far have not shown that supplements prevent depression or ease mild depressive symptoms; whether they help people with low levels who are being treated for depression has not yet been tested (NIH ODS).
Athlete: Supports bone health and normal calcium metabolism; trials on muscle strength have been inconsistent (NIH ODS), and a 2026 meta-analysis of randomized trials found no clear effect on testosterone in adult men (Paez-Allendes et al., 2026), so take it for bones and deficiency, not as a testosterone booster.
DTC multivitamins with vitamin D included
For users who prefer a single daily multivitamin rather than separate vitamin D supplementation, the direct-to-consumer multivitamin segment has matured significantly in the last few years. Ritual is the most-cited option in this category. Their Women's Multivitamin 18+ and Men's Multivitamin 18+ (formerly Essential for Women and Essential for Men) include 2,000 IU of vegan vitamin D3 (lichen-derived), along with omega-3 DHA, folate, B12, and a small number of other targeted nutrients. Usefully for this page, the women's formula also includes vitamin K2, so it is a D3+K2 product in its own right, and it carries USP Verified and Non-GMO Project Verified marks (and the same warfarin caution as our K2 picks applies to it). The formulations are deliberately minimalist (no megadosing, no "kitchen sink" mega-multis), third-party tested, and the brand publishes named clinical trials on their full formulation. Pricing runs around $33/month ($1.10/day), which is more than buying a dedicated D3+K2 and individual ingredients separately. The tradeoff: convenience and adherence (one capsule, not five). For users likely to skip a multi-bottle stack, a complete-but-modest DTC multivitamin like Ritual often produces better real-world outcomes than a more sophisticated stack the user doesn't take consistently.
Safety: upper limits and drug interactions
More is not better. The adult upper limit is 4,000 IU a day from all sources, and vitamin D toxicity, almost always from supplements, causes high blood calcium, nausea, vomiting, muscle weakness, excessive thirst and kidney stones, and in extreme cases kidney failure and heart rhythm problems (NIH ODS). Taking calcium supplements alongside vitamin D can raise the risk of some of these effects.
Check your medications. Vitamin D can interact with orlistat, statins (high doses may reduce the potency of atorvastatin, lovastatin and simvastatin), corticosteroids such as prednisone, and thiazide diuretics, which together with vitamin D can push calcium too high, especially in older adults (NIH ODS). And because both of our top 2 picks contain vitamin K2: if you take warfarin (Coumadin) or a similar blood thinner, vitamin K can interfere with it, so keep your vitamin K intake consistent and talk to your doctor before starting or stopping a D3+K2 product (NIH ODS, vitamin K).
Sources
- NIH Office of Dietary Supplements. Vitamin D: Fact Sheet for Health Professionals. Accessed 19 October 2026.
- NIH Office of Dietary Supplements. Vitamin K: Fact Sheet for Health Professionals. Accessed 19 October 2026.
- Demay MB et al. Vitamin D for the Prevention of Disease: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2024. Accessed 19 October 2026.
- Jolliffe DA 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. Accessed 19 October 2026.
- Martineau AR et al. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ. 2017. Accessed 19 October 2026.
- Forrest KY, Stuhldreher WL. Prevalence and correlates of vitamin D deficiency in US adults. Nutr Res. 2011. Accessed 19 October 2026.
- von Essen MR et al. Vitamin D controls T cell antigen receptor signaling and activation of human T cells. Nat Immunol. 2010. Accessed 19 October 2026.
- Paez-Allendes L et al. Vitamin D Supplementation, Total Testosterone, and Androgen Bioavailability Markers in Adult Men: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. Nutrients. 2026. Accessed 19 October 2026.
Bottom line
Vitamin D is the highest-impact, lowest-cost supplement on Health Britannica. At $0.03-0.17/day, there is no financial reason to be deficient. Sports Research D3+K2 at $0.17/day is the optimal combination product. NOW Foods at $0.03/day is the cheapest effective option. Get your blood levels tested if you are in a risk group, supplement accordingly, and take it with a fat source (your omega-3 or breakfast) for better absorption. This is the one supplement where the cost of NOT taking it when you are low, more respiratory infections and bone loss, dramatically exceeds the cost of taking it.
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