The short answer: From about October to March, northern sunlight is too weak for your skin to make much vitamin D, so a D3 supplement is the simplest way to keep your levels up. Vitamin D and K2 both work on calcium. D helps you absorb it, and K2 activates the proteins that help bind it into bone. Choose K2 as MK-7, and take both with a meal that includes healthy fats.

On a cloudless December day, at about the same latitude as Boston, you'd need more than an hour of noon sun on your bare face and hands to make about 220 IU of vitamin D. That's what a 2013 study estimated for Sapporo, Japan. Not many of us spend our December lunch hour standing in the sun.

By November, most of us are leaving for work in the dark and getting home in the dark. Your skin has already stopped making much vitamin D, so now's the time to sort it out.

Personally, I take D3 and K2 all year round. They're part of the foundation of my daily routine for bone, muscle and heart health, and the beet root in each capsule is a bonus.

Why can't your skin make vitamin D in winter?

Your skin makes vitamin D from UVB light. In winter, the sun sits too low in northern skies for much of that UVB to get through.

Researchers first measured this in the late 1980s. On clear days, winter sunlight produced no vitamin D precursor at all in skin samples in Boston (about 42°N) from November through February, or in Edmonton (52°N) from October through March. Further south, at 34°N and 18°N, production carried on all winter.

Newer work backs it up. A 2016 study modelled ten years of real sunlight across Europe, clouds and all, and found a "vitamin D winter" of four to five months a year at about 47 to 55°N, getting longer the farther north you go. At 35°N, there wasn't one.

Fatty fish, egg yolks and fortified milk help, but few people eat enough of them every day to make up the difference.

D3 or D2?

D3. Head-to-head trials have found that D3 raises blood levels of vitamin D more than D2, especially when people take a big dose now and then instead of a little every day. With daily doses the gap is smaller, but a large 2021 review still found D3 came out ahead. It's also the form your own skin makes.

Why take K2 with vitamin D?

Vitamin D supports the maintenance of healthy bones and teeth, mostly by helping you absorb calcium from food. It also supports normal immune and muscle function.

Absorbing calcium is only part of it, though. Your body relies on proteins like osteocalcin to help bind calcium into bone, and vitamin K is what switches them on. The way I think of it: D brings calcium in, and K2 helps put it where it belongs.

The longest trial of K2 at a typical supplement dose followed 244 postmenopausal women for three years. Those taking 180 mcg of MK-7 a day lost less bone at the lower spine and the top of the thigh bone than the placebo group, and the top of the thigh bone kept more of its strength.

Not every K2 trial has matched that result, and the research is still young. But vitamin K's role in putting calcium to work is well established.

Does the form of K2 matter?

It does. If a K2 label doesn't say MK-7, I'd ask before buying it.

MK-7 has a very long half-life, so it hangs around in your blood far longer than other forms of vitamin K, and levels stay much steadier. With daily use, it reached levels seven to eight times higher than vitamin K1.

MK-4 clears fast.

In one small study, a single 420 mcg dose of MK-4 couldn't be detected in the blood at all, while the same dose of MK-7 peaked at six hours and stayed detectable for up to two days. At a typical 60 mcg a day for a week, MK-7 raised blood levels and MK-4 didn't. That's why ours is MK-7.

How should you take them?

Both are fat-soluble, so take them with a meal that includes healthy fats. In one small clinic study, people whose vitamin D levels weren't budging switched to taking it with their largest meal, and their average levels rose by about 57% over two to three months. There was no comparison group, so it's not proof.

Same meal, every day. Breakfast works for most people. Let it float and it becomes something you remember twice a week.

You'll want enough magnesium too, since the enzymes that convert vitamin D into the forms your body uses appear to need it. Which forms of magnesium are worth taking is a post of its own, and it's coming up. And watch your total. The commonly cited upper limit for adults is 4,000 IU of vitamin D a day from all sources, so count any multivitamin or fortified foods.

Heads up: if you take a blood thinner such as warfarin, talk to your doctor before starting any vitamin K, including K2. Research on MK-7 suggests doses of 50 mcg a day or more may interfere with these medications.

How long until it works?

In a study of men taking daily D3 through a winter, blood levels in the higher-dose groups were only approaching a plateau after 20 weeks. That's longer than most people expect. Your levels tend to be highest at the end of summer and drift down through the winter, so the best time to start is early fall, while you still have what the sun gave you. If you're reading this later, start now and keep going until spring.

How we make ours

I kept D and K2 as separate capsules on purpose, so you can take them as a pair or adjust either one on its own. Our Vitamin D gives you 2,500 IU of D3 a day, and our K2 gives you 120 mcg of MK-7 a day. Each one has a bit of beet root in it as a whole-food pairing. It isn't filler. We make them in small batches, as needed, in a GMP-certified compounding lab. Every ingredient is third-party tested.

Common questions

Can I take vitamin D and K2 at the same time? Yes. Take them at the same meal.

Should I take vitamin D all year? Many people only need it from fall to spring. I take mine year-round. If you spend little time outdoors or always wear sunscreen, year-round can make sense. A blood test is the only way to know your level.

Is 2,500 IU a day too much? For most healthy adults it sits well within the commonly cited 4,000 IU upper limit. Just add up what's in your multivitamin and anything fortified.

Can I get K2 from food? Some, from aged cheeses and fermented foods like natto. But amounts vary a lot, and most of us don't eat natto.

If you're not sure whether D and K2 belong in your routine, or what else should be in it, our free Health Assessment takes about five minutes and gives you a plan based on your answers.

Take the Health Assessment →

Kevin A.

Every study cited here was checked against the original paper.

This article is for general information and isn't medical advice. Talk to your healthcare provider before starting a new supplement, especially if you take medication.

References

1. 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):373-8. PubMed 2839537
2. O'Neill CM, Kazantzidis A, Ryan MJ, et al. Seasonal changes in vitamin D-effective UVB availability in Europe and associations with population serum 25-hydroxyvitamin D. Nutrients. 2016;8(9):533. doi:10.3390/nu8090533
3. Miyauchi M, Hirai C, Nakajima H. The solar exposure time required for vitamin D3 synthesis in the human body estimated by numerical simulation and observation in Japan. J Nutr Sci Vitaminol. 2013;59(4):257-63. doi:10.3177/jnsv.59.257
4. 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):1357-64. PubMed 22552031
5. Balachandar R, Pullakhandam R, Kulkarni B, Sachdev HS. Relative efficacy of vitamin D2 and vitamin D3 in improving vitamin D status: systematic review and meta-analysis. Nutrients. 2021;13(10):3328. doi:10.3390/nu13103328
6. Knapen MH, Drummen NE, Smit E, Vermeer C, Theuwissen E. Three-year low-dose menaquinone-7 supplementation helps decrease bone loss in healthy postmenopausal women. Osteoporos Int. 2013;24(9):2499-507. PubMed 23525894
7. Rønn SH, Harsløf T, Oei L, Pedersen SB, Langdahl BL. The effect of vitamin MK-7 on bone mineral density and microarchitecture in postmenopausal women with osteopenia, a 3-year randomized, placebo-controlled clinical trial. Osteoporos Int. 2021;32(1):185-191. doi:10.1007/s00198-020-05638-z
8. Schurgers LJ, Teunissen KJ, Hamulák K, et al. Vitamin K-containing dietary supplements: comparison of synthetic vitamin K1 and natto-derived menaquinone-7. Blood. 2007;109(8):3279-83. PubMed 17158229
9. Sato T, Schurgers LJ, Uenishi K. Comparison of menaquinone-4 and menaquinone-7 bioavailability in healthy women. Nutr J. 2012;11:93. PubMed 23140417
10. Mulligan GB, Licata AA. Taking vitamin D with the largest meal improves absorption and results in higher serum levels of 25-hydroxyvitamin D. J Bone Miner Res. 2010;25(4):928-30. PubMed 20200983
11. Uwitonze AM, Razzaque MS. Role of magnesium in vitamin D activation and function. J Am Osteopath Assoc. 2018;118(3). PubMed 29480918
12. Heaney RP, Davies KM, Chen TC, Holick MF, Barger-Lux MJ. Human serum 25-hydroxycholecalciferol response to extended oral dosing with cholecalciferol. Am J Clin Nutr. 2003;77(1):204-10. PubMed 12499343