Vitamin D Cofactors: What Helps Vitamin D Absorb and Work

Vitamin D absorbs best when you take it with a meal that contains some fat, and it works alongside a few cofactors: magnesium, vitamin K2, boron and zinc. In a controlled study, a vitamin D3 dose taken with a meal containing fat produced a 32 percent higher peak in the blood than the same dose with a fat-free meal (Dawson-Hughes et al., 2015). Once absorbed, vitamin D is converted in the liver and then the kidney into its active form (NIH ODS), and magnesium status influenced vitamin D levels in a randomized trial (Dai et al., 2018).

Key Takeaways

• Take vitamin D with fat: the peak blood level of vitamin D3 was 32 percent higher with a fat-containing meal than with a fat-free one (Dawson-Hughes et al., 2015).
• Magnesium: in a randomized trial of 180 adults, magnesium raised 25-hydroxyvitamin D when levels started near 30 ng/mL and lowered it when they started higher, which the authors read as magnesium helping to optimize vitamin D status (Dai et al., 2018).
• Vitamin K2: vitamin K is the coenzyme that activates vitamin K-dependent proteins involved in bone metabolism (NIH ODS), which is why it is often paired with vitamin D3.
• Boron: low boron intakes appeared to reduce blood levels of 25-hydroxyvitamin D (NIH ODS), and most adults get only about 1 to 1.5 mg a day.
• Zinc: needed for the activity of hundreds of enzymes and for immune function (NIH ODS).

What Is a Vitamin D Cofactor?

A cofactor is a nutrient that another nutrient needs to do its job. Vitamin D is a good example of why they matter. The vitamin D you swallow or make in your skin is not active yet. The liver first turns it into 25-hydroxyvitamin D, the form a blood test measures, and the kidney then turns that into 1,25-dihydroxyvitamin D, the active form (NIH ODS). Enzymes run each step, and enzymes depend on minerals.

So "what helps vitamin D absorption" really has two parts: getting vitamin D from your gut into your blood, and then converting and using it. Fat at the meal helps the first part. Magnesium, boron and vitamin K2 come into the second part. You can read more about the vitamin itself on our vitamin D3 ingredient page and in our vitamin D3 supplement guide.

Vitamin D Cofactors at a Glance

Cofactor	Role with vitamin D	Adult daily need	Upper limit (adults)
Dietary fat	Helps absorb vitamin D from the gut	Some fat at the meal you take vitamin D with	Not applicable
Magnesium	Affects parathyroid hormone and the active form of vitamin D	310 to 420 mg (RDA)	350 mg from supplements
Vitamin K2	Activates vitamin K-dependent proteins in bone	90 to 120 mcg vitamin K (AI)	None set
Boron	Low intakes linked to lower 25-hydroxyvitamin D	No RDA set	20 mg
Zinc	Needed by hundreds of enzymes	8 mg (women), 11 mg (men)	40 mg

Figures from the NIH Office of Dietary Supplements fact sheets for magnesium, vitamin K, boron and zinc.

Take Vitamin D With a Meal That Has Fat

Vitamin D dissolves in fat, and NIH notes that fat in the gut at the same time enhances its absorption, although some vitamin D is absorbed without it (NIH ODS).

The clearest test comes from a study of 50 healthy older adults. After an overnight fast, each took a single 50,000 IU dose of vitamin D3 with breakfast. One group's breakfast was fat free; the other two got 30 percent of calories from fat. The peak blood level of vitamin D3 was 32 percent higher in the fat groups. The type of fat (more monounsaturated or more polyunsaturated) made no difference (Dawson-Hughes et al., 2015).

The size of the meal may matter too. In a small clinic study, 17 people whose vitamin D levels had not risen on their usual supplement were asked to take the same supplement with their largest meal of the day. Their average 25-hydroxyvitamin D rose from about 30.5 to 47.2 ng/mL (Mulligan and Licata, 2010).

In practice, take vitamin D with a regular meal rather than with black coffee or on an empty stomach. Eggs, yogurt, nuts, avocado, olive oil or fish all bring enough fat. Our list of foods high in vitamin D covers the foods that carry the vitamin itself.

Magnesium

Magnesium is the cofactor with the most direct research. NIH notes that magnesium affects the levels of both parathyroid hormone and the active form of vitamin D (NIH ODS).

A double-blind randomized trial of 180 adults aged 40 to 85 tested magnesium against placebo, with each person's dose set by how much magnesium they already ate. Magnesium raised blood 25-hydroxyvitamin D in people who started near 30 ng/mL, and lowered it in people who started higher, at about 30 to 50 ng/mL. The authors concluded that good magnesium status may be important for optimizing vitamin D status (Dai et al., 2018).

Adults need 310 to 420 mg of magnesium a day from all sources. Keep magnesium from supplements at or under 350 mg a day; that limit does not include magnesium from food (NIH ODS). For more, see vitamin D and magnesium and foods high in magnesium.

Vitamin K2

Vitamin K works as a coenzyme for vitamin K-dependent carboxylase, the enzyme that activates proteins involved in blood clotting and bone metabolism, including matrix Gla protein in bone, cartilage and blood vessel walls (NIH ODS). Vitamin D helps you absorb calcium, so many people take vitamin D3 and K2 together.

The adequate intake for vitamin K is 120 mcg a day for men and 90 mcg for women, and no upper limit has been set. Vitamin K can interact seriously with blood thinners such as warfarin, so anyone on one should keep their vitamin K intake steady and talk to their clinician before adding K2 (NIH ODS). Our guides to vitamin D3 and K2 and MK-4 vs MK-7 go deeper.

Boron

Boron is a trace mineral found in fruit, vegetables, nuts and legumes. No RDA has been set for it. In dietary studies, low boron intakes appeared to reduce blood calcium and 25-hydroxyvitamin D, and NIH notes that boron may affect steroid hormones, including vitamin D and estrogen (NIH ODS).

Median intakes for US adults are about 1.0 to 1.5 mg a day including supplements, and the upper limit for adults is 20 mg a day (NIH ODS). Our boron benefits guide covers the human studies and food sources.

Zinc

Zinc is needed for the catalytic activity of hundreds of enzymes and plays a role in immune function (NIH ODS). It often rounds out a vitamin D routine because both nutrients support normal immune function; our immune system hub covers the rest.

Adults need 8 mg (women) to 11 mg (men) a day, and the upper limit is 40 mg. Doses of 50 mg or more for weeks can block copper absorption, and very high doses (142 mg a day) interfered with magnesium absorption in one study (NIH ODS; NIH ODS). At normal doses, magnesium and zinc can be taken together; see magnesium and zinc together, how much zinc per day and our zinc bisglycinate ingredient page.

A Simple Vitamin D Routine

1. Pick a meal with some fat, ideally one you eat every day, and take your vitamin D with it.
2. Take the cofactors at the same meal. Magnesium, boron and zinc at normal doses can go together.
3. Count every source. Add up vitamin D, magnesium and zinc from all your supplements and multivitamins so you stay under the upper limits. For vitamin D, that limit is 100 mcg (4,000 IU) a day for adults (NIH ODS).
4. Get tested if you are unsure. A 25-hydroxyvitamin D blood test shows where you are, so you and your clinician can decide on a dose.

Where Our Cofactor Capsules Fit

We made Vitamin D Cofactor Capsules to sit next to vitamin D3 at the same meal. Every 2 capsules contain:

Nutrient	Per 2 capsules	Form
Magnesium	100 mg	Magnesium bisglycinate
Zinc	10 mg	Zinc bisglycinate
Boron	3 mg	Boron glycinate

These are gentle chelated forms, and each amount sits well under the adult upper limit. The capsules are made to pair with our Vitamin D3 + K2 Drops: vegan vitamin D3 from lichen, 2,000 IU (50 mcg) per drop, with vitamin K2 as MK-7 200 mcg and MK-4 100 mcg, taken once a day with a meal that contains some fat. The drops are available now; the capsules are coming soon, and you can join the waitlist on their page to hear when they launch.

Frequently Asked Questions

What are the cofactors for vitamin D?

The ones most often paired with vitamin D are magnesium, vitamin K2, boron and zinc, plus some fat at the meal you take it with to help absorption.

What helps vitamin D absorption the most?

Taking it with a meal that contains fat. In one study the peak blood level was 32 percent higher than with a fat-free meal (Dawson-Hughes et al., 2015).

Do you need magnesium to absorb vitamin D?

Magnesium is not needed to absorb vitamin D from the gut, but it affects the active form of vitamin D, and in a randomized trial magnesium raised vitamin D levels in people who started lower and lowered them in people who started higher (Dai et al., 2018).

What should you not take with vitamin D?

Avoid stacking several products that each contain vitamin D, which can push you past the 4,000 IU adult upper limit. If you take a blood thinner, talk to your clinician before adding vitamin K2.

Is it better to take vitamin D in the morning or at night?

The meal matters more than the clock. Pick a daily meal that contains fat and take it then.

Can I take vitamin D, magnesium and zinc together?

Yes, at normal doses. Very high zinc doses, far above the 40 mg upper limit, interfered with magnesium absorption in one study (NIH ODS).

How much boron should I take with vitamin D?

No RDA is set for boron. Most adults get about 1 to 1.5 mg a day, and the upper limit is 20 mg (NIH ODS). Our capsules provide 3 mg per 2 capsules.

Is vitamin D3 better than D2?

See our guide to vitamin D vs D3, and vegan vitamin D3 for plant-based sources.

Sources

1. Dawson-Hughes B, Harris SS, Lichtenstein AH, et al. Dietary fat increases vitamin D-3 absorption. J Acad Nutr Diet. 2015. PubMed 25441954
2. Mulligan GB, Licata A. Taking vitamin D with the largest meal improves absorption and results in higher serum levels of 25-hydroxyvitamin D. J Bone Miner Res. 2010. DOI
3. 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. PubMed 30541089
4. National Institutes of Health, Office of Dietary Supplements. Vitamin D: Fact Sheet for Health Professionals. NIH ODS
5. National Institutes of Health, Office of Dietary Supplements. Magnesium: Fact Sheet for Health Professionals. NIH ODS
6. National Institutes of Health, Office of Dietary Supplements. Vitamin K: Fact Sheet for Health Professionals. NIH ODS
7. National Institutes of Health, Office of Dietary Supplements. Boron: Fact Sheet for Health Professionals. NIH ODS
8. National Institutes of Health, Office of Dietary Supplements. Zinc: Fact Sheet for Health Professionals. NIH ODS

Our products are not intended to diagnose, treat, cure or prevent any disease. This article is for education only and is not medical advice.