Vitamin D2 vs D3 and Why K2 Matters: What the Forms Actually Mean

If you have ever compared two vitamin D supplements and noticed one listed D2 (ergocalciferol) and the other D3 (cholecalciferol), you may have wondered whether the difference is meaningful. It is.

D2 vs D3: The Basic Difference

D2 comes from plant sources and irradiated yeast. D3 comes from animal sources, most often lanolin derived from sheep's wool, though some brands now offer lichen-sourced D3.

Both forms raise serum 25-hydroxyvitamin D (25(OH)D) levels when taken orally. Multiple meta-analyses have found that D3 tends to raise and sustain blood levels more effectively than D2 at equivalent doses. The effect is most pronounced over longer supplementation periods.

For people choosing between the two, D3 is the form found in most clinical trials on vitamin D supplementation.

Why K2 Appears on the Same Label

Vitamin K2 (menaquinone) is a fat-soluble vitamin that activates proteins involved in calcium transport. Two of those proteins, osteocalcin and matrix Gla-protein, require K2-dependent carboxylation to function.

When vitamin D increases calcium absorption from food, K2 activates the proteins that govern where calcium is deposited. Because both vitamins are fat-soluble and their functions intersect in calcium metabolism, supplement manufacturers often combine them.

The two most common forms of K2 on labels are MK-4 and MK-7. MK-7 has a longer half-life in the bloodstream than MK-4, meaning it stays at detectable levels longer at lower doses.

What to Look for on a Label

When reading a vitamin D or D+K2 supplement label, these are the details worth checking:

• Form of D: D3 (cholecalciferol) rather than D2 (ergocalciferol) is the form used in most vitamin D clinical trials.
• Form of K2: MK-7 and MK-4 are both used; MK-7 is more common in newer formulations due to its longer half-life.
• Dose: D3 is commonly sold in 1,000 IU, 2,000 IU, and 5,000 IU doses. K2 in MK-7 form typically appears between 90 and 200 mcg per serving.
• Fat content: Both vitamins are fat-soluble. Taking them with a meal that includes fat supports absorption.

For a look at reviewed vitamin D and K2 supplements, see the supplements recommendations page.