Should You Worry About Getting Enough Vitamin K2?

[This is an edited reprint of a post I did for another blog I ran a few years back. I’ve seen vitamin K (and K₂) popping up around the news again, so I thought it might be a good time to put this article out again, especially since we’ve been talking about micronutrients anyway!]

Vitamin K is not as simple as it may seem. We only measure total vitamin K, but the vitamin can really be divided into multiple forms with multiple functions. Vitamin K₁ is not the same as vitamin K₂—they have different structures and different purposes—so it can seem like a gross oversight to measure them under the same category. Some individuals have decided that this means we have a discrete need for both, and that we should either supplement with vitamin K₂ or alter our diet to get enough. They’re wrong, though; there’s a legitimate reason why we consider them together and not separate.

Vitamin K₁ and K₂ Are Different, but End Up the Same

In terms of human health, there are two important forms of vitamin K: K₁ and K₂. We get vitamin K₁ from plants and vitamin K₂ from bacteria and animals.

Vitamin K₁ has but a single form known as phylloquinone, but Vitamin K₂ is actually comprised a group of compounds (menaquinones) with different chemical tail lengths. Different foods contain different types of vitamin K₂, but our body only appears to use a single type of K₂ called menaquinone-4 or MK-4 for short. All other forms of vitamin K₂ (collectively known as MK-n) must be converted into MK-4 to affect our health.

At times, we find vitamin K₂ in the spotlight as an important nutrient for bone health. And it’s true biologically that vitamin K₂ is important for bone health—but that doesn’t mean anyone needs to go out of their way to find it or consume it.

For example, there’s a hypothesis that eating more grass-fed animal products will improve our intake of vitamin K₂. It seems reasonable enough—animals convert the vitamin K₁ they get from grass into vitamin K₂, so animals who eat more grass should have higher vitamin K₂ levels. The problem is, we don’t actually know whether this is true because there has never been a single study done to assess the difference in vitamin K₂ levels between grass-fed and grain-fed meats; it’s premature to make this claim.

What we do know is that vitamin K turns over rapidly (around once every 1.5 days),¹ which means that any vitamin K₂ the animals gained from munching grass in a sunny field would be gone in less than half a week if switched off a grass-fed diet. Moreover, it means we could mimic the effects of pasturing animals by giving them a diet high in vitamin K just before slaughter. This, at least, is true: commercial chicken meat is one of the richest sources of vitamin K₂ (in the MK-4 form) because we feed those chickens a vitamin K supplement that they subsequently convert to vitamin K₂.

This is all aside from the real point, though, which is that we really don’t need to get vitamin K₂ from our diet at all. Instead, we merely need to get vitamin K in general, which we can then convert into vitamin K₂. This vitamin K can be 100% phylloquinone or 100% menaquinone (of any tail length) or a mixture, because once we absorb it our body will convert it to the bioactive menaquinone-4 form, just as cows grazing upon grassy fields convert the phylloquinone in the same way.^{2,3,4,5,6,7,8}

[Quick Aside: No study that I am aware of has shown vitamin K₁ to K₂ conversion in humans; however, animals as different as rats and chickens use the same pathway, so it likely exists in humans as well. Humans accumulate vitamin K₂ in the exact same tissues as rats, and in rats it has been shown that it accumulates in those tissues because those are the tissues converting vitamin K₁ to K₂. Thus, even though no study has demonstrated vitamin K conversion in humans, it’s reasonable to assume it happens.]

This means that vitamin K₂ is, at best, conditionally essential. We need it for bone health, but we can make it ourselves as long as we get enough vitamin K₁ (or MK-7, or MK-9, or whatever).

Our body only uses a single type of vitamin K₂—MK-4.^8,9 All vitamin K ends up as MK-4, whether we get it as vitamin K₁ or vitamin K₂. The MK-4 form of vitamin K is barely found in our diet: an egg yolk contains about 5.5 micrograms, meat typically contains less than 5 micrograms, and dairy products contain about 2 to 3 micrograms per serving.^10,11 Compare this to the 1,062 micrograms we get from a cup of cooked kale.¹¹

Fermented foods and animal products may contain higher amounts of other types of vitamin K₂—mostly MK-7, MK-8, and MK-9—¹⁰ but these long-tailed vitamin K₂s must be converted to MK-4 just the same as vitamin K₁.⁹

Now, these vitamin K₂s stick around in our blood longer,¹² so we might get more opportunity to convert them to bioactive MK-4. But then again, the opposite could also be true—perhaps they stick around longer because they don’t get converted well (i.e., they aren’t being taken up to be converted), and therefore aren’t so beneficial for bone health. At the end of the day, it’s probably inconsequential, if academically interesting; as long as your overall vitamin K intake is adequate, it won’t make a difference either way.

Don’t Worry About the Differences Between Vitamers for K

The scientists who determine how much vitamin K the average human needs are well-aware of the research on how vitamin K₁ and K₂ differ. When they decided not to measure vitamin K₁ separately from vitamin K₂, it was based on all the research available, which includes research suggesting humans convert vitamin K₁ into K₂ in the amounts they need—provided total vitamin K intake is adequate.

It could turn out that vitamin K₂ is an important dietary nutrient, or has greater or different activity than vitamin K₁, kind of like how the carotenoids serve health-related functions aside from their ability to be converted into vitamin A. We don’t have any evidence that suggests this, though, so it’s premature to say we should be eating more vitamin K₂ specifically or buying supplements, at least as long as your dietary intake of vitamin K is adequate (which it probably is; vitamin K deficiency is extremely rare).