The Two Types of Vitamin K in Egg Yolks
When asking how much vitamin K is in one egg yolk, it is important to distinguish between the two primary forms of this fat-soluble vitamin. Vitamin K comes in two main types: phylloquinone (K1) and menaquinone (K2). While a standard egg contains some K1, the most significant and variable vitamin K content is the K2, specifically the MK-4 subtype. The amount of K2 in the yolk is directly linked to the diet of the hen that produced the egg, which is why there can be a wide range of reported values.
Phylloquinone (K1) is the type of vitamin K found predominantly in green leafy vegetables, and a very small amount is present in the egg yolk. On the other hand, menaquinone (K2), especially the MK-4 subtype, is primarily found in animal products like egg yolks and organ meats. Its presence is a direct result of the hen's consumption of vitamin K, which is then passed into the egg.
Standard vs. Pasture-Raised Egg Yolks: A Vitamin K Comparison
The most critical factor influencing the vitamin K2 content of an egg yolk is the diet of the hen. This leads to a significant nutritional difference between conventionally raised eggs and those from pasture-raised hens.
- Conventionally Raised Hens: These hens are typically fed a grain-based diet, which provides a lower level of vitamin K precursors. As a result, the eggs they produce contain minimal amounts of K2. Research has shown that egg yolks from biofortified eggs (hens fed a K2-enriched diet) may contain between 46–51 µg of K2 per 100g, while standard, grain-fed hens have lower levels.
- Pasture-Raised Hens: Chickens that forage on pasture consume a varied diet of grasses and insects. This natural diet, rich in vitamin K1 from the greens, enables them to convert it into vitamin K2 (MK-4), resulting in a much higher concentration in their yolks. Studies show that egg yolks from pasture-raised hens can contain a substantially higher and more beneficial amount of vitamin K2.
Comparison of Vitamin K Content
| Nutrient | Standard Grain-Fed Egg Yolk (approx.) | Pasture-Raised Egg Yolk (approx.) |
|---|---|---|
| Vitamin K1 (Phylloquinone) | 0.12 mcg | ~0.12 mcg (Similar) |
| Vitamin K2 (Menaquinone) | 1–3 mcg (Estimated low value) | 37–60 mcg per 100g, highly variable |
| Total Vitamin K | Very Low | Significantly Higher |
This table highlights why simply knowing the amount of vitamin K1 is misleading. The true health benefits associated with vitamin K in eggs are driven by the K2 content, which is dramatically higher in pasture-raised eggs.
How Egg Preparation Affects Vitamin K
As a fat-soluble vitamin, vitamin K is quite stable during cooking. Hard-boiling, poaching, or frying an egg will not significantly degrade its vitamin K content. The fat in the yolk also aids in the absorption of vitamin K, meaning the cooking method is less important than the source of the egg itself. The most important factor remains the hen's diet.
Why Egg Yolk's Vitamin K2 is Important
While vitamin K is widely known for its role in blood clotting, K2 has specific, crucial functions beyond this. Research suggests that K2 helps regulate calcium metabolism, directing calcium toward bones and teeth and away from soft tissues like arteries and kidneys. This has major implications for bone and cardiovascular health. A diet rich in K2 can help to:
- Support bone health: K2 activates proteins like osteocalcin, which helps bind calcium to the bone matrix, increasing bone density and reducing fracture risk.
- Protect heart health: By preventing the calcification of arteries, K2 is linked to a lower risk of heart disease. Some studies have shown a significant reduction in severe arterial calcification and heart disease risk in populations with higher K2 intake.
- Maintain dental health: Similar to its role in bone health, K2 helps mineralize teeth, potentially strengthening them against cavities.
How Eggs Compare to Other Sources of Vitamin K
Eggs are a valuable, but not the most concentrated, source of vitamin K compared to others. The highest concentrations of vitamin K1 are found in green leafy vegetables, while K2 can be found in a variety of fermented and animal-sourced foods. For example, the Japanese fermented soybean dish natto is one of the richest sources of K2. The inclusion of eggs in a varied diet is a simple way to boost K2 intake, especially when combined with other nutrient-dense foods.
- Leafy green vegetables (e.g., kale, spinach) are the primary source of K1.
- Natto (fermented soybeans) is an exceptional source of K2.
- Certain hard cheeses, like Gouda and Emmental, contain moderate levels of K2.
- Organ meats, such as liver, are another good source of K2.
Conclusion
A standard, grain-fed egg yolk contains a very small amount of vitamin K1, but its potential contribution to a person's overall vitamin K intake is minimal. The true value of eggs as a vitamin K source lies in the more beneficial K2 (MK-4), and its concentration is highly variable depending on the hen's diet. Opting for eggs from pasture-raised hens can provide a much higher dose of this crucial nutrient, which plays an important role in bone and cardiovascular health. For those seeking to maximize their vitamin K intake, incorporating pasture-raised eggs along with other K1- and K2-rich foods is the best strategy.
