Vitamin K1 (Phylloquinone): Clotting, Greens & Absorption

Vitamin K1, or phylloquinone, is the plant form of vitamin K and the primary form involved in blood coagulation. It's found abundantly in green leafy vegetables, where it participates in photosynthesis. Despite being widespread in plant foods, many people don't consume enough because they simply don't eat enough greens.

Role in Blood Clotting

K1's best-established function is as a cofactor for the enzyme gamma-glutamyl carboxylase, which activates clotting factors II (prothrombin), VII, IX, and X in the liver. Without adequate vitamin K, blood clotting is impaired, leading to excessive bleeding. This is the mechanism behind warfarin (Coumadin), an anticoagulant drug that works by blocking vitamin K recycling.

K1 vs. K2

While K1 is primarily used for clotting, vitamin K2 has distinct roles in calcium metabolism and bone health. Some K1 is converted to K2 (specifically MK-4) in various tissues, but this conversion is limited and variable. Getting both forms from the diet is the most reliable approach.

Food Sources & Absorption

Kale, spinach, collard greens, Swiss chard, broccoli, and brussels sprouts are the top food sources. K1 in leafy greens is bound to chloroplast membranes, and absorption from whole leaves is relatively low (5-15%). Chopping, cooking, and adding fat significantly improve bioavailability. A salad with oil dressing delivers far more usable K1 than raw, undressed greens.

Like other fat-soluble vitamins, K1 requires dietary fat for absorption. People on very low-fat diets or with fat malabsorption conditions may be at risk of poor K1 status.

External resources: NIH — Vitamin K Fact Sheet