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food source · 3
Dark leafy greens are among the main food sources of vitamin K1, and spinach ranks near the top. K1 dissolves in fat, so it is absorbed better when eaten with a little oil.
Mechanism · Darker leaves, more K1
K1 is a part of the chain that passes electrons along during photosynthesis in the chloroplast, so the darker the leaf, the more K1 it holds. Cooked spinach has about 540 µg per 100 g, and a small bowl far exceeds an adult's adequate intake for a day. K1 dissolves in fat and needs a little oil to be absorbed well.
Safety · On warfarin, keep it steady
In healthy people, eating more greens does not thicken the blood. The people who need a roughly steady daily amount of K1 are those taking the blood thinner warfarin: K1 helps the liver make clotting factors, and that is exactly the step warfarin blocks, so the goal is a steady intake, not avoiding greens. The other form of vitamin K, K2, comes mainly from fermented and animal foods, and spinach has almost none.
←Kale
Raw kale has about 450–817 µg of vitamin K1 per 100 g, among the highest of any vegetable by weight. K1 does not dissolve in water and needs a little fat to be absorbed well.
Mechanism · Fat-soluble, so it needs oil
K1 and the carotenoids do not dissolve in water; they have to ride in micelles formed with fat before they can reach the gut wall. A plate of boiled kale and a plate stir-fried with garlic and olive oil differ not in what they contain but in how much of it gets absorbed.
Safety · On warfarin, keep it steady
Raw kale leaves have about 450–817 µg of vitamin K1 per 100 g, while the adult adequate intake (AI) is about 90–120 µg a day; cooking changes the figure a little, and it stays high. K1 is needed to make clotting factors, the very step the blood thinner warfarin blocks, so people taking warfarin need not give up kale but should keep the amount they eat each day roughly steady.
Dark green cruciferous vegetables such as broccoli are also a source of vitamin K1. K1 dissolves in fat, so it is absorbed more easily with a little oil.
Mechanism · Fat-soluble, so it needs fat
K1 is plentiful in the dark green florets; it helps make clotting factors and is also involved in bone. It does not dissolve in water, and the contents of the small intestine are basically a pot of water, so K1 first has to dissolve into tiny droplets formed with fat, called micelles, before it can reach the gut wall. A little olive oil or a few nuts is not fussiness; it is what gets K1 absorbed.
Numbers · The K1 in a serving of broccoli
Cooked broccoli has about 140 µg of vitamin K1 per 100 g, less than kale or spinach, but a modest serving comes close to an adult's adequate intake for a day. People taking the blood thinner warfarin should keep their daily K1 roughly steady rather than avoid it.
antagonism · 1
High-dose vitamin E supplements may interfere with activating the vitamin K–dependent clotting factors and raise bleeding risk; people on anticoagulants such as warfarin, or recovering from surgery, need particular care. Vitamin E from food does not cause this problem.
contrast · 1
Vitamins K1 and K2 share the same naphthoquinone ring and differ in their side chain. K1 comes mainly from leafy greens and is used mostly by the liver to activate clotting factors; K2, such as the MK-7 in natto, reaches tissues outside the liver more and helps activate osteocalcin and other proteins that decide where calcium goes. Their jobs overlap; the emphasis differs.