There are two natural forms of vitamin K: K1 & K2.
K1 - Plant source (phylloquinone aka phytomenadione aka phytonadione)
K2 - Bacterial and animal source (menaquinone - MK-n, n=4..10, ‘n’ determined by the number of prenyl side chains). The best bio-available form.
K3 - Synthetic (Menadione)
K4 and K5 - Synthetics, not used in human diets.
Mammals can synthesize K2 MK-4 from K1. Vitamin K2 is produced by animal tissues, including the mammary glands, from vitamin K1, which occurs in rapidly growing green plants.
K1: Spinach, Brussels sprouts, Swiss chard, green beans, asparagus, broccoli, kale and mustard greens. Very good sources include oats, potatoes, tomatoes, asparagus, green peas. Found in the green tissues of plants, tightly embedded within the membrane of the photosynthesizing organelle called the chloroplast.
K2: Egg yolk, grass-fed butter, cow liver, certain cheeses and fermented soybean products such as natto. Natto is by far the best food source for vitamin K2. Vitamin K2 is also produced by lactic acid bacteria, although bacteria produce forms of the vitamin that are chemically different from those that animals produce, and researchers have not yet established the differences in biological activity between these forms.
MK-4 is found in meats, MK-7, MK-8, and MK-9 found in fermented food products like cheese and natto.
The beta-carotene associated with vitamin K1 will also impart a yellow or orange color to butterfat; the richness of this color therefore indirectly indicates the amount of both vitamins K1 and K2 in the butter.
Absorption of K1 is from the gut (duodenum and jejunum) via the lymphatic system. As with other fat-soluble vitamins, absorption depends on the presence of bile and pancreatic juices and is enhanced by dietary fat. Impaired absorption occurs in individuals with gastrointestinal disorders, fat malabsorption or liver disease, or after prolonged antibiotic therapy coupled with compromised dietary intake. Absorption of vitamin K may be decreased by mineral oil, bile acid sequestrants (cholestyramine, colestipol) and orlistat (weight loss medication).
Although the liver contains menaquinones synthesised by intestinal bacteria, the absorption of menaquinones and their contribution to the human vitamin K requirement have not yet been fully elucidated.
When you eat vitamin K1 in your food, only 5-10% of ingested K1 is absorbed and reaches your blood, but almost 100% of K2 is absorbed into your blood stream. Humans do not possess the ability to absorb much more than 200 micrograms of vitamin K1 per day from vegetables.