What are the two forms of vitamin K, where are they found in food, and what dietary factors can lead to deficiency?
Vitamin K is a group of structurally similar, fat-soluble vitamins that the human body needs for post-translational modification of certain proteins required for blood coagulation, and in metabolic pathways in bone and other tissue. This group of vitamins includes two natural vitamers: vitamin K1 and vitamin K2.
Three synthetic types of vitamin K are known: vitamins K3, K4, and K5. Although the natural K1 and all K2 homologues (versions) have proven nontoxic, the synthetic form K3 (menadione) has shown toxicity. K3 is nonetheless used in the pet food industry (!) and K5 is used to inhibit fungal growth.
Vitamin K1, also known as phylloquinone, phytomenadione, or phytonadione, is synthesised by plants; being directly involved in photosynthesis, it is found in highest amounts in green leafy vegetables. It is active in animals and may also be converted to vitamin K2 in the body.
Vitamin K2, the main storage form in animals, has several subtypes called menaquinones.
Menaquinone-4 (also known as MK-4 and menatetrenone) is the most common type of vitamin K2 in animal products because since MK-4 is normally synthesised from vitamin K1 in certain animal tissues (arterial walls, pancreas, and testes); This homologue of vitamin K2 may have enzyme functions that are distinct from those of vitamin K1.
Bacteria in the colon can also convert K1 into vitamin K2; in fact, all forms of K2 other than MK-4 can only be produced by bacteria, which use these forms in anaerobic respiration.
Food sources of vitamin K
Green vegetables such as dandelion leaves, spinach, asparagus, spring onions, Brussels sprouts and broccoli
- Fermented vegetables, especially natto and sauerkraut
- Herbs such as parsley
- Chili powder, curry, paprika, and cayenne
- Vegetable oils
- Strawberries, prunes, kiwi, grapes and avocado
- Meat (especially organs) and dairy foods in smaller quantities
Often the absorption is enhanced when eaten with fats such as butter or oils.
When absorption may be compromised
Chronic vitamin K deficiency is less obvious than acute deficiency. It is actually more dangerous because there are no alarming symptoms and the results – impairments in bone, cardiovascular health, and other disease of aging – might be severe. It had been long believed that vitamin K deficiency is rare. Requirements could be easily met via diet and microbial biosynthesis by bacteria living in the gut. However, recent scientific data show that the amount of vitamin K is not as abundant in the diet as once thought. Even a well-balanced diet might not provide vitamin K in the amounts sufficient for satisfying the body’s needs, especially as it has been shown mean dietary intake of vitamin K is currently significantly lower than it was 50 years ago,.
Vitamin K deficiency uptake and processing may occur because of:
- Poor gut flora (eg, from sugar-rich diet or prolonged use of antibiotics)
- Heavy damage to the colon
- Low fat diet:
- Fewer fat-soluble vitamins available in the first place
- Vitamin K is less bioavailable from plants when fat isn’t present
- Poor diet (as in having few other vitamins which are needed to help with vitamin K uptake, synthesis and use)
- Those on strict or faddy diets
- Use of anticoagulant drugs including long-term aspirin use
- Diseases such as gallbladder disease, Crohn’s disease, cystic fibrosis, liver diseases, pancreatic disease
- Diseases or surgical removal of the small intestine
In addition, sterile conditions introduced by international standards of food manufacturing stop microorganisms, including beneficial flora, from multiplying and penetrating the human body. That said, it is thought all K vitamins are absorbed in the small intestine, but bacterial colonies producing menaquinones are located in the colon, where the bile salts required for vitamin K uptake are not present.
NOTE: Newborns are low in vitamin K because it is not transported sufficiently across the placenta, and the newborn gut is sterile at the beginning meaning there are no bacteria to produce the required amount of vitamin K.
 For example, cooked spinach has a 5% bioavailability of phylloquinone; however, fat added to it increases bioavailability to 13% due to the increased solubility of vitamin K in fat.
 Warfarin, for example, works by blocking recycling of vitamin K, so that the body and tissues have lower levels of active vitamin K, and thus a deficiency of the active vitamin. The newer anticoagulants dabigatran and rivaroxaban have different mechanisms of action that do not interact with vitamin K, and may be taken with supplemental vitamin K.