Vitamin K is a type of fat-soluble vitamin that is stored in the liver. There are actually two types of vitamin K, vitamin K1 and vitamin K2. Let's try to understand their function, their dietary sources and what is the daily requirement.
Vitamins K1 and K2, food sources and functions
Vitamin K1 plays a role in blood clotting processes, it is the most abundant vitamin K in the diet but also the least active. Vitamin K1 is mainly contained in leafy greens, especially kale (145 micrograms in 100 grams of food), broccoli sprouts (177 microg / 100 g), broccoli (180 microg / 100g) and spinach (380 microg / 100g), but also olive oil and soy oil (193 microg / 100g) (Dinicolantonio et al, Open Heart, 2015). Vitamin K2, instead, protects the bones and the heart, in minimal part it is produced by the bacteria of the intestinal flora but it is also found in foods such as fermented soy beans, called natto, (10 microg / 100 g) and in small quantities in chicken, egg yolk, butter and cheese (Dinicolantonio et al, Open Heart, 2015).
Daily requirement of vitamin K, diet and supplements
The recommended daily requirement of vitamin K is 90 microg for women and 120 microg for men (National Academy of Science Food and Nutrition Board). Since this vitamin is fat soluble and is accumulated in the liver, the diet is the main and preferable source of this vitamin. However, in some special cases, such as after a treatment with antibiotics for more than 10 days, a low-fat diet, disorders of the intestinal tract or other situations such as increased risk of osteoporosis and in the elderly, it may be useful to take vitamin K in the form of supplements. In any case, however, the treatment must always be supervised by a doctor to avoid overdosing. The benefits of vitamin K supplementation, especially K2, have been observed with regard to skeletal health. In fact, scientific studies have shown that the intake of at least 110 microg of K2 in women has reduced the risk of pelvic fractures by 30% and the risk of breaking any bone by 80%, it is protective in case of osteoporosis, especially if taken together with vitamin D and calcium, and recent data are also indicating its role in osteoarthritis prevention (Dinicolantonio et al, Open Heart, 2015 - Groeber et al, Dermatoendocrinol., 2014). Moreover, vitamin K2 proves to be protective for heart health by reducing the risk of heart disease by 40% (Groeber et al, Dermatoendocrinol., 2014). As for vitamin K1, it does not only participate in the coagulation processes but also seems to have a protective role against the risk of developing diabetes. K1 supplements, always under medical supervision, are also connected to a lower risk of Alzheimer's as well as improving cognitive function (Schwalfenberg, J Nutr Metab, 2017). These last results are still preliminary and need further investigation.
Vitamin K, how much is really absorbed by the diet and some advice
Although vitamin K1 is more abundant than K2 and is easily found in vegetables, its absorption by the body represents only a minimal part of the vitamin actually ingested. For example, when you eat cooked vegetables like spinach you get from these vegetables only up to 10% of the total vitamin K1. However, when vegetables are combined with fat, such as olive oil, the amount of vitamin can double. For what concerns vitamin K2, since it mainly accumulates in the fatty parts of foods, such as dairy products, it is instead absorbed entirely (Vermeer et al, Food Nutr Res, 2012).