Vitamin K belongs to the group of the so-called vitamins fat-soluble, that is to say that they dissolve in fats, and it is essential for blood clotting, a process that serves to repair wounds and prevent bleeding (blood loss).

More specifically, vitamin K ensures the proper functioning of an enzyme that allows the synthesis of some proteins involved in coagulation. The deficiency of this vitamin, which can occur, albeit rarely, as a result of diseases that prevent intestinal absorption, or prolonged treatment with antibiotics, therefore leads to bleeding. Vitamin K plays a role of primary importance for the health of the cardiovascular system, as it also reduces the risk of calcification in the blood vessels and the formation of atherosclerotic plaque (arteriosclerosis). In addition, mobilizing calcium from vessels and soft tissues to the bones helps maintain bone health.

Two forms of vitamin K have been described, both of which are important for clotting:

  • vitamin K1 (phylloquinone), is the natural form of the vitamin taken through food.It is mainly present in green leafy vegetables and actively participates in the maintenance of the blood coagulation system. It also helps prevent calcifications in the arteries and allows bones to retain calcium and develop properly
  • vitamin K2 (menaquinone), is produced inside the body (endogenous production), starting from the K1 form, by bacteria normally present in the intestine. It is also found in fermented foods such as cheese, eggs and butter. The natto, a Japanese food derived from the fermentation of soy beans, is the richest food of this vitamin. Vitamin K2 is the one that seems to bring more health benefits because, compared to vitamin K1, it has a greater protective effect against cardiovascular and degenerative diseases of the nervous system. In addition, it helps prevent osteoporosis by acting in synergy with other fat-soluble vitamins such as vitamin A and vitamin D

Nutritional indications

To avoid vitamin K1 deficiency, the recommendation is to take at least two hundred grams of vegetables every day. Furthermore, it would be desirable to eat fermented foods (yoghurt, cheeses) on a daily basis, in order to also introduce a certain amount of vitamin K2. The supply of the latter should be ensured by its production by intestinal bacteria present inside the organism (endogenous production). However, the altered balance, often present, between the species of bacteria (dysbiosis) can alter production by making it insufficient.

The average requirement (or minimum necessary quantity) of vitamin K has not been defined (vitamins).However, according to the LARN (Reference levels of intake for the Italian population) drawn up by the Italian Society of Human Nutrition (SINU), the quantities of vitamin K corresponding to an adequate intake in the different age groups, regardless of sex, are the following:

(on a daily basis)


Vitamin K
(micrograms, μg)


6-12 months




1-3 years



4-6 years



7-10 years



11-14 years



15-17 years




18-29 years old



30-59 years



60-74 years



≥75 years








Italian Society of Human Nutrition (SINU). LARN - Reference levels of intake for the Italian population: VITAMINS

Sources / Foods rich in Vitamin K.

The most important natural sources of vitamin K1 are green leafy vegetables:

  • spinach
  • cabbages
  • broccoli
  • lettuce

It is also present in legumes, vegetable oils, fruit (blueberries, strawberries, kiwis, figs) and, to a lesser extent, in some foods of animal origin (meat, eggs, liver). Fermented products such as cheeses and whole yogurt, on the other hand, contain vitamin K2, albeit in lower quantities. However, while only 10% of the vitamin K1 found in food is absorbed, the vitamin K2 from food is almost completely absorbed.

Risks related to deficiency or excess

Vitamin K deficiency

A vitamin K deficiency can occur in the newborn or in the first 2-3 months of life following:

  • reduced transfer from mother to fetus during pregnancy
  • low in breast milk
  • intestinal bacterial flora of the newborn not yet developed

Deficiency can cause various types of bleeding (known as "vitamin K deficiency bleeding"):

  • bleeding from the mucous membranes
  • blood loss in the intestines and at the navel level
  • brain hemorrhages, serious but rare

These events, which could occur in the first week of life, are effectively prevented by administering a single (intramuscularly, most effective) or multiple (oral) doses of vitamin K soon after birth. Following the latest guidelines on childbirth from the World Health Organization (WHO), the Italian Society of Neonatology (SIN) recommends a single administration of 0.5-1 milligram (depending on weight) of intramuscular vitamin K in all newborns, including those born before the term. In exceptional cases, parents can choose oral administration.

Prophylaxis (prevention) at birth also serves to reduce the risk of late bleeding, which could occur between the first and third month in breastfed infants, as breast milk is poor in vitamin K. For the latter, however, it is recommended to continue supplementation with 25 micrograms (mg) per day orally from the 15th day for 12 weeks. The use of infant formula, already enriched in vitamin K, does not, however, require any supplementation.

Vitamin K levels can also be low in children and adults with chronic diseases that reduce intestinal absorption capacity, such as ulcerative colitis, cystic fibrosis or celiac disease, as well as following prolonged treatment with antibiotics. . Very low levels can also occur in older people with atrial fibrillation or other heart problems or who suffer from venous thrombosis (clot formation in the blood vessels) and are therefore on anticoagulant therapy. Medicines with anticoagulant action (for example, warfarin), in fact, reduce blood clotting by blocking the activity of this vitamin. In these cases, regular monitoring of coagulation levels is recommended.

Extremely low levels of vitamin K can cause bleeding (blood loss) and bleeding of varying degrees:

  • nosebleeds
  • bleeding from the gums
  • bruises
  • severe brain bleeding

Vitamin K deficiency also causes:

  • weakening of the bones
  • osteoporosis
  • increased risk of fractures

A balanced diet, on the other hand, will never lead to a deficiency of vitamin K.

Excess of vitamin K.

There are no indications that an excess of vitamin K can be harmful, except in the case of taking anticoagulant therapy. In this case, the excess of vitamin K deriving from food could counteract the action of the drug. It is therefore advisable to control the daily intake of vitamin K by limiting the consumption of foods that are rich in it.

Vitamin K and pregnancy

There is no indication that a greater or lesser intake of vitamin K is required during pregnancy.

However, attention must be paid to the use of "dicumarolic" oral anticoagulants (for example, warfarin), which block the activity of vitamin K. They cross the placenta and induce malformations in the fetus (teratogens). and the twelfth week of gestation can induce fetal malformations (warfarin embryopathy) manifesting in the form of reduced development of the nose, eye or skull, decreased vision or mental retardation of varying degrees. they may be an increased risk of miscarriage, stillbirth of the fetus, neurological problems of the newborn, and fetal and maternal haemorrhages.Dicumarol therapy appears, however, to be reasonably safe in the second and third trimester of pregnancy.


Chen HG, Sheng LT, Zhang YB, Cao AL, Lai YW, Kunutsor SK, Jiang L, Pan A. Association of vitamin K with cardiovascular events and all-cause mortality: a systematic review and meta-analysis [Summary]. European Journal of Nutrition. 2019; 58: 2191-2205

Shah K, Gleason L, Villareal DT. Vitamin K and bone health in older adults [Summary]. Journal of Nutrition in Gerontololy and Geriatrics. 2014; 33: 10-22

In-depth link

European Food Safety Authority (EFSA). Dietary reference values: EFSA publishes opinion on vitamin K

Italian Society of Gynecology and Obstetrics (SIGO). The thromboembolic risk in pregnancy and the puerperium. Recommendations

Italian Society of Human Nutrition (SINU). VITAMINS - Recommended Intake for the Population (PRI) and Adequate Intake (AI)

Bambino Gesù Children's Hospital. Vitamin K

Mayo Clinic. Vitamin K (Class) (Oral route, Parenteral route) (English)

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