Vitamin K refers to one of several compounds that have vitamin K activity. One form, called phylloquinones, comes from plants, while another form called menaquinones is produced by intestinal bacteria. There is also a synthetic type of vitamin K called menadione that's made in a laboratory.
The best known function of vitamin K is to help with blood clotting. The liver produces clotting factors. Before these clotting factors are released into the bloodstream, vitamin K modifies them. So important is vitamin K for blood clotting that it's used to reverse the effects of pharmaceutical blood-thinning medications and to stop bleeding related to medications that damage the stomach lining. Vitamin K is also essential for forming other proteins in the body. Vitamin K plays a role in bone health and the prevention of osteoporosis since people deficient in vitamin K have an increased risk of bone fractures.
Phylloquinones, the form of vitamin K that comes from plants, is found in a variety of plant-based foods including spinach, Swiss chard, collard greens, Brussels sprouts, broccoli, parsley, kale, turnip greens, asparagus, beef liver and romaine lettuce. Deficiency of vitamin K isn't common since intestinal bacteria make the menaquinone form of vitamin K. People who use antibiotics for a long period of time are at higher risk for deficiency since the antibiotics destroy intestinal bacteria that produce vitamin K.
Because of vitamin K's role in blood clotting and its importance for maintaining healthy blood vessel function, it's sometimes used to treat spider veins. Spider veins are clusters of dilated capillaries or tiny veins that look like spider webs on the surface of the skin. They occur when valves that prevent the backflow of blood become damaged and the capillaries begin to leak. Factors that contribute to them include pregnancy, trauma, standing and sitting for long periods of time and obesity. Vitamin K appears to strengthen the walls of veins and capillaries so they're less likely to leak.
Vitamin K is also used to treat dark under-eye circles. According to a study published in the Journal of Cosmetic Dermatology, participants who used a gel containing 2% vitamin K cream, vitamin E, vitamin C and retinol experienced moderate improvement in their under-eye circles. Combining vitamin K with vitamin C has additional benefits since vitamin C stimulates collagen production. Increased collagen production helps to reinforce and thicken the thin skin around the eyes, making dark, under-eye circles less visible, while vitamin K strengthens the underlying capillaries.
Vitamin K is also beneficial for treating bruising after pulsed-dye laser surgery. This type of surgery causes significant discoloration and bruising. According to a study published in the Journal of the American Academy of Dermatology, participants that used a vitamin K cream after their laser treatment experienced less bruising. Vitamin K also has anti-inflammatory effects, which makes it beneficial for relieving redness and discoloration after laser surgery. It also helps to reduce redness and dilated capillaries in people with the inflammatory skin conditioned rosacea.
According to one study, vitamin K may be helpful for fighting skin wrinkling due to aging. This study showed that topical vitamin K reduced wrinkling in people suffering from a hereditary condition called pseudoxanthoma elasticum. People with this condition have abnormal elastin, a protein in skin that helps skin "bounce back" and plays a role in preventing wrinkles. Vitamin K appears to be involved in keeping elastin in the skin healthy. Therefore, it may also prevent wrinkles in healthy people, although more research is needed.
Is it safe? Vitamin K appears to be safe orally and topically for people who are otherwise healthy. It can interact with certain medications and supplements when used orally and shouldn't be used by anyone taking blood thinners, antacids, aspirin or antibiotics or anyone on dialysis for kidney disease. Vitamin K is safe in the amounts found in cosmetic and skin care products. It usually doesn't cause significant skin irritation or redness and is well tolerated even by people with sensitive skin.