Vitamin K is an essential nutrient and one of four fat-soluble vitamins (including A, D and E). There are two main kinds of vitamin K — vitamin K1 (phylloquinone) found in plants like leafy green vegetables, and vitamin K2 (menaquinone), which is naturally produced in the intestine.
Bacteria in the gut can synthesize vitamin K1 into K2 and make about 10% of our body’s vitamin K supply. You can also find small amounts of K2 in fermented foods like sauerkraut and kimchi, as well as liver and egg yolks.
“Vitamin K is most known for its role in blood clotting and it also plays a role in bone formation and maintaining heart and eye health,” says registered dietitian Caroline Passerrello, a spokesperson for the Academy of Nutrition and Dietetics and a faculty member in the Dietitian Nutritionist Program at the University of Pittsburgh.
“Greens — like collard, spinach, kale, broccoli, soybeans, and soybean oil are all excellent sources of vitamin K. For a generally healthy adult, 90-120 micrograms (mcg) per day is the adequate intake (AI). Half a cup of collard greens provides 530 mcg.”
Here, we’ll reveal more about how vitamin K works, the benefits, sources and signs of deficiency.
What are the benefits of vitamin K?
Caroline Passerrello is a spokesperson for the Academy of Nutrition and Dietetics, a faculty member in the Dietitian Nutritionist Program at the University of Pittsburgh, and is co-author of Human Nutrition: Science for Healthy Living (3rd ed.).
Without vitamin K, the body cannot produce prothrombin, a protein that is necessary for blood clotting, bone metabolism and wound healing. According to the Harvard School of Public Health, the vitamin helps produce four of the 13 proteins needed for blood clotting.
Vitamin K helps to make osteocalcin, a protein that produces healthy bone tissue. It also teams up with other vitamins.
“Vitamin K works with vitamin D to ensure that calcium finds its way to the bones to help them develop properly,” says Dr. Sherry Ross, women’s health expert at Providence Saint John’s Health Center in Santa Monica, California.
Studies have shown that people who eat a lot of vitamin K-rich foods have stronger bones and are less likely to break a hip than those who don’t, but more research is needed in this area.
Upon attaining her medical degree from New York Medical College, Dr. Sherry spent her Ob/Gyn residency at the University of Southern California School Of Medicine. Dr. Sherry is on the board of Planned Parenthood, Los Angeles. She also acts as spokesperson ambassador for the American Heart Association.
A growing body of research suggests that vitamin K is important for cardiovascular health. A study by Edith Cowan University in Australia found that people with a vitamin K-rich diet had a 34 percent lower risk of developing atherosclerosis-related cardiovascular disease (conditions affecting the heart or blood vessels).
Vitamin K may benefit the eyes as well. In a small study of 935 people in their forties, researchers from the University of Leuven in Belgium found a link between their levels of matrix Gla protein or MGP (a marker of vitamin K deficiency) and the retinal microvascular diameter 11 years later.
Put simply, the study highlighted the possibility that vitamin K supplementation could promote eye health and better vision.
Higher levels of vitamin K have been linked with improved episodic (long term) memory in healthy older adults, and boost cognitive health (the ability to think, learn and remember).
Vitamin K2 is unique because it is produced by beneficial microbes within the large intestines, although as we know, you can also get it from some foods. So eating gut-friendly fermented foods like sauerkraut can maintain healthy levels of K2 in the body and promote microbial diversity in the microbiome.
How much vitamin K should you consume?
The amount of vitamin K you need depends on your age and sex, and the recommended daily allowance (RDA) can vary between countries. The National Institutes of Health (NIH) recommends that American men consume 120 micrograms (mcg) and women 90 mcg.
Just one cup of raw kale has 472mcg of vitamin K — about seven times the RDA.
Is there a toxic dose of vitamin K? Passerrello told Live Science: “Most micronutrients (vitamins and minerals) have guidelines for minimum recommended (Recommended Dietary Allowance or Adequate Intake), and maximum intake levels (Tolerable Upper Limit). Vitamin K does not have a tolerable Upper Limit.
“In its report, the Food and Nutrition Board stated that ‘no adverse effects associated with vitamin K consumption from food or supplements have been reported in humans or animals’.”
- Related: Nine sources of vitamin C
What are the best sources of vitamin K?
Good sources of vitamin K include:
- Dark leafy greens
- Hard cheeses
- Soybean oil
- Egg yolks
- Blackberries & blueberries
Passerrello says: “Fat-soluble vitamins are better absorbed by the body when eaten along with dietary fat – so massage kale in olive oil, add soybean oil to collard greens, and toss some nuts into a spinach salad for maximum absorption of vitamin K.”
Signs of vitamin K deficiency
“A sign of vitamin K deficiency is excessive bleeding, since vitamin K is essential for the production of blood-clotting factors,” says Passerrello. This can also manifest itself in the form of frequent bruising, small blood clots under nails, blood in urine and stools.
While vitamin K deficiencies in the US are uncommon, you may be at higher risk if you:
- Have a gut-related medical condition that affects absorption
- Are on antibiotics
- Are taking anticoagulants (blood thinners)
- Are severely malnourished
- Have a high alcohol consumption
- Have had bariatric (weight loss) surgery
“People who have severe gastrointestinal disorders such as gallbladder disease, cystic fibrosis, celiac or Crohn’s disease are unable to properly absorb vitamin K, so they are more susceptible to being deficient,” says Ross. “Supplements are useful for these medical conditions.”
“Folks on blood thinners need to maintain a consistent level of vitamin K intake – they should not avoid vitamin K. Working with a registered dietitian nutritionist is a great way to determine a meal plan to meet their individual needs and they will likely have routine blood work to monitor their blood levels of vitamin K,” says Passerrello.
She adds: “Since 1961, the American Academy of Pediatrics has been recommending that all newborn babies receive Vitamin K supplementation immediately after birth. This is because Vitamin K is not effectively transported across the placenta and breast milk contains low levels of Vitamin K.”
Anybody who has undergone a long course of antibiotics is also at risk of a vitamin K deficiency. This is because the antibiotics kill off their gut flora, preventing them from producing vitamin K.
As long as you have a healthy gut flora and no problems with fat absorption, you are very unlikely to experience a vitamin K deficiency. If you think you are deficient, speak to your health care professional about taking a supplement.
This article is for informational purposes only and is not meant to offer medical advice.
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Maddy has been a writer and editor for 25 years, and has worked for some of the UK's bestselling newspapers and women’s magazines, including Marie Claire, The Sunday Times and Women's Health. Maddy is also a fully qualified Level 3 Personal Trainer, specializing in helping busy women over 40 navigate menopause.
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