What is vitamin K?
Vitamin K is a fat-soluble nutrient best known for its importance in blood-clotting, without which it would be easy to bleed to death from even a tiny cut.
There are two basic forms: vitamin K1, or philloquinone; and vitamin K2, or menaquinone. Vitamin K1 is found mostly in dark-green leafy vegetables such as kale, spinach and cabbage, while K2 is present in meat, cheese and fermented products like natto (made from soybeans).
Both forms appear to play important roles in keeping our bones strong and our hearts healthy.
A number of studies show that people low in vitamin K1 are likely to suffer bone fractures-a common problem in the elderly and postmenopausal women. One study looking at more than 2800 Norwegian men and women aged 71-75 years found that those with the lowest vitamin K1 intakes had a roughly 60-per-cent greater risk of hip fracture compared with those with the highest intakes (Bone, 2011; 49: 990-5).
Similarly, in the US Nurses' Health Study, a 10-year follow-up of over 72,000 middle-aged women, those who consumed moderate-to-high amounts of vitamin K (nearly all from vegetables, so vitamin K1) had a 30-per-cent lower risk of hip fractures-even when other factors such as calcium and vitamin D were taken into account. It didn't take much to achieve this protective effect-just around 100-150 mcg/day, mostly from one or more servings of lettuce each day (Am J Clin Nutr, 1999; 69: 74-9).
Vitamin K1 levels are also linked to osteoarthritis (OA) and, specifically, the bony outgrowths called 'osteophytes' that commonly develop in OA. One US study found that people with lower levels of vitamin K1 in the blood had an increased risk of OA of the hand as well as a greater number of osteophytes (Arthritis Rheum, 2006; 54: 1255-61).
What about supplements? Can supplementing with vitamin K1 protect against such bone problems?
A few placebo-controlled trials have attempted to figure this out. In one, 5 mg/day of vitamin K1 improved bone formation and reduced the risk of fractures in postmenopausal women with lower-than-normal bone mineral density (BMD) (PLoS Med, 2008; 5: e196). In others, K1 boosted bone strength as well as reduced fracture risk (Nutr Res, 2009; 29: 221-8).
Meanwhile, the K2 form of the vitamin is showing even more promise as a bone-building supplement.
According to one review of the published data, vitamin K2 exerts a more powerful influence on bone than K1 does (Altern Med Rev, 2005; 10: 24-35). Test-tube studies also show that vitamin K2 is far more active in both bone formation and in minimizing bone loss (Bone, 1995; 16: 179-84), while human studies confirm the potential of K2 as a treatment for osteoporosis.
This treatment is already in use in countries such as Japan, Korea and Thailand. The Japanese in particular have pioneered the use of K2 supplements for osteoporosis. Over more than a decade, numerous clinical trials have confirmed the safety and efficacy of K2 for treating a variety of forms of the disease.
In one two-year study, 45 mg/day of vitamin K2 improved BMD and reduced spinal fractures in osteoporosis sufferers as effectively as the drug etidronate (J Orthop Sci, 2001; 6: 487-92). Other studies suggest that K2 works in synergy with bisphosphonate drugs. A randomized trial of nearly 100 postmenopausal osteoporotic women found that combining etidronate and vitamin K2 was more effective for reducing fracture rates than etidronate alone (Altern Med Rev, 2005; 10: 24-35).
Most studies have focused on postmenopausal osteoporosis, but vitamin K2 has also prevented the bone loss typically seen in kidney-dialysis patients (J Int Med Res, 2002; 30: 566-75). In addition, further studies have confirmed the effectiveness of K2 for improving the loss of BMD caused by Parkinson's disease (Bone, 2002; 31: 114-8), cirrhosis of the liver (Am J Gastroenterol, 2002; 97: 978-81), stroke (Bone, 1998; 23: 291-6) and anorexia (Psychiatry Res, 2003; 117: 259-69).
Vitamin K2 also appears to be important for heart health. Animal studies have shown that it can inhibit calcification of arterial plaque-one of the features of atherosclerosis (hardening of the arteries). This effect was seen with both high (100 mg/kg body weight) and low (10 mg/kg body weight) doses of K2, although the low dose had less of an impact (Int J Vitam Nutr Res, 1996; 66: 36-8).
In humans, clinical trials are scarce, but the Rotterdam Study of 4807 people followed for 10 years found that those with a high K2 intake (more than 33 mcg/day) had a much lower risk of suffering from coronary heart disease compared with those with lower intakes (less than 22 mcg/day). The high-K2 group also had a reduced risk of dying from a heart attack (J Nutr, 2004; 134: 3100-5).
Another Dutch study but much larger, involving around three times the population size of the above study and only including women, similarly reported that low K2 intakes came with a significantly greater risk of coronary heart disease (Nutr Metab Cardiovasc Dis, 2009; 19: 504-10).
Vitamin K1 does not appear to be associated with the same heart-healthy effects, although one study found a "weak association" between vitamin K1 intake and CHD risk in women that was dose-related: the lower the intake, the higher the risk (Eur J Clin Nutr, 2005; 59: 196-204).
What's more, vitamin K also appears to be beneficial for a number of other conditions, including cancer, dementia and diabetes (see box above).
Most people get enough vitamin K from their diets to maintain adequate blood-clotting, but probably not enough to protect against health problems like osteoporosis and heart disease. According to a review of the research by several vitamin K experts across Europe, the optimal daily dietary intakes for bone health appear to be between 200 and 500 mcg/day, but few of us are actually getting this amount (Eur J Nutr, 2004; 43: 325-35). Also, most dietary vitamin K intake is in the form of K1 rather than K2.
Natto is the chief dietary source of K2, containing 100 times the amount found in various cheeses. But natto consumption in Britain is virtually non-existent.
Vitamin K supplements may therefore be a worth-while investment for many of us-particularly those at particular risk for bone and heart problems. A dose of 100 mcg/day is what the European vitamin K experts came up with but, for treating disease, much higher doses are likely to be necessary. In adults, as few-if any-adverse effects are seen with high-dose vitamin K, those with osteoporosis could take up to 1000 mcg/day of K1 or K2. In fact, several trials of osteoporotic postmenopausal women have safely used K2 dosages as high as 45 mg/day (Am J Health Syst Pharm, 2005; 62: 1574-81).
Because of its blood-coagulating properties, however, vitamin K should not be taken with the blood-thinner warfarin. Also, dietary fat should be taken with K to maximize its absorption.
For bone health, taking vitamin K in combination with other bone-building nutrients such as vitamin D, magnesium and possibly calcium may have a better effect than taking vitamin K alone.
Factfile: Other benefits
- As a cancer-fighter. In one study, 30 patients with hepatocellular carcinoma, a type of liver cancer, were given 40 mg of oral vitamin K1 daily. The disease stabilized in six patients, seven had a partial response and seven others had improved liver function (Altern Med Rev, 2003; 8: 303-18). Another study found that K2 can kill a variety of lung cancer cells in the lab, including small-cell carcinomas, adenocarcinomas, squamous cell carcinomas and large-cell carcinomas (Int J Oncol, 2003; 23: 627-32). Others have shown that vitamin K2 intake can lead to a reduced risk of prostate or any type of cancer and of death due to cancer (Am J Clin Nutr, 2010; 91: 985-92; 1348-58).
- As an antidiabetic. In a three-year study from Boston, MA, 355 non-diabetic men and women were randomly assigned to one of two groups: one took 500 mcg/day of vitamin K1 and the other took a placebo. Although there were no significant differences between the two groups among women, in men, those taking vitamin K showed improvement in insulin resistance-when naturally produced insulin (a hormone) becomes less effective at lowering blood sugar (Diabetes Care, 2008; 31: 2092-6).
- As a brain-booster. Evidence is accumulating that vitamin K has important functions in the brain and may have the potential to prevent and treat Alzheimer's disease (Med Hypotheses, 2001; 57: 151-5). One study found that patients with early-stage Alzheimer's consumed significantly less vitamin K than the healthy controls did (J Am Diet Assoc, 2008; 108: 2095-9).
WDDTY vol 23 no.6