Is vitamin K essential for strong bones?
A handful of recent studies suggest that vitamin K2, one of several types of vitamin K, can help build bone, while a few studies suggest it may have no impact. Given the critical importance of bone health for those with celiac disease and other malabsorption issues, we talked with the experts at Tufts University’s vitamin K research lab about the confusion.
Vitamin K doesn’t get nearly the attention of vitamin D or other “mainstream” vitamins but it plays a major role in blood clotting. And recently it’s been studied in connection with breast cancer, prostate cancer, heart disease, metabolic syndrome and osteoporosis. Here we ask Kyla Shea, PhD, and Sarah Booth, PhD. about its potential role in strengthening brittle bones. Both are vitamin K experts with the Jean Mayer Human Nutrition Research Center on Aging at Tufts University, home of the only dedicated vitamin K nutrition research laboratory in the United States.
Vitamin K isn’t one vitamin. Can you explain?
Yes, there are a number of different forms of vitamin K. Phylloquinone, more commonly known as vitamin K1, is the primary form in our diet and is found almost exclusively in plant-based foods, especially dark leafy greens like kale, spinach, Swiss chard and broccoli. Menaquinones, vitamin K2, are a class of vitamin K compounds that are primarily produced by bacteria. At the present time, we don’t know what the biological implications are regarding one form of vitamin K versus another.
How does gut bacteria relate to vitamin K?
The bacteria in our gut make some forms of menaquinone (vitamin K2). However, we don’t know how much of the menaquinones made in our gut contribute to maintaining our vitamin K nutritional status. This is an area that needs to be studied.
A number of studies have now looked at vitamin K in connection with bone health. What does the data show?
Based on the available studies, it doesn’t appear that vitamin K adds any additional benefit to bone health when an individual is calcium and vitamin D-replete. We conducted a clinical trial that compared bone mineral density between older men and women who received vitamin K1 supplementation with calcium plus vitamin D and older men and women who received calcium plus vitamin D without vitamin K1. After 3 years, both groups had higher bone mineral density but the amount their bone density increased did not differ. This confirms that calcium and vitamin D are important to bone health but vitamin K1 supplementation didn’t confer additional benefit.
Many people with celiac disease and others with malabsorption issues are deficient in calcium and vitamin D. Would vitamin K offer bone benefits for them?
There are currently insufficient data on the role of vitamin K and bone independent of vitamin D and calcium to be able to comment.
In your study, you supplement with vitamin K1, not vitamin K2. Why did you choose K1?
We chose vitamin K1 because it’s the primary dietary form of vitamin K in the U.S. diet. Vitamin K2 has been studied with respect to bone in the United States by another group and it did not affect bone health in that study.
But some studies with vitamin K2 seem to show the opposite. A recent study from the Netherlands and several animal-based studies from Japan reported improvements in multiple markers of bone health with a type of vitamin K2 supplement called menaquinone-7 (MK-7).
There is also a U.S. animal-based study that found no effect of MK-7, MK-4 [a form of vitamin K2] or vitamin K1 on bone health. Given the inconsistent results, it’s premature to recommend any form of vitamin K supplementation for bone health at this time.
Apart from bone health, how do you know if you need extra vitamin K?
We don’t have an established recommended dietary allowance (RDA) for vitamin K, so we don’t have a measure of what is or is not enough. Our laboratory is focused on developing the physiological and biochemical basis for establishing an RDA for vitamin K.
So what are the percentages that appear on a vitamin K supplement label referring to?
There’s not an RDA. It’s what’s known as an adequate intake (AI). This is an estimate that’s used when there are not enough reliable data to set an RDA. The AI for women is 90 mcg/day and 120 mcg/day for men.
Is it possible to overdo vitamin K?
There are no known risks associated with high intakes of vitamin K unless a person is taking warfarin, a medication used to slow blood clotting. Vitamin K can impact its effectiveness.
There are more questions than answers when it comes to vitamin K. Why is that?
Vitamin K was discovered in the 1930s. Until recently, the focus was on its role in blood coagulation. Technological advances are now allowing us to explore additional roles.
Gluten-free supplements that contain vitamin K are available from Carlson Labs, Garden of Life, Puritan’s Pride and Solgar. Consult a knowledgeable dietitian or physician about supplements and proper dosages.