Researchers Are Keeping Track of Vitamin K
Dietary Vitamin K
Much of what is known about the content of vitamin K in the U.S. food supply comes from research conducted in the Vitamin K Laboratory at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University. Extensive databases now exist for the food content of one type of vitamin K, phylloquinone. Synthesized by plants, phylloquinone makes dark green leafy vegetables the richest source of vitamin K in the American diet. Lab Director Sarah Booth, PhD, and her USDA colleagues, for the first time reported data on the content of the two other major types of dietary vitamin K - menaquinones and dihydrophylloquinone - in more than 500 commonly consumed meats, dairy foods, fast-foods, grains, cereals and baked goods. Assessing the natural and synthetic forms of vitamin K content in foods is important because of its possible links to a number of conditions such as osteoarthritis and coronary heart disease.
"We know that meats, dairy foods and many cereals and grain products contain relatively low amounts of phylloquinone," says Booth, also an associate professor at the Friedman School of Nutrition Science and Policy at Tufts. "Our analysis of the two other forms of vitamin K in these foods confirms that no single food item in the meat, dairy, cereal or grain categories is a rich dietary source of any form of vitamin K. However, since many meats, dairy foods, grains and cereals are often consumed in large quantities, they may be important contributors to total vitamin K intake."
Vitamin K, which is a fat-soluble vitamin, is essential in blood clotting and cellular growth. It is also involved in building and maintaining bone mass. "These are known functions of phylloquinone in the diet," Booth notes, "but research is continuing to uncover other potential roles for this form of the vitamin." As an example, Booth mentions the results of a study published in the April 2006 issue of Arthritis & Rheumatism, in which she and colleagues collaborated with Tuhina Neogi, M.D., a scientist from Boston University School of Medicine. The researchers observed that higher blood levels of phylloquinone were associated with lower risk of osteoarthritis in the hand and knee.
Of the other forms of vitamin K, Booth says, "We are just beginning to study the potential biological activity of menaquinones and dihydrophylloquinone. As we learn more, quantifying the content of these other forms of vitamin K in foods is important," she adds. Booth and other nutrition scientists conducted this research under the direction of the Agricultural Research Service (ARS), which is the chief scientific research agency of the USDA.
Although several types of menaquinones exist, Booth and colleagues specifically measured levels of menaquinone-4 (MK-4). "We found that the meat and dairy foods we analyzed have moderate amounts of MK-4. Some of the foods with the highest levels of MK-4 include chicken, cheddar cheese and egg yolks," says Booth.
"MK-4 is relatively high in brain tissue," Booth notes, "but its exact functions are not known. In the future, if we identify functions unique to MK-4, the presence of this form of the vitamin in meat and dairy foods may prove to be important."
While phylloquinone and menaquinones are found naturally in foods, dihydrophylloquinone is only formed during the processing of plant oils. Booth explains, "During a commercial food processing technique called hydrogenation, phylloquinone is converted to dihydrophylloquinone. Commercial hydrogenation also results in the formation of trans fats, which are now known as 'bad fats.'" Last year, Booth was part of a study that revealed that dihydrophylloquinone is detectable in the blood, and that levels go up with increased dietary intake. In this study, men and women with higher blood levels of dihydrophylloquinone also consumed greater amounts of trans fatty acids.
Now, Booth and USDA colleagues elaborate on the previous vitamin K research by reporting in the Journal of Agricultural and Food Chemistry that dihydrophylloquinone was not detectable in most samples of meat and dairy foods, but was found in low levels in fast food items, such as hamburgers, tacos and chicken sandwiches. Booth explains, "We suspect these foods have more phylloquinone and dihydrophylloquinone than the meats with which they are made because they are cooked in plant oils." In research reported in the Journal of Food Science, Booth and colleagues reported that dihydrophylloquinone was found in a wide range of levels in fast-foods and baked goods. "This does not mean that they are good sources of vitamin K," she says, "but depending on how much an individual consumes, it may be important to consider them in assessing that person's overall vitamin K intake."
Booth's past research has also suggested that high levels of dietary phylloquinone may be an indicator of lower risk of coronary heart disease, but not stroke, in women. Among participants in the ongoing Nurses' Health Study, those with healthier dietary and lifestyle patterns consumed higher levels of phylloquinone and tended to have lower rates of heart attack and death from coronary heart disease.
Booth notes, "It is too soon to say if our findings will affect dietary recommendations in the future." She and her colleagues emphasize that more research is needed to better understand the functions of the different types of vitamin K and which foods contain them, particularly because studies show intricate relationships between vitamin K levels and adverse health conditions affecting large populations. Booth summarizes, "We already know some of the health benefits of phylloquinone. More research is needed to understand the possible benefits of menaquinones and to understand the health ramifications of converting phylloquinone to dihydrophylloquinone."