Vitamin K in the U.S. diet

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Vitamin K in the U.S. diet

http://www.news-medical.net/?id=18313

Much of what is known about the content of vitamin K in the US food

supply comes from research conducted in the Vitamin K Laboratory at

the 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 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, MD, 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. "

http://www.tufts.edu