New study adds to knowledge about
natural forms of vitamin K in dietary sources, their appreciable presence in
commonly consumed foods
Newswise, July 10, 2017—Vitamin K, with its multiple forms, is
among the lesser known nutrients. Now, new research from scientists at the Jean
Mayer USDA Human Nutrition Research Center on Aging (USDA HNRCA) at Tufts
University sheds new light on the vitamin and its significant presence in some
dairy products available in the United States.
In the study,
published June 1 in Current Developments in Nutrition, researchers
quantified the activity of two natural forms of vitamin K in dairy products of
various fat contents and found that common U.S. dairy items, including milks,
yogurts and cheeses, contain appreciable amounts of multiple forms of vitamin
K. Vitamin concentrations varied by fat content.
Vitamin K, which helps the blood to clot, is most commonly thought
to come from leafy greens such as spinach, kale and broccoli.
In fact, dietary sources of vitamin K are found in two natural
forms: phylloquinone (PK, or vitamin K1), which is widely distributed through
plant-based foods, and menaquinones (MK, or vitamin K2), which appear to be
primarily in animal products and fermented foods.
Almost all MK forms are also produced by bacteria in the human
gut. Not much is known about MK amounts in U.S. dairy products.
“Dairy foods contain minute amounts of PK, the best known of
the vitamin K forms, and so dairy is not commonly considered a rich dietary
source for this nutrient. However, when it comes to MK forms, we found that
dairy items already found in many peoples’ refrigerators are indeed a good
dietary source for vitamin K,” said Xueyan Fu, Ph.D., first and corresponding
author and scientist in the Vitamin K Laboratory at the USDA HNRCA.
Guidelines for adequate vitamin K intake are based only on PK
intake without consideration for other forms of vitamin K. MK differ from PK in
structure in that they are compounds with different numbers of isoprenoid units
in the side chain, designated as MK4 through MK13. Which forms of MK are
present reflects which bacteria might be in the dairy products. Lactic acid
bacteria, for example, are widely used in dairy and fermented foods.
To understand the presence of MK and PK in dairy products, the
researchers used 50 nationally collected dairy samples provided by the USDA
Nutrient Data Laboratory and 148 dairy samples purchased in 2016 from Boston
area retail outlets.
The products were divided into categories based on dairy types
and fat content: milks, yogurts, Greek yogurts, kefirs, creams, processed
cheeses, fresh cheeses, blue cheeses, soft cheeses, semi-soft cheeses, and hard
cheeses. The effect of fat content on total vitamin K in all forms was compared
using a two-sample T-test.
The vitamin K content of cream products, for which the
researchers had a smaller sample size, was analyzed using a general linear
model, with heavy cream as the reference group.
Among the findings:
- All
full-fat dairy products contained appreciable amounts of MK, primarily in
the forms of MK9, MK10 and MK11. Combined, these three forms of MK
accounted for approximately 90 percent of total vitamin K present in the
foods tested.
- In
cheeses, the total vitamin K content varied by type, with soft cheese
having the highest concentration, followed by blue cheese, semi-soft
cheese, and hard cheese. All of the cheeses contained MK9, MK10 and MK11,
and modest amounts of PK, MK4, MK7, MK8 and MK12. Little MK5, MK6 or MK13
was measured in the majority of cheeses.
- In
milk, the vitamin K concentrations varied by fat content; both total
vitamin K and individual MK concentrations in full-fat milk were
significantly higher than in 2 percent milk. PK was only detected in
full-fat milk. Only MK9-11 were detected in milk.
- In
yogurts, full-fat regular and Greek yogurts exhibited similar vitamin K
concentrations as in full-fat milk; neither MK nor PK were detected in
fat-free yogurt.
“Estimated intakes of PK and MK in dairy-producing countries
in Western Europe suggest that between 10 and 25 percent of total vitamin K
intake are provided by MK, and primarily from dairy sources.
Additionally, observational data from Europe suggest that MK
from dairy products have a stronger association with heart health benefits
compared with PK intakes. This data from other countries highlights the need to
analyze MK in commonly consumed foods in the U.S.,” said Sarah L. Booth, Ph.D.,
last author on the study. Booth is senior scientist and director of the Vitamin
K Laboratory at the USDA HNRCA, interim director of the USDA HNRCA, and
professor at the Friedman School of Nutrition Science and Policy at Tufts
University.
Additional research is needed to determine the role of
microbes used in production of dairy products, and their impact on MK content.
The researchers also say there is a need to determine the relative
bioavailability of all MK forms given their abundance in the U.S. diet.
The researchers acknowledge limitations of the study,
including the reliance on food labels for fat content instead of direct
measurement of fat content. Additionally, whereas the dairy product samples
obtained from the USDA Nutrient Data Laboratory were geographically representative
of the U.S. diet, those purchased in the Boston region were not. However, items
purchased locally were selected from retail outlets with national
representation.
Additional authors on this study are Stephanie G. Harshman and
Xiaohua Shen, Ph.D., Jean Mayer USDA Human Nutrition Research Center on Aging
at Tufts University; David B. Haytowitz, Ph.D., Beltsville Human Nutrition
Research Center, U.S. Department of Agriculture’s Agricultural Research
Service; J. Philip Karl, Ph.D., alumnus of the Friedman School of Nutrition
Science and Policy at Tufts University and formerly of the Jean Mayer USDA
Human Nutrition Research Center on Aging, now at the U.S. Army Research Institute
of Environmental Medicine; and Benjamin E. Wolfe, Ph.D., department of biology,
School of Arts and Sciences at Tufts University.
This work was supported by the U.S. Department of
Agriculture’s Agricultural Research Service and the National Dairy Council.
