1a. Objectives (from AD-416):
1. Determine the amounts of individual dietary forms of vitamin K in nationally representative samples of frequently consumed U.S. foods and dietary supplements. 2. Characterize the effects of dietary and non-dietary factors, such as age, lipid profile and body fat, on the bioavailability and utilization of different forms of vitamin K in humans. 3. Identify mechanisms of action for vitamin K, other than its classic role as an enzyme cofactor, using cellular and animal models. 4. Determine the associations between vitamin K nutritional status and risk of abnormal non-skeletal calcification.
1b. Approach (from AD-416):
Laboratory analysis of different forms of vitamin K will be conducted in selected foods obtained through collaboration with the USDA Nutrient and Data Laboratory (NDL), as part of the Food and Nutrient Analysis Program. Priorities for food analysis will include dietary supplements, food purchased in family style restaurants, foods common to the Hispanic/Latino diet, and foods associated with high calorie diets. Food composition data will be transferred to the NDL for entry into national food composition databases. To identify dietary and non-dietary factors that determine how much vitamin K obtained from foods is utilized, we will apply stable isotope techniques to measures of vitamin K metabolism. Data obtained from ongoing metabolic studies in men and women, in addition to pilot feasibility studies, will be used to refine the study design to test the response of these measures to intake of different vitamin K-rich food sources. Animal models will be used to identify tissue-specific effects of interactions between vitamin K and other fat-soluble vitamins, with an emphasis on vitamins A and D. To identify mechanisms of action for vitamin K other than its classic role as an enzyme cofactor, urinary and serum levels of vitamin K metabolites will be measured in response to vitamin K supplementation using archived samples from human and animal studies. We will then focus on the role of different forms of vitamin K in inflammation through the inactivation of nuclear receptors in macrophages.
3. Progress Report:
To establish dietary recommendations for vitamin K, it is important to identify factors that influence individual response to vitamin K intake. There is well-documented variation in the absorption of vitamin K among adults. Possible factors attributed to these variations are age and sex differences, influence of plasma triglycerides on transport of vitamin K, and body size. To assess this variation among healthy adults, collard greens labeled with stable isotopes were fed to 42 younger and older adults who were pre-conditioned to either a vitamin K-depleted or a vitamin K-repleted state. Variations in age, sex, body size, and triglycerides accounted for 49% of the variability in vitamin K absorption. Specifically, older adults, adults with a smaller body size, and adults with high plasma triglycerides had greater absorption of vitamin K compared to younger and overweight/obese adults. Of interest was the observation that adults absorbed vitamin K as efficiently when in a vitamin K-depleted state as when in a vitamin K-repleted state. Future research is required to identify the remaining ~50% of the variation in vitamin K absorption among healthy adults. Bacterially-synthesized forms of vitamin K, menaquinones, may also contribute to vitamin K nutriture but are not accounted for in current vitamin K dietary recommendations. There are limited data on inter-individual variation in gut bacteria synthesis of menaquinones and their relation to serum vitamin K concentrations, which are critical to understand for design of future studies on vitamin K requirements. Serum and fecal vitamin K concentrations were assessed in 13 healthy adults following the consumption of a standardized diet for 14 days. Fecal content of phylloquinone, the primary dietary form of vitamin K, accounted for less than 10% of total fecal vitamin K content, and was not correlated with serum phylloquinone concentrations. In contrast, there was an abundance of multiple menaquinone forms in fecal samples that varied in form and content among individuals. Menaquinones were not detectable in serum. Based on these findings, we conclude that the majority of menaquinones are present in the human colon, with evidence of large variation among individuals.