Low plasma phylloquinone was associated with high circulating concentrations of proinflammatory markers (CRP and IL-6) and low concentrations of osteoprotegerin in older men and women. Our cross-sectional findings are generally consistent with in vitro (11
) and observational (16
) data, and collectively they suggest a potentially protective role for vitamin K in inflammation.
Osteoprotegerin is an antiresorptive cytokine that is believed to protect against bone resorption. High %ucOC was associated with low osteoprotegerin, which is consistent with high plasma phylloquinone being associated with low osteoprotegerin in the same cohort. In vitro, vitamin K treatment increased osteoprotegerin production in bone marrow cells (25
), and, in one small study of patients being treated with glucocorticoid medication for kidney disease, vitamin K treatment prevented a glucocorticoid-induced reduction in serum osteoprotegerin (26
). In contrast, an inverse association between plasma phylloquinone and serum osteoprotegerin was previously reported in a community-based cohort (16
). Circulating osteoprotegerin increases with age (27
), and our cohort was older (mean age: 68 y) and within a narrower age range (60–81 y) compared with the community-based cohort, in which the mean age was 59 y (range: 35-89 y) (16
). This age-related increase would also explain the significant 3-y increases in circulating osteoprotegerin we observed in both the treatment and nontreatment groups.
When vitamin K status is low, osteocalcin is not fully carboxylated, so a high %ucOC is indicative of poor vitamin K status. Therefore, the association between high %ucOC (ie, poor vitamin K status) and low circulating IL-6 does not support our overall hypothesis that improved vitamin K status is associated with less inflammation. Circulating IL-6 and CRP were significantly inversely associated with total osteocalcin in our cohort, as was reported elsewhere (29
). Measurement of total osteocalcin captures both carboxylated and uncarboxylated forms of osteocalcin, so it is not a measure of vitamin K status. Instead, it is a measure of bone formation, independent of the posttranslational role of vitamin K as an enzyme cofactor. Therefore, we cannot dismiss the possibility that the total osteocalcin was responsible for an inverse association between %ucOC and IL-6. In contrast, total osteocalcin was not significantly associated with circulating osteoprotegerin, whereas low osteoprotegerin concentrations were consistently associated with low phylloquinone and high %ucOC in our study.
Phylloquinone supplementation did not change cytokine concentrations in this cohort of older men and women. The null findings of our intervention trial, which are not in agreement with the cross-sectional associations, are not readily explained by the in vitro studies (12
). Changes in circulating cytokine concentrations were small in our cohort in comparison to other nutrient intervention studies that have reported improvements in inflammatory cytokines (30
) and serum osteoprotegerin (26
) among persons with heart and kidney diseases respectively. This attenuated response may reflect a selection bias because participants in our study were generally free of chronic disease because of the exclusion criteria used for acceptability into the study. This is consistent with the results of other supplementation studies in which the participants were generally healthy and inflammatory cytokines remained unchanged (31
). That neither group in our study showed a detrimental change in circulating cytokine concentrations after 3 y may also contribute to the lack of measurable effect of phylloquinone supplementation on these cytokines. In contrast, this does not explain why we observed a consistent, cross-sectional association between vitamin K status and concentrations of circulating cytokines. One possible explanation is that biochemical measures indicative of a poor vitamin K status are consistently associated with a less healthy diet and lifestyle compared with those of persons with high serum concentrations of phylloquinone and low %ucOC (33
). The lack of cytokine response to supplementation of 500 μ
g phylloquinone/d suggests that in the cross-sectional data the biochemical measures of vitamin K are surrogate measures of other dietary or lifestyle factors that may influence circulating cytokine measures in this cohort. In addition, although a supplemental dosage of 500 μ
g/d exceeds the current adequate intake of 90–120 μ
), those studies that reported a beneficial effect of phylloquinone supplementation on health outcomes supplemented with a dosage of 1000 μ
). The dosage chosen for the current study may have been insufficient to confer any beneficial effect on cytokine concentrations. Furthermore, dietary intakes of phylloquinone were above the current recommended adequate intakes at baseline and did not change significantly throughout the study. It is plausible that the high intakes attenuated any beneficial effect of vitamin K on our measures of inflammation.
Although a role for inflammation in bone resorption was suggested (7
), we did not observe associations between plasma IL-6 or CRP and BMD either cross-sectionally at baseline, before randomization (data not shown), or during the 3 y of follow-up. It was suggested that the predictive effect of serum IL-6 on bone loss in women is attenuated after the first decade of menopause (38
). Because the women in our study were, on average, >10 y postmenopausal, the possible association between circulating IL-6 and BMD may have been attenuated.
Associations between serum osteoprotegerin concentrations and BMD are inconsistent in men (39
) and women (40
), which may be related to the potential modulation of osteoprotegerin by sex hormones (40
). Because the clinical significance of the measure of circulating osteoprotegerin remains to be clarified (42
), the meaning of the association between vitamin K status and circulating osteoprotegerin with respect to bone health is also uncertain.
All participants received daily supplementation with 600 mg elemental calcium and 400 IU vitamin D, which has been shown to reduce bone loss in older men and women over 3 y (43
) The attenuation in bone loss because of the calcium and vitamin D supplementation, in addition to the nonstatistically significant changes in circulating IL-6 and CRP from baseline to year 3, may partially explain our findings of no association between circulating proinflammatory cytokine concentrations and the 3-y change in BMD. Furthermore, the influence of cytokines on bone is local, so circulating measures may not be sensitive enough to reflect the cytokine activity in the microenvironment of skeletal tissue (38
In summary, poor vitamin K status was associated with high concentrations of the cytokines, IL-6 and CRP, in a cross-sectional analysis of older men and women. The use of %ucOC as a measure of vitamin K status was limited by the potential confounding effect of cytokine concentrations on total osteocalcin. However, supplementation with phylloquinone (vitamin K1
) at a dosage of 500 μ
g/d for 3 y did not confer a decrease in inflammatory cytokine concentrations in our cohort. Similarly, although cross-sectional results suggest a positive association between vitamin K status and plasma osteoprotegerin, phylloquinone supplementation did not change plasma osteoprotegerin in this cohort. Because markers of vitamin K status are reflective of an overall healthy diet, the cross-sectional observations may reflect a general influence of healthy dietary patterns on serum IL-6, osteoprotegerin, and CRP. Alternatively, in this generally healthy cohort of older men and women, the concentrations of IL-6 and CRP did not change during the 3-y study, so any putative effect of vitamin K supplementation would have been attenuated. The health status of this cohort may also partially explain the lack of association between circulating cytokines and BMD changes over 3 y. However, the role of vitamin K supplementation in modulating measures of circulating cytokines among persons at greater risk of inflammation, such as the elderly (45
) or those with cardiovascular, metabolic, or rheumatic diseases (30
), merits investigation.