DISCUSSION

We demonstrated that log-transformed hair Na and K concentrations positively correlated with the FRS independent of major CAD risk factors. Also, the hair Ca/Na ratio was inversely associated with CAD risk.

A previous study showed that hair Na and K concentrations were significant higher in patients with metabolic syndrome.(Park et al., 2009) In this study, hair Na and K concentrations correlated with metabolic syndrome components such as TG and fasting glucose.

The importance of dietary Na restriction to lower CAD risk is widely recognized. High Na intake not only expands plasma volume, but also elevates vascular tone. High serum Na level raises the concentrations of endogenous cardiac glycosides that inhibit the Na+, K+-ATPase on the plasma membrane of arterial smooth muscle, and results in accumulation of Na+ in the subplasma-membrane space.(Iwamoto and Kita, 2006) The retention of Na+ in the arterial smooth muscle facilitates Ca2+ entry through Na+/Ca2+ exchanger on the plasma membrane and causes vasoconstriction.

Also, dietary Na increases shear stress on arterial endothelial cells by fluid retention.

Increased shear stress stimulates Transforming growth factor-β1 production and nitric oxide (NO) synthesis in endothelial cells through augmentation of NO synthase expression.(Ying and Sanders, 1999) Transforming growth factor-β1 is a fibrogenic growth factor and overexpression produces a reversible cellular- and matrix-rich neointima in the arterial wall.

NO plays critical role in vascular remodeling as well as in BP regulation. Therefore, chronic high Na intake causes fibrosis of arteries and loss of elasticity which ultimately exacerbates CAD risk, independently of BP.(Safar et al., 2009) This may explain how the effect of hair Na is independent of BP or other CAD risk factors in our study.

It is well known that high K intake has cardio-protective effects by modifying CAD risk factors such as BP(Tunstall-Pedoe, 1999) and fasting glucose.(Chatterjee et al., 2010) However, K homeostasis is influenced by the levels of other minerals and hormones as well as dietary K intake. In a small cross sectional study, the hypotensive effect of high K intake was neutralized by concurrent Na supplementation.(Rebacz-Maron et al., 2013) Our result also showed disappearance of the relationship between hair K concentration and CAD risk,

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after adjustment for hair Na level. Aldosterone is a major endocrine regulator of K homeostasis. While aldosterone decreases plasma K level by activating Na+, K+-ATPase in the renal tubule, aldosterone stimulates intracellular K uptake(Syed and Qureshi, 2012) in the same manner and results in an elevated hair K concentration. Insulin stimulates K uptake into cells, as well. It has been confirmed that obesity and insulin resistance activates the renin-angiotensin-aldosterone system which leads to fluid retention, endothelial cell dysfunction and atherosclerosis.(Lamounier-Zepter and Ehrhart-Bornstein, 2006; Syed and Qureshi, 2012) These observations are consistent with the observations in this current study which showed that hair K concentration correlates with BMI, WC, serum insulin and HOMA-IR.

Several studies have reported the association of hair Ca with CAD risk, which still remains inconsistent.(MacPherson and Bacso, 2000; Park et al., 2009) Bacso et al. reported an inverse relationship between hair Ca level and aortic calcification(Bacso et al., 1986) and between hair Ca level and CAD mortality.(MacPherson and Bacso, 2000) Another study included hypertensive obese patients with insulin resistance who inherently had a significantly higher hair Ca concentration compared to healthy controls.(Suliburska et al., 2011) These results do not show concordance probably because Ca metabolism is regulated by numerous factors such as oral Ca intake, bone turnover, and endocrine status,(Jeruszka-Bielak and Brzozowska, 2011) which were not fully considered as variables in the previous studies. Hair Na level also impacts hair Ca level as high Na intake facilitates intracellular Ca uptake through Na+/Ca2+ exchanger as mentioned above. This may support our observation that CAD risk has a significant negative correlation with hair Ca/Na, but not with hair Ca itself.

Hair mineral analysis provides information regarding the intracellular levels of minerals such as Na and K, reflecting not only dietary intake but also pathologic and physiologic conditions. Because minerals accumulate in hair at least 10 times more compared to minerals concentrated in serum and urine, hair is more sensitive and reliable for detecting mineral imbalances in the body compared with serum and urine analysis.(Combs et al., 1982) The current study is the first study to demonstrate the relationship between hair Na, K concentration and CAD risk. This relationship persisted, independent of other major CAD risk factors such as age, BMI, BP, lipid profiles and fasting glucose. These data suggest that

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hair mineral content may be a non-invasive predictor of CAD risk.(MacPherson and Bacso, 2000)

This study has a few limitations. First, to obtain a more precise representation of Na and K metabolism in the body, more endocrine factors and life-style factors need to be assessed such as food analysis and aldosterone level. However, our analysis did include serum insulin level which contributes to the regulation of hair K concentration. Second, data were obtained from periodic health check-ups of men without any apparent sign of cardiovascular diseases and few adults with 10 year CAD risk higher than 20% were included in this study. In spite of these limitations, we successfully demonstrated distinctive differences between the low and high CAD risk groups in hair mineral levels and CAD risk factors.

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