**1. Introduction**

86 Biomarker

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The traditional approach of nutritional assessment is to survey the amount of nutrients consumed by dietary assessment. Although this method can provide approximate intake, this approach often makes misreporting, and can't determine nutritional status. Especially, to determine micronutrient intake by dietary assessment is difficult because of high variations in habitual micronutrient intake. A nutritional biomarker can be an indicator of nutritional status with respect to intake or metabolism of dietary constituents. The nutritional biomarkers can be designated into one or more of three categories, 1) a means of validation of dietary instruments, 2) surrogate indicators of dietary intakes, or 3) integrated measures of nutritional status for a nutrient (Potischman & Freudenheim, 2003). Recent validation studies have developed the urinary compounds as nutritional biomarkers to estimate nutrient intakes. For example, 24-hr urinary nitrogen has been established as a biomarker for protein intake (Bingham, 2003), same as urinary potassium and potassium intake (Tasevska et al., 2006), and urinary sugars for sugar intake (Tasevska et al., 2005).

Water-soluble vitamins are absorbed from the digestive tract after ingestion, stored in the liver, delivered to peripheral, and then excreted to urine (Food and Nutrition Board, Institute of Medicie, 1998). Urinary water-soluble vitamins or their metabolites decrease markedly as vitamin status declines, and they are affected by recent dietary intake (Food and Nutrition Board, Institute of Medicie, 1998). Urinary excretion of water-soluble vitamins such as thiamin, riboflavin and niacin has been used for setting Dietary Reference Intakes (DRIs) in USA and Japan (Food and Nutrition Board, Institute of Medicie, 1998; The Ministry of Health, Labour, and Welfare, 2009). Although pharmacological dose of watersoluble vitamin intake such as vitamin B2 (Zempleni et al., 1996), nicotinamide (Shibata & Matsuo, 1990) and biotin (Zempleni & Mock, 1999) dramatically increase urinary vitamin levels, a few study had studied about the relationship between several oral dose correspond to dietary intake and urinary excretion of vitamin C (Levine et al., 1996, 2001). Thus, little attention had been paid to assess the quantitative relationships between intakes and urinary excretion of water-soluble vitamins. However, only a single study had investigated urinary vitamin as a possible marker for intake until 2007. Individuals' 30-day means of thiamin intake are highly correlated with their mean 24-hr urine thiamin levels under strictly controlled condition, showing 24-hr urinary thiamin as a useful marker for thiamin intake under strictly controlled conditions (Tasevska et al., 2007).

Urinary Water-Soluble Vitamins as Nutritional Biomarker to Estimate Their Intakes 89

with urine volume even on the day before, the day of, and the day after intake (Fig. 1B-D). These results clearly showed that urinary excretion of vitamin B12 was dependent uponurine volume, but not on intake of vitamin B12.Vitamin B12 is different from other B-group vitamins with respect to main excretion route, which is through the bile, and <10% of the total loss of vitamin B12 from the body is through urine (Shinton, 1972). These results suggest that the change in the level of urinary vitamin B12 is too small to evaluate intake of vitamin B12, and thus urinary vitamin B12 was unavailable to be used as biomarker for estimation of its intake. To excrete vitamin B12 into urine, vitamin B12 binds to carrier protein transcobalamin (TC) in serum (Allen, 1975), the TC–vitamin B12 complex is filtered in the glomeruli, and the proximal convoluted tubule reabsorbs this complex via a receptormediated system (Birn, 2006). Megalin is an essential receptor for reabsorption of the TC– vitamin B12 complex in the proximal tubule (Birn et al., 2002), binds to the TC–vitamin B12 complex with an estimated affinity (*K*d) of ~183 nmol/L (Moestrup et al., 1996). This high affinity may explain why urinary loss of vitamin B12 is very low. However, little is known about how water regulation mediated by regulatory factors such as aquaporin, vasopressin

Fig. 1. Effect of administration of a pharmacologic dose of cyanocobalamin on urinary concentration of vitamin B12 (A) and the correlations between urinary vitamin B12 and urine volume on the day before cyanocobalamin intake (B), the day of intake (C) and the day after

intake (D) (Fukuwatari et al., 2009).

and angiotensin is linked to reabsorption of vitamin B12.

In the present review, recent findings from our intervention and cross-sectional studies are described to contribute to the establishment and effective use of urinary water-soluble vitamins as potential nutritional biomarkers. Furthermore, we propose the reference values for urinary water-soluble vitamins to show adequate nutritional status based on the findings. Our findings suggest that urinary water-soluble vitamins can be used as nutritional biomarkers to assess their mean intakes in groups. More accurate estimation of individuals' water-soluble vitamin intakes based on urinary excretion requires additional, precise biological information such as the bioavailability, absorption rate, and turnover rate.
