Content area
Full Text
1. Introduction
Water is the main component of the human body and is essential for human survival. Water plays important roles in body metabolism, maintaining electrolyte balance, modulating normal osmotic pressure, and regulating body temperature. Optimal hydration status is important for maintaining health. Both hyperhydration and hypohydration have negative impacts on health. If fluid intake exceeds the capacity of renal excretion (700-1000 mL/h) [1], it can result in acute water intoxication, and even hyponatremia. Water intoxication, along with the symptoms of brain cell swelling, cerebral edema, and increased intracranial pressure, can result in headache, nausea, and memory loss, even progressive mental retardation, trance, coma, convulsions, and death. Hypohydration occurs when the fluid intake is insufficient to replace the free water output. It has been reported that hypohydration may impair cognitive performances, such as short-term memory [2,3], psychomotor skills, vigilant attention, choice reactions, and perceptive discrimination [4]. Hypohydration also reduced the ability to perform physical activities [5,6]. In addition, hypohydration increased the incidence and the prevalence of urinary system diseases [7,8,9,10]. A clear relationship between the hardness of water and cardiovascular diseases was also reported [11,12,13]. It is crucial to maintain optimal hydration in order to ensure the normal functions of human body.
A few methods can be applied to assess hydration status, including serum osmolality, urine volume, urine osmolality, and other urine biomarkers. Serum osmolality has been suggested as a good marker for acute hypohydration but was insensitive to mild hypohydration with ad libitum access to fluid [14]. Urine biomarkers reflecting total fluid intake [15] were useful and biologically significant for assessing mild hypohydration [16,17]. Some studies have reported that 24-h urine biomarkers were strongly correlated with daily total fluid intake [18,19,20]. However, few studies about the association between fluid intake and related biomarkers have been conducted in China.
Only two surveys regarding fluid intake were carried out in 2010 and 2011. One survey for assessing fluid intake among 1483 adults from four cities in China found that approximately 32% of the subjects drank less fluid than the amount recommended by the Chinese Nutrition Society in 2007 (1200 mL/day). Another survey for assessing fluid intake among 5868 primary and middle school students from four cities reported that nearly two-thirds of the subjects drank less than...