Digestion, Hydration, and Energy Balance

Water is the single largest component of the human body by mass, accounting for roughly 50 to 60 percent of total body weight in adults, and it is essential for regulating body temperature, transporting nutrients and waste products, lubricating joints, and serving as the medium in which virtually all metabolic reactions occur. Fluid balance is maintained through a tightly regulated system involving the hypothalamus, which senses changes in blood osmolality and triggers thirst, and the kidneys, which adjust urine concentration and volume under the influence of antidiuretic hormone (vasopressin). Water is lost continuously through urine, sweat, respiration, and feces, and these losses must be replaced through both drinking fluids and consuming water-containing foods, since even fruits and vegetables can be more than 80 percent water by weight. Even mild dehydration, corresponding to a loss of as little as 1 to 2 percent of body weight in water, can measurably impair cognitive performance, mood, and physical exercise capacity, while more severe dehydration disrupts electrolyte balance and can become life-threatening; conversely, drinking excessive amounts of water in a short period can dilute blood sodium levels and cause a rare but dangerous condition known as hyponatremia.