Human metabolism, nutrient transport, circulation, temperature regulation, contraction of skeletal muscles, nerve impulse transmission, regulation of fluid-electrolyte balance, and waste excretion would be impossible without water because it is the medium for all cellular and systemic processes. Thus, total body water is defended by multiple neuroendocrine mechanisms. But even though water is essential to life and optimal human performance, (a) no scientific or clinical consensus exists regarding precise values for the daily water requirements of men and women,1 and (b) surprisingly, few data exist regarding the hydration status of healthy, free-living, sedentary adults who are not under environmental or physiological stress.2 This dilemma exists, in part, because human body water balance is complex (ie, residing in interconnected intracellular and extracellular fluid compartments) and dynamic (ie, both volume and concentration change in sinusoidal patterns due to fluid and food intake, excretion, metabolism, and respiration).3 Unfortunately, the published data that describe human water requirements contain recognized limitations.4 This means that precise measurements of adult water consumption patterns are needed.
During the past decade, increasing international consumption of sugar-sweetened beverages has sparked debates about the potential negative health consequences of high-calorie beverages. For example, data5 spanning the period 1999 to 2001 indicate that beverages accounted for approximately 21% of all calories consumed in the United States. Schulze and colleagues6 associated a higher consumption of sugar-sweetened beverages with a greater weight gain and an increased risk of type 2 diabetes in women, possibly due to excessive calories and large amounts of rapidly absorbable sugars. The rapid absorption of glucose induces a sequence of metabolic and hormonal changes that promote excessive food intake.7 Other research has emphasized a parallel growth of diabetes and sugar consumption in the United Kingdom and the United States,8 as well as a new hypothesis that excessive fructose intake (ie, in sweetened beverages) and excess uric acid may promote type 2 diabetes. These recent trends prompted one panel of experts to recommend that adults and children should consume fewer sugar-containing beverages and use water as the foundation of a healthy hydration program.9
From a different perspective, investigators have hypothesized that consumption of water, before or during meals, may help overweight individuals manage their body weight by increasing satiety or altering food intake.10,11 One study of women reported that satiety increased during, but not after, a meal if water was consumed before the meal.12 A subsequent study compared energy intake when water, diet cola, regular cola, orange juice, and 1% milk were consumed with the midday meal.11 The total energy intake of this meal was greater (P < .0001; mean, 652 kJ) when a caloric beverage was consumed (vs a noncaloric fluid including water), but satiety ratings were not different among the fluids. Although promising, these studies suggest that further research is required to discern the subtle interactions between water intake, food energy content, and satiety.
Boschmann and colleagues13,14 provide a different focus: water-induced thermogenesis. These investigators measured a 30% increase of metabolic rate after obese men (n = 7) and women (n = 7) consumed 500 mL of water13; this increase (ie, approximately 100 kJ) occurred within 10 minutes and reached a maximum at 30 to 40 minutes. Similarly, a 24% increase of metabolic rate was observed when a different sample of obese men (n = 8) and women (n = 8) consumed 500 mL of water;14 however, no change of metabolic rate was measured when these subjects consumed either 50 mL of water or 500 mL of isotonic saline. These researchers concluded that this inexpensive intervention might be a useful adjunctive treatment for obese individuals, to attain an increase in energy expenditure.13 Although these were short-term studies, a report15 evaluated the effects of drinking water on weight loss, in overweight dieting women, at 3 points across 1 year. Mean daily intakes of water, noncaloric fluids, unsweetened caloric fluids (eg, 100% fruit juice, milk), and sweetened caloric fluids; food energy; and nutrients were tracked using 24-hour diet recalls at 2, 6, and 12 months. Absolute volume (in grams) and relative (percentage of all fluids) increases in drinking water were associated with a significant loss of body weight and fat over time, independent of several other factors that were statistically covaried. The authors concluded that drinking water may promote weight loss in overweight dieting women.
Although confirmatory research is required, a laboratory study16 suggested that water intake (to the point of euhydration) reduced the mean resting heart rate of healthy men who were moderately hypohydrated (3.4 % loss of body mass). Similarly, water consumption reduced their mean resting plasma osmolality (ie, a physiological index of hydration state) from a hypertonic level of 296 mOsm/kg when hypohydrated to a normal level of 287 mOsm/kg when euhydrated. The long-term health effects of chronic mild physiological strain are unknown.
Dehydration also has been shown to have behavioral effects. Studies indicate that degradation of cognitive function and mood begins at a point between 1% and 2% body mass loss.17 Such mild dehydration is experienced by most adults, at some point during their weekly activities,3 but it is unlikely that they are aware of subtle cognitive deficits.
Low dietary water intake has been associated with a number of diseases including coronary heart disease and bladder cancer.18,19 Additionally, the beneficial effects of increased water consumption on colon cancer incidence have been reported independently by 2 research teams, utilizing multivariate statistical analyses.20,21 A strong inverse dose-response relationship was reported. Women who consumed more than 5 glasses of water each day had a 45% decreased risk of colon cancer than those who consumed 2 or fewer glasses per day. Also, epidemiological evidence describes a strong association between increased water intake and a decreased risk of kidney stone formation and recurrence.22,23 For example, a group of investigators, observing 107 men and women with idiopathic calcium oxalate kidney stone disease23 recommended increased water intake to decrease the risk of nephrolithiasis. The underlying goal of this increased water intake was to produce a dilute urine.
The studies above (ie, involving nutrition, metabolism, obesity, epidemiology, physiological responses, and disease) provide ample rationale for renewed emphasis on water consumption, distinct from other beverages. With this in mind, the present supplement to Nutrition Today presents critical reviews and identifies future research directions. The scientific community, across a range of specialty areas, will benefit by considering the important theme of this issue: improving public health by optimizing water intake.
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