Water is life’s mater and matrix, mother and medium. There is no life without water.
—1937 Nobel Prize Recipient, Albert Szent-Gyorgyi1
The importance of water for life is obvious, but its role in promoting human health has been slow to be recognized. This collection of articles attempts to shed some light on the importance of water in promoting optimal health. One of the major challenges in addressing water requirements is lack of consistency in global recommendations. Shirreffs2 reviewed global patterns of water intake for 5 different countries and found that it is difficult to compare intakes among countries because of differing methodologies used to collect water intake data. Data collection ranged from conducting 24-hour recalls to 7-day dietary records. Shirreffs points out that water requirement is based on water losses, and perhaps we should consider body water losses as the basis for recommendations for water intake rather than intake data. Shirreffs also reminds us that because body water needs are predicated on environmental temperature, physical activity, and unique individual physiology, “a low water intake will not necessarily be associated with hypohydration, and a high water intake will not necessarily be associated with euhydration or overhydration.”
Another area of great concern in all countries is that of overweight and obesity. Could water play a role in curbing obesity? Older adults who consumed 16 oz of water 30 minutes before an ad libitum meal reduced the number of calories consumed at that meal.3 Akers and colleagues4 have shown that water consumption is an integral part of maintenance of weight loss and weight maintenance. As a follow-up to a weight loss study, where participants consumed 16 oz of water 30 minutes prior to breakfast, lunch, and dinner, researchers continued the study with a weight maintenance phase for 12 months. Two groups of women (average age, 63 years) were assigned to follow with the intervention diet (1200–1500 cal/d plus 16 oz of water prior to main meals) or control diet (1200–1500 cal/d). In addition, the participants set goals for the weight maintenance phase including daily monitoring of body weight, walking 10 000 steps or more, and eating 5 or more servings of fruits and vegetables; the intervention group continued to drink 16 oz of water before meals. At the end of the 12-month intervention, success was defined as a body weight no more than 3% weight regain from baseline. Using that definition, 80% of participants were successful in maintaining weight loss. Ingesting water prior to meals provided an additional benefit, and those in the intervention group lost more weight than did those in the control group.
The interest in water for a variety of health concerns is explored in this supplement. The articles cluster around 4 themes:
1. how the body regulates water and the challenges in measuring hydration status
2. water requirements
3. hydration issues throughout the life cycle, including disease prevention
4. behavioral changes to improve water intake and hydration
HOW THE BODY REGULATES WATER AND THE CHALLENGES IN MEASURING HYDRATION STATUS
Thirst is the physiological driver of water intake, but in the modern world other factors, such as availability and palatability of fluids, are also important. I have often observed athletes playing basketball, dripping with sweat and in obvious need of fluids, who go thirsty rather than walking a few steps to quench their thirst at the water fountain; however, when those same athletes are handed a bottle of fluids by an athletic trainer, they eagerly drink! Kavouras, in the article, “Thirst: Survival Instinct or Sensitive Fluid Balance Homeostatic Mechanism,” differentiates between thirst for survival and adequate hydration. He presents evidence that even when active people start physical exercise in a hypohydrated state, they do not drink enough fluids to enhance hydration. So, although thirst may be important for survival, it is not enough to keep us well hydrated.
Hormonal control of thirst and kidney control of water balance are addressed by articles authored by Thornton and Bankir. Thornton reviews the hormonal control of water balance through the secretion of antidiuretic hormone in response to increased osmolality (initiated by intracellular dehydration) and angiotensin II release in the kidney in response to extracellular dehydration. Normally, thirst corrects both intracellular and extracellular dehydration; however, population studies of fluid consumption reveal that behavior does not always match the need for fluids and can override normal thirst mechanisms. Bankir targets the kidney’s control of fluid balance through its role in concentrating urine. Antidiuretic hormone or vasopressin plays a key role in urine concentration, thereby allowing more water to be reabsorbed in the kidney tubules. For most of our history, the need to conserve water was paramount to survival, but today, with adequate fluids for most people, the kidney works hard to excrete solutes, especially urea/nitrogenous wastes. Consuming adequate fluids helps the kidney flush the wastes and reduces the risk of hyperfiltration by the kidney, which could lead to kidney disorders. Many active people, and athletes in particular, overconsume protein (both dietary protein and protein from supplements), so adequate fluids to dilute wastes are important for renal function and long-term health.
Johnson and Armstrong compared small- and large-volume water drinkers and the ability to regulate body water. Low water drinkers and high water drinkers participated in 4 days of modified water intake where the low drinkers and high drinkers “switched” drinking habits, and measures of hydration were assessed, as well as thirst. The researchers found that both the habitually low and high water drinkers maintained normal hydration during the experiment. Habitually low water drinkers may have an enhanced ability to concentrate urine to maintain water balance even in the face of chronic low water intake.
Perrier and Klein describe the short-term effects of increased water intake and the challenges of measuring hydration in a clinical setting. Although no criterion standard exists for measuring hydration status, various measures including 24-hour urine collections, plasma and urine osmolality, urine specific gravity, and urine volume and color have all been used to assess hydration. Participants in Perrier and Klein’s study were grouped into “low” and “high” drinkers and completed a 5-day crossover intervention following prescribed fluid intakes. All participants were inpatients to better control timing and volume of fluid intakes. Plasma osmolality was measured twice a day. The authors concluded that plasma osmolality is well controlled over a variety of hydration conditions, but urine osmolality and urine specific gravity and color might be useful to track daily hydration and could be helpful as a screening tool in a clinical setting.
Clark and colleagues ask the question: Is drinking at least “8 glasses of water a day” needed to be healthy? This question was asked in a 2002 article by Valtin,5 who concluded that despite the widespread belief that 8 glasses of water a day were needed for good health there was little scientific support for the statement. In this supplement, Clark asked the same question but in relationship to the progression of chronic kidney disease, and he suggests a randomized controlled trial is needed to fully understand the relationship of water intake to kidney disease.
HYDRATION ISSUES THROUGHOUT THE LIFE CYCLE, INCLUDING DISEASE PREVENTION
In articles that look at 3 life-cycle stages, the authors discuss research on issues surrounding hydration in children and adolescents, women, and the elderly. Moreno et al describe water and beverage intake in more than 2700 European adolescents using 24-hour recalls and found that sugar-sweetened beverages were the most consumed beverages in young adolescents, and only 12% of the study population was drinking water during the 24-hour period. Gomez and colleagues describe an interesting intervention in French women to increase water intake. In this study, women who were classified as low water drinkers (<1.2 L of total fluid intake and <300 mL of water/d) were encouraged to increase water intake through the home delivery of bottled water and instructed to gradually increase water intake over a 3-week period. Bottled water continued to be delivered for an additional 4 months coupled with online education on the benefits of water. At the end of 1 year, the low water drinkers had increased their water consumption by 163%, long after the free water delivery had stopped. Mentes reviews concerns about hydration in the elderly and notes that both physiological changes and behavioral issues compound the problem of inadequate fluid intake. Older adults have blunted thirst in response to dehydration, drink less water as they age, and clinicians have a difficult time assessing hydration in this age group. As the world’s population ages, the need for more research on hydration in older adults and a rapid assessment tool for hydration are needed.
Yotan presents data on hydration and the prevention of kidney stones, as there is very little attention paid to primary prevention of this disease. Fluid intake is inversely related to kidney stone formation, and as we have seen from other articles in this supplement, fluid intakes in most populations are inadequate. Yotan estimates that high fluid intakes could cut the risk of kidney stones in half, thereby saving a considerable amount of healthcare dollars on treating this disorder.
BEHAVIORAL CHANGES TO IMPROVE WATER INTAKE AND HYDRATION
Lafontan summarized an Expert Working Group Consensus for healthy hydration. The recommendations include the following:
* Include information on healthy hydration as part of dietary and lifestyle choices.
* Include information on water’s importance as a macronutrient in national dietary guidelines.
* Develop guidelines that are simple and encourage effective strategies for drinking water.
* Emphasize water during physical activity.
* Encourage more research on water needs of specific stages of the life cycle.
It is clear that there is much to be learned about water intake and a healthy life, and the articles in this supplement provide direction to further that work.