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Executive summary and conclusions from the European Hydration Institute expert conference on human hydration, health, and performance D. Benton, H. Braun, J.C. Cobo, C. Edmonds, I. Elmadfa, A. El-Sharkawy, J. Feehally, R. Gellert, J. Holdsworth, M. Kapsokefalou, W.L. Kenney, J.B. Leiper, I.A. Macdonald, C. Maffeis, R.J. Maughan, S.M. Shirreffs, P. Toth-Heyn, and P. Watson On April 7–8, 2014, the European Hydration Institute hosted a small group of experts at Castle Combe Manor House, United Kingdom, to discuss a range of issues related to human hydration, health, and performance. The meeting included 18 recognized experts who brought a wealth of experience and knowledge to the topics under review. Eight selected topics were addressed, with the key issues being briefly presented before an in-depth discussion. Presented here is the executive summary and conclusions from this meeting.
INTRODUCTION
DEFINITIONS AND ASSESSMENTS
Body water content is defended within narrow limits by regulation of both water intake and water loss. However, the regulation is not perfect, and dyshydration, which is a positive or negative deviation from the state of euhydration, can occur. Transient mild hypohydration is common, and probably of little consequence, but acute or chronic severe hypohydration has adverse effects on both health and performance. Hyperhydration is less common, but may not be as benign as is usually assumed. An understanding of the factors that affect water balance is critical to the development of strategies to optimize health as well as physiological and cognitive function.
Water loss from the body is continuous, in the form of respiratory and transcutaneous losses, urine, fecal loss, and sometimes sweat, but intake is episodic, so the body water content is never constant. Euhydration is assumed to be the normal state for the majority of the population over a typical 24-hour period. Water balance can be achieved at high or low turnover rates provided intake is sufficient to match losses. Deviations from euhydration can be assessed in various ways, but the definition and the method of assessment will depend on the context. For most purposes, it is appropriate to use a global clinical assessment that includes a variety of markers of hydration status: these might include serum
Affiliation: D. Benton is with the University of Wales Swansea, UK. H. Braun is with the German Sports University, Cologne, Germany. J.C. Cobo is with The Coca-Cola Company, Belgium. C. Edmonds is with the University of East London, UK. I. Elmadfa is with the University of Vienna, Austria. A. El-Sharkawy is with the University of Nottingham, UK. J. Feehally is with the University of Leicester, UK. R. Gellert is with the Szpital Bielan´ski, Poland. J. Holdsworth is with the European Hydration Institute, UK. M. Kapsokefalou is with the Agricultural University of Athens, Greece. W.L. Kenney is with the Penn State University, USA. J.B. Leiper is with the University of Dundee, UK. I.A. Macdonald is with the University of Nottingham, UK. C. Maffeis is with the University of Verona, Italy. R.J. Maughan is with the School of Sport, Exercise and Health Sciences, Loughborough University, Loughborough, UK. S.M. Shirreffs is with Glaxo SmithKline, United Kingdom. P. Toth-Heyn is with Semmelweiss University, Hungary. P. Watson is with Loughborough University, UK. Correspondence: R.J. Maughan, School of Sport, Exercise and Health Sciences, Loughborough University, Loughborough, LE11 3TU, UK. E-mail:
[email protected]. Key words: European Hydration Institute, executive summary, hydration, public health. C The Author(s) 2015. Published by Oxford University Press on behalf of the International Life Sciences Institute. All rights reserved. For V
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doi: 10.1093/nutrit/nuv056 Nutrition ReviewsV Vol. 73(S2):148–150 R
and/or urine osmolality and 24 h urine volume. It is important to recognize that individual variability exists and this, perhaps, precludes the use of exact boundaries between euhydration and dyshydration. Water intake includes water from all sources, including the water content of all drinks and foods. The water of oxidation generated by substrate oxidation should be included in intake, though it is often ignored. EFFECTS OF FLUIDS AND FOODS ON WATER AND SALT BALANCE In humans, water and sodium balance are tightly regulated by neuroendocrine control systems. These finely tuned physiological mechanisms adjust thirst and sodium appetite as well as renal excretion of fluid and sodium to maintain homeostasis. However, there is large individual variation, which comes from a variety of sources, including physiology, culture, and behaviour, and so-called “normal” values should be treated with caution. Stimulated by changes in the volume or electrolyte content of the extracellular fluid, these control systems enact appropriate compensations to bring sodium balance back to within a narrowly defined range. Typically, these systems are highly accurate and work in concert, with changes in intake compensated for by changes in output. The kidneys cope well with extremes of salt and water intake, or salt and water loss. In health, these control mechanisms allow a range of urine sodium excretion from 400 mmol/24 h, a range of water excretion from 400 ml to >10 000 ml/day, and a range of urine osmolality from 50 to 1200 mosm/kg. Exceeding these limits may adversely affect renal function and health. However, in specific instances of pathological dysregulation of these sensitive mechanisms, the resultant hyper- or hypo-osmolality of the extracellular fluid may lead to acute and/or chronic disease. FATE OF INGESTED FLUIDS: FACTORS AFFECTING GASTRIC EMPTYING AND INTESTINAL ABSORPTION The volume of fluid ingested for rehydration is critical in determining the restoration of euhydration. However, the formulation of ingested beverages is also important as this will affect the rate at which the water content of the drink enters the body water pool and its subsequent disposition, thus modifying its effect on hydration status. There is no net absorption of water in the stomach, so the rates of gastric emptying and small intestinal absorption of consumed drinks critically affect the speed at which the water portion of a beverage is assimilated into the water pool. Both a high volume and a low energy content of a drink improve the rate of gastric emptying. Net water uptake from the proximal Nutrition ReviewsV Vol. 73(S2):148–150 R
intestinal lumen occurs along osmotic gradients that are promoted by the absorption of solute. The active cotransport of solutes such as glucose and amino acids with sodium enhances water absorption, and moderate hypotonicity of the luminal contents promotes water absorption. Coingestion of food will also influence the fate of ingested beverages. EFFECTS OF DEHYDRATION ON MOOD, WELL-BEING, AND MENTAL PERFORMANCE When dehydration reduces body mass by over 2% there are consistent reports that mood is influenced; fatigue is greater and alertness lower. The effects on cognition have been less consistent, perhaps because of inadequate methodologies. There has been virtually no attempt to look at the small changes in hydration status within the range that occurs in those living a sedentary life in a temperate climate. Few studies have looked at females and there has been little attempt to consider hormones that influence kidney functioning. There are, however, four acute intervention trials reporting that the cognition of children responds positively to water consumption, and three such trials in adults. There is little or no information on older adults, who may be at particular risk of dehydration. ACUTE AND CHRONIC EFFECTS OF HYDRATION STATUS ON HEALTH There is a growing body of evidence supporting a link between states of fluid imbalance and disease. However, in some cases the evidence is largely associative and lacks consistency owing to varying definitions of euhydration, dehydration, and overhydration, as well as a limited number of randomized trials. There are reported associations between acute and chronic hypohydration and a range of urological, gastrointestinal, circulatory, and neurological disorders, although the evidence is, at times, inconsistent. There is good evidence linking iatrogenic salt and water overload in surgical patients with generalized edema, gastrointestinal dysfunction and poor postoperative outcome. There is an urgent need for well-designed studies with appropriate methodologies to further clarify the effect of dyshydration on health. EFFECTS OF ACUTE AND CHRONIC HYPOHYDRATION ON KIDNEY HEALTH AND FUNCTION Renal tract calculi produce morbidity – pain, infection, and need for intervention to remove obstructing stones. Undiagnosed and untreated renal tract stones are also associated with development of renal failure. Despite 149
interest in understanding and controlling underlying metabolic predispositions to stone formation, the single clinical factor known reliably to reduce stone recurrence is high water intake. Stones are a common problem and their management leads to a high health economic burden. There may be a significant reduction in health costs with implementation of a high fluid intake strategy. Maintaining a fluid intake of >2 L/day is estimated to reduce recurrence risk by 55%. High fluid intake may reduce infection risk in individuals with recurrent urinary tract infection, whether or not there is a structural or mechanical predisposition to infection, e.g., renal tract calculi or bladder outflow obstruction. There is no definitive evidence that low fluid intake on its own results in chronic kidney disease. IMPLICATIONS OF BEVERAGE CHOICES FOR METABOLIC AND CARDIOVASCULAR HEALTH There is continuing interest in the potential beneficial or detrimental effects of various beverages on health, but the current views are based primarily on observational studies. In particular, claims have been made that the flavonoids in tea and cocoa affect cardiovascular function and reduce the risk of cardiovascular disease, whilst positive and negative effects of orange juice and milk have been reported. Whilst alcohol appears to increase the risk of high blood pressure (BP), red wine has been linked with reduced risk of cardiovascular disease. Sugar-sweetened beverages and fruit juice contribute to total energy intake, but the intake of these beverages varies greatly among individuals. IMPLICATIONS OF ACTIVE LIFESTYLES AND ENVIRONMENTAL FACTORS FOR WATER NEEDS Water loss from the body at rest in a cool environment is generally small and water requirements are, therefore, correspondingly small, with a daily basal water requirement of about 1000-1500 mL/d. Water generated by oxidative metabolism will contribute about 300-400 mL/d, so the requirement for liquid intake is rather small. Several factors will increase the rate of water loss from the body and, therefore, cause corresponding increases in requirement. While metabolic water may represent a significant fraction of the requirement when losses are
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small, this does not increase in proportion to losses. Sweat rate may reach 2–3 L/h or 10–12 L/d in occupational or recreationally active individuals in hot environments, and such losses can be sustained on a daily basis without development of cumulative hypohydration. Hypohydration, if sufficiently severe, will adversely affect all aspects of physical and mental performance.
PATTERNS OF WATER INTAKE ACROSS THE EUROPEAN UNION Water and other beverages are generally available to the population without limitations, but consumption patterns show some variability across Europe, with drinking patterns and quantities being influenced by age, gender, diet, and activity level among other factors. In nutrition surveys from the Netherlands, Denmark, Spain, Austria, and Germany, the average fluid intake was generally within the recommended range of 1500– 2000 mL/d. The amount consumed increases with the frequency of intake, but some population groups consumed less than others. Daily water intake and beverage choice are influenced by various factors including taste, habit, culture, the composition of the diet, and other factors. A diet rich in fruit and vegetables is associated with higher water intakes and better hydration status. Though water contributes the largest part to total fluid intake in all countries, tap water consumption was highest in the northern European countries (Finland, Denmark) and lowest in the central-eastern European countries with the exception of Austria. Germany, in particular, had a low intake, but was leading in fruit and vegetable juices, soft drinks, and bottled water.
CONCLUSION There is evidence to support an association between aspects of hydration and outcomes relevant to human health and performance, but there are many inconsistencies in the available evidence. This inconsistency is the result of variations in the definitions used, lack of a standard method for assessment of hydration status, use of inappropriate methodologies, and the large individual variation in physiological function.
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