In a 2007 article, Lawrence Armstrong1 of the Human Performance Institute at the University of Connecticut remarked that, when it came to assessing hydration status, we were still searching for the "elusive gold standard." Hydration is one of those topics, like the weather, that everyone talks about but no one can make accurate predictions about. This seems to hold true for water in the form of rain[horizontal ellipsis] and also the water in our bodies that hydrates us. Nevertheless, athletes want to know exactly how much water they need under various exercise conditions, parents want to know if certain sugar-containing beverages will make their children obese, clinicians want to know how hydration affects chronic disease risk, and everyone wants to know if they should be carrying around a water bottle all day long! But all we can often do as nutrition scientists and exercise physiologists is to give general responses because the topic remains so "elusive."

In this supplement, the authors report on what is known, or in many cases what is unknown, about hydration. Some of the articles are controversial and pose theoretical questions that may take decades of research to answer, and some review the recent advances with suggestions for future work.

In this issue, the reader will find that the "madness is in the methodology." As Armstrong points out in the article on hydration biomarkers, we will very likely never find a single biomarker that is valid and reliable for measuring hydration. However, because human beings respond to both internal and external stimuli to control water balance, maybe it is time that we should realize that assessing hydration at a single point in time is not very useful for everyday life. Armstrong suggested that those who are interested in assessing hydration should use 2 or more methods more than once a day to estimate hydration.

In the Vergne article, we learn that it is a challenge to talk about how much fluid we need when we do not know how much fluid is ingested. Surveys on food intake do not truly capture intake as most of the surveys are designed to study energy or nutrient intakes. Could a specific, 7-day fluid diary accurately capture what people are drinking? Perhaps this methodology will allow researchers to find out not only how much fluid is consumed but also the sources of the fluids. Time will tell.

Another area of hydration where few data are available is on the hydration status of active young people. It is known that exercising in the heat is more dangerous for children because they have a smaller thermoregulatory capacity while exercising, compared with adults. In his article, Kavouras argues that more research on fluid needs of children athletes is needed to help keep youth safe while exercising.

Whereas in the United States we have recommendations for water intake,2 most countries do not. The findings from the UK Fluid Intake Study, reported by Gandy, reveal that about a third of adults and more than half of the children do not meet European adequate intakes for water. The study also showed that children consumed an average of 175 kcal/d as sugar-containing beverages. The causative role of sugary beverages as opposed to excess energy from any source in rising rates of obesity is controversial. As Pereira3 notes in a review of the possible role of sugar-containing beverages in promoting obesity, "the complexity of our food supply and of dietary behavior, and how diet relates to other behaviors" makes it exceedingly challenging to find a causative link to obesity. Indeed, the very definition of "sugar-sweetened beverages" can be confusing; some use the term for sugar-sweetened soft drinks, disregarding sugar-containing fruit juice, fruit drinks, energy drinks, coffee drinks, tea drinks, and several other beverages that contain sugar. More standardized definitions would be helpful.

Johnson, Thomas, and Lanaspa take the sugar-sweetened beverage issue one step further by presenting animal data on the role of fructose in beverages as a potential culprit in not only obesity but also diabetes and kidney disease. High-fructose corn syrup is the most commonly used sweetener in the United States and is about half fructose and half sucrose, and sugar-sweetened beverages represent the major source of fructose in the diet.4 Sugar-sweetened beverages containing sucrose and fructose can be consumed rapidly, leading to an increase in fructose delivered to the liver. The metabolism of fructose in the liver can affect ATP production and lead to higher concentrations of uric acid, which could have deleterious effects on the kidney. Animal data are not a substitute for well-controlled human trials, however, the theory is intriguing and supports general guidelines calling for reduced sugar intake in the general population. Lafontan5 reviewed the research on sugar-containing beverages and obesity and noted that although there are many observational studies reporting an association between sugar-sweetened beverages and obesity, the interventions studies are fewer in number and not without methodological problems including the dose of fructose used in animal studies that far exceeds any that would be consumed by humans.4 However, a recent, short-term (10-week) intervention study in adults showed that adults consuming 25% of energy from sucrose- or fructose-containing beverages had more negative metabolic outcomes from fructose consumption (dyslipidemia, decreased insulin sensitivity, and increased adiposity). Lafontan5 concludes his review by saying, "results of intervention trials are promising, although they remain quite rare, difficult to settle, and expensive."5 Most importantly, the mechanism purported by Johnson and colleagues lacks strong human evidence for the mechanism whereby fructose is a direct cause of obesity or kidney disease.6

An intriguing article presented by Pross relates to hydration and mood. We know that mild dehydration can impair exercise performance,7 but can it also affect mood? When women were in a fluid-deprived condition, they reported more fatigue and sensations of sleepiness while also being less alert compared with women who were adequately hydrated.

In total, the articles in the issue pose some interesting questions regarding hydration[horizontal ellipsis] from issues of measurement, to obesity and chronic disease, and even to mood states. Julia Child is reported as saying, "Water is the most neglected nutrient in your diet, but one of the most vital," and that sentiment easily sums up this special issue on hydration!

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