Response to: "No doubt that dehydration impairs exercise performances and risks health."
We agree with Dr Stachenfeld that the medical community should consider exercise a priority for research and practice; however we feel it is important to address a concern raised in her response, which questioned the relative importance of systematic reviews compared to an individual randomized trial.
Systematic reviews of randomized controlled trials are now widely considered to be the highest quality evidence, and the gold standard, to determine whether treatments work or not.  This is perhaps the most fundamental tenet of evidence-based medicine- that a "fair test"  of a treatment is required to know whether it has the effects it is purported to have- and that combining multiple trials improves the strength of the evidence by providing additional observations on more people in different situations. By obtaining all the available trials in an objective and reproducible manner, it is possible to further understanding of whether the results in an individual trial constitute a "one-off" or if they are representative of a larger pattern of effectiveness, i.e., the true effect.
In essence, a systematic review ensures that all the available evidence is taken into account, and reduces the chance that certain trials with positive (or negative) effects have been "cherry-picked" to support a certain position. Failure to conduct systematic reviews in medicine has had disastrous consequences; the delay in wide spread acceptance to give steroids to women in preterm delivery is just one of many examples. 
This is why "position papers" that are not supported by systematic review are considered the lowest quality of evidence. It is easy enough for 'experts' in a certain area to engage in this "cherry-picking" (perhaps albeit unconsciously) that is detrimental to the scientific method and violates basic principles of evidence-based medicine. Reviewing evidence in this way, potentially eschewing negative or ambiguous results in service of some outcome selected a priori, does not answer the question of whether a product is likely to have its purported effect for a certain person in a satisfactory manner.
The quality of evidence depends on the question being asked.  When testing treatments or products, the highest quality evidence involves reporting and/or combining (through meta-analysis) the results of as many randomized controlled trials as possible.
Expertise is valuable but subjective; systematic review seeks to solve this problem by ensuring that all available evidence is taken into account. When researchers misunderstand this fundamental principle of evidence-based medicine, they fail to fulfill their obligation to help clinicians, scientists and most importantly the general public make educated decisions.
It is increasingly important for clinical epidemiologists and the medical community at large to take an interest in exercise as a preventive measure against non-communicable diseases and as part of a healthy lifestyle.  By encouraging a widespread understanding of the fundamentals of evidence-based medicine we hope to support the research community's efforts to do so in a rigorous and appropriate manner.
1. OCEBM Levels of Evidence Working Group: JH, Paul Glasziou, Trish Greenhalgh, Carl Heneghan, Alessandro Liberati, Ivan Moschetti, Bob Phillips, Hazel Thornton, Olive Goddard and Mary Hodgkinson. The Oxford 2011 Levels of Evidence. 2011;http://www.cebm.net/index.aspx?o=5653.
2. Evans I, Thornton H, Chalmers I, Glasziou P. Testing Treatments: Better Research for Better Healthcare. 2011/12/16 ed. London: Pinter and Martin, 2011.
3. Glasziou P, Vandenbroucke JP, Chalmers I. Assessing the quality of research. BMJ 2004;328(7430):39-41.
4. Lee IM, Shiroma EJ, Lobelo F, Puska P, Blair SN, Katzmarzyk PT. Effect of physical inactivity on major non-communicable diseases worldwide: an analysis of burden of disease and life expectancy. Lancet 2012;380(9838):219-29.
Conflict of Interest:
No doubt that dehydration impairs exercise performances and risks health.
I was pleased to see that physicians were at last taking an interest in physical activity as a method of preventive medicine. Further, I could not agree more that there are many claims made by manufacturers regarding improving performance that are unsupported by the literature (if there is any literature at all). In this note, I will not address anything other than claims made regarding fluid regulation, and do not support any specific claims made by manufacturers. I am not an expert in the area of manufacturers' claims. I am an expert in the study of fluid regulation and exercise.
It was a disappointment to me to see the puzzling methodology used in the paper. For example, since when is a Review of randomized trials of higher evidence than the randomized trial? I recognize that sometimes using reviews can be more convenient than going to the original source, but the original source (where the data are presented) is the highest level of evidence, a concept agreed upon by individual scientists as well as all of their governing bodies. That way, the individual reader (hopefully an expert in the area) can decide for him or herself whether the conclusions reached by the investigators are valid. In Reviews, this process has already been done by the Reviewer and may not be the same as the individual's. For clarification, I suggest reading the American College of Sports Medicine (ACSM) Position Stand on fluid and electrolyte requirements. This paper explains in detail the sources of the positions taken in the position stand, and the quality of evidence to support those positions.
Years and years of research by excellent and diligent scientists has gone into understanding the impact of hypohydration during physical activity. Much of the research regarding fluid and hydration has been done in US Army laboratories with the intention of keeping soldiers safe. Research has also been conducted in exercise physiology laboratories in the US supported by the National Institutes of Health. Continuous water loss inhibits sweating and therefore increases the risk of heat injury. This concept is not really debatable at this point any more so than are evolution or climate change. There are those that continue the debate, and that is healthy--this continues to keep researchers active in this area. Aspects relating the causes and cures for hypohydration continue to be debated, and the serious debates continue on a high scientific level, with papers published routinely journals of the American Physiological Society, Journal of Physiology (Lond) and as well ACSM--this is apparent from the journals referenced in the above mentioned Position Stand.
A more in depth review of the scientific literature will show that thirst is an important indicator of fluid needs, but the idea that "thirst sensation to gauge the need for fluid replacement maximizes cycling time trial performance" is a limited and minority view of the regulation of fluid intake and physical activity. To explain, a time trial typically lasts about 60 minutes. Marathons last double that, for the best in the world. Thirst sensation may be adequate for elite athletes running marathons or doing time trials. However for a more typical individual, who requires closer to four hours to complete a marathon, thirst sensation is not sufficient, especially in the heat.
I do not know what journals are used to support claims made by manufacturers. I do know that the science is strong regarding fluid regulation, drinking (with electrolytes) heat injury and physical activity. I hope that the authors and readers find the time to read the ACSM position stand, or go deeper into the literature. Doing this will demonstrate the challenges faced by athletes and active people, and also to understand the seriousness with which we take our scientific endeavors.
Thank you to the authors for their time and efforts.
Nina Stachenfeld, PhD Associate Fellow John B. Pierce Laboratory Associate Professor Obstetrics Gynecology and Reproductive Sciences Yale University School of Medicine 290 Congress Avenue New Haven, CT 06519
(203) 562-9901 x219 firstname.lastname@example.org fax (203) 624-4950
Conflict of Interest:
Have had funding from PepsiCo.