Posted on 29 Jun 2009 00:21
Sports drinks, sweat and electrolytes
Part V and a Half (Theories and Fallacies of Muscle Cramps)
BY Ross Tucker, PhD and Jonathan Dugas, PhD
The Science of Sport
The actual data from the lab and the field
In the last article we introduced you to Randy, our imaginary 70 kg average male runner, and we created some potential scenarios regarding his fluid and sodium losses and replacement. The biggest take home message was to listen to your body and to drink to thirst, as this has been shown again and again in the field and the lab to keep people from drinking either too little or too much. We have received tons of feedback and discussion, and as we stated in the comments to that post we are pleased that so many of you are participating in the discussion, sharing your stories, and asking relevant and insightful questions.
Just as we will admit that field studies are not the perfect experiment but play an important role, it is the same with prediction equations. We can predict all we want from imaginary scenarios, and sometimes the equations are pretty accurate, but there is no substitute for the real thing, and many readers wanted to see more actual data and references that demonstrated why sodium ingestion is not necessary, and how both sports drinks and water can cause a fall in sodium concentration. So while it is important to go through the exercise in the prior article, the next logical step is to look at the actual data.
Baker LB, Munce TA, Kenney WL. "Sex differences in voluntary fluid intake by older adults during exercise." Med Sci Sports Exerc. 2005 May;37(5):789-96
This study examined ad libitum fluid ingestion in older adults during intermittent exercise in the heat. Basically, they had continual access to water in one trial and Gatorade in another, and they had to cycle for 15 min and then rest for 15 min. The total time of each trial was two hours (four work/rest bouts) followed by an additional 30 min recovery period. A number of variables were measured, but we will focus on the sodium concentrations. One limitation of this study for our purposes here is that it was performed in older adults, and there is a well-documented effect of age on the thirst mechanism so that as you become older you become less sensitive to thirst. That is, your plasma osmolality rises higher before thirst kicks in.
According to the authors, their main findings were (and we quote):
1. When cool palatable fluids were readily available, active adults aged 54–70 yr drank enough to match sweating rates and maintain their body mass;
2. Their fluid intake behavior was repeatable;
3. CES [note: Gatorade] promoted greater voluntary fluid intake and restored PV losses faster than water;
4. There were sex differences in the fluid intake behavior of older active adults, with women drinking more water per kilogram of BM than men
As we tried to explain in many of our prior articles, the ingestion of any hypotonic fluid in excess of thirst will cause a fall in the sodium concentration. In this case "in excess" means drinking more than to your thirst. This occurs even though sports drinks contain some sodium because they still have much less when compared to the body fluids. Therefore the end result is a fall in sodium concentration. The data from this study show that these older adults, even when drinking to thirst, experience a fall in sodium concentration when ingesting water or Gatorade:
What we see is the time on the x-axis and the sodium concentration on the y-axis. The black dots represent the Gatorade trial, and the white (open) dots represent the water trial. All the subjects started with a sodium concentration of 142 mM per Liter, and in both trials the average concentration fell over time to approximately 139-140. There were no differences between the groups, and the symbols you see on the graph means that those values are significantly (statistically) different from the baseline value. So water and Gatorade ingestion produced a similar effect, and so Gatorade did not prevent a fall in sodium concentration in these subjects.
However one female subject ingested 2.8 L of water and 2.7 L of Gatorade in the respective trials. She gained weight in both instances, indicating an excessive fluid ingestion, and here are the data that support the conclusion that ingesting Gatorade will not prevent hyponatremia:
The problem is that the authors herald this as proof that ingesting Gatorade is much better than water:
"Furthermore, this woman’s data support the notion that a CES [Gatorade]
is superior to water in limiting reductions in serum sodium during exercise-heat
stress. During the CES trial, this female subject consumed 2.7 L and had a final
serum sodium of 131 mmol per L. Therefore, although she consumed similar
amounts of CES and water, serum sodium was maintained above that of
symptomatic hyponatremia during the CES trial."
While the authors are entitled to their interpretation of the data, we disagree and conclude that both fluids are producing a steady fall in sodium concentration, and that the 131 value in the Gatorade trial is just marginally outside the symptomatic range (< style="font-weight: bold;">ingesting either water or Gatorade produced a nearly identical fall (~2-3 mmol) in the sodium concentration.
The take-home part: Sports drinks do not prevent hyponatremia
In fact Jonathan tried to apply this finding to a more "real world" situation in a letter to the British Journal of Sports Medicine. In that letter he argued that since the mean finishing time of women marathoners in America is five hours, and if the ingestion of Gatorade at rates similar to those found in the study is advocated by races, coaches, scientist, etc., then there would likely be many women (and probably men, too) presenting with hyponatremia. These data demonstrate that sports drinks do not prevent this condition as their ingestion in these subjects and at these rates causes a fall in sodium concentration.
Again one limitation to this study is it was done on older subjects, who are less sensitive to thirst, and what we might see in younger subjects would be a slight rise in sodium concentration when ingesting Gatorade and a maintenance of sodium concentration when ingesting water. The evidence for that statement comes from a 1992 study by Robert Cade, the inventor of Gatorade who incidentally died this week at the age of 80. In that study three groups of runners completed a marathon. One ingested Gatorade, another "half-Gatorade" (50% water, 50% Gatorade), and the third group water:
So in fact ingesting Gatorade to thirst in younger subjects results in a rise in sodium concentration, which is why you drink more—-you never lower your osmolality below the thirst threshold and therefore are thirstier when ingesting a sports drink, whereas with water you maintain the osmolality right around the thirst threshold and drink and abstain as your thirst comes and goes. With sports drinks you instead just get thirstier, which seems kind of ironic since their slogan is "The thirst quencher!"
So those are some of the data that support our conclusions, and we hope that helps to clarify some of our interpretation(s) and conclusions. We will still post a "wrap-up" for the series on cramping in which we will try to briefly summarize the main points but more importantly leave you with some practical advice on this complex topic!
Used with permission. All rights reserved by the authors.
This page created 29 Jun 2009 00:21
Last updated 27 Feb 2016 21:51