 Being that most of us are in the midst of summer, at least in the northern half of the world, now would probably be a good time to discuss hydration, and which on-the-bike hydration strategy will lead to the best performance. If you thought this was going to be a fairly straightforward and uncontroversial video, well, you'd actually be wrong. Yes, I was surprised by this fact as well. This is one of those cases where the science is a bit mixed to say the least, and every time I make a video with an inconclusive conclusion, I'll often get comments about how I didn't answer the question and I just wasted their time. Huh, look at that. He actually reads my comments. Better to be honest, though, than to just make something up. And even if this is the case, this doesn't mean that we can't learn a thing or two by looking at the available research. In fact, I'd argue that that's the best way to learn about most topics. So let's go ahead and jump into what the science has to say on this hydration question. We all know that dehydration is not a good thing if you're trying to ride your bike fast, but at what point does dehydration start to negatively affect your performance? After all, we can expect some dehydration when you're riding and racing, but if we can stay above that point, we should be good. Fortunately, we have research that seeks to answer this question. This study on mild dehydration in the heat had subjects perform a five-kilometer cycling test in either a hydrated or dehydrated state where they had lost 1% of their body mass. What they found was that power and speed was reduced in the dehydrated group and body temperature was higher. The study concludes that mild dehydration decreases cycling performance possibly by inducing greater thermal and cardiovascular strain. It isn't surprising that dehydration decreases performance that we already knew, but what is surprising is that such a mild case of dehydration seems to decrease performance. A 1% loss in body mass does not take long to achieve on a hot day at all. And this 1% figure has been found to be detrimental to performance in other similar dehydration studies as well. However, there's also plenty of research out there that shows that you need a lot more body water loss before performance is affected. For example, this study, which straight up calls out the current hydration guidelines in the title, calling them erroneous and making the bold claim that dehydration does not impair exercise performance in the heat. It seems as though clickbait is not just a problem here on YouTube. The study's methods are good though and they come up with some very interesting findings. They dehydrated cyclists to negative 3% body mass by performing 2 hours of submaximal exercise in the heat before re-infusing them with saline to replace 100%, 33%, or 0% of fluid losses, leaving them 0%, negative 2%, or negative 3% dehydrated respectively. This way the treatment could be participant blinded. The subject's not knowing how hydrated they are is important. We know the power of the placebo and a subject just thinking that they're dehydrated could negatively affect their performance and hence the results of the study. A 3% loss of body weight is pretty significant dehydration. And yet, while body temperature was higher in the negative 3% versus 0% conditions, no other differences between trials were shown. That being said, this study using blinded and non-blinded participants and the same 3% loss of body weight found that dehydration of around 3% body mass impairs time trial performance in the heat, regardless of knowledge of hydration status. You know, it would be nice if we could get a consensus on this point, but the research simply isn't showing that. This review of many studies on hydration and physical performance states that there is little doubt that performance during prolonged continuous exercise in the heat is impaired by levels of dehydration greater than negative 2% body mass. And there is some evidence that lower levels of dehydration can also impair performance. And yet we have a meta-analysis like this one stating that the 2% dehydration rule comes from studies done under fixed intensity. Evidence indicates that less than 4% body weight loss is very unlikely to impair endurance under real-world exercise conditions like a time trial. This disagreement about the point at which dehydration starts to negatively affect performance also plays into the disagreement in the scientific literature about which hydration method is optimal. What are these different hydration methods you ask? This counter-view on drinking to thirst lays out four modes of intake. There is, of course, drinking to thirst or relying solely on one's personal sensation of thirst as the only stimulus or guide to drinking. Basically, if you're thirsty, drink. It's that simple. Some people may say that this method is too simple. There's no way that proper hydration can be caveman brain level easy. But there actually is quite a bit of evidence in support of this hydration method and we'll get into that in a minute. The next of the four approaches is ad libenum drinking or whenever or in whatever volume is desired. I know, I know, this method sounds just like the first one just phrased in a more pretentious way. The subtle distinction is that with drinking to thirst you drink when you're thirsty and with ad libenum drinking you drink as much as you want regardless of thirst. Both of these methods are a little bit up for interpretation as the review points out. These are opposed, if you will, to our next two strategies which get a little bit more systematic and what we might typically think of as, well, a strategy. There's an individualized drinking plan or drinking based on past experiences and simulations of a specific sport or activity including one's measured sweating rate and program drinking where individuals drink a specified volume per unit of time. Volume or rate may be adjusted on the basis of exercise intensity or type of activity. If you've ever heard somebody say that you should be drinking a bottle per hour which is pretty common advice when it comes to hydration on the bike then this would fall under this category. Of course there are more sophisticated ways of going about this method that take into account more factors. Yeah, but where's the option for I just take one half filled bottle of water with me on a five hour ride to save some grams and then end up having to bum water off of my riding buddies 40 minutes in. So how exactly have researchers tried to go about determining which one of these methods is optimal? This study on half marathon performance and fluid intake had runners do two half marathon time trials while either drinking the thirst or to maintain body water loss of less than 2% which is the recommendation by the American College of Sports Medicine. What they found was that despite a greater level of dehydration, higher body temperature and a higher heart rate in the drink to thirst group running pace and time over the half marathon which is what we actually care about did not differ significantly between the trials. This review on drinking to thirst supports these results stating that drinking ad libenum appears to optimize performance and safety during exercise in many situations. The presence of thirst not of water loss may be the biological signal that impairs exercise performance in those who drink less than their thirst dictates during exercise. No study found that drinking more than ad libenum during exercise produced any biological advantage but it could cause exercise associated hyponatremia. For those that don't know hyponatremia occurs when sodium levels in the blood are too low. This is actually a real concern for athletes especially when they're drinking just plain water and not replenishing lost sodium. Sodium needs for cyclists is a topic for a whole nother video so I'll leave it at that because we still haven't gotten to the bottom of how much fluid should you be taking in in the first place. These studies seem to make a compelling case that you shouldn't overthink it and you should just drink when you feel thirsty. There are however a number of studies that suggest that this method is inadequate. Take for example this study on prescribed drinking versus ad libenum drinking which had subjects perform three 10 kilometer cycling tests in which they either drank as much water as they wanted or drank water every one kilometer to match fluid losses. When doing this subjects were only dehydrated to half a percent on average as opposed to 1.8 percent when they were told to drink as much as they wanted. This led to a 5 percent difference in speed in the third test between the two conditions. So when subjects drank the amount that they wanted that wasn't enough fluid which goes against the case that you can rely on your thirst to regulate your hydration level and there are other studies like this that come to the same conclusion. For example this study had subjects perform a two hour cycling protocol followed by a five kilometer time trial in the heat under hydrated and dehydrated conditions. In both trials thirst was matched by drinking 25 milliliters of water every five minutes but in the hydrated trial sweat losses were fully replaced by water via a nasogastric tube. By the end of the test body mass had dropped 0.7 percent in the hydrated trial and 2.9 percent in the dehydrated trial and this produced a significant difference in cycling speed and power between the two trials. The study concluded that dehydration decreased cycling performance independent of thirst as subjects were unaware of their hydration status. This is evidence against the notion that the negative effects of dehydration are mostly psychological or if you're thirsty you won't go as hard. It does appear that there are some significant physiological factors at play as well. So where does this leave us on the question of what is the optimal hydration strategy? This article makes the argument that drinking to thirst is sufficient. They state that the widespread belief that dehydration of at least 2 percent of body mass impairs performance comes from controlled laboratory studies arriving at such conclusions using methodologies that render them invalid in determining the real impact of dehydration during outdoor conditions. The fastest long distance triathletes, marathoners and ultramarathoners often complete events with body mass losses of 4 percent to 9 percent. Thus, mass losses in excess of 2 percent to 3 percent during prolonged exercise are well tolerated by elite athletes. Field studies have also shown that drinking to thirst even during prolonged exercise up to 30 hours in hot conditions will allow maintenance of proper hydration while over hydration can lead to GI distress. In the conclusion they state, nevertheless even without considering the scientific evidence related to this essential question one would have to believe it remarkable that the human species would exist if thirst were inadequate to guide drinking during exercise. Good point. If our ancestors got by just drinking when they felt thirsty then it's probably a foolproof system. Then again our ancestors weren't riding bicycles as fast as they could for hours on end. Speak for yourself bro. My ancestors were stealing KOMs from the Neanderthals as soon as they invented the wheel. And of course there are good arguments against drinking to thirst as well. Going back to this counter view on drinking to thirst they state that it's unlikely that it can be determined whether any athlete is solely relying on thirst alone or a combination of thirst and other influences. The concept of drinking to thirst is vague and may be easily misinterpreted by athletes. For example should I drink so that thirst is always absent or should I drink only when my sensory perceptions indicate that I am thirsty? Ah yes, the age old question. Moreover the discussion often revolves around individuals with average sweat rates. Differences in sweat rates should be taken into account. They suggest that the right strategy is situation and individual specific. There is a continuum for hydration recommendations. For shorter events people with lower sweat rates, cooler days or lower intensity rides drinking to thirst is probably fine. For hotter days, longer events and heavy sweaters a hydration plan may be needed. Even though the research here is very mixed I do think that this last review had a good take away and in my personal experience as a coach I have found this to be true. Proper hydration is very individual. For some riders drinking to thirst is probably more than adequate while other riders constantly struggle with dehydration. And for these riders following a set hydration plan whether they're thirsty or not is necessary. This is going to require more work on the part of these riders to determine how much water they're losing while they're riding. And then I would say to be on the safe side make sure that they're drinking enough so that they're not losing more than 2% of their body weight. Thanks for watching. If you enjoyed this video be sure to give it a like. Subscribe for more science based cycling videos just like this one and share this video with your cycling friends. I'll see you in the next one.