How much hydration is enough hydration during long rides?

Question

I have been doing some experiments to see if I'm hydrating enough during rides of moderate length and intensity. Basically, I weigh myself (naked) and my water bottles before and after a ride, to measure my weight loss and water consumption. Also keeping track of ride time, average speed of the ride, and the outside temp (per my phone's weather app), and any snacks I eat.

My questions is: if I'm hydrating enough, should I expect my body weight to stay approximately the same before and after a ride? Or would I expect to lose some measurable amount of weight independent of water loss (i.e. from respiration of CO₂). For example, this weekend I did a 48km ride in ~2:00 hours, over which I consumed ~44oz of water (with electrolytes and sugar mixed in, no other snacks). After the ride, I was about 1 lb (0.45kg) lighter than when I started, though I wasn't feeling especially parched or exhausted. Is this difference mostly water weight? And if so, should I be drinking an extra half liter of water on similar rides? Or is it possible this weight difference was mostly from calorie burn, and my hydration level was adequate?

Obviously it isn't too much trouble to make up a small water deficit post ride, but I'm training for my first 200k brevet, and trying to establish a good baseline for staying hydrated over a substantially longer time-frame. For context, I'm a fairly overweight dude at ~225lbs. So I may need more water than other riders would on a similar ride.

Answer 1

When I first started doing longer distance rides, the rule-of-thumb I was told was "drink a bottle every hour."

Obviously, that's a very rough figure, and it's going to depend on a lot of factors, including how much you're sweating (a combination of workload and heat+humidity), how much you weigh, and just generally how much fluid you personally need. The first time I did a century (160km/100mi) I was loaded touring and it was very hot; that day I drank 9l of water on the road, and still needed to keep rehydrating that evening.

But to get to my point. After many years of riding, I've found one of the best indicators that I'm getting adequate hydration is that I keep regularly needing to urinate; if I find I'm going more than 2.5h without needing to relieve myself, I'm getting dehydrated. Again, there's a certain amount of knowing your own body here, and my numbers won't be the same as yours. But if you're doing a long ride, you're going to need nutrition; metabolizing nutrients (especially protein) produces water and metabolic waste products you need to get rid of. If you're not doing so regularly, it means that your body is starting to hoard water and you need to drink more.

Answer 2

Good luck on your brevet attempt! The answer has been modified a bit from the original. I'm assuming that the OP's going to be riding for at least 7-8 hours, not including breaks.

Note: I initially recommended that the OP drink ahead of thirst. In comments, @whatisname contended that this advice is based on a myth. On further research, I modified it.

The quick summary is that thirst perception is a good guide. If you're riding in very hot conditions, it may be worth it to drink ahead of thirst, and you also want to manage your electrolyte consumption. You do not need to replace weight lost 1 for 1. With time, riders will develop an intuitive sense of their water and electrolyte needs over time. if you're craving salt during or after a ride, then you can add more salt to your drinks.

Thirst perception

Just drinking to thirst is sufficient most of the time, but also consider electrolyte intake (described further below).

The OP may not exactly be in a normal situation. This is a very long ride, and it may be hot. If true, it may be worth drinking ahead of thirst. This issue may be more prominent later in the ride. This is based on personal experience, not anything empirical. Our thirst perception declines with age, so athletes over about age 65 should err on the side of drinking more as well.

Individual fluid consumption varies a lot. You probably develop a sense of your needs over time. One can obviously start from a rule of thumb, e.g. one bottle per hour as proposed by @DavidW, and adjust based on your perception of thirst.

For example, I drink a lot more than my ride buddies. Some of them start a weekend with just one bottle, even in the heat, but I always carry two full sized ones. In temperatures of 85F/30C and higher, I find that I can easily drink a 25oz/750ml bottle per hour. By two hours, my less thirsty friends have finished their lone bottle.

Drinking ahead of thirst isn't necessary

British Cycling recommended that cyclists drink ahead of thirst, rather than just to thirst. They argued that thirst is a lagging indicator of your hydration status; when you feel thirsty, you are in a significant deficit. However, Allen Lim, who has worked extensively with professional road cyclists, countered on a podcast that these athletes were good at listening to their perception of thirst. He felt that perceived thirst was enough for them, and that they would generally hydrate themselves enough based on drinking to thirst. That said, these are professional athletes, and they train very extensively, so their perceptions of their bodies are well-developed.

Andy Blow, an exercise scientist writing for the TrainerRoad blog, argued that thirst alone is a good general guide in most exercise conditions. He argues that heat and exercise duration may modify this. I based my argument above on his post.

If you want to measure your fluid intake

It's not strictly necessary, I think, but you can measure your fluid intake like the OP outlined. That is, weigh yourself before and after the ride. But you need to account for your fluid and food intake if you do this. Not accounting for food, you have 1) the difference in your start and finish weight, and 2) the amount of fluid consumed. The total is the amount of fluid lost. Divide that by time, and you have an estimate of sweat rate.

You don't need to replace all the lost weight

Precision Hydration (NB: they sell hydration products) points out that in a very long event, the amount of fuel you burn (and expel as CO2) can start to confound the measurement. For example, at zone 2, I can burn around 400 calories (estimated, probably at least a 10% margin of error in either direction) per hour. If it were half carbs and half fat, that's 50g carbs and 22g fat (NB: you can't practically measure your carb:fat oxidation ratio on a ride, so I assumed). For a 2h ride, I'd ignore the difference. But for a 5h ride, that's 375g, a bit over 0.8lb. A relevant abstract describes some other mechanisms the weight difference can be confounded.

In this blog post, Precision Hydration reviews some case studies. Since 2007, the American College of Sports Medicine issued a guideline to keep water loss under 2% of body weight. However, a lot of elite Ironman or marathon athletes lost 1-6% of bodyweight during their events.

Clearly, it's possible for at least some people to tolerate over a 2% bodyweight loss. It seems likely to me that you may need conditioning for this, and there's almost certainly individual variation in how much loss is tolerable.

The pre-2007 ACSM guideline was full replacement. This, plus the fact people didn't consider sodium levels, may have led some athletes in long and hot events to develop hyponatremia, i.e. sodium levels too low, caused by dilution.

The role of electrolytes, particularly sodium

The OP clearly recognized the need for electrolytes, but I felt that more info would help others.

When I started cycling, I'd drink plain water, only to find that I'd crash at the end of long rides in the heat. This changed considerably when I started using electrolyte drinks. The rate at which we excrete sodium is highly variable - Precision Hydration says they have seen some athletes who lose 200mg Na/liter of sweat, and some who lose 2,300mg or more. Everyone's drinks should contain some sodium.

You can develop a sense for how much salt you need to take in over time. After rides, do you crave salty food? That means you need more. Also, if you have very salty sweat, you will see significant salt deposits on your exercise gear after a long workout, or dogs may lick you after a workout. Your sweat stinging your eyes is another indicator. Personally, I get some salt deposits in some circumstances, but not a lot. I infer that I'm probably near the average in terms of salt concentration, although my sweat volume is very high.

On my last 5h endurance ride at about 90 degrees F, I took about 1000mg sodium plus 250mg potassium in a drink, plus I dissolved a bit of salt in some maple syrup. This seemed sufficient, and I didn't have an unusual craving for salt afterward. I might experiment with higher salt content in the future.

If you know nothing about your own sodium needs, maybe start with a pre-mixed drink that's got about 400mg sodium. Aim to do a bottle of that every few hours. Titrate your intake from there. It probably isn't necessary to replace potassium, even though the mix I bought had it. You can actually just use table salt, as I discussed here. 1 teaspoon table salt has about 2,320mg sodium. I add this to my flask of maple syrup, or you can add it to a non-electrolyte drink, or you can make sure you eat food with salt (e.g. jerky).

This ties into the role of carbohydrates in sports drinks

Carbohydrate in your drink may affect how well you absorb the fluid, but you do not want your drinks too sweet. Lim argued that a drink that is too concentrated (i.e. has too much sugar and other material dissolved) actually inhibited how well athletes could absorb the water content. Basically, the sports drinks had a higher concentration than our blood, and due to chemistry, this tends to pull water into your digestive tract rather than into your blood. Lim reported that many of his athletes developed bloating or other gastric distress from too-concentrated drinks. While you need to replenish carbohydrate stores while riding, I would not rely on sports drinks to do this.

Lim runs a company called Skratch Labs, and their drink mix contains about 20g of carbs (i.e. 80 calories) in a 16oz / 500ml drink, plus 400mg sodium and a bit of other electrolytes. I'm unable to find consistent nutrition information for Gatorade on the web, but I believe this is half or less the amount in an equivalent Gatorade serving.

Specialist cycling hydration mixes are available, but they can be pricey, and you can brew your own. The section at the end of the answer has some suggestions.

How much fuel?

While the OP didn't ask, the discussion about carbs does naturally tie into the role of fueling during exercise. Also, sodium with carbs does increase the drink's absorption rate. Cycling for very long distances burns a huge number of calories, and even Gatorade alone wouldn't be able to replenish them adequately. For example, my power meter estimates that I burn about 400-500 calories per hour at Z2-ish pace. I'm relatively small, and a larger rider probably burns more than this.

Your body stores carbohydrates in the form of glycogen. You will deplete your glycogen stores after about 3 hours of cycling. This will cause a bonk, which is very unpleasant.

You obviously don't need to replenish all the carbs (and fat) you burn. Lim proposed that athletes could aim to eat about half the calories they actually burned per hour. And as far as I know, riders on a diet should still have simple carbs when doing a long ride. The metabolic consequences of ingesting a lot of sugar don't apply because you aren't ingesting excess sugar.

Pre-made ride food has advanced a lot. But the modern stuff can be expensive, and is probably not necessary for a lot of people. Fresh fruit, if available, contains water as well as carbohydrates, and it can make for a good healthy snack at rest stops (but note that too much fiber on a long ride can upset your stomach, so experiment to see if you can tolerate this). Confections like chocolate bars, cookies, gummies, and candy bars will also work, although note that their fat content may also cause gastric upset (so experiment). Or there's your favorite full-calorie soft drink. A lot of people straight up get sick of eating later in the ride, so you may need to put some effort into feeding yourself. Seek variety if you need it.

One of my weaknesses is that when I'm solo, sometimes my planned stop doesn't have anything I regard as palatable, so I skip eating there. This happened last week. Then I'm underfueled in the last hour or two. More egregiously, there's a Burger King about 1.5 hours away from home on one of my routes. I thought the fat from the french fries would slow digestion, but in fact I could have got a small order of fries. Or if not that, I should have stashed an extra gel in my handlebar bag. Try not to fall into traps like this.

Notwithstanding what I said above about gels and the like, they're convenient and palatable. The mix I took lets you carry 60g of carbs in one bottle, possibly more. That said, you can actually DIY this using table sugar and table salt.

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Drinking ahead of thirst not actually necessary

This answer initially advised to drink ahead of your thirst. This is actually a myth.

Tim Noakes, interviewed in Outside magazine, is an exercise scientist who advised on the dangers of over-hydrating during long endurance events. Indeed, if you take in too much water without also taking in sodium, your blood sodium levels can drop too low (i.e. hyponatremia). This can be dangerous. In fact, a number of marathoners have died from hyponatremia.

Blow, on the TrainerRoad blog cited earlier, responded that Noakes may be exaggerating the evidence. Blow countered that there is significant evidence that many successful athletes take on supplemental sodium during exercise. Blow did agree that drinking plain water (i.e. no electrolytes added) was generally sufficient, emphasis mine:

And for many people training or racing shorter events in cool to moderate conditions, drinking water to thirst will be sufficient most of the time.

He also added that we have some room for error; if you under-hydrate on a two hour ride, you can push through and just drink more afterward.

However, he cautioned that if you're in a long event on a hot day, that's not really a normal situation. You may also have trained hard up to that event, and thus depleted your existing reserves. In this case, drinking plain water alone to thirst is not likely to be sufficient. Thus the extra advice.

For those of us who need low sodium intake due to blood pressure, I would advise consulting your physician.

Answer 3

This is just some additional tips.

One thing I do on really long rides (first on a 400km brevet) is set a periodic reminder on my phone to drink every 15 minutes, also to stretch, and to consume some calories every half hour. Also a proper drink before and after each snack.

On a 200 a proper stop is a good idea, and popular: walking around, sitting on something other than a saddle for a few minutes, and having a big drink and something light but proper to eat. While it's only a couple of hours longer than a century, this stop is much more likely to be needed.

It's important at this distance to know where you can get water. This may mean a tap or a cafe that will refill bottles, or it may mean buying a big bottle. Normal facilities may still not be operating normally at the moment. What you don't want to do is set out onto a dry stretch without much water, then struggle.

If you're drinking several litres in a day, and mostly losing that to sweat, you should consider your electrolyte intake too.

Added later: Your need for hydration depends on your energy source. Specifically as you switch to burning more fat later in the ride, your need will go up, as well as any reserve drunk just before setting off being depleted. This means you can't easily extrapolate from shorter rides even in the same weather. As an example I forgot my bottle on a 50km ride last week, and was thirsty but fine even at the end. A few days later, in similar but wetter conditions, I did 83km and drank about 0.7l, nearly all of that in the 2nd half.

Coincidentally this got bumped just as I got back from a 200km in early winter (7C at the start, 3C at the end). I went fully self sufficient, carrying 2 litres of water, 0.75 litres of electrolyte drink, and 0.5 litres of coffee in a vacuum flask. I drank nearly all of it. (shops weren't completely ruled out, but our localised restrictions are complex and I was trying to stick to the spirit of minimising unnecessary contact; I'd also done something similar a couple of weeks ago, passing very near home just in case).That would never work in summer. Note that if you're cold on the surface, vasoconstriction leads to urination.

Answer 4

Medically, dehydration is usually detected/defined as a 5% increase in the concentration of solutes in your blood. Thirst sets in at about 2%, so you'll always feel strong thirst setting in long before you're dehydrated. This is basically a way of measuring the fact that your blood volume is low. Body weight is not a reliable way of measuring this. Your body weight changes for a lot of reasons, some of which have nothing to do with hydration. For example, when you exhale CO2, you're getting rid of the mass of those carbon atoms. It also changes when you eat, drink, sweat, urinate, or poop.

Fortunately we don't need a doctor with a syringe to tell us when we're dehydrated. Thirst is a powerful sensation. People who are actually dehydrated know it because they're extremely thirsty. Folk wisdom about hydration has diverged from the scientific evidence, especially with some of the silly or even dangerous bro-advice about drinking very large amounts of water (which can cause hyponatremia). Drink if you're thirsty.

Pop culture would have us believe that dehydration sneaks up on us like a silent killer. Actually this is a pretty good description of heat illness, which can be insidious and very deadly, and can happen regardless of water consumption. Heat illness can fry your brain and make you behave irrationally or fail to do important, obvious things like ceasing exercise or seeking shade.

gschenk asks in a comment:

would you clarify the following: (a) at which amounts of water intake one might begin to worry about hyponatremia? (b) how does direct Na loss through perspiration factor in.

For a complete answer, with references to scientific papers, see this SE answer. In the case described there resulting in death, an army recruit drank 20 liters of water in one morning because his drill instructor told him he had to.

Ivan McA says in a comment:

I think your (and @whatsisname)'s "you only need to drink when you are thirsty" is the bro science here.

This is a valid complaint, since I didn't provide any references to scientific evidence in the original version of this answer. I've added some references below.

References

Almond et al., Hyponatremia among Runners in the Boston Marathon, 2005, NEJM, DOI: 10.1056/NEJMoa043901

Noakes et al., "The danger of an inadequate water intake during prolonged exercise," European Journal of Applied Physiology and Occupational Physiology 57 (1988) 210

Heinz Valtin, "'Drink at least eight glasses of water a day.' Really? Is there scientific evidence for '8x8'?," Am J Physiol Regul Integr Comp Physiol 283: R993-R1004, 2002. http://ajpregu.physiology.org/cgi/content/full/283/5/R993

Answer 5

You need more water when it's hot—-particularly on the days your body isn't you yet adapted to the heat. You need less water if the effort is lower. Some days that's a lot of water per hour and some days it's almost none.

I hate to answer with a product when you're asking for a rule of thumb, but I've been really impressed with the hydration tracking feature on my Garmin 530. It monitors temperature, elevation gain, speed, duration, and heart rate and power data (if applicable) and then alerts you when it's time to drink as well as milestones for how much you should have consumed by now.

https://www8.garmin.com/manuals/webhelp/edge530/EN-US/GUID-E5AEF3C5-2139-42FF-8E5E-069D09D1D80D.html

Answer 6

The answer is simple:

Drink when you're thirsty.

And, it has a similar counterpart: Eat when you're hungry.

Yes, it doesn't have to be any more complicated than that.* We're creatures that are the result of millions of years of evolution, constantly tweaking the biochemical pathways that make us work. "Sports medicine" and practices more complicated than "drink when thirsty" are barely a century old.

Human beings have lived for millions of years doing strenuous activities without complicated formulas and schedules indicating when they should drink. Wild animals are born able to know when to drink, they don't need to be instructed. We don't either.

I understand that modern life has caused many of use to learn to ignore those signs. such as jobs where we have limited breaks, or we've learned to ignore them because we want to "power through it". And if you think you are having a problem hydrating, maybe that's what you've done. But you can listen to those signals again.

*Unless you are in a competition where you are in it to win it, or you are riding in extreme heat. You've probably seen numerous advice saying things along the lines of "if you're thirsty, you're already dehydrated". That is good advice if you are a serious competitor, but it is totally untrue for normal activities. Unless one of those conditions is satisfied, your own biological feedback systems are more than capable of signaling when you need to drink.

Answer 7

I have been doing some experiments to see if I'm hydrating enough during rides of moderate length and intensity.

It only matters for rides of high length or high intensity. For rides of moderate length and intensity, hydration is not required during the ride as the human body has enough spare water for a 50 km ride built in.

Long time ago, I used to do a daily 50 km ride. I never once needed water during the ride.

if I'm hydrating enough, should I expect my body weight to stay approximately the same before and after a ride?

For long-distance riding, this is the rule. The body weight is reduced by exhaled carbon dioxide and evaporated water. Of these, the evaporated water explains most of the difference.

However, for moderate-distance riding, you have to allow some flexibility. Nobody cares whether you weight one or two kilograms or half a kilogram less, or zero kilograms less for that matter, after the ride.

There is spare water built in to the human body. I suspect the reserve of spare water is around 2 liters (2 kg). A healthy human that was not thirsty before the ride can lose this amount of water (weight) during the ride without any ill effects.

Or would I expect to lose some measurable amount of weight independent of water loss (i.e. from respiration of CO2).

Yes, as I explained CO₂ removes some carbon from your body.

A human producing moderate power can produce 200 watts of power, burning 800 watts of hydrocarbons to do that. A 2-hour ride burns thus at most 1.6 kWh = 5760 kJ of energy.

If we assume half of the energy comes from hydrogen and half from carbon, 2880 kJ of carbon is burned. Carbon has 32.8 MJ = 32800 kJ of chemical energy per kilogram. You thus need 2880 / 32800 = 0.0878 kg of additional weight reduction for those 2 hours.

You said you were 0.45 kg lighter and consumed 44oz of water. If I calculated correctly, 44 oz is 1.23 kg so your weight reduced by 1.68 kg. Of this, 0.0878 kg was due to carbon burning to carbon dioxide and the rest 1.5922 kg was due to evaporated water. Carbon dioxide exhaling therefore explains only 5.5% of your weight loss. The rest, 94.5%, is explained solely by water evaporation.

By the way, the 1.5922 kg evaporated water is entirely within the safe limits, so the length of your ride does not need you to carry 1.5922 kg of water with you. 1.5922 kg is the difference between weight of an expensive bike and a cheap bike. Save some weight and drink only after the ride.

For longer rides, let's say 70-100 km, you may need to drink during the ride.

And if so, should I be drinking an extra half liter of water on similar rides?

No. As I explained, a human can lose 2 kilograms of water easily with no ill effects.

Or is it possible this weight difference was mostly from calorie burn, and my hydration level was adequate?

No. As I explained, only about 0.0878 kg is explained by calorie burn.

Obviously it isn't too much trouble to make up a small water deficit post ride, but I'm training for my first 200k brevet, and trying to establish a good baseline for staying hydrated over a substantially longer time-frame.

For this your experiments are useful. While a healthy human can do a 50 km ride without drinking any extra water during the ride, 200 km is way above the limits for riding water bottle free. In fact, I would argue that during a 200 km ride, you need to not only drink but also eat.