Our body is built around water. We can go a long time without food (though we’ll cover that problem next time), but we cannot survive without water. So, let’s go over hydration.
Water to Sweat
Sweat is pretty impressive. A single, bead of sweat can cool nearly 1 liter of blood by about 1° F. The thing that makes sweat work so well is that it’s made of water. It takes advantage of the large heat of evaporation of water; to keep it non-technical, that means a lot of heat is released into the air as water evaporates from a surface (taking the heat from that surface with it into the surrounding air).
Now here’s where it becomes a problem: about 60% of our body is comprised of water. About 55% of our blood is fluid, with 95% of that fluid being water. That means over half of our blood is made from water. When you sweat, water is removed from the body. It’s gotta come from somewhere, and that somewhere is mostly your blood. The more you sweat, the lower your blood volume. As your blood volume decreases, it becomes more difficult for the body to shed heat.
Heat is conducted away from the core of the body by blood moving to the outer-limits of the body (skin), and then sweat conducts that heat away from the skin upon vaporizing. The lower your blood volume, the less sweat you produce (less water to pull), which in turn means the less heat you can eliminate. Your blood also becomes thicker (more viscous), which means it’s harder to pump through the body. If blood doesn’t flow as quickly to the muscles, it can’t bring in as much fuel (nutrients and oxygen) and carry away as much waste and heat.
So there’s a whole feedback loop of increasingly negative effects that occur as you dehydrate.
Effects of Dehydration
From a performance standpoint, dehydration is a rather large factor. I’ll use myself for some numbers. I weigh about 165 lbs. If I lose about 1.5 L/h of sweat during a hard/hot effort (top-end sweat for an average person; can be as high as 2L or more in conditioned athletes), that is about 50 ounces per hour, or more than the two of the large water bottles I carry on the bike. Now, here’s some numbers to remember:
- 50 ounces of water weighs about 3.25 lbs
- 2% of my body weight is 3.3 lbs
Why is this important? Because once you’ve lost about 2% of your body weight from fluids, your performance starts to decline.
- 2-3% – impaired ability to regulate body temperature
- 3-4% – reduced muscular endurance (by as much as 30%)
- 4-6% – headaches, irritability, light-headedness or or dizziness, reduced muscular strength, reduced endurance, heat cramps
- >6% – severe heat cramps, heat exhaustion, heatstroke, coma, death
So let’s say you’re drinking about half of what you’re losing. In an hour, you’re down by maybe 1% of your body weight – not too bad. Within 2 hours, you start to heat up, even in cold weather. You’re breathing harder. If you stop there, you’ll probably be okay.
In 3 hours, your power starts to drop off pretty rapidly (what little you may have). After 4 hours, you’ve lost power, endurance, and things are starting to cramp up. You could even be dizzy, which makes it dangerous to ride. Now, you’ve been drinking water so you think you’re okay, but since you haven’t been drinking enough to make up for the losses, you’re going to be suffering the effects of dehydration, even while trying to stay hydrated.
Refilling the Reservoir
So, let’s say I’m not going all-out, and I’m losing about 0.75L/hr, or about one 24 oz. water bottle worth of water per hour. Does drinking one water bottle completely replace my loses?
Unlike food (we have a maximum caloric uptake of about 250 Calories/hr), water absorption is very rapid, and sometimes starts within seconds of drinking it. But the majority of water is absorbed through the small intestine. And though absorption is fast compared to caloric processing, the rate can vary a lot.
If your stomach is empty, the water you drink now will start showing up in your bloodstream within five minutes. Temperature can affect the absorption rate (colder fluids tend to empty from the stomach faster than warmer fluids), but not by much. If your stomach is full of food, it could be anywhere from 45-120 minutes before the water passes through the stomach and starts being pulled in through the intestines. Since we’re usually not riding on a full stomach (though we should be eating while riding), the water you drink now should end up in the body within about 10 minutes of drinking it.
So what does all this mean? Well, let’s do the math:
- I’m losing about 24 oz / hour, or about 4 ounces every 10 minutes
- It could take up to 10 minutes to absorb the water I drink right now
- To keep myself topped-up water-wise, I should drink about 4-6 ounces (a small glass or couple gulps from a bottle) of water ten minutes before I start to ride, and keep doing that once every ten minutes.
- I should run through about one 24 oz bottle every hour
NOTE: Because it can take up to 2 hours to absorb water on a full stomach, make sure you drink water with that pre-ride meal that you ate 1.5-2 hours before your event.
Over Hydration and Mineral Loss
Lest you think, “Hey, I’ll just pound water like a fish!” Slow down. It isn’t just water that we’re pouring out of our skin. With that water comes minerals, including sodium, potassium, calcium, magnesium, and other trace elements. The amount varies greatly from person to person. But those minerals are important to keeping our bodies functioning properly.
When you drink too much water (or don’t supplement your mineral loss), you start to dilute your system, and this can lead to something called hyponatremia. At this point, your body’s cells start to swell to maintain balance between the inside and outside of the cells, and you can end up with some seriously impaired bodily functions.
Eating foods containing sodium, potassium, calcium, and magnesium can help maintain this mineral balance in the body, or talking supplements containing those minerals. It’s hard to tell if you’re over-hydrating. But a good way to check would be to weigh yourself before and after a ride. If you’ve gained weight on a ride while drinking lots of water, you were probably over hydrating.
[DISCLAIMER: I AM NOT A HEALTH EXPERT. I’M NOT A DOCTOR, PHYSIOLOGIST, KINESEOLOGIST, OR ANY OTHER “OLOGIST”. WHAT’S WRITTEN HERE IS FROM STUDIES I’VE READ, MEDICAL JOURNALS, AND ANECDOTAL EVIDENCE FROM MYSELF AND OTHER CYCLISTS. WATER CAN BE DANGEROUS! YOU CAN SLIP ON IT, CHOKE ON IT, AND EVEN DROWN! PLEASE CONSULT YOUR DOCTOR BEFORE DRINKING WATER. FOLLOWING ANY OF THE ADVICE HERE IS SOMETHING YOU DO OF YOUR OWN FREE WILL, NOT BECAUSE OF SOME JEDI-MIND TRICK. OH, AND DON’T SUE ME FOR ANYTHING WRITTEN HERE. ;)]