Rehydrating

Took this from http://www.pponline.co.uk/encyc/0138.htm

But there's an art to rehydrating yourself - and also a science. And the scientific research tells us that many athletes rehydrate themselves the wrong way. After competitions and workouts, for example, you see lots of sports-minded people guzzling down Gatorade, Isostar, plain water, or colas. Except for the caffeinated variety of the latter, those beverages aren't bad for rehydration, but they're definitely not the best!

Why aren't they so good? For one thing, those quaffables are pretty low in salt. Salt?!!! I know, it's a bit of a paradox, but even though dehydrating exercise tends to make your body fluids more - not less - salty, it's a good idea to have some extra salt in your rehydration fluid. Follow along and we'll show you why, and we'll also tell you how to make sure you are rehydrating yourself properly

How salt won
The advantages of extra salt in a rehydration drink were detected several years ago by Dr. Hiroshi Nose and his colleagues at Yale University when they asked a group of volunteers to dehydrate themselves by exercising for 90 to 110 minutes under hot conditions. The rugged activity caused the exercisers to lose about 3.5 pounds of weight, most of which was simply water lost through the sweat glands. A loss of 3.5 pounds works out to a deficit of over an ounce for each two minutes of exercise - and almost a half-gallon in total.

After their pyrexic exertion, the athletes rehydrated themselves by drinking as much plain water as they wanted over a three-hour period. However, for every 100 ml (about 3.4 ounces) of water they swallowed, the athletes also had to gulp down a capsule containing .45 grams of salt. The volunteers had no idea what was in the capsules, nor could they taste the salt as they drank.

On a separate occasion, the athletes exercised at the same intensity under similar temperature and humidity conditions and drank as much water as they wanted for three hours after the bout of exercise. Again, they were given one capsule for each 100 ml of water, but this time the capsules contained small amounts of sugar, instead of salt. As in the first case, the subjects weren't aware of what was actually in the pills ('Role of Osmolality and Plasma Volume during Rehydration in Humans,' Journal of Applied Physiology, vol. 65(1), pp. 325-331, 1988).

And how did it turn out? Well, it wasn't even close: salt was the winner, helping the athletes rehydrate themselves considerably more effectively, compared to the sugar-plus-water drink! One of the benefits of the salt was that it stimulated greater amounts of drinking. When the athletes took the salt rather than sugar capsules, they freely drank about 1.22 litres of fluid during the three hours post-exercise, versus only 1.1 litres of the sugary stuff. Of course, getting more water into your body is a key component of the rehydration process (the other part is keeping the water there once it's returned).

Why does salt make you want to drink? Perched under your brain is a little mass of tissue called the hypothalamus, a crafty collection of nerve cells which - among other things - checks the salt concentration of your blood. If your blood is too salty, the hypothalamus 'tells' the conscious part of your brain that you need to drink. Once you've located a water source and started gulping down H2O, the incoming flow of water dilutes the blood (makes the salt in the blood less concentrated), quietens down the hypothalamus, and decreases your desire to keep drinking.

However, remember that after a bout of dehydrating exercise, you actually have two problems: (1) your blood is too salty, and (2) your total blood volume is too paltry, since a bucketful of your plasma has percolated into your sweat glands and worked its way out of your body.

If you drink plain water to rehydrate, you address problem no.1 very nicely; the incoming water dilutes your blood so that salt concen-trations come back to normal. Unfortunately, that stupid old hypothalamus of yours thinks that everything is okay and accordingly shuts down your desire to drink, even though your total blood volume is still too low (the hypothalamus has no 'dipstick,' so it can't really tell how much water is in your blood; it has to rely on electrolyte concentrations to try to keep things regulated, and thus it can be fooled fairly easily).

In contrast, if you drink salty water, salt will be absorbed into your blood along with water, and your hypothalamus will call out to you, 'Hey! Drink some more; your blood is still too concentrated'. By drinking more, you address the big problem (no. 2 from above) and bring your blood volume back up to normal. Later on, any excess salt can topple into your urine, and everything will be great. You've got enough water in your body - and the right salt concentration in your blood, too. Salty drinks are just better for quick rehydration!

Urine, diet colas, and sports drinks
And salty rehydration drinks are also helpful in another way. If you drink plain water and simply dilute your blood, you actually encourage urine production, causing you to lose body water at the exact time you are trying to build it back up again. On the other hand, if you take in salty fluid, your kidneys become repelled by the thought of peeing, and you keep more of your precious water on board. In Nose's study, rates of urination were considerably lower in the salt-water drinkers, compared to the sugary people. Overall, net fluid gain was 27-per cent greater when the athletes sipped salty potables!
But what about caffeinated diet colas and sports drinks such as Gatorade, which are often used by athletes to rehydrate? A couple of years ago, researchers at the University of Texas at Austin asked 19 college students to exercise at an intensity of 60 to 80 per cent of VO2max under calescent conditions (32.5 degrees Centigrade, 40 per cent humidity) until they had lost 2.5 per cent of body weight (slightly over four pounds). After the exercise, the subjects sat for two hours under more comfortable conditions while drinking either plain water, Gatorade, or Diet Coke. Instead of quaffing the drinks freely, the students drank two approximately one-litre 'boluses' of fluid - one right after exercise and another 45 minutes later. This meant that they drank almost exactly the quantity of fluid (66 ounces) which they had lost via sweating ('Rehydration after Exercise with Common Beverages and Water,' International Journal of Sports Medicine, vol. 13 (5), pp. 399-406, 1992).

In this study, the subjects didn't rehydrate themselves particularly well, even though they drank almost twice as much fluid as Dr. Nose's volunteers. For example, Diet-Coke drinkers restored only 54 per cent of their body weight by drinking the two boluses of Coke, while water-drinkers got back 64 per cent and Gatorade drinkers returned 69 per cent. The big problem with Diet Coke was that it encouraged far too much urine production - about 227 ml more urine, in fact, compared to Gatorade (the caffeine in the Diet Coke may have been the culprit). Even plain water caused more urine production than Gatorade, probably because it diluted the blood more quickly.

Why did Gatorade fare better than water and Diet Coke? Well, Gatorade contains a bit of sodium (more than water and Coke, at least), and sodium works its magic in a couple of ways. First, as we continue to mention, sodium keeps your hypothalamus from prematurely thinking that fluid levels are okay. In addition, when sodium and sugar are present together inside your small intestine, they are 'co-transported' across the wall of the intestine - and into your body. This transportation plan creates an osmotic gradient between the fluids in your tissues and the solution which is inside your small intestine. Since the sugar and sodium have rushed over to the tissue side, water tries to flood along after it, osmotically attempting to dilute the salty, syrupy environment in your body. You benefit because you get to 'keep' much of that water that flows inward.

Are sports drinks like Gatorade the absolute-best rehydrating drinks then? Well, not exactly. Although sports drinks are sometimes touted as electrolyte replacers, the things are actually pretty low in sodium. They do have more sodium than cola, fruit juices, and (of course) water, which all have really puny salt contents, but their salt concentrations are actually quite modest. Unless you force yourself to drink like a bulldog, you're likely to get better dehydration with something a little saltier.

No, you don't have to drink actual salt water (although 5 tablespoons of sugar and 1 teaspoon of salt, added to a quart of plain water, make a pretty good rehydrating - but not tasting - drink, and one which has about three times as much salt as Gatorade). All you need to do is ingest some salty food (pretzels, low-fat potato crisps) along with your Gatorade or other sports drink as you rehydrate post-exercise (of course, don't load up on salt if your doctor has advised you against it because of specific medical problems). Or, you can toss a couple of pinches of salt into your sports beverage - and drink away!
As you rehydrate, remember that you can't just drink what you sweated out and expect everything to be fine. For example, if you lost three pounds during your workout/race, you might assume that taking in 48 ounces of fluid would get you back to normal again without delay. That strategy just won't work! It ignores the fact that you continue to sweat, lose water from your respiratory system, and produce urine as you are rehydrating, all of which cancel out some of the intaken water. About an hour or so after your exertion, you should be producing light-colour urine of normal volume; if your urine is dark in colour, stinks, or is small in quantity, you are simply not rehydrating adequately. Exercise physiologists sometimes recommend that you drink at least a quart-and-a-half to two quarts more than the quantity of water you think you lost by sweating. If in doubt - drink!

Solid food can help you rehydrate, too
Don't forget that electrolyte-rich meals can also help with the rehydration process. In research carried out recently at the University of Aberdeen, five male and three female cyclists cycled in a steamy environment (34 degrees Centigrade, 55 per cent humidity) until they had dehydrated them-selves by about 2 per cent of body weight. After the exercise, the athletes ingested either a carbo-hydrate-electrolyte sports drink or else a standard meal comprised of 53 per cent carbohydrate, 28 percent fat, and 19 percent protein, along with water (at a volume 1.5-times greater than the amount actually lost during exercise).

Total urine output was significantly lower after the meal, compared with the sports drink, probably because of all the electrolytes (especially sodium and potassium) naturally found in the food, which helped hold water in the body. When only the sports drink was ingested, the subjects' body-water levels were still down by about 10 to 11 ounces six hours after the exercise had concluded, while the meal-and-water combination restored fluid balance to normal ('Restoration of Fluid Balance after Exercise-Induced Dehydration: Effects of Food and Fluid Intake,' European Journal of Applied Physiology, vol. 73, pp. 317-325, 1996). Obviously, combining good water intakes with consumption of electrolyte-rich foods can be a great way to rehydrate.

Finally, don't forget that drinking at the right rate DURING your workout or race can also make it easier to rehydrate, because there will be less lost fluid to replace post-exercise. Sipping 10 to 12 ounces of sports drink 10 minutes before your effort and then five to six ounces every 15 minutes as you exercise is the best way to keep your tank topped off. That will mean lower heart rates during exercise, calmer body temperatures, better blood flow to your leg muscles, higher performances, and - of course - quicker rehydration and return to 'status quo' once your exertion is over.

During spells of warm weather, remember to use salt liberally and eat electrolyte-rich foods between workouts, but not JUST BEFORE OR DURING training or competition, when low-sodium sports drinks are called for. Never rely solely on your thirst to determine if your hydration status is okay; thirst is a very imperfect indicator of your body's water levels (remember how easily your hypothalamus can be fooled). In hot weather, you should make a conscious, determined effort to drink extra fluids and take in additional electrolytes. In the two- to three-hour period after a dehydrating bout of exercise, try to drink at least one and one-half times the weight of the water you lost during exercise. For example, if you lost three pounds while exercising, that means you sweated out roughly 48 ounces of fluid, and you should attempt to drink at least 72 ounces of rehydrating fluid within the two- to three-hour 'window' after your workout.

John Beveridge