My friend Eddie is a competition cyclist, and a sports coach specialising in diet and nutrition.
I asked him one day whether it's true that, on hot days in the summer, glider pilots "should drink a litre an hour"
He didn't pretend to know what's right for pilots. But he was happy to tell me, what he tells cyclists, and speculate a bit on what might be right for a glider pilot. (I told him someone could spend 2 - 3 hours rigging and getting launched, then fly for 4/5 hours with an occasional cool-down in the shade of a cloud).
Cyclist: On a hot day a cyclist riding hard can lose 1.5 litres an hour. Unfortunately, he can only reabsorb about 1.0 litre an hour. A human being idling in his armchair can absorb more than 1.0 litre an hour. But the cyclist isn't exactly lounging about - his muscles are pumping and demanding blood. When the muscles want blood, the gut has to go short of blood. So the gut can't work "at peak" and the rate of water-absorption gets limited to that "about 1.0 litre". If our hard working cyclist drinks more than his litre-per-hour the "excess" just sloshes around in his stomach! Apparently in races like the Tour de France the riders water-deficit can get so severe they have to spend most of the evening drinking and sometimes even have to be topped up on an intravenous drip. So our cyclist cousin certainly does observe the 1.0 litre an hour rule. But should pilots?
Pilot: A pilot is unlikely to sweat out so much water as the cyclist: and will not have the same limitation on the gut. The water intake actually required to maintain full hydration will obviously vary according to temperature, and there will probably be days when a pilot needs to drink that litre an hour. But unlike the cyclist the pilot need never go into deficit because he has the option of drinking more!
Cyclist: With 2% dehydration, endurance suffers by 22%. And a 4% loss of water results in a 48% loss of time on the road.
Pilot: Physical endurance is not really the issue, (is it?) But as perception and judgement are crucial and also known to be impaired by dehydration it's every bit as important to stay topped up.
Our instincts have been hard wired by evolution, to make us do, what we have to do, (to ensure the species survives). It comes as something of a surprise to most people therefore, to find, that the wish to drink does not kick in at quite the "right" time. Unfortunately by the time you feel thirsty you are already overdue, and have begun to dehydrate. If you don't want something nasty to happen and pull the plug from your gene-pool, the Lesson seems to be: Don't Wait Until You Are Really Thirsty. When Drink Comes To Mind - Have One!
Cyclist: Eddie advises his cyclists to make sure they start competitions fully hydrated and drink their 1 litre an hour throughout the event. And also to spray or pour water over their heads. Eddie calls it "giving the body with free water". This "free water" acts just like sweat. As it evaporates it cools the body. Sweat however is (very) "expensive water" (you had to drink it, and your body process it, and in the case of the cyclist he can't even afford it!)
Pilot: Pouring water is obviously impractical but mopping the head with soaked cloths will be effective. The head is particularly sensitive to heat (one of the reasons it has such a good blood supply is to regulate temperature) and Eddie was pleased to know we all wear sun hats. He was interested whether we wore shorts and short sleeved shirts and thought he would prefer long sleeves and trousers himself, as these contribute marginally to reducing water loss - though perhaps at the cost of feeling a bit damper and a bit warmer. Long sleeves and trouser legs also reduce UV exposure and should be light coloured and closely woven . "Rohan" sell clothes which are rated with sun protection factors. Long clothing also evens out the temperature fluctuations between shade/cloud and full sun.
Cyclist: The main groups of interest to the competitive cyclist are protein, fat and carbohydrate. Protein: little usable energy is derived from protein. It is used for repairing cell damage and growth. Sedentary people need only about 0.8 grams per kilogram of bodyweight a day. An athlete may need up to 2 grams per kilo but no more. Fat: this provides the lions share of energy. It's energy dense, about 2.25 more so than carbohydrate. Unfortunately fat requires more oxygen than carbohydrate to give up a unit of energy. When exercising gently the cyclist processes fat OK but as he cycles harder less oxygen is devoted to these processes, and consequently the energy which can be obtained from carbohydrate becomes more important. Carbohydrate: is not so energy rich as fat, but more oxygen-efficient. It's handicap is that it can only be stored in association with water. We cannot store large amounts of carbohydrate-energy (only enough for 2 hours hard riding) so replacement is essential. Another component of food is fibre and though now recognised as essential for whole-life health, it can be a bit of a nuisance during and immediately after exercise because it delays absorption by the gut and hence delivery of energy to the muscles. During and after a race cyclists take non-fibrous carbohydrate drinks. These provide energy and protein and maintain hydration. It is important to have these available after the race because the body repairs cell damage more rapidly immediately after exercise. And the body will also burn up its fat if nothing else is available. [Not a bad thing for a lot of the population but not a good idea if you are a skinny cyclist!]
Pilot: Eddie advises having a snack after rigging - no need for protein just bun(s) and a soft drink. And for the flight, more buns, and bananas or apples. Fat will not be needed. Being hungry will not affect judgement so badly as being thirsty (dehydrated). Feeling "lightheaded" happens when blood sugar gets low; this can be put right quickly with a sweetish drink. Carbohydrate "sports drinks" can be bought (like Isostar, from Boots) or made up for virtually nothing. Dilute a fruit cordial to the strength you like, then add a pinch of salt! Have a look at the footnote for the history of this knowledge.
Eddie also explains the difference to our bodies between sprint events and endurance events. Most exercise is aerobic; producing energy as a chemical reaction with oxygen. Sprint exertion demands peak energy at whatever cost; this is achieved without using oxygen and is called anaerobic. It is not efficient. It also produces by-products which quickly affect the muscles. Within a few seconds lactic acid levels rise considerably and inhibit/stop further activity. Recovery is slow as the body uses bicarbonate to gradually absorb the lactic acid: this process produces carbon dioxide. Our rate of breathing is determined by the level of carbon dioxide in the blood (the more, the deeper and quicker we breathe). Sports commentators often refer to this oxygen-debt when you see the athletes prostrate and gasping after a short race, [you can see the same effect on someone opening a Council Tax Bill ].
Footnote:
Cholera kills by provoking massive and ultimately fatal dehydration. Over many years, various means of rehydration had been experimented with, with differing success. As the dehydration associated with Cholera also causes a rapid loss of salt (which causes nerves to misfire, and subsequently convulsions) the early treatments attempted to administer salt-and-water via the mouth.
It was not until the 1960s however that it was discovered that, neither, a solution of salts, nor a simple solution of sugar, can, on their own, be absorbed by the lining of the gut. And it was only in 1968 that doctors applied the first successful treatment in Calcutta, by mixing glucose with salt(s) in equal proportions.
When the doctors thought about their discovery they decided they should not have be surprised. Because in real life, after all, one does not usually eat purely salty-foods or purely sugary-foods, but foods that contain a mixture of the two!
And, it seems, this simple rehydration therapy has saved thousands of lives.
And can save you many pennies on "Sports Drinks". Just make up something like your Robinson's Barley Water and add a pinch of salt!!