To drink or not to drink

[CL]

Mike T wrote an article on the 'Long Mynd Valley' thread, suggesting that below 20degrees one need not drink at all during a marathon. This is it:

"Running a marathon, at 100 calories a mile, takes approximately 2600 calories. About 25% of this results in work/movement but 75% just results in heat which has to be dissipated to the environment or we would quickly become hyperthermic and die. 75% of 2600 is 1850 calories of heat to lose, and as long as the ambient temperature is less than 20C it is estimated that sweating only has to dissipate half of this ( or less as it gets colder ) as the rest is lost through radiation and convection. 1ml of sweat loses about 0.6 of a calorie so 925 calories needs about 1.5 litres of sweat. But glycogen breakdown liberates water at 2.7mls/gm - if half of the energy of the marathon is derived from glycogen at 4 calories/gm (and the rest from fat ) then about 800 mls of water is liberated and about another 400mls is generated as a by product of energy use ( carbohydrate plus O2 being converted to CO2,H2O and energy ). So the body has come up with 1.2litres of the 1.5 needed. But during exercise our temperature rises, so we don't need to lose all the heat we generate. A typical temperature rise would be from 36C to 39C, and as each degree of temperature increase is about 60 calories, or 100mls of sweat, we can subtract 300 from 1,500 leaving 1,200 - so there is no need to drink at all!"

So to those who understand the science in this article how many will now stop drinking during their marathons?

[Alexandra]

As a mouth breather, I reckon I lose some moisture that way even if I'm not sweating at all - like a dog!

[CL]

Mike T I thought I'd bring your excellent post onto a thread of its own and query some of your theory. My concern is with the glycogen part of your calculation. You state glycogen yields 2.7mls of water per gram of glycogen. You also state that glycogen yields 4kcal per gram. My question is: was the glycogen in both parts of the calculation hydrated or dehydrated?

The other concern is with the amount of glycogen available during the marathon. Since the liver stores about 150gms(600kcal in dry form) of glycogen and little glycogen is used in the muscles during steady state exercise, where will the 1300kcal of glycogen come from?

[Brotherton Lad]

I did about 30 marathons fairly seriously down to 2:30. Most of the water I picked up went over my head or on to my legs, less than a pint made it to my stomach. Only on very hot days would I deliberately drink more.

[Rob Furness]

I don't have any idea whether I "need" to drink on longer runs (haven't done a marathon yet so can't comment there) but I do know I feel better and am able to refuel on the way round more comfortably if I do have some water. That said I did 19 miles this morning and took a 275ml bottle with me and only drank about a quarter to a third of it. I think sometimes it's more to do with comfort then actual need, but either way I won't be stopping to take any water on long runs as a result of some calculations.

Although I suspect this thread is more an attempt to wind up another forum member than a serious question

[Brotherton Lad]

When doing my long training runs on roads, I never took any water or food. I deliberately trained 'dry and empty'.

I've found with long fell running, though, that I do need to take food on board and for that I also need water. I'm not sure whether that's a function of fell running or my age and lack of training.

[Mike T]

Using glycogen both liberates water stored with it AND generates water when it is metabolised to CO2, H2O, and energy. Steady state exercise at very low levels like walking uses mainly fatty acids, but marathon like events completely deplete the slow twitch fibres and even some of the fast twitch ones of their glycogen stores (Noakes, Lore of Running, 3rd edition, pge 67) - so there is plenty of glycogen used to liberate/generate water. Even at only 30% VO2max, the main energy source is glycogen until after the 3 hour mark, when fat takes over (pge 76).
I am no great runner, and need all the help I can get in races - if those around me are willing to handicap themselves by carrying/consuming unnecessary water I suppose I should let them get on with it - but part of the joy of running is seeing others improve/succeed with fairly simple advice - such as an appropriate and scientific water strategy. I am not suggesting that everyone runs marathons without drinking, but as long as it is not hot it certainly works for me.

[shaunaneto]

Horses for courses, I sweat like a pig when fell running, whether it's hot, cold, mild, wet. If I don't drink water it all goes to pot very quickly.

[Mike T]

Some people sweat an amazing amount - at our local track are several who have water dripping off their noses after a few gentle warm up laps - and if you run behind them you get almost rained on. Each runner has to sensibly work out their water strategy, taking into account sweat rate/temperature/duration/access to water on the route and so on. Weigh yourself before and after a long run and work out your sweat rate, knowing that weight loss and dehydration are not the same thing. The only time I suffered from not drinking was on a 20 mile run on a hot day - high 20s - this was a deliberate experiment - I lost 7 lbs - just over 5% of my body weight - and was getting dizzy and slow towards the finish.

[Mike T]

As a mouth breather, I reckon I lose some moisture that way even if I'm not sweating at all - like a dog!

We all lose moisture this way, and roughly the same amount - exhaled air is close to 100% humidified whether we mouth or nose breathe - just like sweat it contributes to heat loss.