Dehydration II

There have been some interesting questions raised in this discussion of hydration/energy replenishment.

When I am monitoring athletes in training, I advise them that hydration and energy replenishment is not just for this workout, but for the next 48-56 hours' workouts as well. Ideally we want to minimize the negative impact of any given workout (i.e. fatigue, muscle soreness, dehydration, etc.) so that subsequent workouts can accomplish their desired effects. However, there are times when we do want fatigue (say after a 4:00 hour overdistance training paddle), but not dehydration or severe glycogen depletion as we may need to take more days off from quality training than ideal.

Myself I advise that in races of 45 minutes or less you can do without fluids in warm weather, possibly up to an hour in cooler conditions (i.e. the water bottle or hydration system hose freezes solid). However, this is racing alone, in warm up I always carry sufficient fluids for the pr-race hydration and then race "dry". I have often seen fit athletes fade in a 30 minute hot weather race because they didn't rehydrate during a 20-30 minute warm up prior to racing.

Here is a quick reference table fort how much fluid you can loose at different temperatures based on a 70 kg male. Keep in mind that it is not just sweating that causes us to loose fluids, but breathing out hydrated air from the lungs every single breath. Bigger people will loose more, smaller less.

temperature (C)	fluid loss/hour
-05	0.6-1.4 litres
10	1.2-1.5
20	1.6-2.5
30	2.0-2.8

For me I know I can't drink 2 L per hour to replenish that fluid (assuming that energy drink is about the same density as water where 1 kg = 1 L), so I am minimizing losses from the start at 20ºC and above.

BODY MASS LOST	EFFECT
1%	increased core temperature
2	decreased plasma volume
	decreased muscle volume
	decreased heart stroke volume
	increased heart rate
	decreased blood pressure
3	decreased blood flow to skin
4	decreased blood flow to muscles
	increased muscle metabolite concentration
	increased probability of heat exhaustion
5	increased probability of heat stroke
	decreased muscle mineral concentration increased possibility of muscle spasms and cramps
6	increased urine specific gravity (yellower and smelly)
	increased urine acidity
7	increased urine protein decreased blood flow to kidneys
	decreased nutrient and O2 supply to kidneys
	increased concentration of toxins in blood

The relationship between % of body mass lost to dehydration and loss of performance is dramatic. At a 1% loss performance dropped by 1-5%, at 2% performance losses could be as high a 5-10%, and by 3% most athletes lost 25-20% if they could still "race" and were not reduced to a death march.

For me personally, I know I can't drink 2 L per hour to replenish that fluid (assuming that energy drink is about the same density as water where 1 kg = 1 L), so I am minimizing losses from the start at 20ºC and above. The longer I can hold of a 2% loss of mass the better I am likely to perform.

Keep in mind that it is not just sweating that causes us to loose fluids, but breathing out hydrated air from the lungs every single breath.

This fear of dehydration is what compels some athletes to hyperhydrate and develop low blood and muscle sodium levels (hyponatremia) which can be fatal. However, as previously mentioned you need to be obsessive to drink that much water: over 2-4 l/hour for hours on end.

Long term repeated dehydration can cause severe medical problems, a number of elite Ironman triathletes have lost portions of their large intestines to dehydration, other case studies have shown kidney problems (i.e. kidney stones to reduced function), increased joint damage due to repeated loss of lubricating fluid, etc.

Alan Carlsson