Water intake speed vs absorption

Deepunder

I recently saw a training video (non cycling) and the guy starts his day by downing 2 pints of water to boost hydration.

Does anyone know if necking a decent volume in one hit will actually be absorbed, or does the body vent off excess (even if needed by your system) as urine, i.e. the absorption rate has a 'maximum' intake rate and the extra is lost?

I would think if the water is needed the body wouldn't ditch it - not a good survival mechanism....I sometimes am guilty of not keeping up my water intake, especially at work, so try to catch up later in day/evening, especially before rides. Am I wasting my time. I understand of course gradual intake over the day is best but interested if anyone actually knows the science?

jehannum5

The answer is complex. Essentially, you can preload water by downing, as you say, 2 pints. It will be absorbed through the stomach and intestines into the large veins of the body. Once the volume in the circulation reaches a certain point the kidneys will remove the excess by dumping it into the urine (making the urine more dilute). There is a maximum intake rate, in that your digestive system can only absorb a certain amount per hour (usually calculated to be no greater than 1.25l per hour). I worked previously as a medic in the Australian army and our policy dictated that it is better in hot weather to preload of an evening and drink steadily throughout the day. There are other variables at play here but essentially your kidneys and circulatory system and it's associated pressure and concentration receptors will regulate the concentration of water and electrolytes in your blood. the other aspect to be aware of is cumulative dehydration overanumber of days and the effect of illness, both of which contribute greatly to heat illness.

Deepunder

Thanks for your reply, its interesting you mention pre-loading in the evening. I find if I drink for example a litre in the evening (when feeling dehydrated) I wake up in the night or morning needing a good wee....what I don't understand is, if I'm dehydrated why is my body venting off fluid as urine in the morning i.e. not absorbing overnight? Surely I need it.

Like you say it may be complex but just doesn't make sense to me??

jehannum5

Good question. The answer is that when you drink of an evening before bed you have drunk sufficiently to replace your fluid deficit. The fluid absorbed goes into the venous portion of your circulation, it reaches an equilibrium with your cells (which are presumably dehydrated from the day's activities) and any extra that you have ingested fills out your veins. The kidneys sense via pressure receptors that the veins are full and excrete the extra into the urine. Your kidneys don't 'know' that you ar erplanning another big ride tomorrow and would love some extra on board. If you wake up weeing during the night and/or pass a nice full dilute stream in the morning than you can be assured thsat you've hydrated enough. Another tip, during the day, is to drink enough to keep passing clear urine. Also look at the veins on the back of your hand if they're full you have sufficient on board (as a rough guide, this tends to wax and wane).

Trev Magill

Dont drink cold fluid straight from the tap or fridge as the body finds it harder to absorbe into the system. Drink the fluid at room temp. The body absorbs fastet hence better rehydration.
Your urine should run clear at least once a day, that way you know you are hydrated. It also depends on the weather for the amount of fluid needed to stay hydrated. You will need more fluid in warm weather than in cold weather because the body looses moisture quicker when it gets warmer. A good guide is, the darker the urine the more dehydrated your body is , and the clearer it is the more hydrayed you are .