Last Word is New Scientist鈥檚 long-running series in which readers give scientific answers to each other鈥檚 questions, ranging from the minutiae of everyday life to absurd astronomical hypotheticals. To answer a question or ask a new one, email lastword@newscientist.com
Is it more efficient for me to carry water in a bottle or in my stomach? (Continued)
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Mark Dirnhuber
Bristol, UK
If this purely depended on biomechanics, then the stomach would be more efficient because the load is nearer to the body鈥檚 weight-bearing axis. But the stomach is seriously leaky: a drink of a few hundred millilitres of water will be absorbed into the body within an hour, some directly from the stomach but most from the small intestine.
The absorbed water lowers plasma osmolality (a measure of the concentration of dissolved particles in the blood plasma 鈥� the liquid part of blood) and suppresses anti-diuretic hormone secretion, so the kidneys will excrete most of that excess water within 4 hours. So carry your water in a plastic bottle, lest you end up needing to recycle the lost water from your stomach.
The stomach is seriously leaky: a few hundred millilitres of water in it gets absorbed into the body within an hour
Gary Trethewey
Leigh Creek, South Australia
First, look at the notion of efficiency. This refers to reducing the use of one resource 鈥� water 鈥� at the expense of another resource 鈥� effort. For example, a car鈥檚 fuel efficiency might be improved by changing its size, weight or shape, but this can come at the cost of speed or comfort. Conversely, fast speeds can be achieved by burning more fuel.
So, where to carry water? Our bodies are very sensitive to water balance, so we need to have the right amount within ourselves. Any notion of saving water by storing it in our backpack and going thirsty is dangerous. On the other hand, because of that same sensitivity, any over-hydration simply makes us pee, wasting what we carried on our back yesterday. So, the most efficient way to carry water is to have the right amount in our body and bring the rest in the backpack.
Pat French
Longdon-upon-Tern, Shropshire, UK
Water leaves your stomach in about 15 to 20 minutes. Therefore, if your journey is short and you are likely to find water available on arrival, there is little need to carry it under normal conditions. Harsher conditions, such as high heat or humidity, will raise the rate at which this stomach reserve will be depleted.
Beyond your stomach, water may be considered 鈥渋nvolved in the vital process鈥� rather than being stored, and it will need to be replaced with water from your stomach reservoir as the process continues. If you have only a little water, you may as well drink it as carry it. It will be better for your body to use it before becoming affected by thirst than trying to remedy those effects afterwards.
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