Wikipedia:Reference desk/Archives/Science/2018 March 28

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March 28

Too much water can kill you?

Why does drinking an enormous amount of water lead to hyponatremia? Couldn't the tap water actually increase the amount of sodium in the blood? Does it make any difference if it's tap water or mineral water? --Doroletho (talk) 01:43, 28 March 2018 (UTC)

See Water intoxication. Almost all tap water or mineral water has a much lower sodium content than the human body fluids. Graeme Bartlett (talk) 01:58, 28 March 2018 (UTC)

The article doesn't really go to first principles, so I'll explain a little. The fundamental issue here is that you can't simply unmix a rum and cola. Entropy describes the tendency of random distribution of chemical compounds in solution to become uniform, much as randomly shaking salt over a meal tends to lead to a roughly even salting. But more importantly, entropy actually affects the free energy of a system, which is to say, it actually requires energy to unmix it. Conversely, dissolving ethanol, or more to the point salt, in water tends to release heat energy.

A consequence of this is that desalination takes energy to do, whether it is done in an industrial setting or in the human body. The more that urine would differ from ordinary body fluids, either in being hypertonic or hypotonic, the more energy it takes to do that separation. Even sweat is not purely evaporation of water, but brings salt out with it. The human body hasn't evolved to put in as much effort as it takes to secrete crocodile tears or pure distilled water; it assumes that the fluid taken in will be in a range of salinity that implies avoiding drinking seawater and also implies that large amounts of pure water consumed will need to be compensated with an occasional salty meal. Wnt (talk) 03:42, 28 March 2018 (UTC)

To put some numbers on that: The (non-enforceable) EPA recommendation is that tap water have no more than 60mg/L sodium, which is equivalent to about 1.7mM. Most internal bodily fluids (though not the interior of cells) have sodium concentrations around 140mM. As Wnt is getting at, the kidneys simply can't expel excess water without also expelling some sodium. The water that winds up in your bladder has to pass through your blood, which has sodium in it. The kidneys will try to pump sodium back into the blood if that's needed, but they can only do this with up to 98% efficiency[1]. So what with tap water probably being at most 1.25% as salty as blood... well, you can work out the rest. Unless your local water authority provides some amazingly salty tap. Someguy1221 (talk) 03:47, 28 March 2018 (UTC)

Water softening will change the calculus though ([2]). A 1997 study with N=59 found a range of 46-1219 mg/L with 17% over 400 mg/L ([3]). EvergreenFir (talk) 06:37, 28 March 2018 (UTC)

To elaborate on the physiology, the core issue is that excreted water always takes some electrolytes out of the body with it via osmosis. If you drink massive amounts of water, the body has to excrete the excess to control blood volume and pressure; said water then takes electrolytes such as sodium with it. The kidneys pump some electrolytes from the filtrate back into the body if necessary, but they can only do so much. --47.146.60.177 (talk) 07:14, 28 March 2018 (UTC)

@Doroletho: see the article Leah Betts for sociopolitical implications. —SerialNumber54129 ^{paranoia /cheap shit room} 07:34, 28 March 2018 (UTC)

This is probably off-topic, but pure water is dangerous as well. ⌤TheMitochondriaBoi⌤^(☎) 15:43, 30 March 2018 (UTC)

Nevermind, didn't read discussion. ⌤TheMitochondriaBoi⌤^(☎) 15:44, 30 March 2018 (UTC)

Drinking too much water can kill you. See http://www.nbcnews.com/id/16614865/ns/us_news-life/t/woman-dies-after-water-drinking-contest/#.Wr7K4Mi-lE4 Ohanian (talk) 23:40, 30 March 2018 (UTC)

Maximum energy content in food

I was getting a little bit confused when I was reading a report (page 68) about eating insects, they claim that the insect green ant from Australia has an energy content of 1272kcal/100g, in my world that seems impossible due to the highest possible energy content for edible food shuold be fat with around 900kcal/100g, is it possible that an insect has this high energy content? /155.4.145.251 (talk) 09:44, 28 March 2018 (UTC)

Took me a while to figure this out. They just messed up their data transclusion. In the original source, BioFoodCompV4, the number for Green Ant is the same, but the units are kJ/100g. Someguy1221 (talk) 10:35, 28 March 2018 (UTC)

Thank you very much Someguy1221!/155.4.145.251 (talk) 11:33, 28 March 2018 (UTC)

Reminds me of an elementary school math problem that was marked wrong cause my teacher forgot (or maybe never knew?) that food calories are kilocalories. Sagittarian Milky Way (talk) 19:00, 29 March 2018 (UTC)

That sucks. ⌤TheMitochondriaBoi⌤^(☎) 15:45, 30 March 2018 (UTC)