Abstract 
 
 BACKGROUND. The idea that increasing salt intake increases drinking and urine volume is widely accepted. We tested the hypothesis that an increase in salt intake of 6 g/d would change fluid balance in men living under ultra-long-term controlled conditions. 
 
 METHODS. Over the course of 2 separate space flight simulation studies of 105 and 205 days’ duration, we exposed 10 healthy men to 3 salt intake levels (12, 9, or 6 g/d). All other nutrients were maintained constant. We studied the effect of salt-driven changes in mineralocorticoid and glucocorticoid urinary excretion on day-to-day osmolyte and water balance. 
 
 RESULTS. A 6-g/d increase in salt intake increased urine osmolyte excretion, but reduced free-water clearance, indicating endogenous free water accrual by urine concentration. The resulting endogenous water surplus reduced fluid intake at the 12-g/d salt intake level. Across all 3 levels of salt intake, half-weekly and weekly rhythmical mineralocorticoid release promoted free water reabsorption via the renal concentration mechanism. Mineralocorticoid-coupled increases in free water reabsorption were counterbalanced by rhythmical glucocorticoid release, with excretion of endogenous osmolyte and water surplus by relative urine dilution. A 6-g/d increase in salt intake decreased the level of rhythmical mineralocorticoid release and elevated rhythmical glucocorticoid release. The projected effect of salt-driven hormone rhythm modulation corresponded well with the measured decrease in water intake and an increase in urine volume with surplus osmolyte excretion. 
 
 CONCLUSION. Humans regulate osmolyte and water balance by rhythmical mineralocorticoid and glucocorticoid release, endogenous accrual of surplus body water, and precise surplus excretion. ||||| Salt makes people drink less, not more, over a period of several months, according to a study of a simulated mission to Mars. 
 
 The amount of water needed will be a key consideration for space agencies with a round trip to the Red Planet expected to take about 510 days. 
 
 It has long been thought that salt increases people’s thirst, particularly because it is associated with a rise in the amount of urine produced. 
 
 But a team of scientists from Germany, the US and elsewhere found that a high-salt diet had the opposite effect on their test subjects, 10 men kept in sealed living quarters and given a strictly controlled diet for a period of 205 days. 
 
 Over the course of the study, they drank less and retained more fluid in their bodies despite producing more urine. 
 
 They also reported being hungrier when their food contained a lot of salt compared to lower levels. The study was repeated in mice with the same results – those on the high-salt diet drank less but were also hungrier. 
 
 One of the researchers, Professor Friedrich Luft, of Vanderbilt University Medical Centre in the US and the Max-Delbrueck Centre for Molecular Medicine in Germany, told The Independent in an email they had tested three levels of salt intake: two estimates of what people normally eat (12 grams and 9g) and “what the guideline folks want us to eat” (6g). 
 
 “We observed that our subjects – all 10, watched for 205 days – actually drank less, and not more on high-salt rather than low-salt even though they made more urine,” he said. 
 
 “These amounts are small; you will not notice them at the urinal. However, it takes 510 days to get to Mars and back.” 
 
 However Professor Luft said pubs were “not necessarily wrong in feeding you salt pretzels”. 
 
 “These elicit secondary, short-term drinking responses so that you drink more beer. We are talking about long-term responses,” he said. 
 
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 Writing in the Journal of Clinical Investigation, the scientists described their findings as “seemingly counter-intuitive”. 
 
 They said the kidneys acted as a highly efficient “biological barrier designed for water conservation”. 
 
 “This water accrual process was so effective that a body water surplus was available at the 12g-a-day salt intake level and could be excreted and thereby increase the urine volume,” the paper added. 
 
 The body’s production of fluid is not simply a reflection of the amount of fluid consumed as liquid or in food. 
 
 This is because when the body ‘burns’ food and drink to create energy this process produces carbon dioxide, water and other by-products that must then be excreted. The way this process happens has a significant effect on the amount of by-products. ||||| Salt makes you less thirsty over time, according to new studies published in The Journal of Clinical Investigation. (Photo: wideonet, Getty Images/) 
 
 It's conventional wisdom as timeless as beer taps: Eating salty foods makes you thirsty. 
 
 But if you inhale fistfulls of bar snacks on a regular basis, you may find the opposite to be true. Diets high in salt can lead the body to retain more water, decreasing thirst and ramping up hunger, according to newly published research in The Journal of Clinical Investigation. 
 
 An international group of scientists from places such as the Vanderbilt University and the German Aerospace Center conducted the first-ever study on salt intake and drinking habits, according to the Max Delbrück Center for Molecular Medicine, which also took part. 
 
 To create a controlled environment with fixed diets, researchers used two simulated Mars missions involving 10 male volunteers. One "mission" lasted for 105 days and the other for 205 days. The men all ate the same diets, but with differing levels of salt. 
 
 At first, the hunch of every bartender held up: Salty foods increased thirst in the short term, according to the Max Delbrück Center. But over the long term, those on salty diets drank less and complained about being hungry more. That's because the salt set off a mechanism that keeps water in the kidneys, researchers found. 
 
 While many theorized that salt essentially gloms onto water molecules and pulls them into the urine, the results showed salt staying with the urine while water moved back into the kidneys and body. 
 
 It caused scientists to take a closer look at urea, a bodily compound thought to be a waste product, said Friedrich C. Luft, M.D., a doctor with the center. 
 
 "Its function is to keep water in when our bodies get rid of salt," Luft said. "Nature has apparently found a way to conserve water that would otherwise be carried away into the urine by salt.” 
 
 But making urea takes a lot of energy, which leads to hunger — in mice, at least. That could explain why high-salt subjects experienced increased hunger. 
 
 Scientists behind the research said it could shape understandings of how the body achieves proper balance, or homeostasis. 
 
 © 2017 USATODAY.COM
– An international team of scientists studying the effects of salt intake on our bodies has discovered a surprise: High-salt diets seem to make us less thirsty over time. Reporting in the Journal of Clinical Investigation, they confirm previous findings that more salt makes humans pee more, but added "seemingly counterintuitive" findings that humans on higher-salt diets also consume less water and feel hungrier. The findings, part of the Mars500 project to research space travel, may not only help nutritionists plan what meals astronauts traveling 510 days to Mars and back should eat, but could help us better understand how our bodies achieve homeostasis, or balance, reports USA Today. It seems that bartenders aren't wrong in putting out salty snacks—they make us thirsty in the short term. But after a while, the opposite appears to hold true. In their study, researchers observed 10 healthy men in a highly controlled environment for 205 days and tested three levels of daily salt intake: 6 grams, 9 grams, and 12 grams, reports the Independent. Over time, the more salt consumed, the less water the men drank and the hungrier they became. So what's going on? The answer apparently revolves around the bodily compound urea. "Its function is to keep water in when our bodies get rid of salt," says one of the researchers. "Nature has apparently found a way to conserve water that would otherwise be carried away into the urine by salt.” But because producing urea is a taxing process that requires energy, it results in feelings of hunger. The latter point was confirmed in a separate but related study of mice, per Popular Science.