Researchers tracking physiological processes in a Mars mission simulation have come up with a surprising discovery: eating salty foods makes you hungry, not thirsty.
The conclusion stems from observations of two groups of ten male volunteers who spent 105 and 205 days, respectively, cordoned off in a mock spaceship to simulate the rigours of space travel to Mars. The subjects were given identical diets except for variations in their salt intake over periods of a few weeks at a time.
Researchers tested three different salt levels commonly associated with contemporary diets and dietary guidelines: 12 grams, 9 grams, and 6 grams per day and found that the subjects actually drank less during the higher salt periods than the lower ones and that those put on the higher salt diet complained more often of being hungry.
The standard claim (never-before tested, apparently) has been that the sodium and chloride ions in salt bind with water molecules in the body and effectively pull more H2O out of the body, expelled as urine. In fact, the new results show that the body actually conserves water as a result of higher salt intake.
Through follow-up experiments with mice, the researchers found that higher salt intake causes the production of more urea, a substance formed in the muscles and liver which accumulates in the kidneys and causes them to retain more water. Eventually, this retention process as a result of salt intake creates a surplus of water which is then expelled, creating over time the increase in urine.
“This water accrual process was so effective that a body water surplus was available at the 12-g per day salt intake level and could be excreted and thereby increase the urine volume,” say the study’s authors, an international team of scientists including researchers from the German Aerospace Center (DLR), the Max Delbruck Center for Molecular Medicine and Vanderbilt University.
As to why the extra salt made the fake cosmonauts hungrier, that’s a result of the extra energy needed for the body’s muscles and liver to produce the urea. “Nature has apparently found a way to conserve water that would otherwise be carried away into the urine by salt,” said Prof. Friedrich C. Luft, MD of the Charité and Max Delbruck Center and study co-author, in a press release.
The researchers say the new results call for a change in the way we understand not just salt intake but how the body achieves water homeostasis. “We now have to see this process as a concerted activity of the liver, muscle and kidney,” says Prof. Jens Titze, MD of the University of Erlangen and Vanderbilt University Medical Center and study co-author.
The findings are published in a pair of studies in the Journal of Clinical Investigation.
Health Canada’s Food Guide says that adults should aim for an adequate daily intake of 1500 mg per day of sodium (roughly 4 grams of salt) without going over an upper limit of 2300 mg per day (roughly 6 grams). Over 85 per cent of men and 60 to 80 per cent of women have sodium intakes exceeding recommended amounts, according to the 2004 Canadian Community Health Survey.