Salt makes you…. hungry?

International Space Station photo courtesy of NASA Image Library

Who says space exploration is a waste of money?

Over the years, plenty of useful information and technologies developed for space exploration programs has been translated to uses for everyday people.  Now we have another rather unexpected piece of information.  Salt does not make you thirsty.

Here’s how this piece of .. useful?.. information was uncovered.  Human missions to Mars are being discussed, but before anyone can go, scientists have to know how those astronauts are going to survive on the long voyage.  Water and food are critical pieces of that puzzle.  The assumption is that salt makes you thirsty and you drink more water.  We don’t want astronauts craving more water, if the supply is tightly controlled.  So the effect of salt intake on water requirement was added to an ongoing study of simulated space flight.

Two groups of male volunteers were sealed into a mock spaceship.  One group for 105 days, the other for 205 days.  Their diets were the same, but for several weeks during the study period, three different amounts of salt were added to their food: 6 grams, 9 grams and 12 grams.  This corresponds to 1 tsp, 1.5 tsp and 2 tsp, not much considering the usual intake of people who eats lots of processed food.  The 2 tsp/12 gram level has just over 2300 mg of sodium, which is in line with sodium limit recommendations of some health organizations.  So these men were not eating a high sodium diet by any means.

During the 12 gram higher salt intake period, the subjects did excrete more sodium in urine.  But they did not drink more water.  In fact, they drank less water.  Somehow higher salt intake triggered the kidneys to conserve water while dumping excess sodium.  The scientists were surprised, and wanted to find out how this happened.  They had some clues from mouse studies.  Urea, normally thought of as a waste product produced by muscles and liver, was accumulating in the kidneys, where it drew water back into the body.  Urea production uses up energy, leading to another unexpected outcome: mice in these studies ate more than low salt mice.  And in fact during the high salt intake period, astronaut study subjects reported being hungrier.

Well that’s unexpected: salt makes you hungrier.  It also flies in the face of all the presumptions about the effects of eating salty food.  We assume salty foods will make us drink more.  Now it turns out salty foods might make us eat more.  Lest you think salt is a new diet aid, leading to excess calorie burning, consider this: the extra energy used to produce urea in the mouse studies came from breakdown of protein, or muscle mass.  Another limitation: the mouse studies used amounts of water and salt that would be completely unrealistic for a human to consume.  At most you could take away this thought: if you’re trying to control your food intake to control weight, eating a high salt diet, or eating very salty foods for a short period of time, might make you hungrier, making it hard to stick to your plan.  Certainly that should be a consideration for the astronauts.  They might not need extra water to balance higher salt intakes, but they might need extra food, which could also be a logistical problem.

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