When we send people to Mars (not because we spoiled this planet but due to our spirit of adventure), we need to make absolutely sure they don't run out of necessities such as food and water. So scientists wanted to put some numbers to the conventional wisdom that salty food makes you drink more.
What they learned turned the common lore on its head. Over the long run, participants fed a saltier diet actually drank less! When they went looking for a reason to explain this unexpected observation, they got an even bigger surprise: salt makes you hungrier.
The controlled diet fed to participants in simulated space missions makes an ideal opportunity to study nutritional science. Every aspect of the participants' lives can be controlled and recorded. In this case, scientists at the Max Delbrück Center for Molecular Medicine studied 10 men in two flight simulations of 105 days and 205 days duration, respectively. They gave the participants 3 levels of salt intake, maintaining all other nutrients constant. And then they tracked just how much fluids went into and out of each participant, in addition to some other fun stuff like aldosterone and cortisone, two of the hormones involved in helping us maintain our water-salt balance, which is critical to life.
The common sense of what should happen when we eat salt goes wrong in two ways. First, even the less scientific among us knows that salty snacks at the bar make patrons drink more - that correlation between the salty taste and the sip of beer promotes the misconception that salt makes us more thirsty. Second, for those who remember science classes, the basic principle of osmosis implies that if you have saltier urine inside a bladder membrane, it should pick up more water in a natural process. That should make more urine volume, and therefore more thirst to encourage replacement of the water lost.
The study found the exact opposite: while participants were on the higher salt diet, their bodies started working to conserve water. The scientists suggest that at some point of very high salt intake the body might need to force you to dilute your diet, but the reaction to conserve water starts at well below the maximum levels where a thirst might have to be induced.
The new mystery caused the team to go looking for the biochemical mechanisms behind this unexpected activation of the body's water conservation process. They found that the kidneys can use urea, previously thought to be a waste produced as a way for the body to rid itself of nitrogen, to help transport water out of the urine, preserving it for further use by the body. The process of making and using urea this way takes a lot of energy. While the "cosmonauts" were under a strict dietary intake control, the mice tested were permitted unrestricted access to food. And guess what? The high-salt diet mice ate more, in fact 1.2 times the amount the controls ate.
It would be an inappropriate leap to launch the next diet craze by imagining that eating less salt will be linked to weight loss; in fact, the added energy demand that causes the increased hunger might even offset any weight gain from additional food intake. Most nutritionists will emphasize that salty foods are bad for you because these are the ones that tend to be high in fat and the tastiness of salt makes you want to eat more in spite of your hunger level. If thirst is associated, the salt gets blamed for the beer and soft drink calories rather than for inducing weight gain by triggering hunger.
However, finding the body's reaction to salt intake much different than we had hypothesized will certainly lead to a better understanding of how salt fits into our overall fitness picture. The link between salt, energy metabolism, and fluid homeostasis may also extend concerns about high salt diets to potential to damage the cardiovascular system even in patients without the high blood pressure traditionally associated with salt-related disease as well as link salt to diabetes and osteoporosis.
This should make the DASH diet (Dietary Approaches to Stop Hypertension) even more popular than it already is though, because whether it is the good food or the low salt, the diet is credited with weight loss success and may stop more diseases than just high blood pressure. Focusing on "tastier" foods like vegetables and fruits instead of on the high-fat, sugar and salt triumvirate also favors more environmentally-friendly foods over the super-processed, packaged foods or high protein approach (anyone eating meat without salt would welcome meatless Monday!)
Read the studies in the Journal of Clinical Investigation:
Increased salt consumption induces body water conservation and decreases fluid intake. Journal of Clinical Investigation, 2017; DOI: 10.1172/JCI88530
High salt intake reprioritizes osmolyte and energy metabolism for body fluid conservation. Journal of Clinical Investigation, 2017; DOI: 10.1172/JCI88532