A refresher: When you burn a fuel, say wood, in the presence of oxygen, it gives off energy (in the form of heat and light), and it yields gas (e.g. carbon dioxide) and water.
In our body, that combustion process is called cellular respiration, the fuel can be fat or glucose. In the process of respiring, glucose and oxygen yield the same carbon dioxide and water. The reaction also gives off energy, some is trapped or stored in the bonds of a chemical called adenosine triphosphate (ATP), and some energy is given off as heat (which keeps our bodies warm).
Glucose + Oxygen → Carbon Dioxide + Water + ATP
So, our body can actually make water. We make it by burning a fuel.
With that background … Here’s the thing I just read about salt:
It reminds us of this, which I was taught, and always believed:
If you eat a lot of salt — sodium chloride — you will become thirsty and drink water, diluting your blood enough to maintain the proper concentration of sodium. Ultimately you will excrete much of the excess salt and water in urine.
But when researchers gave Russian cosmonauts a lot of salt:
Instead of drinking more, the crew were drinking less in the long run when getting more salt. So where was the excreted water coming from?
“There was only one way to explain this phenomenon,” Dr. Titze said. “The body most likely had generated or produced water when salt intake was high.”
So, the water that the body uses to flush out extra sodium doesn’t come only from what we drink, but also from what we make, either through respiration or water conservation (e.g. reabsorption in the kidney). These processes use energy, which might make people hungry?
Another puzzle: The crew complained that they were always hungry on the high-salt diet.
There was some discussion about how high-salt diets could lead to weight loss because they increased energy expenditure:
Experiments found that mice burned more calories when they got more salt, eating 25 percent more just to maintain their weight.
But trying to capitalize on this as a way to lose weight seems fraught. The hormones involved in water conservation can also lead to osteoporosis, muscle loss, and type 2 diabetes (according to study author and lead researcher Dr. Jens Titze). That’s a high cost.
All of this seems like a lot of trouble to go to just to find water in the body. But it would work if it happened that no water was coming in … which is one way animals like camels get fluids without actually drinking.
Here are the studies that this article was based on:
High Salt Intake Reprioritizes Osmolyte And Energy Metabolism For Body Fluid Conservation, The Journal of Clinical Investigation, Online 17 April 2017
Increased Salt Consumption Induces Body Water Conservation And Decreases Fluid Intake, The Journal of Clinical Investigation, Online 17 April 2017
Editorial: Salt And Water: Not So Simple, The Journal of Clinical Investigation, Online 17 April 2017
So, it’s not true that eating salt will make you thirsty. In fact, it does the opposite, it makes you less thirsty, because internal water-making kicks in. But it’s true that eating salt makes you hungry, for the same reason: water-making uses calories that you are primed to replace.
You know who will love these studies? Food manufacturers. Put a little extra salt in their products and it will have us coming back for more.