A High Salt Diet Is Bad, Right?
Most of us probably think salt is bad for us. Our doctors will most likely have prescribed a low salt diet if we suffer from high blood pressure or heart disease. Yet, recent studies shed a whole new light on how our bodies adjust to a high salt diet. And maybe its not all bad…
It had always been thought that if we ingest an extra load of salt, our kidneys will immediately try to restore the balance of salt and water in our blood. According to this simplistic concept – when we eat more salt, we drink more water. The imbibed water then flushes out the excess salt, allowing the levels of both salt and water to return to normal.
But some recent studies, including observations on astronauts, whose intake and outgo could be monitored carefully, show that this is not what happens when we increase our salt intake.
What Really Happens On A High Salt Diet
Instead, taking in extra salt triggers a complex shift in our metabolism. We do not deal with salt loads by drinking more water – instead our kidneys conserve water by excreting a very salty urine using only a small volume of water. In fact, the astronauts on a salty diet actually drank less water than did their colleagues who were on a lower salt intake.
This gets complicated here, but hold on, because how it all happens is quite surprising.
In essence, the body (mostly the liver and muscles) generates more urea, which allows the kidneys to trade urea for salt, excrete the salt, and reabsorb the urea. As the urea reabsorbed, it’s osmotic force draws water along with it and back into the system. This mechanism allows for maximal salt excretion with a minimal loss of water. But the generation of urea is energetically costly, so the body burns fat, sugar and/or protein to provide the needed energy.
Now here is what’s key these energy sources – sugars, fats, and proteins – when combusted liberate a lot of ‘metabolic water’. In fact, burning a single molecule of glucose generates up to 42 molecules of metabolic water!
Ok, so now the astronauts who ingested a salty diet have mobilized some additional water from body stores as they respond to the demand for energy to generate urea – what happens next? The extra, metabolic water dilutes the extra salt, and then the salt and some of the metabolic water get excreted together through their kidneys. The rest of the metabolically-derived water is reabsorbed, and so, our astronaut is less thirsty. The excess salt is gone, no need to find a supply of water to drink, and a little fat, perhaps, burned in the process.
If a high salt diet results in increased liberation of metabolic water by burning fat – does this have implications for those of us who could stand to loose a little weight? Would caloric restriction be more effective as a weight loss strategy if salt intake was not also restricted, but instead encouraged? Interesting idea – but those of us with high blood pressure or heart disease should wait for more studies before we embark on what is likely to be a risky road.
What does this have to do with skin?
Good question and the answer is perhaps, nothing directly. However, our readers know that the primary role of skin is to provide a barrier against the loss of water. And water conservation is absolutely essential for survival as a terrestrial species. Indeed, the authors speculate that the reason why the body responds to excess salt intake by these complicated metabolic maneuvers, rather than by just ‘telling’ us to drink more water, is that, teleologically, it makes sense not to be absolutely dependent upon always being able to find water to drink if we happen upon a salty meal. It would be too risky to consume salt, if water were scarce in the vicinity.
Indeed the authors – when they observed that the astronauts generated internal supplies of water when given a salt load – found two possible explanations. Either metabolic water was being generated through oxidative metabolism OR the skin barrier, like the kidneys, becomes more efficient in retaining water when we are exposed to a salt overload.
That concept – a tighter skin barrier to retain more water in response to salt overloading- has not been tested, but we are skeptical. Our studies to date have shown that the skin’s permeability barrier is regulated by external conditions – such as the humidity of the surrounding atmosphere. To date, there is no evidence that the our internal water and salt status regulates how avidly our skin barrier retains water, but then, has anyone looked? Perhaps this could be a subject for future research.