Salt Marks and Blood Sodium 101
Transcribed from video:
We lose salt in our sweat. So let me just preface some things that people get really freaking confused about. We talk about electrolytes all the time. We talk about electrolytes being things that conduct electrical currents and how they're totally important for our brain to talk to the rest of our nervous system without electrolytes like sodium, potassium, magnesium chloride, iron, calcium. We would be dead in the water. They provide you all sorts of different functions in the human body. There's also a lot of water in the human body. When you think about where that water is and you think about where that electrolyte is. There are some real important distinctions that I think we're never taught in. I don't know, health class. We're taught about condoms and cucumbers, but we're not taught about anything else.
Here's the deal. There is water inside of cells and there's water outside of cells. So effectively, there are two water compartments. Right. This is the human body. We have water in cell. And that's referred to as intracellular space intracellular. And then there's water out and that is are extra cellular space. Water in/out.
What's important about intracellular versus extracellular water is that the electrolyte content is not the same. Inside of cells, we mostly have potassium. That's K+, potassium. Tons of potassium. Yeah. Inside the cells. Potassium outside of cells. We have good old fashioned table salt, sodium chloride ions. Okay. So salt outside of cells. Potassium inside of cells. (watch video for what this is referencing) This is about two thirds of our water volume. This is about one third.
The outside water. Right. We'll expand this drawing up a little bit. The outside water is also where our heart is and where our vascular system is. That circulates, you know, between our heart and lungs, oxygen and other vital nutrients throughout our body to get to our cells. So what's really important about the extracellular water is that that's where our cardiovascular system lies.
Also, if this is skin, this is also where we have sweat glands that are like big ole tunnels connecting the outside world with the inside wall. So we've got a bunch of little tunnels. We have this high amount of potassium inside the cells. We have this high amount of sodium chloride outside. And so when we sweat, right ... when we sweat, what comes out of our sweat gland is not a lot of potassium or magnesium, which is typically in muscle or calcium, which is typically in bone when we sweat,
Mostly getting is good old fashioned table salt or sodium chloride. Right. But how much and why? There's a lot of misnomer as to what is happening here. Some people think that it's because we eat too much salt. All of this salt comes out of our sweat. Other people think that when they see the sweat, they must be losing a lot, so they have to actually eat and replace more.
So what is going on? Well, what's going on is this is the amount of salt in our extracellular space or in our bloodstream is somewhere between 3300-3500 milligrams of sodium per liter. Okay. So 3300 to 3500 milligrams of salt. That's like maybe when soup starts to taste good, right? That's like when we start to like are chicken noodle soup.
That's what we like to do. We start to like, broth. So your blood is about as salty as a decent bowl of soup. Okay. But we don't lose all of the salt in our bloodstream because when we go through the sweat gland. And plasma at this concentration goes through, there are all these little escape hatches through this sweat gland.
Think of a sweat gland as a big tunnel that you're driving through. And you can see these doors on either end. Right. And these escape hatches or these doors basically pull chloride, I mean, sodium back into the body. Okay. But depending upon how fast the sweat is going through and depending upon how many escape hatches you have, if you don't have a lot of escape hatches, more sodium is going to leak through.
The number of these escape hatches and sweat glands is determined by this gene called cf1 or this gene set called cf1, which actually stands for cystic fibrosis in cystic fibrosis. There are none of these little escape hatches in the sweat gland that can pull sodium chloride back into the body. And this gene set is the most mutated gene set in the human body, meaning there's more variation in this trait than any other trait in the human body.
For example, at the very low end, you might find some people at 200-300 milligrams of sodium per liter. In the middle, maybe about a thousand milligrams of sodium. On the far extreme maybe 2000. In a disease state like cystic fibrosis, 3000 to 3500 milligrams of sodium per liter. So there's this wide range of sodium loss in sweat that is, for the most part, genetically determined because of the number of these escape hatches and the sweat glands.
Okay. If you were to have a big salty meal and then you exercised, some of that salt might leak out and you might get a higher sodium sweat, but your kidneys, the rest of your body will create equilibrium with that sodium very, very quickly. The amount of salt in your bloodstream is quite fixed and it's a tightly controlled homeostatic variable.
For example, if you were to eat an excessive amount of salt and there are now more sodium molecules than potassium molecules inside of the cell water would begin to shift out to dilute that concentration out. So even being chronically on a high sodium diet is not necessarily going to cause you to lose more sodium. You'll equib with shifting water so that that concentration stays constant.
But that's also why that watershift might cause high blood pressure. In any case, a couple of things. One, a highly variable genetic factor. Two is just good old fashioned salt because it's coming from the extracellular space. There's very little potassium, very little magnesium, very little calcium. So 90% of what is in our sweat is good old fashioned salt.
Remaining 10% is a little bit of potassium, a little bit of, you know, calcium, a little bit of magnesium.
How much comes out is not necessarily related to what you see. There are other factors like the total volume of sweat, not just the concentration. So if you have a low concentration but sweat for a long time, it might appear that you have a lot of salt on your clothing or your helmet straps, but it doesn't necessarily give you an indication of the concentration.
The best way to figure that out is if you did a little sodium sweat test where we actually take a non evaporative sample of sweat and measure the actual concentration. If you can replace the concentration that you lose ends up happening is thirst becomes a pretty good barometer for what you need to drink. You know, even if you don't get it perfect, replacing at least a good percentage of the sodium you lose when you're sweating is going to help drive the thirst that allows you to replace water in a way that creates both better water balance as well as maintaining electrolyte balance.
So that's why we put sodium in our drink. You might notice if you're a consumer who reads labels that we use sodium citrate, which is sodium bound deacidified citric acid, not chloride ion. The reason we do that is because you can replace, if you don't replace the sodium. There are a lot of negative effects, but if you don't replace the chloride ion during exercise, there are very few, if any, negative effects, if anything.
By not replacing the cholride ion, we get a buffering effect with that sodium citrate. And so for the context of exercise, that's why we do what we do. We place the sodium, not the chloride ion. And, uh, that's it. That's why you see the salt. It's because of the fact that we lose about a third of the salt in our blood through our sweat.
And depending on your genetics, you may lose more, or you may use less. I think one of the best ways to know if you need more salt besides doing the sodium sweat test is if you're drinking to thirst, which we always recommend. Don't drink beyond your thirst. But if you're drinking to thirst and you find yourself still losing a lot of water by weighing yourself pre and post exercise instead of trying to force yourself to drink more water, add more sodium to your drink mix that will in turn drive you to drink more and you can almost self calibrate to the optimal hydration during exercise, during sweating by incrementally increasing your sodium amount such that when you drink by thirst, you're not losing more than two or 3% of your body weight.