When running long distances, like a marathon, the three main nutrients your body needs to keep going strong are fluids, electrolytes, and carbohydrates. Salt, particularly sodium, is the primary electrolyte lost when you sweat. (Potassium, magnesium, and calcium are lost as well, but to a lesser extent). Electrolytes are important because they direct fluid to the right places both inside and outside of your cells, leading to optimal hydration.
Additionally, too much water consumption without adequate electrolytes during long endurance events can sometimes lead to a medical condition called hyponatremia, characterized by headache, nausea, impaired balance, and even seizures or coma (rare). Lastly, salt increases our desire for fluids (hence why they give you salty snacks at a bar), so it helps ensure we drink enough fluids to minimize risk of dehydration.
Now that you know the reason why electrolytes (salt) are important, let’s figure out how much you need. General recommendations vary widely based on how salty your sweat is.
On the low end, it could be as little as 300 mg of sodium per liter (32 oz.) of fluid, and sometimes as high as 700 mg or more of sodium. For perspective, a small salt packet that you get at a cafeteria usually has about 200 mg of sodium. A quarter teaspoon of salt has about 500 to 600 mg of sodium.The other factor to consider is how much you sweat. Heavy sweaters who are also salty sweaters probably need closer to the higher end—around 700 mg of sodium per liter of fluid—versus those who are not heavy or salty sweaters and end up closer to 300 mg. There are methods on the market to measure the concentration of electrolytes in your sweat, although the accuracy of these tests is still questionable.
You are likely a salty sweater if:
You have white streaks on your clothing after a run
You get little crystals/grit on your skin after your sweat dries
Your sweat stings your eyes
Many sports nutrition products have their sodium content listed on their nutrient facts panel, so you can figure out how much to have based on how much fluid you take in per hour and how much salt you need based on your sweat type/rate.
For example, if you drink about half a liter (16 oz.) of fluid per hour when running and are a salty, heavy sweater, then you may need 350 mg of sodium (700 mg x ½ liter) in your sports drink/gel/chews/etc. If you are not able to consume adequate sodium from your sports products, you can use an electrolyte supplement (tab/capsule/etc.).
If all of these calculations make your head spin, it may be worth reviewing your fueling plan with a board certified sports dietitian (CSSD).
Jason Machowsky is a board-certified sports dietitian and certified strength and conditioning specialist at the Tisch Sports Performance Center at Hospital for Special Surgery. He received his Masters of Science degree in applied physiology and nutrition from Teachers College at Columbia University.