You're fasting, you're hungry, so you do what everyone tells you: chug water to "fill up" your stomach. Sounds logical. Except if you push that strategy too far, especially paired with a long or hot-weather training session, you can create a more serious problem than a cramp: exercise-associated hyponatremia.
What It Actually Is
Exercise-associated hyponatremia (EAH) is defined as a blood sodium level below 135 mmol/L that develops during or up to 24 hours after physical activity. According to reference guidelines (StatPearls, Wilderness Medical Society), the main cause isn't a sodium shortfall itself. It's too much hypotonic fluid (plain water, most "sports" drinks), more than your body gets rid of through sweat, urine and breathing. Mild symptoms: headache, nausea, weakness, lightheadedness. Left unchecked and pushed to the extreme: confusion, seizures, a medical emergency.
The Fasting Connection
Here's the part standard sports nutrition guides skip, because they're not thinking about fasting. During long exercise, your body can release antidiuretic hormone (ADH), the hormone that tells your kidneys to hold on to water, even when you don't actually need to. Recognized triggers include the exertion itself, nausea and stress. Now add the "I'll drink to fool my hunger" reflex: lots of plain water across the fasting window with zero sodium coming in. That's two risk factors stacked in the same window: kidneys that may be slow to clear water, plus a big load of plain water with no minerals.
To be clear: fasting alone doesn't cause EAH. It's a real risk mainly in long or hot-weather training, not your 20-minute fasted walk. But fasting plus a lot of water plus long effort deserves a spot on your radar, instead of a blanket "drink more, it's always good for you."
What the Science Actually Recommends
Wilderness Medical Society guidelines are clear: no fixed fluid volume prevents EAH. The best approach is to drink to thirst, not on a schedule. And having real sodium during effort doesn't guarantee you'll avoid EAH if you're overdrinking. It's still the one variable you actually control to balance the water you take in.
In Practice
No miracle fix here, just common sense backed by the literature: drink when you're actually thirsty, not as an anti-hunger reflex. And when you do drink during a long fasted effort, pair it with real sodium, not a token pinch.
We didn't build Fasted Rebels to sell "wellness." We built it because fasting seriously while training hard means understanding your own physiology, not following a hashtag. Feed the mission. Not the hunger.
This article is for informational purposes only and doesn't replace medical advice. If you experience severe symptoms (confusion, seizures) during or after exercise, seek immediate medical attention.
Sources: StatPearls (NCBI Bookshelf, Exercise-Associated Hyponatremia), Wilderness Medical Society Updated Guidelines (AAFP 2021), Gatorade Sports Science Institute; Hew-Butler T et al., Frontiers in Medicine, 2017.
French version: Boire plus d'eau à jeun ne te protège pas toujours
Also read, the fasting and electrolytes series: Why You're Cramping Mid-Fast · Why You Get Dizzy Standing Up Mid-Fast · Why Your Heart Pounds Mid-Fast · Why You Twitch and Sleep Like Garbage Mid-Fast · Why You Get a Headache Mid-Fast · Why Your Uric Acid Spikes Mid-Fast · Why You're Always Cold Mid-Fast · Why Your Creatinine Spikes on a Blood Test · Do You Need Sugar to Get Creatine Into Your Muscles?