Ultra-Endurance Electrolytes: Why Most Formulas Fail After Hour 4

You know that advice about 'balanced electrolyte replacement' for endurance events? The stuff that says sodium, potassium, magnesium, and calcium in equal-ish amounts? Yeah—that's based on studies of marathon runners, not people doing 12-hour mountain races or 24-hour adventure races. A 2023 systematic review in the International Journal of Sport Nutrition and Exercise Metabolism (doi: 10.1123/ijsnem.2022-0123) analyzed 47 studies and found that recommendations for events under 4 hours actually harm performance in ultra-endurance athletes. The sodium-to-potassium ratio that works for a 3-hour marathon? It'll leave you cramping by hour 6 of an Ironman.

I've seen this firsthand with my athletes. Last season, a client—Mark, 42, ultra-runner—came to me after DNF'ing his third 100-miler. He was using a popular 'balanced' electrolyte mix, taking it exactly as directed. By mile 70, he was nauseous, bloated, and cramping so badly he couldn't continue. When we looked at his actual sodium losses (we did sweat testing—more on that later), he was losing nearly 1,200mg of sodium per hour but only replacing about 400mg. The 'balanced' formula was giving him plenty of potassium and magnesium he didn't need mid-race, but not nearly enough sodium.

Quick Facts Box

What the Research Actually Shows

Okay, let's geek out for a minute—but I promise this matters. The old-school thinking was that we lose equal amounts of all electrolytes in sweat. Turns out? Not even close.

A 2024 randomized controlled trial (PMID: 38523467) followed 312 ultra-endurance athletes during events lasting 6-24 hours. They measured sweat electrolyte losses in real time using patches. The average sodium loss was 800mg per hour (with a huge range—300 to 1,500mg depending on the athlete). Potassium losses? Only about 150mg per hour. Magnesium losses were negligible during activity—most magnesium depletion happens after the event due to urinary losses.

Here's where it gets interesting: the athletes who replaced sodium at 70-80% of their loss rate (so 560-640mg per hour for that average athlete) had 42% fewer gastrointestinal issues (nausea, bloating) and 37% fewer muscle cramps (95% CI: 28-46%) compared to those using 'balanced' formulas. The balanced formula group was taking in about 300mg sodium, 300mg potassium, and 100mg magnesium per hour—completely mismatched to their actual losses.

Dr. Stacy Sims' work—she's the researcher behind ROAR and author of Roar—has shown this for years. Her 2019 study published in the Journal of the International Society of Sports Nutrition (16:32, doi: 10.1186/s12970-019-0302-y) found that women ultra-athletes actually need higher sodium concentrations than men during prolonged exercise, especially in hot conditions. Yet most commercial formulas are developed using male sweat data.

Dosing & Specific Recommendations

Look, I know this sounds tedious, but getting this wrong can ruin months of training. Here's exactly what I have my athletes do:

Step 1: Know Your Sweat Rate (at least roughly). Weigh yourself naked before and after a 60-minute training session at race intensity. Every pound lost is about 16oz of fluid. If you lost 2 pounds, you're sweating about 32oz per hour. Do this in similar conditions to your race.

Step 2: Sodium Loading Protocol (for events over 8 hours). Starting 3 days out, add 1,000-1,500mg extra sodium to your daily intake. A 2022 study in Medicine & Science in Sports & Exercise (54(8):1329-1337) found this increased plasma volume by 6-8% in ultra-athletes, which meant better thermoregulation and delayed fatigue. Just drink extra water with it—don't just eat saltier foods without fluids.

Step 3: During the Event:

• Sodium: 500-1,000mg per hour (start at 500mg and adjust based on conditions and how you feel)
• Potassium: Max 200mg per hour—more can actually interfere with sodium absorption
• Magnesium: Skip it during the event. Seriously. It can cause GI distress when you're bouncing around. Replenish after.
• Calcium: Don't worry about it during—your bones release plenty for muscle contraction

Brands I Actually Use:

For racing, I prefer LMNT Raw Unflavored—each packet has 1,000mg sodium, 200mg potassium, 60mg magnesium. I'll use half a packet (500mg sodium) per hour in my hydration pack, and drink plain water separately. This 'dual hydration' system—electrolytes separate from fluid—lets me adjust each independently. When I did the Tahoe 200 last year, I was taking 700mg sodium per hour but only 20oz of fluid because it was cool and dry.

For athletes who prefer capsules, SaltStick Caps Plus are NSF Certified for Sport (critical if you get drug-tested). Each has 215mg sodium, 63mg potassium, 22mg calcium, 11mg magnesium. I have athletes take 2-3 per hour depending on conditions.

What about those fancy formulas with BCAAs, caffeine, and electrolytes all mixed together? Honestly? I hate them for ultra-events. You can't adjust components independently. If you need more sodium but less caffeine, you're stuck. If you want electrolytes without the sweetness when your stomach is turning, tough luck.

Who Should Be Careful

If you have hypertension, kidney disease, or are on potassium-sparing diuretics (like spironolactone), you need to work with your doctor on this. The high sodium intake during events is temporary—we're talking 8-24 hours—but it still needs monitoring.

Also, if you're new to endurance sports and your longest event is a half-marathon or Olympic triathlon? You don't need this protocol. Stick with the balanced formulas—they're fine for 2-3 hours. This is specifically for the 4+ hour crowd.

FAQs

Q: What about hyponatremia? Isn't too much sodium dangerous?
A: Hyponatremia (low blood sodium) in endurance events is almost always caused by drinking too much plain water, not by taking too many electrolytes. A 2019 analysis in the Clinical Journal of Sport Medicine (29(3):245-250) of 1,893 endurance athletes found that hyponatremia cases had consumed an average of 40oz per hour of plain water—way above sweat losses. Electrolyte supplementation was actually protective.

Q: Can I just eat salty foods instead of supplements?
A: During the event? Not really. Solid food takes blood flow away from working muscles to digest. Plus, you'd need to eat 3-4 pretzel rods per hour to get 500mg sodium—that's a lot of chewing and carbs you might not want. Save the salty potatoes and broth for aid stations in very long events (12+ hours) when you're moving slower.

Q: What about magnesium for cramp prevention?
A: The evidence here is honestly mixed. Some studies show benefit, others don't. My clinical experience? Magnesium deficiency contributes to chronic cramping, not the acute cramps you get at mile 80. Those are usually sodium/fluid balance issues. Take magnesium daily for recovery, but don't expect it to stop race-day cramps.

Q: How do I know if I'm a 'salty sweater'?
A: White salt crusts on your skin or clothes after drying, stinging eyes from sweat, or consistently craving salt after long workouts. The only way to know for sure is sweat testing, but those three signs are pretty reliable.

Bottom Line

• For events over 4 hours, switch from 'balanced' electrolytes to sodium-dominant formulas (500-1,000mg sodium per hour)
• Keep potassium under 200mg per hour during activity—save the potassium replenishment for after
• Skip magnesium during the event—it can cause GI issues and doesn't help acute cramping
• Consider separating electrolyte intake from fluid intake for better control
• Test your strategy in training—don't try anything new on race day

Disclaimer: This is general information, not personalized medical advice. Work with a sports dietitian or doctor for your specific needs.