I bonked at mile 18 of my third marathon. Not a slow-down bonk — a sit-on-the-curb, can’t-feel-my-quads bonk. I’d been drinking water religiously, hitting my gels, doing all the things you read in the running magazines. What I hadn’t done was the actual sodium math.

Most runners — most outdoor athletes, most people who sweat through hot summer afternoons — are operating on a hydration plan they got from a vague magazine article in 2008. Drink half your bodyweight in ounces.Sip every fifteen minutes.Listen to your thirst. All directionally fine. None tells you the actual number that matters: how much sodium you’re losing per session, and what that looks like in real product terms.

Here’s the math I wish I’d had at mile 18. And the routine I run now.

The short answer: the average adult loses 500-1,500mg of sodium per liter of sweat, depending on heat, exertion level, and individual sweat composition. A “salty sweater” can hit 2,000mg per liter. Over a hot 90-minute run, that’s 1,000-3,000mg of sodium gone — more than the entire RDA for an entire day, in 90 minutes of exercise.

The longer answer involves understanding why sodium is the bottleneck specifically (vs. potassium, magnesium, or just plain water), and what to actually do about it on different exertion days.

Why Sodium Is The Bottleneck

Sweat is mostly water, but the dissolved minerals matter more than the volume for performance. Of the major electrolytes you lose:

• Sodium: 500-1,500mg per liter of sweat (highest concentration)

• Potassium: 150-300mg per liter

• Calcium: 30-100mg per liter

• Magnesium: 5-20mg per liter

Sodium dwarfs the others. It’s also the one your body uses to regulate fluid balance — without it, the water you drink isn’t held in your bloodstream and tissues. You can drink a gallon of plain water during a long run and still be functionally dehydrated if your sodium has dropped below the threshold where your body can hold onto fluid. Worse: drinking a lot of water without sodium can push you into mild hyponatremia — low blood sodium — which is more dangerous than dehydration in serious cases.

This is why “just drink water” is the wrong advice for anyone exercising more than an hour in heat. The water without the sodium is a bigger problem than the deficit alone.

The Sweat Sodium Self-Test

Easy field test: do a 60-90 minute workout in your normal training clothes. After, look at the front of the shirt.

• No visible salt rings → you’re a low-sodium sweater (~500mg/L). Standard hydration works.

• Light salt residue → average sweater (~700-1,000mg/L). Add electrolytes for sessions over 60 min.

• Visible white salt crusts → high-sodium sweater (~1,200-1,500+mg/L). Electrolytes for any session over 30 min.

• Tastes salty when you lick your skin → confirmed high-sodium. Replace aggressively.

Most outdoor athletes underestimate their sodium loss by half. When in doubt, assume you’re losing more than you think.

The Actual Replacement Math

For a 60-minute moderate workout in moderate heat, the average athlete needs roughly:

• 400-700mg sodium

• 150-300mg potassium

• 30-60mg magnesium

• 400-600ml of fluid

For a 90-minute hard workout in real heat (above 85°F), double those numbers. For a 3-hour event in serious heat, you’re talking 2,000+mg of sodium and 1.5L+ of fluid.

Most popular sports drinks fall woefully short on the sodium math. Standard Gatorade has 160mg of sodium per 12oz bottle — you’d need 4-5 bottles to cover a moderate hour, and you’d consume 80g+ of sugar in the process. LMNT goes the other direction: 1,000mg of sodium per stick is overkill for most sessions and uncomfortable to drink during effort.

The sweet spot — for anyone who isn’t an ultramarathoner — is 400-600mg of sodium paired with the full electrolyte profile (potassium, magnesium, calcium) at roughly the ratios you actually lose them. Honestly, very few products are dosed for the realistic athlete.

The one I keep coming back to is VitaWild.