Boston Marathon
The oldest annual marathon. Cool spring conditions, qualifying-only field, downhill quad-trashing first half, Newton Hills back half.
Course Profile
Where The Sodium Math Bends
Schematic profile · Cramping windows tend to cluster around major climbs and the descents that follow them
Climate Window
40-65°F · moderate humidity
Climate
cool
Humidity
moderate
Cutoff
6h after final wave start
Why This Race Is Hard For Sodium
The Cumulative Deficit Window
Boston is a cool-weather marathon where the standard advice ("you need less sodium when it is cool") is wrong for a large fraction of the field. BAA qualifiers are typically lean, well-trained, and high sweat-rate athletes. Many of them are also salty sweaters. The Newton Hills late in the race (miles 16 to 21) are where sodium-driven cramping most often hits, particularly the climb up Heartbreak Hill. Most athletes overdrink water in the first 10 miles when they feel cool, then run a sodium deficit into the back half.
Key Considerations
- Salty sweaters should treat Boston like a warmer race for sodium intake. Cool ambient temperature does not lower sweat sodium concentration.
- The downhill first 16 miles is deceptively heat-generating; quad work raises core temperature regardless of air temp.
- Hyponatremia risk is real at Boston due to ambitious mid-pack runners drinking aggressively in cool weather. Stick to fluid targets.
- Pre-race sodium loading the night before (salty dinner) supports plasma volume before a cool-weather start when thirst signals are blunted.
Plan the five systems
Free Tools, Pre-Filled For Boston
Tap any tool. We pre-load your event, climate, and sweat profile. Adjust your weight and finish target and the plan generates instantly.
Sodium plan
Per-hour sodium and fluid targets for Boston conditions.
Carb fueling
Hourly carb targets and a fuel menu calibrated to your weight and the Boston duration.
Heat protocol
10-14 day adaptation plan if you're racing into a warmer climate.
Hydration audit
Weigh-in test that converts a 60-minute training session into a measured sweat rate.
Caffeine timing
Pre-race dose, mid-race top-ups, and race-week taper protocol for Boston timing.
Your Boston Plan,
Built From Real Data.
Pro unlocks: distribution of sodium intake per hour for sub-3, sub-3:30, and sub-4 BAA finishers, your historical April long-run sweat rates from Strava, and a mile-by-mile schedule that pre-doses for the Newton Hills.
- Measured sweat rate from a Strava ride or weigh-in test
- Per-leg sodium schedule keyed to the course profile
- Multi-event race calendar across the season
- Post-race feedback capture so the next plan is sharper
Per race
Best value
Unlimited race plans, Strava connection, multi-event calendar.
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Common Questions
About Boston
Do I need less sodium because Boston is cool?
Not necessarily. Your sweat sodium concentration is a fixed individual trait. Cool weather lowers total sweat volume but does not change the mg per liter you lose. If you are a salty sweater, your hourly target stays high even at 50F.
When in the race should I take my pre-loaded sodium?
A 600 mg dose 60 minutes before the start, paired with 16-20 oz of fluid, supports plasma volume through the first hour. The first on-course intake should happen within the first 30 minutes of running, before thirst signals appear in cool conditions.
How much fluid should I take in cool Boston weather?
For most athletes targeting a sub-4 hour finish, 16 to 20 oz per hour is the working window. Resist the urge to drink at every water stop in the first 8 miles; cool-weather overdrinking is the leading hyponatremia cause at Boston.