Summer Fueling and Hydration Guide for Running: What Actually Works in the Heat

Episode: S6E21 | Guest: Stevie Lyn Smith, RD, Stevie Lyn Nutrition | Aired June 25, 2026 | 1 hr 11 min

Author: Nick Klastava

Podcast Hosts: Nick Klastava & Amanda Katz

Listen: Spotify | Apple Podcasts | YouTube

Summer running comes with a whole different set of fueling and hydration demands. Coach Amanda and I sat down with registered dietitian and sports nutrition specialist Stevie Lyn Smith to break down exactly how to adjust your hydration, sodium, and carbohydrate strategy for hot and humid conditions, plus a deep dive into caffeine dosing and the truth about iron supplementation.

Key Takeaways

•             Proper hydration regulates body temperature and blood volume, and it becomes especially critical, though never irrelevant, in hot and humid conditions.

•             More electrolytes isn’t automatically better. The right amount is individual, based on your unique sweat rate and sodium losses.

•             A basic at-home sweat test (weigh yourself before and after a 60-minute run, accounting for fluid intake) gives a solid starting benchmark for hydration needs.

•             Heat increases your carbohydrate burn rate and can lower gut tolerance at the same time, making summer training the ideal window to practice race-day fueling.

•             Caffeine timing and dosing are highly individual. Practice both well before race day to avoid unpleasant surprises.

•             Chronically low ferritin is often a symptom of under fueling or excessive training load, not simply a signal to add more iron supplements.

Hydration Running Guide Doesn't Disappear in the Off-Season, But Summer Raises the Stakes

Proper hydration supports blood volume, body temperature regulation, and cardiovascular function year-round, but heat and humidity add real strain, which is why running feels harder even at an easy effort once temperatures climb. Sweat rate and sweat sodium concentration vary significantly between individuals, which is part of why generic “drink more electrolytes” advice can actually backfire. More sodium isn’t automatically better. Finding your appropriate amount based on your actual losses is what matters. This will help you make your summer running hydration guide.

How to Test Your Own Sweat Rate at Home

A simple, accessible sweat test: weigh yourself without clothes before a 60-minute run, track how much fluid you drink during that run, then weigh yourself again immediately after (skip the bathroom in between to keep the math simple). Each pound of weight lost equals roughly 16 ounces of sweat. From there, a general assumption of about one gram of sodium lost per liter of sweat gives you a reasonable starting range, and organizations like the Gatorade Sports Science Institute offer free calculators to do the math. The goal during a run is to replace roughly 50 to 75 percent of your fluid losses, adjusting upward in hotter, more humid conditions.

Why Carb-Based Sports Drinks Help Absorption

Carbohydrate-containing sports drinks (not the zero-calorie versions) trigger a sodium-glucose transport mechanism that pulls water into the body’s cells faster than plain water alone, while simultaneously delivering the energy your muscles need. In hot conditions specifically, your body burns through stored muscle glycogen faster because it’s working harder to cool itself, which is part of why staying on top of daily carbohydrate intake, not just during workouts, matters so much heading into summer training and racing.

Heat Lowers Gut Tolerance, So Practice Now

Heat redirects blood flow away from your digestive tract toward your skin to help cool your body, which can make fueling that felt easy in cooler months suddenly cause GI distress in summer conditions. The upside: since most races increasingly happen in warm conditions, summer training is a genuine opportunity to build confidence and gut tolerance for your actual race-day fueling plan, rather than something to dread.

Caffeine Dosing Is Individual, So Test It First

Caffeine can meaningfully reduce perceived pain and fatigue late in a race, but individual response varies enormously based on how quickly your body metabolizes it. Effects are generally felt within 15 to 45 minutes, peak between 30 and 90 minutes, and the average half-life runs three to five hours. The generally accepted max effective dose is 3 to 6 milligrams per kilogram of body weight, but tolerance is highly personal. Practicing caffeine dosing and timing in training, not for the first time on race day, prevents unpleasant surprises like heart palpitations from an unexpectedly strong dose.

Low Ferritin Usually Isn’t Just an Iron Problem

Chronically low ferritin is a common and frustrating issue for endurance athletes, but jumping straight to iron supplements or infusions often misses the underlying cause. Inadequate carbohydrate and overall caloric intake, high training volume without enough recovery, and inflammation can all impair how the body absorbs, utilizes, and stores iron. Iron infusions can even lead to iron overload, which carries its own uncomfortable symptoms and potential for organ strain, without necessarily fixing the root issue. Addressing training load and overall intake first, rather than treating supplementation as a permanent fix, tends to produce more lasting results.

Guest Bio

Stevie Lyn Smith, RD, board certified in sports nutrition, is the founder of Stevie Lyn Nutrition, a virtual private practice helping endurance athletes fuel for both performance and health. She is the co-author of “Power Up: A Young Woman’s Guide to Winning with Sports Nutrition” and hosts the Real Fuel with SLS podcast. She is also a triathlete and ultrarunner. Find her on Instagram at @stevielynlyn or at stevielynrd.com.

Frequently Asked Questions

How do I calculate my sweat rate as a runner? Weigh yourself without clothes before a 60-minute run, track your fluid intake during the run, then weigh yourself again immediately after (skip the bathroom to simplify the math). Each pound lost equals about 16 ounces of sweat. This gives you a personalized baseline for how much fluid to replace during future runs.

How much sodium do runners need during exercise? A common starting estimate is about one gram of sodium lost per liter of sweat, but individual needs vary widely based on your unique sweat rate and sweat sodium concentration. Symptoms like excessive salt residue on gear, headaches, or cramping can indicate you need more sodium than a standard sports drink provides.

Why is it harder to eat and digest food while running in the heat? Heat causes your body to redirect blood flow away from the digestive tract toward the skin to help regulate temperature, which can reduce gut tolerance and increase the risk of GI distress during exercise. Practicing your fueling strategy specifically in hot conditions helps build tolerance ahead of race day.

Should I take an iron supplement if my ferritin is low? Not necessarily without further investigation. Chronically low ferritin is often linked to inadequate carbohydrate or overall caloric intake, high training volume, or inflammation, rather than a simple lack of dietary iron. Addressing training load and intake first, alongside guidance from a qualified professional, often resolves the issue more sustainably than supplementation alone.