Fueling Endurance: What the Science Actually Says

Whether you're logging trail miles in the foothills above Salt Lake City or training for your first race around the valley, endurance performance comes down to more than just putting in miles. According to The RP Diet for Endurance, how you manage body composition, fuel, and hydration has just as much impact on your race times as your training plan itself.

Leaner Isn't Always Better — But It Usually Helps

Endurance performance favors leanness more than almost any other sport, with optimal body fat ranges around 5–10% for men and 10–15% for women. But there's a limit — pushing leanness too far can compromise fueling and health. Muscle matters too, just less than in most other sports: extra muscle mass burns more oxygen and fuel per unit of distance and can actually hurt running, cycling, or swimming economy. The goal for endurance athletes isn't "more muscle" — it's just enough muscle to support performance without adding unnecessary metabolic cost.

Carbs Are Non-Negotiable

Carbohydrates are the primary fuel for nearly all endurance efforts, and the book recommends a simple approach: eat enough carbohydrate to cover the calories burned in training, on top of normal daily intake. Skimping here doesn't just cost you in the moment — under-fueled training leads to chronically low glycogen stores, which quietly erodes training quality over weeks. As a rough guide, a 60kg runner burns about 1 calorie per kilogram of body weight per kilometer covered, regardless of pace, which makes daily carb needs fairly easy to estimate and adjust for training volume.

Hydration Is About More Than Cramps

Contrary to popular belief, hydration's biggest role isn't preventing cramps — it's maintaining cardiovascular function and your body's ability to cool itself. Losing just 2% of body weight in fluid can measurably reduce cardiac output and performance. But overcorrecting with too much plain water carries its own risk: hyponatremia (diluted blood sodium), which shares symptoms with dehydration — headache, confusion, fatigue — making it easy to misdiagnose and treat incorrectly. The practical fix is straightforward: weigh yourself before and after training to estimate fluid loss, and replace it with 1 to 1.5 liters of fluid per kilogram lost, using a fluid with a modest carbohydrate and electrolyte concentration rather than plain water alone.

The Takeaway

Endurance training success isn't just about logging miles — it's about pairing that training with a body composition, fueling, and hydration strategy that supports it. Get those dialed in, and the mileage you're already putting in starts paying off faster.

If you're training for a race in the Salt Lake City area — whether it's your first 10K around Liberty Park or a big climb up in the Wasatch — dialing in your nutrition and training plan together makes a real difference. Reach out if you'd like a personalized plan built around your goals.

Reference

Harrison, A., Howe, M., & Davis, M. (2019). The RP diet for endurance: Fueling, hydrating, and managing electrolytes for performance. Renaissance Periodization.