Endurance Nutrition FAQs: Iron Levels, Carbs Per Hour and Stinky Sweat

Every endurance athlete wants to perform at their best, but some nutrition factors can make or break your training. In this episode of the Endurance Eats podcast, the ALN team of expert dietitians tackle three frequently asked questions, from iron levels to the signals your sweat may be sending you.

• How do iron levels impact endurance performance, and how can you check if yours are low?
• Should carb intake be based on energy expenditure during exercise?
• Why does sweat sometimes smell like ammonia, and what does it mean for your nutrition?

1. Iron Levels & Endurance Performance: Why They Matter

Iron is one of the most overlooked factors in endurance nutrition. It plays a critical role in oxygen transport—meaning if your levels are low, your performance will suffer.

📌 Symptoms of Low Iron in Athletes:

• Persistent fatigue
• Poor recovery from workouts
• Heavy legs during training
• Increased susceptibility to illness
• Lightheadedness or dizziness
• Mood changes & lack of motivation

Many endurance athletes struggle with low iron due to:

• Red blood cell breakdown from high-impact sports (like running)
• Sweat and urine losses
• Inadequate dietary intake (especially for plant-based athletes)
• Menstrual blood loss in female athletes

💡 How to Get Your Iron Levels Checked

Most standard blood tests do not include ferritin, which measures iron storage. You’ll likely need to specifically request a ferritin test from your doctor. If you get pushback, there may be companies in your area that you can get labs done for a reasonable price. In Minnesota, we have Any Lab Test Now which offers a standalone ferritin test for around $40.

🎯 Optimal Ferritin Iron Levels for Endurance Athletes:

• In our experience working with endurance athletes, we find that 50+ ng/mL is ideal for sustained performance
• Anything below 30 ng/mL can lead to performance declines
• If your levels are low, dietary changes and supplementation may be necessary

📌 Iron-Rich Foods to Include:

• Heme iron (best absorbed): Beef, poultry, shellfish
• Non-heme iron (plant-based): Lentils, spinach, fortified cereals

👉 Pro Tip: Pair iron-rich foods with vitamin C (like citrus or bell peppers) to enhance absorption, and avoid taking iron with calcium or caffeine, which can block absorption.

If supplementation is needed, work with a dietitian to choose the right type—some iron supplements cause GI distress, and timing your intake matters.

⏳ How Long Does It Take to Replenish Iron?

It can take 6-12 months of consistent iron intake to restore optimal ferritin levels. Be patient and track progress through regular lab work.

2. Should You Calculate Carbs Per Hour Based on Energy Expenditure?

This is a question we get often, especially from athletes using sports watches and power meters to estimate calorie burn.

💡 The short answer? No, most everyday athletes do not need to calculate carbs based on precise energy expenditure.

For elite cyclists and professional endurance athletes, fueling can be calculated down to the gram. However, for most endurance athletes, a simpler approach works best:

🎯 General Carb Guidelines for Endurance Training:

• 60-90g carbs per hour = A good target for most endurance athletes
• Above 90g per hour = Beneficial for high-powered cyclists, long-distance runners and triathletes.
• 120g+ per hour = Only for athletes who have trained their gut for extreme fueling

📌 Why Sports Watches Aren’t Reliable for Fueling

Research shows that sports watch calorie burn estimates can be up to 40% inaccurate due to factors like:

• Age, gender, and body composition
• Fitness level & efficiency of movement
• Temperature & altitude
• Menstrual cycle phase

Rather than relying on estimates, athletes should use the general guidelines and gradually increase their carb intake during training.

💡 How to Know When to Increase Carbs Per Hour

• Still feeling hungry mid-workout? You may need more.
• Struggling with recovery or energy crashes? Increase intake.
• GI distress? Slow down adjustments and test different fuel sources.

One of our athletes started at 90g per hour but still felt sluggish. We gradually increased him to 120g per hour—resulting in stronger training sessions, better recovery, and a much stronger Ironman race.

Key takeaway: Instead of obsessing over calorie burn, focus on fueling consistently, testing out different carb amounts, and fine-tuning what works best for your body.

3. Why Does My Sweat Smell Like Ammonia?

Ever finished a workout and noticed your sweat smells like cat pee? This ammonia-like odor can be a sign that your body is burning protein for fuel instead of carbohydrates.

🚨 What Causes Ammonia-Smelling Sweat?

• Low carbohydrate intake
• Poor hydration
• High protein breakdown during exercise

When the body runs out of stored glycogen, it starts breaking down amino acids (protein) for energy, leading to the production of ammonia. This ammonia is then expelled through sweat, creating that distinct smell.

💡 How to Prevent Ammonia Sweat

• Increase carbohydrate intake—before, during, and after training
• Hydrate properly—dehydration concentrates sweat and can amplify the smell
• Eat balanced meals—aim for a mix of carbs, protein, and fat to support recovery nutrition.

If ammonia-like sweat is a persistent issue, it may also be worth checking kidney function and overall protein intake with a healthcare professional.

Final Thoughts: Dialing in Your Endurance Nutrition

These three topics—iron levels, carb intake, and ammonia sweat—all highlight how important proper fueling is for endurance performance.

💡 Key Takeaways:

• Check your ferritin levels annually to prevent iron-related fatigue
• Skip complex fueling math—start with 60-90g of carbs per hour and adjust based on feel
• Ammonia-smelling sweat? Eat more carbs and hydrate better

Feeling overwhelmed with endurance nutrition? My team and I specialize in helping endurance athletes fuel smarter. If you’re ready to optimize your nutrition and maximize performance, head to Alex Larson Nutrition and apply for one-on-one coaching today.