Hydration in long trail running is not just about drinking water. It is about replacing enough fluid and sodium to stay functional as conditions, duration, and sweat losses increase. This episode explains why runners can drink plenty and still feel flat, thirsty, heavy, or cramp-prone when sodium intake does not match the demands of the session. It also unpacks the simple idea that water replaces volume, but sodium helps you retain and use that fluid effectively.

The episode then shows how this appears in real training, why many runners get it wrong by using the same plan in every condition or by drinking too much plain water, and how to build a more practical strategy based on weather, duration, and individual sweat loss.

Main takeaway: Hydration is not just water. On long runs, replace what you sweat, not just what you drink.

Key references:

1. American College of Sports Medicine. Exercise and Fluid Replacement.

2. World Athletics. Fluid Needs for Training, Competition, and Recovery in Athletes.

3. Gatorade Sports Science Institute. Hydration and Nutrition Considerations for Endurance Exercise in the Heat.

4. Gatorade Sports Science Institute. Sodium Ingestion, Thirst, and Drinking During Endurance Exercise.

5. Gatorade Sports Science Institute. Sweating Rate and Sweat Sodium Concentrations in Athletes: A Review of Methodology and Intra-/Interindividual Variability.

6. ACSM. 9 Facts About Hydration & Electrolytes.

7. Gatorade Sports Science Institute. Normative Data for Sweating Rate, Sweat Sodium Concentration, and Sweat Sodium Loss in Athletes.

8. Gatorade Sports Science Institute. Sodium-Free Fluid Ingestion Decreases Plasma Sodium During Exercise in the Heat.
   

HRV, or heart rate variability, is not a score that tells ultrarunners whether they are fit or unfit on a given day. It is a signal of how well the body is handling overall stress. That stress comes not only from training, but also from sleep, work, travel, fuelling, illness, and life outside running.

The key idea in the episode is that HRV is most useful when viewed as a trend, not as a single daily number. One low reading does not mean much on its own, but a drop over several days, especially alongside tired legs, poor mood, or bad sleep, can be an early warning sign that recovery is slipping.

The episode also explains that runners often misuse HRV by treating it like a green-or-red traffic light. Instead, HRV should be combined with how you feel, your sleep, resting heart rate, and how training is going. It can be especially useful for deciding whether it is the right day for a hard session or whether recovery should come first.

The main takeaway is this: don’t chase a perfect HRV score, use HRV to understand whether your body is absorbing training or just accumulating stress.Key references:

• Buchheit, M. (2014). Monitoring training status with HR measures: do all roads lead to Rome? Frontiers in Physiology.
• Esco, M. R., Flatt, A. A., Nakamura, F. Y., et al. (2025). Monitoring Training Adaptation and Recovery Status in Athletic Populations Using Heart Rate Variability. Sports Medicine / PMC review.
• Düking, P., Zinner, C., Reed, J. L., et al. (2021). Monitoring and adapting endurance training on the basis of heart rate variability monitored by wearable technologies: A systematic review with meta-analysis. Journal of Science and Medicine in Sport.
• Manresa-Rocamora, A., Sarabia, J. M., Javaloyes, A., et al. (2021). Heart Rate Variability-Guided Training for Enhancing Cardiac-Vagal Modulation, Aerobic Fitness, and Endurance Performance: A Systematic Review with Meta-Analysis. International Journal of Environmental Research and Public Health.
• Schmitt, L., Regnard, J., Parmentier, A.-L., et al. (2015). Monitoring Fatigue Status with HRV Measures in Elite Athletes: An Avenue Beyond RMSSD? Frontiers in Physiology.
• Sammito, S., Böckelmann, I., et al. (2024). Update: factors influencing heart rate variability – a narrative review. Frontiers in Physiology.
• Herzig, D., Eser, P., Omlin, X., et al. (2018). The Association Between Endurance Training and Heart Rate Variability: The Confounding Role of Heart Rate. Frontiers in Physiology.

Training fatigue is normal. Overtraining is not. And very often, what runners call “overtraining” is actually a mix of excessive load, poor recovery, and under-fuelling.

This episode explains the difference between normal fatigue, non-functional overreaching, true overtraining syndrome, and Relative Energy Deficiency in Sport, or RED-S. The key message is that not all fatigue is the same, and the solution depends on the cause.

For ultrarunners, the biggest risks often come from stacking hard training, life stress, poor sleep, and low energy availability for too long. Warning signs include persistent tiredness, loss of performance, poor recovery, low mood, repeated illness, hormonal disruption, and recurring injuries.

The practical takeaway is simple: monitor warning signs early, fuel properly for the work you are doing, and make recovery as deliberate as training. The goal is not to avoid fatigue, but to make sure it is recoverable.

Main takeaway: Your body does not adapt to training you survive. It adapts to training you can recover from.

Key references

• Meeusen et al. ECSS-ACSM consensus on overtraining syndrome.
• Mountjoy et al. 2023 IOC consensus statement on RED-S.
• Stellingwerff et al. Overtraining Syndrome and RED-S: shared pathways, symptoms and complexities.
• Saw et al. Subjective self-reported measures for monitoring athlete fatigue.
• IOC consensus on load in sport and risk of injury.