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How to properly fuel your endurance training? | #14 ft. Aaron Geiser

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In this episode, Aaron and I discuss the importance of substrate utilization in optimizing athletic performance. We highlight the role of gastrointestinal distress as a major cause of DNF in endurance races and the limitations of carbohydrate absorption. We emphasize the need to practice nutrition strategies and the uncertainty of optimal carbohydrate intake. We discuss the benefits of zone 2 training for fat utilization and the individualization of substrate utilization. The importance of metabolic testing and race day nutrition planning is emphasized, along with the impact of caloric intake on performance and recovery. Aaron shares his typical nutrition protocol for long bike rides, highlighting the importance of individualized nutrition strategies.


Gastrointestinal distress is a major cause of DNF in endurance races, highlighting the importance of proper nutrition strategies.

Carbohydrate absorption is limited, and individual differences in absorption capacity should be considered when planning nutrition during exercise.

Zone 2 training promotes fat utilization and can improve endurance performance.

Metabolic testing can provide valuable insights into individual substrate utilization and guide nutrition planning.

Caloric intake should be sufficient to support performance and recovery, and individual needs may vary.

Individualized nutrition strategies are crucial for optimizing athletic performance.

If you have a question or topic you want us to address, leave a comment!

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00:00 – Introduction to Substrate Utilization

The hosts introduce the topic of substrate utilization, emphasizing its impact on athletic performance. They touch on the significance of understanding what the body utilizes as fuel during exercise.

01:12 – Meta-Analysis on GI Distress in Endurance Races

A discussion about a meta-analysis involving individuals who did not finish Ironman races due to gastrointestinal distress. It highlights the surprising frequency of GI issues affecting performance.

02:21 – Carbohydrate Consumption and GI Distress

Exploration of how excessive carbohydrate intake during races can lead to stomach problems and how practicing a fueling strategy is crucial to avoid such issues.

04:00 – Uncertainties in Carbohydrate Absorption

Addressing the ambiguity surrounding the maximum carbohydrate absorption rate and the challenges in accurately measuring individual absorption capacities.

05:03 – Nuances in Carbohydrate Utilization

Discussing the limitations of established carbohydrate intake guidelines and the ongoing uncertainties about actual absorption percentages during exercise.

07:02 – Training Adaptations and Fuel Sources

Explaining the body's use of protein, fats, and carbohydrates as fuel sources during exercise, highlighting how different intensities affect substrate preferences.

11:00 – Fat Max: Clarification on Fat Oxidation

Explaining the concept of "Fat Max," the point where the body maximizes fat oxidation but still relies significantly on carbohydrates for energy production during exercise.

14:08 – Benefits of Zone 2 Training

Delving into the advantages of training in Zone 2, emphasizing the improvement in fat-burning efficiency and its impact on endurance athletes.

17:05 – Mitigating Inflammation and Adaptation

Exploration of how training in fat-burning zones may potentially reduce inflammation and enhance recovery due to the utilization of fats as a fuel source.

20:08 – Individual Variability in Fuel Requirements

Acknowledging the highly individualized nature of dietary needs during exercise and the importance of individual experimentation and metabolic testing to determine optimal fuel strategies.

22:21 – Importance of Metabolic Testing

This section delves into the significance of metabolic testing in calibrating an individual's physiology for training purposes. It discusses the value of understanding different zones, substrate utilization, and the impact of factors like pre-testing conditions on outcomes.

23:33 – Experimenting with Substrate Utilization

The conversation revolves around the suggestion of conducting substrate utilization tests in various conditions (fasted, carbo-loaded, post-recovery) to gauge individual energy use. It emphasizes the importance of understanding these dynamics for training and race day optimization.

25:19 – Adjusting Athlete Nutrition

This part focuses on athlete nutrition strategies, considering different carb intake levels based on the athlete's needs and race scenarios. It highlights the importance of understanding personal nutrition needs for optimal performance.

27:51 – Managing Carbohydrate Absorption

Discussing the limitations of carbohydrate absorption per hour during intense activities like Ironman races, this section emphasizes the necessity of planning nutrition and energy sources for sustained performance.

30:03 – Fat Adaptation for Endurance

The discussion touches upon the advantages and limitations of being a fat-adapted athlete in longer races like Ironman, detailing how maximizing fat utilization can provide prolonged energy.

31:56 – Consequences of Calorie Deficits

This part highlights the repercussions of calorie deficits on an athlete's health, recovery, and performance, stressing the importance of proper calorie intake for sustainable training and overall well-being.

37:21 – Nutrition Protocols for Training

Detailed nutrition protocols during different training phases, outlining fasting periods, introduction of carbohydrates, and preferences for certain types of carbohydrates based on race distance and intensity.

41:13 – Personalized Carbohydrate Intake

Addressing the highly individualized nature of carbohydrate intake during training, it details the amount and timing of carbohydrate consumption per hour, tailored to the individual's needs.

42:57 – Fat Adaptation and Performance

This section discusses the duration it takes for an individual to become more fat-adapted and the distinctions between long-term adaptations and short-term studies on fat oxidation.

44:36 – Fasted State Exercise and Carbohydrate Intake

Discussion about exercising in a fasted state, targeted carbohydrate intake during higher intensity workouts, and the importance of managing glycogen levels.

47:59 – Post-Workout Refueling Strategy

Exploring the post-exercise refueling strategy focusing on protein intake, meal choices, and considerations for sustained energy levels after long workouts.

53:16 – Approaches to Carbohydrate Intake

Differentiating between low-carb diets, the significance of mindful carbohydrate choices, and the impact on overall health and performance.

54:52 – Balancing Fuel Sources and Body Composition

Discussion on fuel sources emphasizing fat utilization, adequate protein intake for recovery, and caloric balance for performance and health.

1:00:04 – Understanding Macronutrients and Caloric Balance

Exploration of macronutrient breakdown, misconceptions around high-carb and high-fat diets, and the significance of tracking food intake for better nutritional choices.

1:03:28 – Challenges in Tracking Fresh Foods

Challenges associated with tracking fresh foods versus processed foods and the importance of awareness in understanding nutritional intake.

1:05:09 – Exercise, Health, and Nutritional Awareness

Reflection on the relationship between exercise, health, and long-term impacts of dietary choices, emphasizing the importance of informed eating habits.

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