Endurance Diet: Energy Provision for Ultra-Distances, Metabolic Flexibility, and Nutritional Support for the Heart

1. Introduction and Relevance of the Topic

Endurance is the body's ability to resist fatigue over a prolonged period. In endurance sports (marathon running, triathlon, cycling, swimming), nutrition plays the role not merely of support, but of the primary limiting factor for performance. Even the most technically proficient athlete with a perfect cardiovascular system will fail to finish if their fuel tanks are empty or if their digestive system fails mid-race. An endurance diet is a delicate interplay between glycogen stores, the ability to oxidize the body's own fat, and the maintenance of water-electrolyte balance.

The relevance of this topic is driven by the rising popularity of ultra-marathons and amateur triathlons (Ironman). Many beginners attempt to apply general fitness nutrition principles to cyclical sports, often with disastrous results. High-protein diets without sufficient carbohydrates, ignoring electrolytes, and failing to "train the gut" to handle food during exertion are the primary mistakes. In this article, we will dissect the physiology of the athlete's "fueling" system, explore carbohydrate loading strategies, and learn how to make the body metabolically flexible—capable of utilizing both sugar and fat with maximum efficiency.

In endurance sports, your digestive system is your "second engine." If it stalls due to incorrect fuel, your primary engine (heart and muscles) will seize as well.

2. History and Evolution of the Issue

Human evolution is a history of endurance. We are among the few mammals capable of prolonged running in high temperatures, thanks to our sweating mechanism and upright posture. Our ancestors practiced "persistence hunting," chasing antelope for hours until the animals succumbed to heat exhaustion. Our metabolism is fine-tuned for long-duration, low-intensity work where fatty acids are the primary energy source.

In the history of modern sport, views on endurance nutrition have changed radically. In the 1960s, Gunther Karlsson invented the "classic carbohydrate loading" method (7 days: 3 days depletion, 4 days loading), which revolutionized marathoning. The 1980s brought the era of sports gels and isotonics, allowing athletes to maintain intensity for hours. Over the last decade, we have seen a renaissance of low-carb strategies (LCHF/Keto) for ultra-distances, where fat becomes the priority fuel. Today, we are in the stage of personalization: elite athletes utilize "carbohydrate availability periodization," training with low glycogen levels to stimulate mitochondria but competing on a high carbohydrate background.

3. Anatomy and Physiology of the Process

The provision of endurance is based on the cardiovascular system and cellular respiration.

Mitochondrial Density

These are the "power plants" of the cells where fuel is burned. Endurance training and proper nutrition increase the number and size of mitochondria in Type I muscle fibers (slow-twitch). The more mitochondria you have, the more efficiently you utilize fat for energy, preserving precious glycogen.

Stroke Volume and Capillarization

The diet must support blood plasma volume and viscosity. Thanks to the extensive network of capillaries that develops in response to load, oxygen and nutrients are delivered directly to every cell. A deficiency in iron or water instantly breaks this delivery chain.

Training the Gut

During prolonged running, blood is diverted from the stomach to the working muscles, making digestion nearly impossible. Endurance athletes must "train" their gut to absorb carbohydrates (up to 90g per hour) under load to avoid cramping and diarrhea.

Physiologically, endurance is limited by glycogen stores (approximately 2,000 kcal). Since a marathon requires 3,000+ kcal, an athlete's survival depends on their ability to tap into fat stores (which are nearly limitless).

4. Biochemical Impact on the Body

The biochemistry of endurance is a balance between aerobic glycolysis and fatty acid oxidation.

- **Fat Oxidation:** This is the most economical pathway. Biochemically, it requires more oxygen than burning carbohydrates. However, at loads up to 65-70% of VO2 max, a well-trained athlete can derive up to 80% of their energy from fat, allowing them to run "forever" without hitting a wall. - **Krebs Cycle and ATP:** All fuel types eventually convert into ATP. This process requires cofactors: magnesium, B vitamins, and iron. A deficiency in any of these causes a biochemical "bottleneck" in energy production. - **Electrolyte Balance:** Sodium, potassium, magnesium, and calcium ensure the transmission of electrical impulses from nerves to muscles. Salt loss through sweat without adequate replenishment leads to muscle cramps and heart rhythm disturbances (hyponatremia).

Biochemically, prolonged exertion also increases the level of free-radical oxidation, requiring enhanced support of endogenous antioxidant systems through the diet.

5. Practical Methodology and Technique

Constructing an endurance diet is based on the principle: "Carbohydrates for work, fats for the foundation."

1. **Carbohydrate Loading (Carbo-loading):** 36-48 hours before a race, increase carbohydrate intake to 10-12g per kg of body weight while minimizing fiber and fats. This allows you to "overfill" the muscles with glycogen. 2. **Intra-race Nutrition:** Start carbohydrate intake by the 15th minute. The goal is 60-90g of carbs per hour (a 2:1 mixture of glucose and fructose to utilize different transport pathways in the gut). 3. **Hydration Strategy:** Drink according to a schedule, not based on thirst. Use isotonics containing 400-800mg of sodium per liter of water. This prevents blood dilution and cramping. 4. **Carbohydrate Periodization (Train Low, Race High):** Conduct some morning low-intensity workouts with low glycogen levels. This stimulates the body to better oxidize fat. However, always perform key speed sessions on a "full tank."

You must eat before you are hungry and drink before you are thirsty. On the course, you are working ahead of your metabolic debt.

6. Progression of Loads and Integration into the Plan

In cyclical sports, progression is determined by the growth of your aerobic threshold and VO2 max. - **Glycogen Replenishment:** The faster you replenish stores post-workout, the more volume you can perform tomorrow. Consuming carbohydrates within the first 30 minutes after a run is mandatory. - **Heart Muscle Support:** An endurance athlete's heart requires vast amounts of energy and antioxidants. Coenzyme Q10, potassium, and magnesium in the diet ensure the stability of cardiac output. - **Body Weight Control:** Every extra kilogram of fat adds an oxygen cost to every step. However, excessively low weight leads to hormonal failure (RED-S syndrome). You must find your "fighting" fat percentage (typically 8-12% for men).

It is recommended to use "nutrition testing": every long weekend workout should be a rehearsal of your race-day diet. You must know how your stomach reacts to specific gels and bars at high heart rates.

7. Analysis of Scientific Research and Evidence Base

A study published in "Medicine & Science in Sports & Exercise" proved that adding fructose to glucose increases the rate of exogenous carbohydrate oxidation by 35-50% compared to consuming glucose alone. This became the foundation for modern sports nutrition formulations. Another study (Asker Jeukendrup) confirmed that athletes consuming carbohydrates during a marathon finish 15-20 minutes faster on average than those drinking only water.

The concept of "metabolic flexibility" is also scientifically supported. Research on elite ultra-marathoners (the FASTER project) showed that athletes on a fat-adapted diet can oxidize fat at a rate of 1.5g per minute—a record for human physiology. However, these same athletes lose the ability for a final sprint and high-intensity work due to decreased activity of carbohydrate-splitting enzymes. This underscores the importance of balance and periodization.

8. Synergy: Nutrition, Nutraceuticals, and Recovery

For the endurance athlete, the following are critically important: - **Iron (Ferritin):** Iron is the oxygen transporter. Even a minor deficiency lowers VO2 max. Pair red meat with Vitamin C. - **Nitrates (Beet Juice):** Proven to dilate blood vessels and improve mitochondrial efficiency, lowering the oxygen cost of exertion. - **Beta-Alanine:** Helps buffer muscle acidity during intense final sprints or uphill climbs. - **Omega-3s:** Reduce blood viscosity and improve red blood cell elasticity, facilitating their passage through the smallest capillaries.

The best synergy is consuming a portion of oatmeal with banana and honey 2-3 hours before a start, followed by a serving of caffeine and beet juice 30-45 minutes before. This provides the ideal energy and vascular start.

9. Common Mistakes, Myths, and Prevention of Injury

The biggest mistake is "race-day novelties." Never try a new product or supplement on the day of the competition.

• **Myth 1: "More water is better."** Excessive intake of plain water without salt leads to hyponatremia, which can cause cerebral edema and death. Salt is as vital as water.
• **Myth 2: "Protein during a run is unnecessary."** On ultra-distances (over 5 hours), adding a small amount of amino acids (BCAA) can reduce muscle breakdown and central fatigue.
• **Mistake 3: Avoiding carbohydrates in everyday life.** This leads to glycogen depletion and chronically elevated cortisol, which compromises the immune system.

If you feel persistent fatigue, shortness of breath at a normal pace, and have pale skin—immediately test your ferritin levels. Iron deficiency is the primary "killer" of endurance results.

10. FAQ: Answers to the Most Common Questions

Can I run a marathon on a keto diet?

You can, but your result will likely be worse than on carbohydrates. Keto is suitable for ultra-distances (100+ km) at a low heart rate where fat is the ideal fuel.

How much salt should I take during a race?

On average, 500-1,000 mg of sodium per hour, depending on your individual sweat salinity and ambient temperature.

Does caffeine help endurance?

Yes, caffeine lowers the Rating of Perceived Exertion (RPE) and mobilizes fatty acids. The optimal dose is 3-6 mg per kg of body weight one hour before the start.

What is "Bonking" (hitting the wall)?

It is a state of complete glycogen depletion where the brain begins to shut down muscles to preserve glucose for itself. The athlete feels sudden weakness and dizziness.

Are protein shakes useful after running?

Yes, but only alongside carbohydrates. The carbohydrate-to-protein ratio post-run should be 3:1 or 4:1.

How should I eat if I feel nauseous while running?

Switch to liquid nutrition, reduce intensity, and try drinking ginger tea. Nausea is often a sign of overheating or dehydration.

Do I need to take magnesium during a race?

It is better to build up magnesium stores in advance. During the race, sodium and potassium are priorities as they are lost much more intensely through sweat.