A reminder on how the bodies digestive system works - truefuels

A reminder on how the bodies digestive system works

The speed read: do you remember it all…?

Your digestion is a coordinated process that converts food into the energy and building blocks your body needs. For endurance athletes, careful nutritional planning fuels performance, supports recovery, and protects long-term health. Key elements include managing energy via carbohydrate intake, maintaining fluid and electrolyte balance, and supplying the right proteins and fats for muscle repair. Is always good to have a solid foundation of whole-food nutrition, but when used appropriately, supplements can add further benefits.

How digestion transforms food into energy

Digestion begins in the mouth, where chewing breaks down food into smaller pieces and salivary amylase initiates the breakdown of carbohydrates. Swallowed food then travels down the esophagus via peristalsis into the stomach. There, gastric acid and pepsinogen convert proteins into peptides and churn food into a semi-liquid called chyme.

Chyme enters the small intestine, where pancreatic enzymes and bile from the liver complete the breakdown of fats, proteins, and carbohydrates. Nutrients pass through the villi as tiny projections that line the intestinal wall and enter the bloodstream as glucose, amino acids, fatty acids, vitamins, and minerals (Furness, De Giorgio, & Costa, 2013). Any remaining material moves into the large intestine, where water and electrolytes are reabsorbed before waste is excreted.

From nutrients to fuel

Once absorbed, glucose circulates as the primary energy source for most cells, especially during moderate to high-intensity exercise. Excess glucose is stored as glycogen in muscle and liver for rapid release when demand rises (Sherman et al., 1981). Fatty acids—broken down from dietary and stored fats—provide a dense, slower-burning fuel. In contrast, amino acids from proteins support tissue repair or can be converted to energy when needed (Rolfe & Brown, 1997).

Vitamins, minerals, and water are essential co-factors for these processes, regulating enzyme activity, fluid balance, and nerve function (Thomas, Erdman, & Burke, 2016). Dietary fiber, though indigestible, promotes healthy transit and supports the microbiome, which in turn aids nutrient absorption.

Why minor differences matter for athletes

Endurance athletes place extreme demands on digestion and metabolism. Rapid carbohydrate absorption delays fatigue by maintaining blood glucose and sparing muscle glycogen (Sherman et al., 1981). Inadequate hydration impairs performance and increases the risk of heat illness (Sawka & Montain, 2000). Poorly timed or overly complex meals can provoke gastrointestinal distress, disrupting fueling during training or events (de Oliveira, Burini, & Jeukendrup, 2014).

After exercise, consuming a combination of protein and carbohydrates within 30–60 minutes accelerates glycogen resynthesis and muscle repair, thereby reducing next-day soreness (Betts & Williams, 2010). Micronutrients such as iron, calcium, and vitamin D support oxygen transport, bone health, and immune function, all areas often challenged by high training volumes (Thomas et al., 2016).

Ten Reasons Why Nutrition Planning Is Critical for Endurance Athletes.

  1. Energy balance: High caloric turnover demands precise fueling to avoid fatigue.
  2. Glycogen management: Maintaining and restoring stores sustains performance and readiness.
  3. Hydration & electrolytes: Fluid and salt losses affect thermoregulation and muscle function.
  4. GI tolerance: Simple, well-practiced meals minimize cramps and nausea.
  5. Recovery support: Consuming protein and carbohydrates promptly helps speed up repair and reduce soreness.
  6. Stable energy supply: Avoid spikes and crashes by matching glycemic response to exercise demands.
  7. Inflammation control: Foods rich in antioxidants and omega-3 fatty acids support recovery and contribute to long-term health.
  8. Immune resilience: Micronutrient sufficiency reduces the risk of infection during intense training.
  9. Mental focus: Balanced nutrition underpins concentration and decision-making.
  10. Injury prevention: Adequate bone and muscle-supporting nutrients reduce stress fractures and strains.

My five practical tips

  • Plan carbohydrate timing around key workouts and races to maintain blood glucose and delay fatigue.
  • Develop a hydration strategy based on your sweat rate, environmental conditions, and electrolyte needs.
  • Include high-quality protein within the first hour post-exercise to enhance muscle repair.
  • Keep fuel sources simple in sessions and events: practice with the exact foods and drinks you’ll use on race day.
  • Monitor micronutrients through diet or targeted supplementation (e.g., iron, vitamin D) to support performance and health.

References:

[1] Betts, J. A., & Williams, C. (2010). Short‐term recovery from prolonged exercise: What do we know? International Journal of Sport Nutrition and Exercise Metabolism, 20(6), 515–525. https://doi.org/10.1123/ijsnem.20.6.515

[2] de Oliveira, E. P., Burini, R. C., & Jeukendrup, A. E. (2014). Gastrointestinal complaints during exercise: Prevalence, aetiology, and nutritional recommendations. Sports Medicine, 44(Suppl 1), S79–S85. https://doi.org/10.1007/s40279-014-0153-2

[3] Furness, J. B., De Giorgio, R., & Costa, M. (2013). The enteric nervous system and gastrointestinal innervation: Integrated local and central control. Advances in Experimental Medicine and Biology, 817, 39–71. https://doi.org/10.1007/978-1-4939-0897-4_4

[4] Rolfe, D. F., & Brown, G. C. (1997). Cellular energy utilization and molecular origin of standard metabolic rate in mammals. Physiological Reviews, 77(3), 731–758. https://doi.org/10.1152/physrev.1997.77.3.731

[5] Sawka, M. N., & Montain, S. J. (2000). Fluid and electrolyte supplementation for exercise heat stress. American Journal of Clinical Nutrition, 72(Suppl 2), 564S–572S. https://doi.org/10.1093/ajcn/72.2.564S

[6] Sherman, W. M., Brodowicz, G. R., Wright, D. W., Allen, W. K., Simonsen, J. J., & Hartmann, B. R. (1981). Effects of exercise and carbohydrate feeding on muscle glycogen utilization in humans. Journal of Applied Physiology, 51(3), 940–944. https://doi.org/10.1152/jappl.1981.51.3.940

[7] Thomas, D. T., Erdman, K. A., & Burke, L. M. (2016). Nutrition and athletic performance. Medicine & Science in Sports & Exercise, 48(3), 543–568. https://doi.org/10.1249/MSS.0000000000000852

 

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