Carbohydrates or Protein: What Fuels Endurance Best?

 

Completing a half marathon is a goal many set for themselves. The training begins with excitement and discipline, but soon diet questions appear. A friend tells you to increase protein. Another claims carbohydrates are essential for endurance. The choice seems simple, yet the science is more precise.

Energy Inside the Muscle

Every step begins with contraction. Inside the muscle, actin and myosin fibers connect to create force. This action depends on adenosine triphosphate, known as ATP. Muscles only hold enough ATP to last about three seconds. The body must then replace it.

Three systems provide that replacement.

1. Creatine phosphate system. It makes ATP quickly but only for ten seconds. Useful for weightlifting, not for long races.

2. Glycogen–lactic acid system. Carbohydrates stored as glycogen are broken into glucose. With little oxygen, glucose becomes lactic acid. This fuels about ninety seconds of work but leaves muscles heavy with fatigue.

3. Aerobic system. Glucose is broken down with oxygen inside mitochondria. It produces large amounts of ATP for as long as glycogen and fat remain available. Protein can also be used, but the process is slow and inefficient.

In order, the body uses glycogen first, then fat, and turns to protein only when the others run low.

Comparing Diets in Endurance Events

Scientists compared high carbohydrate, high fat, and mixed diets. The results reveal a clear difference.

• On a mixed diet, athletes begin with carbohydrates but switch to fat as glycogen falls. Exhaustion comes in about two hours.

• On a high fat diet, glycogen stores are small. Fatigue arrives early, around ninety minutes.

• On a high carbohydrate diet, glycogen reserves remain high. Even after one hour, most energy still comes from carbohydrates. Athletes may not reach exhaustion until four hours into activity. That works out to almost double the time of a mixed diet.

Dr. Fareha Jamal, a Doctor of Pharmacy and research associate at BioNTech, explained to me why the body favors carbohydrates during endurance work: “The aerobic system is powerful, but it still relies heavily on glycogen to get started. Athletes who load properly with carbohydrates create a buffer that keeps fatigue away for much longer.”

Recovery After Training

Endurance is not only about the event. Recovery decides how the body performs next time. Athletes on fat or protein-heavy diets take longer to restore glycogen. In contrast, high carbohydrate diets refill muscle stores quickly within forty hours.

Protein does serve a purpose. It does not increase endurance, but it reduces muscle damage and helps with soreness. When paired with a high carbohydrate intake, protein supports faster recovery.

Maryam Jamal, a fifth-year medical student, told me why this matters: “Runners often forget that soreness is not just discomfort, it is also a sign of microscopic damage to muscle fibers. Carbohydrates repair the fuel, and protein repairs the structure. Ignoring one of them slows down the entire recovery process.”

Three Lessons for Runners

Research leaves three lessons for anyone preparing for a half marathon.

• Focus on carbohydrates. Aim for 6–10 grams per kilo of body weight each day.

• Allow recovery time. Muscles need 48 hours to rebuild glycogen stores, so avoid intense sessions two days before a race.

• Add protein for recovery. A useful intake is 0.25 grams per kilo of body weight per hour of endurance exercise.

---

The conclusion is not dramatic. It is really the carbohydrates that carry you through the race. Protein helps you recover after it. As my daughters remind me, both belong in the story of endurance. The balance between them may be the quiet answer runners have been searching for.