Picture this: you finish a five-hour training day. Maybe that was last Saturday. Maybe it's the day you're reading this. Maybe you've never (ever) done that in your life, and the thought of 300 minutes of physical exertion is menacing.
Maybe it's a long ride followed by a brick run. Maybe it's a double or triple day in the middle of a training camp - heck, maybe it was your last 70.3 race!
You nailed your carbohydrate intake. Your glycogen stores are depleted, but manageable - you might be able to somewhat function for the remainder of the day (walk the dog, go to the fridge, watch some tv, go to the fridge again...). But there's another question lurking underneath:
What happened to your muscles while all that was going on?
A classic study by Koopman and colleagues tackled exactly that question, examining what happens to protein balance during six hours of prolonged endurance exercise.
The answer?
Long sessions don't just burn carbohydrates and fat - they challenge protein balance, too.
Carbs and Fat Still Run the Show
Let's get this out of the way first - carbohydrates and fat remain the primary fuels during endurance exercise - this hasn't changed. But amino acids aren't sitting on the sidelines. Research shows that protein oxidation is measurable during endurance exercise, and as sessions get longer (or glycogen stores become depleted), the contribution of amino acids can become more meaningful.
That's important because endurance athletes often think of protein as something reserved for after the workout. The science suggests the story may be more nuanced.
The Study: Six Hours of Endurance Exercise
Koopman et al. (2004) recruited eight trained male endurance athletes and put them through a brutal protocol:
- 2.5 hours cycling
- 1 hour running
- 2.5 more hours cycling
A total of six hours of continuous exercise.
The athletes consumed either:
- Carbohydrate alone, or
- Carbohydrate plus protein hydrolysate
The protein trial wasn't low-carb, it kept carbohydrate intake the same and added protein on top. That distinction matters, especially within the context of understanding the analysis of this study.
The carbohydrate-only group still finished the session with a negative protein balance. In other words: they fueled the work, but they couldn't fully offset the protein breakdown occurring during exercise.
The carbohydrate-plus-protein group? Their whole-body protein balance improved. The addition of protein reduced the extent of protein loss during the session, not because carbs failed, but because protein provided additional support during an unusually long bout of exercise.
What the Protocol Actually Looked Like
The athletes consumed:
- 0.7 g carbohydrate per kilogram of body weight per hour
- 0.25 g protein hydrolysate per kilogram per hour
- Drinks were administered every 30 minutes
For a 72 kg athlete, that's roughly:
- 50–55 grams of carbohydrate per hour
- 18 grams of protein per hour
Remember: this wasn't a strategy designed for a 60-minute threshold run, no, this was six straight hours of work. Showcasing that, the longer the session, the more the conversation around protein becomes relevant.
What the Study Didn't Show
This is where things get interesting.
The study showed:
✅ Improved whole-body protein balance.
It did not show:
Better next-day performance
Reduced soreness
Superior race outcomes
The ideal timing strategy for every athlete
Science is often less dramatic than social media - but that's okay! Improving protein balance during long sessions gives athletes a better starting point for recovery, and for athletes stacking big days together, that matters.
Add Protein to Carbs. Not Instead.
This may be the most important takeaway of all. The protein group didn't replace carbohydrate; they kept carbohydrate intake the same and added protein.
That's an important distinction because endurance sports still run on carbohydrates. Protein isn't a substitute for fuel, but a complement.
Fuel the work. Support recovery.
The two ideas aren't competing. They're teammates, working together for a shared goal.
Why This Matters for Real Athletes
If you're:
- Racing an Ironman
- Riding for five hours on the weekend
- Stacking long sessions during training camp
- Training twice or three times a day
...protein during or immediately after exercise may help support the recovery process before you even finish the session.
Not because protein suddenly became fuel, but because long endurance work challenges more than your glycogen stores. These all challenge your ability to maintain muscle protein balance, too. So....
Long sessions change the equation.
Carbohydrates remain king.
But for prolonged endurance exercise, adding protein alongside carbohydrates may help support whole-body protein balance and put you in a better position to recover afterward.
Fuel the work. Support protein balance.
Koopman R, Pannemans DL, Jeukendrup AE, Gijsen AP, Senden JM, Halliday D, Saris WH, van Loon LJ, Wagenmakers AJ. Combined ingestion of protein and carbohydrate improves protein balance during ultra-endurance exercise. Am J Physiol Endocrinol Metab. 2004 Oct;287(4):E712-20.
