You’re Burning Protein on Every Long Ride (And Most Athletes Ignore It)

Somewhere around hour three, or maybe hour four-to-five for some of the extremely elite, things start to shift.

The ride settles in a bit more to you than you’ve settled into it; the numbers are still there, but they take more intention. All of a sudden, that power you held for 3x20’ in hour two is tougher to sustain, marginally, all enveloped by a general sensation that your legs feel a touch heavier.

To compensate for your newfound loss of sharpness, fueling gets more deliberate. Every sip, every gel starts to matter just a bit more.

You’re still moving well. But you’re not quite the same athlete you were in the first hour - most of us have a general understanding of what’s happening here.

We think:

• Glycogen is dropping
• Fat oxidation is rising
• The system is under load

That’s all true. To get to this point in the ride, carbohydrates were basically non-negotiable.

But there’s another layer to it - one that rarely gets talked about in endurance-nutrition circles:

You’re burning protein, too.

The Third Fuel Source Most Athletes Forget

Endurance athletes are taught to think in two fuels: carbs and fat.

Carbs for performance – ‘intense efforts are best fueled with quick bouts of glucose’ is commonly recognized as a fact in modern-day fueling, for good reason.
Whereas fat is utilized more so for endurance – prioritizing a lower carbohydrate, higher fat content intra and extra exercise to support more of a steady burn for ultra-endurance events.

It’s clean, it’s simple, and can be dialed in with minimal input by thousands of athletes every year. To repeat – it’s simple, and it works.

But it’s incomplete.

Because during endurance exercise, your body is also breaking down amino acids and oxidizing protein for energy. Not as the primary driver—but as a consistent contributor. Like a slow drip from an IV, protein quietly is utilized by the body as well during endurance activity.

And as the session goes on, that contribution increases.

Protein oxidation rises when:

• Duration extends
• Glycogen availability drops
• Training load accumulates

In other words, the exact moments where training starts to matter most – three hours into the ride, on hour five of your Ironman, or deep into the miles of your long run. These are the moments where this becomes more relevant. Let us explain more.

A Small Percentage That Adds Up Fast

A 2025 systematic review and meta-analysis found that:

~3.28% of energy during endurance exercise comes from protein oxidation.

At first glance, that number feels easy to ignore – 3% doesn’t feel like all too much, right?

That mindset is actually incredibly counterintuitive to the very principles 2026 endurance sports training, nutrition, and performance are built upon. Just think about how super shoes – the carbon-plated phenoms of the running world that took the industry by storm in the late 2010s – tout to give a 1-4% boost in performance on race day (Kobayashi et al., 2026). Why, then, would we knowingly have protein seep out of our system and not neutralize that loss in energy (which, transitively, could offset the marginal gain of something like a carbon-plated shoe).

High-performance running, cycling, swimming, and more are not built on single moments; rather, they’re built on accumulation. And when you zoom out across hours, sessions, and weeks, that small percentage starts to carry real weight that does more than just impact your training load.

Making it Real – With Numbers

Take a fairly typical session, where a well-trained endurance athlete might burn

• ~700 kcal/hour
• ~3.28% from protein

That comes out to:

• ~23 kcal per hour from protein
• ~5–6 grams of protein per hour

Now stretch that across a long ride:

• 4–5 hours → 20–35 grams of protein oxidized

That’s not just background noise, but a meaningful amount of substrate being used—quietly, steadily, the entire time you’re training. And, if you’re a larger athlete, or riding at higher intensities where total energy demand climbs, that number only goes up.

Where Most Athletes Get It Wrong

Most endurance athletes are dialed when it comes to carbs, and pre/post-exercise fueling.

They fuel during, often hitting numbers ranging from 50 grams of carbohydrates per hour, all the way up to 150+ grams for highly-trained cyclists.

And, likewise, these athletes prioritize glycogen refueling after. But almost no one is thinking about the protein they’re actively using as fuel during the session itself.

Protein is usually framed as a recovery tool: something you take after to repair muscle and support adaptation. Go hit a huge session, fuel it adequately with carbs and fluids during, and slap a protein shake on top afterwards and call it a day.

But that framing misses something highly important: some of that protein wasn’t just used to rebuild but was used as fuel.

And if you don’t replace it, you’re not just recovering - you’re trying to climb out of a deeper hole that you just trained yourself into.

Why This Changes the Equation

This is one of the many reasons endurance athletes consistently need more protein than standard guidelines suggest.

Not just because you’re breaking down muscle and harvesting general training adaptations in your week-to-week fitness builds…

But because you’re actively oxidizing protein during the work itself.

It’s subtle, easy to overlook, but is happening every session, week after week, month after month, year after year.

And over time, it compounds—just like everything else in endurance sport.

 

The ADDRA Perspective

At ADDRA, we think about recovery as a continuously flowing stream, all part of a larger reservoir of fitness, training, and health.

It happens during training.
It happens between sessions.
And it happens while you sleep.

Ensuring your body has access to high-quality protein – and the amino acids it needs - across that entire cycle helps support the adaptations you’re working toward every day.

Because recovery is part of a system in endurance sport, not just an isolated event in the trials and tribulations of training and performance.

Every day is recovery day ~

References

Clauss, M., & Jensen, J. (2025). Effect of exercise intensity, duration, and volume on protein oxidation during endurance exercise in humans: A systematic review with meta-analysis. Scandinavian Journal of Medicine & Science in Sports, 35(4), e70038.

Kobayashi, E. N., Toledo, R. R. F. de, Almeida, M. O. de, Sprey, J. W. C., & Jorge, P. B. (2026). Metabolic effects of carbon-plated running shoes: A systematic review and meta-analysis. Frontiers in Sports and Active Living, 7, 1710224. https://doi.org/10.3389/fspor.2025.1710224