Training Sends the Signal. Protein Builds the Change.
Endurance training isn’t just a test of willpower—it’s a stimulus for biological transformation. With every ride, run, or swim, your body initiates changes that make you more efficient, more resilient, and better equipped to go longer and harder.
But these changes don’t just happen on their own. They require building blocks—and the primary one is protein.
Adaptation Is the Real Goal—and It’s Protein-Dependent
Most people think of protein as a recovery tool. And while it’s true that protein helps repair muscle damage after a hard session, its role goes far deeper for endurance athletes.
Every time you train, your body initiates:
- Mitochondrial biogenesis (to improve energy production)
- Muscle fiber remodeling (to make you more fatigue-resistant)
- Enzyme and capillary expansion (to boost oxygen delivery and fuel use)
All of these processes require a supply of amino acids - which come from dietary protein. Without adequate protein intake, the signal to adapt is there—but the resources to make it happen optimally may not be.
You Don’t Get Fitter During Training—You Get Fitter During Remodeling
Let’s reframe how we think about fitness progression. Fitness gains = the sum of daily muscle and other body protein changes.
This graphic illustrates the basic rhythm of training: you stress the body (red), then replenish (green), and with good rest and nutrition supporting remodeling (white), you come back stronger (higher green curve). But if protein intake is insufficient during the rebuild phase, the slope could be lower or even flatten. You don’t adapt. You don’t get fitter.
The key takeaway? It’s not just the training that drives fitness—it’s the remodeling. And remodeling requires protein and, of course, adequate rest.
New Research Shows We’ve Been Underestimating Protein Needs
For years, endurance athletes and coaches focused on carbs and protein was an after thought. Many simply followed the general protein RDA: 0.8 g/kg/day, which is based on the amount of protein required to prevent deficiency in most sedentary individuals. But that baseline doesn’t hold up when you are undergoing endurance training and your body is in constant adaptation mode.
Recent research has corrected this.
- A 2023 study by Williamson et al. measured optimal protein needs in trained endurance athletes using the Indicator Amino Acid Oxidation (IAAO) method. The results? 1.8 g/kg/day was required to best support post-exercise recovery, with no significant differences between male and female athletes¹.
- A 2025 review by Witard et al., just published in Sports Medicine, confirms these recommendations and explains that endurance athletes—especially during phases of high training load—require significantly more daily protein than the general population to support both muscle repair and mitochondrial adaptation².
If you’re a 160-pound athlete (72.5 kg), that means aiming for ~130 grams of protein per day — more than double the RDA.
This Isn’t Just About Recovery—It’s About Getting Better
It’s a common mistake to view protein as something you need only on hard training days, or only after workouts. In fact, there is evidence that optimal protein intake is even higher on rest days than training days3. And many of the benefits of protein show up in how your body adapts over time with proper training:
- Better oxygen utilization
- Greater efficiency
- Faster recovery between sessions
- Reduced injury risk
Inadequate protein doesn’t just mean sore muscles—it means dampened training gains.
The Bottom Line for Endurance Athletes
If you're training consistently, you’re signaling your body to adapt. But unless you’re giving it the protein it needs—about 1.8 g/kg/day—you’re leaving gains on the table.
And as the above visual shows, the upward slope of your fitness depends on what you do during the "remodel" phase. Don’t waste the window.
Need Help Meeting Your Goals?
At ADDRA, we make protein bars with endurance athletes in mind—portable, leucine-boosted, and built to help you meet your daily protein needs in the real world. Because the work doesn’t end when your workout does.
📝 Coming Next in the Series...
You’ve seen how protein supports the remodeling process that drives endurance gains.
But are you actually eating enough to fuel that change?
Next up: Why endurance athletes may need more protein than strength trainers—
and what the latest science says about how much you need, when, and why.
🔍 We’ll break down:
- How endurance training burns protein for fuel
- The new gold standard for calculating your daily intake
- What higher protein can do for recovery, soreness, and even your 5K time
👇
Scientific References
- Williamson E, Fung HJW, Adams C, et al. Protein requirements are increased in endurance-trained athletes but similar between females and males during postexercise recovery. Med Sci Sports Exerc. 2023;55(10):1866–1875.
- Witard OC, Hearris M, Morgan PT. Protein Nutrition for Endurance Athletes: A Metabolic Focus on Promoting Recovery and Training Adaptation. Sports Med. 2025 Mar 21. Epub ahead of print.
- Moore DR, Gillen JB, West DWD, et al. Protein requirements may be lower on a training compared to rest day but are not influenced by moderate training volumes in endurance trained males. Appl Physiol Nutr Metab. 2024;49(8):1124-1128.
