An updated look at the protein needs of endurance athletes for better recovery, adaptation, and performance
As a runner, I’ve watched my weekly mileage climb from easy, manageable outings to over 45 miles a week at the peak of marathon training. It was a steady build—early mornings, long runs, and back-to-back training days that challenged my recovery and consistency. And whether you're logging 15 miles a week or building toward your first 26.2, one thing becomes clear: the mileage adds up, and so do the demands on your body.
Most marathoners peak between 30 and 60 miles per week, while many recreational runners stay closer to 15 to 25. But even at lower volumes, the cumulative strain of endurance training is real—and how you fuel can make all the difference in how your body responds and adapts.
For decades, the go-to mantra for endurance athletes has been simple: carbs fuel performance, and protein is for lifters. But a new review published in Sports Medicine this month turns that idea on its head. Titled Protein Nutrition for Endurance Athletes: A Metabolic Focus on Promoting Recovery and Training Adaptation, the paper brings fresh clarity to an area that’s long been murky. While carbohydrates are still king for fueling effort, it turns out that protein plays a far more essential role than previously thought—not just after training, but also before and during exercise, especially when carbohydrate availability is low.
You Need More Protein Than You Think—Even on Rest Days
The RDA for protein—0.8 g/kg/day—has never been particularly helpful for endurance athletes. It was derived from sedentary adults and built on nitrogen balance studies that fail to reflect the increased demands of training, adaptation, and recovery. Now, with the emergence of more sophisticated techniques like the indicator amino acid oxidation (IAAO) method, we can finally quantify what athletes truly need. The answer? More. A lot more.
Recent IAAO studies show that endurance-trained men and women should target 1.8 g/kg/day on standard training days—about 135 grams of protein daily for a 75 kg athlete. That’s nearly double the RDA. And it doesn’t stop there. The protein requirement increases even further—to over 2.0 g/kg/day—on recovery days, when the body is actively repairing muscle proteins and remodeling mitochondria, even without additional training stress. This reflects a key point: protein isn’t just about replacing what’s broken. It fuels the very processes that make you fitter.
What’s more, protein needs may shift depending on sex and hormonal status. For example, research suggests that amino acid oxidation increases during the luteal phase of the menstrual cycle, meaning female athletes may benefit from bumping intake up to around 1.9 g/kg/day during that time. While the science on hormonal fluctuations and protein metabolism is still evolving, the takeaway is clear: protein needs aren’t fixed. They should adjust with training load, recovery demands, and individual physiology.
Protein Timing: Your Most Overlooked Performance Tool
It’s not just about how much protein you eat—it’s about when you eat it. Muscle protein synthesis (MPS), the key process for repair and adaptation, doesn’t run on autopilot. It needs to be triggered—and the window for doing that is tightly linked to training. Emerging research suggests that the most effective way to support MPS is to distribute protein evenly across meals and consume ~0.4–0.5 g/kg body weight per meal, especially after exercise.
But here’s where things get more interesting: while we’ve long focused on post-exercise protein, new findings highlight the potential benefits of consuming protein before and even during exercise—particularly in sessions where carbohydrate availability is low. In these scenarios, protein plays a dual role: reducing muscle protein breakdown (MPB) and providing amino acids for oxidation, without interfering with the molecular signals that drive endurance adaptations.
Take your typical early morning run, for example. After an overnight fast, the body’s glycogen stores are naturally depleted—creating a state of low carbohydrate availability. These sessions can help promote mitochondrial adaptations, but they also increase muscle breakdown. Interestingly, research suggests that consuming 10–20 grams of high-quality protein before or during these workouts can help protect lean tissue and support training adaptations—without blunting the benefits of low-glycogen training. That’s a valuable strategy for athletes who train early, run fasted, or periodize carbohydrate intake.
And this “window of opportunity” isn’t just for lower-carb days. In multi-hour sessions or back-to-back training days, strategic protein intake—whether it’s a pre-run shake or a protein-rich intra-ride snack—can help preserve muscle integrity, reduce soreness, and support long-term gains. We’re not just talking about recovery anymore—we’re talking about real-time muscle preservation and fueling.
Why the Fasted State—or Low Carbohydrate Availability—Demands Smarter Protein Use
The concept of "train low, compete high"—training with low glycogen to boost metabolic adaptation—has grown in popularity among both recreational and elite endurance athletes. But there’s a catch: training in a low-carbohydrate state also increases amino acid oxidation, meaning the body starts using protein—particularly branched-chain amino acids like leucine—as an energy source.
Without sufficient protein, that increased oxidation can lead to muscle breakdown and impaired recovery. But here’s the key insight: adding 10–20 grams of high-quality protein before and/or during low-glycogen sessions doesn’t appear to blunt the adaptive signal. In fact, studies show that moderate protein ingestion during these workouts does not interfere with fat metabolism, mitochondrial signaling, or glucose sparing. Some research even suggests it may enhance pathways linked to fat oxidation and aerobic efficiency. That’s a critical finding for athletes looking to maximize adaptation without compromising lean mass or overloading on carbohydrates.
Even better, this strategy may offer crossover benefits during long-duration races or training days when appetite is suppressed or carbohydrate intake is limited. Protein acts as a metabolic buffer, supplying fuel in the form of amino acids without triggering a large insulin response—an advantage in certain training contexts. For aging athletes or those training in an energy deficit, it may also offer added protection against muscle atrophy.
The takeaway? Training low isn’t inherently risky—but only if your protein strategy is dialed in. Ignore it, and you may turn a smart adaptation into a recovery liability. Get it right, and it could be the difference between a strong training block and a frustrating plateau.
The Practical Protein Playbook for Endurance Athletes
Whether you're a marathoner, triathlete, or cyclist, here's a playbook to consider for your protein intake that matches the demands of your sport and physiology:
Daily Protein Targets
- Standard training days: 1.80 g/kg/day
- Female athletes in luteal phase: 1.90 g/kg/day
- Recovery days (no training): 2.00 g/kg/day
- Low-carbohydrate training days: 1.95 g/kg/day
Timing and Distribution
- Carb-low sessions (i.e. mornings): Consume a moderate protein dose (10–20 g) before and/or during exercise to mitigate MPB without interfering with adaptation
- Post-exercise: Ingest 0.4-5 g/kg body weight of protein to promote contractile muscle protein remodeling (e.g., ~27-34 g for a 68 kg athlete)
- Low-CHO recovery: If unable to ingest 1.2 g/kg/hr of carbs after training, supplement with 0.4 g/kg of protein to assist muscle glycogen resynthesis
- Before bed: Consider a slow-digesting protein like casein on hard training days to sustain overnight MPS
These are not rigid formulas but adaptable guidelines. Use them to build a personalized fueling framework based on your training cycle, hormonal fluctuations, or racing demands. Like any performance tool, consistency matters more than perfection.
The Unsung Hero of Endurance
If there’s one message to walk away with, it’s this: protein isn’t just for recovery—it’s for adaptation. And in endurance sports, where training volume is high and energy availability often dips, optimizing protein intake could be the key to staying healthy, progressing steadily, and avoiding burnout. Whether you’re running your first marathon, chasing a podium spot, or simply trying to train smarter as you age, the evidence is clear: you need more protein, more often, and more strategically.
With new guidelines that emphasize not just how much protein to consume, but also when, why, and under what training conditions, we’re entering a new era of performance nutrition—one that goes beyond carbs and into the molecular machinery that drives adaptation. Endurance athletes now have some interesting target numbers to consider. Protein is no longer the afterthought—it’s part of the plan.