Why endurance athletes may need more protein than strength trainers—and how to fix it
You’re logging the workouts. Clocking the long rides. Nailing every swim split. So why aren’t the fitness gains showing up the way they should?
Many endurance athletes are making a crucial fueling mistake—they’re under-consuming protein, especially in the days and weeks of training when adaptation is happening.
And the consequences aren’t just sore muscles. We’re talking blunted adaptation, reduced recovery, and missed performance potential.
1. Endurance Training Burns Protein as Fuel
Endurance training doesn't just break down muscle tissue—it actually uses protein as a fuel source. On average, athletes oxidize 5–10 grams of protein per hour of sustained aerobic exercise, depending on training intensity and nutritional status¹.
That means you’re not just damaging muscle fibers—you’re burning amino acids, especially leucine, mid-session.
This elevated turnover of amino acids—particularly leucine and the other branched-chain amino acids (BCAAs)—sets endurance athletes apart and explains why their daily protein needs are higher than those of strength athletes.
2. How Much Protein Is Optimal? The IAAO Method Tells Us
To figure out how much protein endurance athletes actually need, scientists used a precise technique: the Indicator Amino Acid Oxidation (IAAO) method. This method measures the point where the body stops burning essential amino acids due to inadequate intake and begins to optimally use them for building new muscle and other important body proteins.
Adapted from: Kato H, Suzuki K, Bannai M, et al. Protein requirements are elevated in endurance athletes after exercise as determined by the Indicator Amino Acid Oxidation method. PLoS ONE. 2016;11(6):e0157406.
In a study by Kato et al. (2016), trained endurance athletes showed a clear breakpoint at 1.65g protein/kg bodyweight/day, with 1.83g protein/kg bodyweight/day identified as the recommended optimal intake for maximizing protein retention and synthesis².
- Below this intake = amino acids are inefficiently oxidized (wasted)
- At this intake = the body has full amino acid availability to build and remodel muscle, mitochondria, capillary networks and other important body proteins.
3. Higher Protein Intake Shows Trends Toward Better Performance & Recovery (Faster 5k)
In a 2019 study by Williamson et al., endurance-trained men consumed diets with either LOW (0.94 g/kg), MODERATE (1.20 g/kg), or HIGH (1.83 g/kg) protein (same as the new recommendations) for four days while also running a total of 65km during those four days. Before and after that training and protein intervention block they perfomed a 5k race simulation³.
Adapted from: Williamson E, Kato H, Vultekman KA, et al. The effect of dietary protein on protein metabolism and performance in endurance-trained males. Med Sci Sports Exerc. 2019;51(2):352–360.
Key findings:
Improvement trends for the HIGH protein group in the following measures
-
5k Performance
→ 0.61% faster from before the 4 days training to after - that’s 7–8 seconds faster with just four days of higher protein - Reduced delayed onset muscle soreness (DOMS)
- Better strength retention (max voluntary contraction or MVC)
The MOD and LOW protein groups trended worse in performance, DOMS & MVC from before to after the 4 days of training.
4. Endurance Athletes May Require More Protein Than Strength Athletes
It may sound counterintuitive, but these new daily protein recommendations are higher for endurance athletes than the latest science-based recommendations for strength athletes4.
And, in a 2023 follow-up study, Williamson et al. confirmed that the earlier findings that endurance-trained athletes require higher daily protein intake and further found no difference between males and females5.
How to Optimize Your Intake
If you’re putting in serious training volume and want to support real gains:
- 🎯 Target 1.8 g/kg/day of high-quality protein
(For a 160 lb / 72.5 kg athlete, that’s ~130g/day) - 🔁 Rebuild what you burn—especially after long sessions
- 💥 Include leucine-rich proteins to maximize synthesis of new proteins
- 🚀 Use protein not just to recover—but to adapt
📝 Coming Next in the Series…
You’ve learned that endurance athletes may need even more protein than strength athletes—and why hitting ~1.8 g/kg/day can unlock recovery and performance.
But how you eat that protein may matter just as much as how much.
Next up: Why spacing your protein intake every few hours can drive better endurance adaptations—backed by new research.
We’ll break down:
- The study that tested identical protein amounts with different timing strategies
- Why grazing or saving it all for dinner doesn’t work
- How hitting your leucine threshold several times a day improves mitochondrial and muscle remodeling
⏱️ If you’re training daily, don’t miss this timing trick to make every session count.
👇
References
- Clauss M, Jensen J. Effect of Exercise Intensity, Duration, and Volume on Protein Oxidation During Endurance Exercise in Humans: A Systematic Review With Meta-Analysis. Scand J Med Sci Sports. 2025 Apr;35(4):e70038.
- Kato H, Suzuki K, Bannai M, et al. Protein requirements are elevated in endurance athletes after exercise as determined by the Indicator Amino Acid Oxidation method. PLoS ONE. 2016;11(6):e0157406.
- Williamson E, Kato H, Vultekman KA, et al. The effect of dietary protein on protein metabolism and performance in endurance-trained males. Med Sci Sports Exerc. 2019;51(2):352–360.
- Morton RW, Murphy KT, McKellar SR, et al. A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. Br J Sports Med. 2018 Mar;52(6):376-384.
- 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.
