Why Endurance Athletes Need More Protein Than They Think

Strength athletes obsess over protein. Endurance athletes obsess over carbs. The ones who do both right are the ones still standing at kilometre 7 of Hyrox.

The endurance world has a protein problem. It’s not that athletes don’t know protein matters. It’s that the culture, the coaching, and the conventional wisdom all underweight it. Carbs are king. Carbs fuel the long run. Carbs fill the glycogen tank. Eat pasta the night before. Gel every 45 minutes during. Recover with a sports drink.

None of that is wrong. Glycogen availability is critical for sustained aerobic performance. But somewhere along the way, the emphasis on carbohydrates became so dominant that protein got pushed to the margin. Something you think about after training, maybe. A shake if you remember. Chicken at dinner.

Meanwhile, the endurance athlete’s body is breaking down muscle protein during every long session, repairing micro-damage across multiple tissue types, and trying to adapt to training stress with inadequate raw materials. And they wonder why they can’t hold onto lean mass during a marathon block.

The research gap

The recommended dietary allowance (RDA) for protein is 0.8 grams per kilogram of body weight per day. This number is the minimum required to prevent deficiency in sedentary adults. It’s not a performance recommendation. It’s not even a health recommendation for active people. It’s the floor below which you start losing muscle.

Most general nutrition guidance for endurance athletes lands between 1.2 and 1.4 grams per kilogram. This is better than the RDA but still based on research that primarily looked at steady-state endurance exercise (cycling, long-distance running) in isolation.

For hybrid athletes who combine endurance and strength work, the International Society of Sports Nutrition recommends 1.4 to 2.0 grams per kilogram. For athletes in caloric deficit or with high training volumes, the recommendation goes up to 2.2 grams per kilogram to protect lean mass.

Here’s the problem: most endurance athletes don’t track protein and when researchers measure actual intake, the numbers consistently come in at 1.0 to 1.2 grams per kilogram. Barely above the sedentary RDA. For an 80-kilogram athlete, that’s 80 to 96 grams when they likely need 140 to 176.

The gap is 40 to 80 grams per day. That’s not a minor shortfall. That’s the difference between maintaining lean mass and slowly losing it.

What endurance exercise does to protein demand

Endurance training creates protein demand through several mechanisms that don’t exist at rest.

Protein oxidation during exercise. During sustained moderate-to-high intensity exercise, your body breaks down amino acids for fuel. This increases with duration and intensity, and accelerates significantly once glycogen stores are depleted. A 90-minute run at moderate intensity can oxidise 10 to 20 grams of amino acids during the session. A 3-hour ride could be double that.

Muscle damage and repair. Running, in particular, causes eccentric muscle damage from the repetitive impact loading. This is why runners experience more delayed-onset muscle soreness (DOMS) than cyclists at equivalent effort levels. The repair process requires amino acids, specifically the branched-chain amino acids leucine, isoleucine, and valine.

Mitochondrial biogenesis. Endurance training triggers the creation of new mitochondria in muscle cells. This adaptation requires protein synthesis, not for growing bigger muscles, but for building the cellular machinery that makes you aerobically fitter. The protein cost of mitochondrial adaptation is often overlooked because it doesn’t show up as visible muscle growth.

Immune function. Heavy endurance training is immunosuppressive. The “open window” theory suggests athletes are more susceptible to illness in the 3 to 72 hours after very hard or very long sessions. The immune system’s response to this vulnerability requires amino acids, particularly glutamine. Athletes in a chronic protein deficit are more likely to get sick during heavy training blocks.

Each of these processes draws from the amino acid pool. Combined, they create a protein demand that significantly exceeds what a non-exercising person needs. But because none of these mechanisms produce visible muscle growth, endurance athletes underestimate the demand.

The Hyrox problem

Hyrox is the perfect case study for the protein problem because it punishes athletes who neglect either energy system.

A Hyrox race involves 8 kilometres of total running split across 8 segments, with a functional fitness station between each running leg: ski erg, sled push, sled pull, burpee broad jumps, rowing, farmers carry, sandbag lunges, and wall balls. The race takes most competitors between 60 and 90 minutes and demands both aerobic capacity and muscular endurance.

Athletes who train primarily as runners can handle the 1-kilometre running legs but fade on the sled, the farmers carry, and the wall balls. Athletes who train primarily for strength can muscle through the stations but suffer on the running legs. The athletes who do well are the ones who maintain both their aerobic base and their lean mass across a training block.

This is where protein becomes a competitive advantage. A Hyrox athlete training 5 to 6 days per week with mixed running and gym sessions has overlapping recovery demands from both modalities. The running creates eccentric damage and oxidises amino acids. The strength work stimulates muscle protein synthesis and requires amino acids for repair. Both windows are open simultaneously.

An athlete eating 1.0 grams per kilogram doesn’t have enough amino acids to fully support both recovery pathways. Something gives. Usually it’s the strength adaptation, because the body prioritises immediate tissue repair (endurance damage) over long-term muscle building (strength adaptation). Over a 12-week Hyrox build, this athlete slowly loses the muscular endurance they need for stations while maintaining enough aerobic fitness for the runs.

The athlete eating 1.8 to 2.0 grams per kilogram has the amino acid availability to support both. Their sled push doesn’t get weaker across the block. Their wall ball endurance doesn’t erode. Their body has the raw materials to adapt to both stimuli simultaneously.

Why “running burns muscle” is wrong (and why it feels right)

The belief that endurance exercise burns muscle is widespread and understandable. Athletes who increase running volume often see their upper body get smaller, their weight drop, and their strength decline. The conclusion seems obvious: running catabolises muscle.

But the research tells a different story. Endurance exercise is catabolic to muscle protein during the session (amino acid oxidation is real), but the net effect over days and weeks depends entirely on protein intake.

Studies comparing matched-calorie, matched-training-volume groups show that athletes with adequate protein intake (1.6+ g/kg) maintain lean mass during endurance blocks, while athletes with inadequate protein (below 1.2 g/kg) lose it. The running didn’t cause the muscle loss. The protein deficit did. The running just increased the demand that wasn’t being met.

This distinction matters because the myth leads athletes to avoid running to protect muscle, when the actual solution is to eat more protein while running. You can have both. Elite hybrid athletes prove this constantly. But you can’t have both on a diet that was designed for someone who sits at a desk all day.

How to close the gap

Calculate your actual target. Body weight in kilograms × 1.8. That’s your minimum daily protein goal if you’re doing both endurance and strength work. At 80 kilograms, that’s 144 grams. At 70 kilograms, 126 grams. Write it down.

Audit your current intake. Track for three days. Most endurance athletes are shocked at how far below target they are. The gap is usually largest at breakfast and around training.

Build protein into your fuelling strategy. Post-run nutrition is typically carb-dominated. Add protein. A recovery meal with 30 grams of protein plus 60 to 80 grams of carbs is better than the same carbs alone. The amino acids support repair while the carbs replenish glycogen. Both matter.

Don’t cut protein when cutting calories. If you’re trying to reach racing weight, protein should be the last macro to reduce. Drop fats first, then carbs (within reason for training quality). Protein protects lean mass during a deficit, and losing lean mass during a cut defeats the purpose.

Pre-sleep protein matters. A 30 to 40 gram serving of slow-digesting protein (casein, Greek yoghurt, cottage cheese) before sleep has been shown to enhance overnight muscle protein synthesis. For athletes with dual recovery demands, this is a simple and effective intervention.

The data integration problem

Protein intake is the invisible variable in almost every wearable-based recovery assessment. Your Whoop tells you you’re recovered. Your Garmin says your training status is “productive.” Your body is quietly losing lean mass because neither device knows you’re eating half the protein you need.

Over weeks, this shows up in body composition scans that confuse both the athlete and their coach. Training load went up. Recovery scores looked good. But skeletal muscle mass dropped and body fat percentage crept up. The data from the wearable said everything was fine. The data from the scale told a different story.

Connecting nutrition data to training and recovery data is the missing link that turns isolated metrics into actionable insight. That’s what P247 is building: the synthesis layer that makes all your data tell one coherent story instead of three incomplete ones.