Protein for endurance: 1,8 g/kg and why it's for runners, not bodybuilders

Ask an amateur distance runner what matters most in their nutrition, and almost everyone will name carbohydrates: gels, porridge, carb-loading before a race. Protein, meanwhile, gets quietly filed under “food for the gym” and under-eaten for years. A new review in Sports Medicine (2025) turns that logic on its head: for an endurance athlete, protein isn’t about biceps — it’s about handling the volume, recovering, and getting faster.

Why an endurance athlete needs protein

During prolonged work the body doesn’t only burn carbohydrates. It oxidizes branched-chain amino acids — leucine above all — straight from muscle protein. Those losses have to be paid back, or recovery stalls.

But it’s not only about “patching holes.” Protein is needed for far more interesting jobs:

Mitochondria. Your aerobic “engine” is made of protein structures that are constantly renewed and grow in response to training. No building material — no adaptation.
Immunity. During intense blocks, adequate protein helps hold your immune status and keeps you from falling apart with colds.
Balance in a deficit. When you train under a calorie restriction, protein slows the breakdown of your own muscle.
Bones and connective tissue — also a protein story, and especially important for runners with their impact loading.

In other words, protein works not for “mass” but for recovery and the quality of adaptation.

How much you need

The review’s key figure for ordinary training days is around 1,8 g/kg per day. That’s roughly 50% more than a sedentary person needs. For a 70 kg athlete that’s ~126 g of protein a day.

The problem is that the typical real intake of endurance athletes is only about 1,5 g/kg. Most fall short systematically.

Then it gets counterintuitive:

On low-carb training days, the requirement rises to 1,95 g/kg.
On rest and recovery days — up to >2,0 g/kg. Yes, on your day off, not on a hard-work day: tissue recovery and remodeling happen precisely when you’re not running.
For women in the luteal phase of the cycle, the target is a touch higher — around 1,9 g/kg.

How to spread it across the day

Hitting your target with one giant dinner is a bad idea. Muscle protein synthesis is better stimulated by even portions spread across 3–4 meals every 3–4 hours.

Practical guidelines from the review:

After exercise — a portion of around 0,5 g/kg (for 70 kg that’s ~35 g) to kick off synthesis of contractile muscle protein. In a low-carb scenario ~0,4 g/kg is enough — and it also helps restore glycogen.
Before bed — slow protein. In one of the studies reviewed, 45 g of casein at night raised the synthesis of mitochondrial proteins specifically over 7 hours of post-workout sleep.

Here’s how it looks on the plate (approximate values):

Chicken breast, 100 g — ~30 g of protein
Two eggs — ~12 g
Cottage cheese, 200 g — ~34 g
Salmon or white fish, 100 g — ~22 g
Lentils/beans, a serving — ~9–15 g
A scoop of whey protein — ~24 g

Reaching 1,8 g/kg from ordinary food isn’t hard: eggs and cottage cheese for breakfast, protein at lunch and dinner, a portion after a long session, and something casein-based at night.

A plant-based diet works, but it takes attention: plant sources are lower in leucine and absorbed slightly less well, so vegans should aim a little above target and be sure to combine sources (legumes + grains + soy) rather than living on lentils alone.

And something important: protein does not replace carbohydrates. They remain the main fuel for prolonged work — protein complements them, it doesn’t substitute for them.

Limitations

A few caveats are worth keeping in mind.

Most of the data comes from cyclists and triathletes, and predominantly from men. Female physiology in this area is an acknowledged gap.
The authors honestly call the optimal distribution of protein around training under-studied.
The myth that “lots of protein wrecks your kidneys” applies to people who already have kidney disease; for healthy athletes there’s no convincing evidence of harm. And the myth that “protein is only for the gym” is one this review closes for good — for endurance it’s no less important.

The bottom line

The target is around 1,8 g/kg/day, not the usual ~1,5 g/kg that most people fall short of.
On low-carb days — up to 1,95 g/kg; on rest days — >2,0 g/kg.
After exercise — a portion of ~0,5 g/kg; spread protein across 3–4 meals every 3–4 hours.
Casein before bed supports recovery and mitochondrial growth overnight.
For a distance runner, protein is about recovery, mitochondria, immunity, and bones — not mass. Carbohydrates, meanwhile, remain the main fuel.

Source: Witard OC, Hearris M, Morgan PT. Protein Nutrition for Endurance Athletes: A Metabolic Focus on Promoting Recovery and Training Adaptation. Sports Medicine, 2025. https://doi.org/10.1007/s40279-025-02203-8