Protein for Endurance Athletes: How Much & Which

Ask a runner or a triathlete about protein and you will usually get an answer about muscle size. That framing sells protein short, and it is the reason a lot of endurance athletes under-eat the one macronutrient that decides how well they absorb the training they are doing.

Protein is structural material. Your body builds well over a thousand distinct proteins from it: the keratin in your hair, the collagen in your skin, the actin and myosin that let a muscle contract, the haemoglobin that carries oxygen, the immunoglobulins that defend you, the enzymes that run every metabolic pathway, and hormones like insulin. Muscle is one line on a very long list.

This is part one of a two-part guide: what protein actually does, how much you need, and where to get it. Part two covers timing and supplements.

Key takeaways

• Protein is not a muscle nutrient. It is structural material for haemoglobin, enzymes, antibodies, collagen and hormones.

• For endurance athletes the case is repair, not size: muscle fibres, mitochondria and the enzymes of energy metabolism are all damaged by long sessions and rebuilt from amino acids.

• Most training athletes land between 1.4 and 2.0 g per kg of body weight per day. The per-serving dose matters nearly as much: roughly 20-40 g, spread every three to four hours.

• Source quality is measurable. DIAAS scores whey, milk, egg and beef above 1.0; soy around 0.90; pea around 0.82; wheat around 0.40.

• You do not need to combine plant proteins at every single meal. Daily variety is enough; the old rule about pairing rice with beans at every sitting has been retired.

Why protein is not just about muscle

The structural list above has a practical consequence that matters more to an endurance athlete than to anybody else. When protein intake falls short while training volume is high, the body does not politely burn only fat. It catabolises structural protein to cover the gap, and haemoglobin is protein. That is one reason under-fuelled endurance athletes can drift toward low haemoglobin even when their iron intake looks perfectly adequate.

There is a broader version of the same point. Muscle mass and strength are routinely treated by clinicians as markers of physiological reserve, and low muscle mass is consistently associated with poorer outcomes in hospitalised and post-surgical patients. Muscle is not decoration. It is a metabolic organ with a buffer function, and protein is what you build it out of.

Three reasons protein does most of the work in a fat-loss phase

If your goal involves losing fat while keeping performance, protein is doing three separate jobs at once.

1. It protects the tissue that keeps your metabolism high

A calorie deficit makes the body draw on both fat and lean tissue. Lean tissue is a large part of what sustains resting metabolic rate, so losing enough of it means the same deficit stops producing the same rate of loss. Raising protein while calories are low is the single most reliable nutritional lever for making sure what comes off is fat rather than muscle. The alternative is the outcome nobody wants: lighter on the scale, softer in the mirror.

2. It keeps you full

Protein lowers ghrelin and slows gastric emptying more than carbohydrate or fat do. Practically, that means a bowl of noodles can leave you hunting for a snack an hour later while the same calories with 35 g of protein does not. Carbohydrate, particularly refined carbohydrate, tends to do the opposite: a fast blood sugar rise followed by a fast crash and a craving for something sweet and fatty.

3. It costs energy to process

The thermic effect of food is the energy cost of digesting, absorbing and storing a nutrient. Protein runs roughly 20-30%, carbohydrate about 5-10%, and fat around 0-3%. Eat three meals a day and the difference compounds into a real number over a month.

None of this makes protein a fat-loss drug. It makes it the macronutrient that protects the things you actually care about while the deficit does its job.

How much: start with grams per kilogram

The International Society of Sports Nutrition position stand on protein and exercise puts active people at roughly 1.4-2.0 g per kg of body weight per day, with the upper end reserved for heavy training volumes and for phases where calories are restricted. Some work expressed per kilogram of fat-free mass goes as high as 2.3-3.1 g/kg of fat-free mass during aggressive fat loss.

Three adjustments matter nearly as much as the daily total:

• Per serving. Roughly 0.25 g/kg, or an absolute 20-40 g for most people, is the dose range associated with a maximal muscle-protein-synthesis response. Beyond it the response plateaus; a meta-analysis of resistance training studies puts the daily plateau at around 1.6 g/kg.

• Leucine. Aim for roughly 700-3,000 mg of leucine per serving. Whey, dairy, egg, meat and soy clear this comfortably. Wheat and collagen do not.

• Spacing. Distribute protein across three or four feedings roughly every three to four hours rather than loading it all into dinner. Work in healthy adults found that an even distribution raised 24-hour muscle protein synthesis by about 25% compared with the same total eaten mostly at the evening meal.

A worked example. A 70 kg athlete at 1.6 g/kg needs about 112 g per day: roughly 30 g at breakfast, 35 g at lunch, 20 g after training and 28 g at dinner. That is far less dramatic than most people expect, and it is much easier to hit from food than from shakes.

Macro splits: three starting points

These are percentages of total daily calories, and they are starting points rather than prescriptions. The honest answer to what ratio is right for you is that you try one for four weeks, watch scale weight, performance and hunger, then adjust. People who manage their body composition well almost always arrive at their own number by iteration.

Training days and rest days do not have to be identical, though the difference is smaller than most people assume. On a rest day, hold protein up and trim the extra carbohydrate and fat you were only eating to fuel the session.

For muscle gain, run the calculation in the opposite direction. Set protein first in g/kg, roughly 1.2-1.6 g/kg, let that determine its share of calories, then solve for carbohydrate and fat around it. It is arithmetic, and after doing it twice you will know your own numbers by heart.

What protein does for endurance athletes specifically

Endurance training damages more than glycogen stores, and the repair bill comes due on a delay.

• Muscle fibre micro-damage, mitochondrial turnover and the enzyme systems of energy metabolism all need amino acids to rebuild.

• Oxidative stress and inflammatory markers commonly peak one to three days after a hard endurance effort, which is exactly when appetite is often at its lowest and the temptation to under-eat is highest.

• Under-replacement flips the body toward catabolism: structural protein synthesis falls, and in a sustained deficit the visible signs are hair shedding, slow healing and a drift in blood markers.

There is also a simple arithmetic problem specific to long-course athletes. The longer the session, the larger the share of energy that comes from protein, and the more of it you oxidise. A 50 km trail race or a long-course triathlon leaves a bigger protein debt than a hard hour, and a diet built around an hour of daily training will not cover it.

Protein by discipline: marathon, trail, triathlon and HYROX

The daily target barely moves between these four. What changes is where the damage lands and when the repair bill arrives. All figures below are starting points inside the 1.4-2.0 g/kg range, not prescriptions.

Two rules hold across all four. Protein does not replace carbohydrate during a session, so it never solves a fuelling problem mid-race. And the highest-value serving of the day is almost always the one after your longest or hardest session, because that is when the repair demand peaks and appetite is usually at its worst.

Choosing a source: plant, animal, and what is new

Plant proteins

Soy, pea, wheat, chickpea and hemp, extracted and concentrated from seeds, legumes, grains and nuts.

• For: no cholesterol, generally low fat, lower cost, a much smaller carbon footprint, and the only real option for vegan athletes and many dairy-avoiding ones.

• Against, quality: single-source plant proteins carry a limiting amino acid. Grain proteins such as wheat and rice are short on lysine; legume proteins such as pea are short on the sulfur amino acids methionine and cysteine.

• Against, digestion: plant protein tends to digest more slowly. That is excellent for satiety and less ideal in the hour after a hard session. Some people also get genuine bloating from soy, and that is worth testing on yourself rather than pushing through.

Animal proteins

Whey in its concentrate, isolate and hydrolysate forms, plus casein, whole milk protein and egg white.

• Complete amino acid profile, high in branched-chain amino acids and especially leucine, which is the trigger for muscle protein synthesis.

• High digestibility, and, honestly, better taste, solubility and stability in a shaker. This is most of why whey became the default.

• Casein clots in the stomach and releases amino acids slowly, which is why it is the classic pre-sleep option and whey is the classic post-session one.

Novel proteins: yeast and beyond

Yeast protein, grown from Saccharomyces cerevisiae, is the most interesting of the new entrants and the one with actual published data behind it.

A 2024 study in Food Chemistry compared yeast protein isolate against whey concentrate, whey isolate, soy, wheat and pea isolates. Indispensable amino acid content came in at 508 mg per gram of protein for yeast, essentially level with whey concentrate at 511 mg/g and whey isolate at 528 mg/g, and clearly above soy at 398, pea at 384 and wheat at 303 mg/g. Yeast protein also had the highest ratio of indispensable to dispensable amino acids of the six.

Two caveats belong next to that number. The same study found yeast protein's digestibility to be comparatively poor, and it was funded by a yeast protein manufacturer, so treat the amino acid data as promising rather than settled. In practice yeast protein also runs roughly 15-20% lower in leucine than whey, so a slightly larger serving delivers a comparable leucine dose.

One property is genuinely useful rather than just novel: yeast protein carries beta-glucans from the cell wall, which slow its digestion and make it behave as a sustained-release protein. That suits it to between-meal or pre-sleep use rather than as the immediate post-session hit.

Insect protein, cricket and silkworm pupa among them, is nutritionally respectable and, for most Western consumers, a psychological hurdle rather than a nutritional one. It is coming. It is not here yet.

This is why Maction does not default to whey. Our protein range is built on yeast protein because it delivers an amino acid profile close to whey without being a dairy derivative, and because its slower digestion suits the way endurance athletes actually eat: steadily, between meals and around long sessions, rather than in one bolus at a single moment.

Protein quality, measured

The score used in the research literature is DIAAS, the Digestible Indispensable Amino Acid Score, recommended by the FAO in 2013 to replace the older PDCAAS. It measures how much of each essential amino acid you actually absorb, then scores the protein on its weakest one.

Two things follow from that table. First, gram for gram a lower-scoring source delivers less usable protein, so an athlete eating mostly plant protein generally needs a higher total intake to absorb the same amount. Second, and this contradicts advice you will still see everywhere, you do not need to combine complementary proteins at every meal. Your body maintains a free amino acid pool that spans meals, and the Academy of Nutrition and Dietetics position is that varied plant protein across the day is sufficient.

Pairing grain with legume in one sitting is still a good habit, rice with tofu, hummus with flatbread, because it lifts that meal's score and tastes better. It is an optimisation, not a requirement.

Whole foods worth building around

From plants

• Soy foods: tofu, tempeh, edamame. Edamame brings fibre along with the protein.

• Lentils, chickpeas and black beans.

• Almonds lead the nut category on protein per calorie; chickpeas and edamame lead the legumes.

From animals

• Skinless white meat. Chicken and turkey breast are the purest protein-per-calorie options in most supermarkets.

• White fish such as perch, tilapia and cod: very lean, very clean protein.

• Oily fish such as salmon and mackerel: roughly 17-18 g of protein per 100 g against about 23 g for chicken breast, with the difference made up by omega-3 fats.

• Lean red meat. Beef is the gym standard for a reason beyond taste: it also supplies creatine and carnitine, and red meat is one of the better sources of heme iron, which matters to anyone watching ferritin.

• Eggs. Still the best protein per unit of money and effort in most kitchens, and they bring vitamin A, vitamin D and riboflavin with them.

• Strained Greek yoghurt, ideally low-fat and unsweetened, at roughly 10 g of protein per 100 g.

Cooking: keep the protein, lose the added fat

• Boil, poach, stew, steam, braise, roast or air-fry. A pressure cooker and an air fryer will cover almost every case.

• Boiled eggs: 5 to 8 minutes. Push past that and the white discolours and the texture turns.

• Fried eggs: a light spray of oil, one side only. Eggs absorb fat extremely efficiently, so a two-sided fry is not the same dish nutritionally.

• Avoid restaurant scrambled eggs. Commercial kitchens often beat one egg with water and a large volume of oil to bulk it out, so a plate that looks like three eggs may contain one.

• Steamed egg custard is the most digestible option available when appetite is low or the stomach is unsettled.

• Omelettes, whole egg or whites only, take vegetables and a little low-fat cheese well.

• Cook it yourself. That is the only reliable way to control how much oil and salt arrives with your protein.

A day that actually hits the target

For the 70 kg athlete needing about 112 g:

• Breakfast: two eggs plus 150 g Greek yoghurt, roughly 30 g

• Lunch: 150 g chicken breast with rice and vegetables, roughly 35 g

• After training: a protein snack or shake, roughly 20 g

• Dinner: 150 g salmon or tofu with vegetables, roughly 28 g

Nothing exotic, no supplements required. The point is distribution, not drama.

What this comes down to

Protein is the least glamorous part of an endurance fuelling plan and the one most likely to be quietly under-eaten. Carbohydrate gets the attention because it decides how a race feels on the day. Protein decides whether you absorb the block you just built, whether the weight you lose is fat, and whether you arrive at the start line with the structural reserve to handle the next one.

Get three things right and the rest is detail: hit a daily total in the 1.4-2.0 g/kg range, split it into 20-40 g servings across the day, and put one of those servings after your hardest session. Where those grams come from matters less than whether they arrive.