Protein during exercise: role, intake and tolerance

Key takeaways

Protein’s role during exercise is different from its role in recovery.
The 5–10 g/h reference concerns ultramarathons, not every endurance activity.
No single number can predict your digestive tolerance.

What does protein do?

Proteins are made up of amino acids. They contribute to building and renewing tissues and to many functions in the body. They can also provide energy, but they are not the fuel to prioritise for every hour of endurance exercise. In this setting, meeting carbohydrate needs generally remains the main priority.

‘Do I need protein in my diet?’ is a different question from ‘Should I add it to every product consumed during my race?’ The first concerns the whole day and your training. The second depends on duration, the food you already plan to eat and your ability to eat comfortably during exercise.

It also helps to distinguish a serving taken during exercise from a meal afterwards. A product marketed for recovery does not automatically meet the demands of a hard climb, a quick intake or a race in the heat. Its advertised purpose is commercial information; composition and conditions of use still need to be considered.

Does it directly improve performance?

Adding protein to a drink can increase its energy content. To attribute a benefit to protein itself, research needs to distinguish this from extra calories and account for the carbohydrate supplied. A trial by Van Essen and Gibala found no improvement in time-trial performance when protein was added to the sports drink tested.^[1]

That result does not mean protein consumption is useless in every event. It answers a particular research question. Exercise duration, the performance test and the amounts consumed affect the conclusions. Research on one drink in one exercise protocol cannot describe every situation in ultra-endurance sport.

In practice, a ‘high protein’ claim alone is not evidence that a product will help you run faster. Ask a more concrete question: does it provide what you need at that moment in a serving you can eat, without making your planned carbohydrate intake or digestive comfort harder to maintain?

Why its role can change in ultra-endurance

During a very long event, eating extends over many hours and sometimes much of the day. Food variety can help you keep eating when some flavours become difficult to accept. Foods containing protein may then fit within a wider eating approach, beyond the choice of a single drink or gel.

The ISSN’s 2019 position stand proposes roughly 5–10 g of protein per hour during a single-stage ultramarathon, within an approach aimed at limiting energy deficits. This reference concerns that particular setting. It is neither a requirement for every runner nor a target to apply to every endurance training session.^[2]

To keep this guidance in context, first consider all the foods you plan to consume. Protein may already come from a bar or an aid station. Adding a dedicated product on top of that intake is not automatically necessary. Variety, practicality and tolerance still matter when you need to keep eating for many hours.

Protein and digestive comfort

A 2017 study compared water, glucose and whey protein hydrolysate in eleven runners completing two hours at 35 °C. Repeated protein intake amounted to approximately 45 g/h during exercise and produced more digestive symptoms. This finding concerns a high dose, a specific protein preparation and hot conditions.^[3]

It does not establish a universal point at which protein becomes problematic. In particular, 15 g/h is not a scientifically established boundary for all athletes. Amount, product form, other ingredients, intake timing and individual sensitivity can change the experience from one outing to another.

When testing a product, avoid changing the number of servings, drink, pace and intake timing all at once. If discomfort occurs, a well-recorded trial is easier to interpret. Also note fullness: food that stops you maintaining planned intake can create a practical difficulty even when it does not cause pain.

Read amounts per serving and across repeated intakes

Start with the reference quantity. Protein per 100 g is not necessarily protein per whole bar. Use the weight of the serving actually consumed, then multiply by the number of servings. Packaging containing two servings deserves particular attention if you intend to eat the whole pack in one go.

Next read carbohydrate, fat, fibre and ingredients. A protein bar may also supply carbohydrate or contain polyols that raise separate digestive considerations. The word ‘protein’ does not replace this reading. To compare products, start with the contribution you want, then examine the other nutrients supplied alongside it.

Keep missing information unknown. A product name or the presence of an ingredient cannot reconstruct an exact protein quantity. If you share a bar or consume only part of a sachet, adjust the calculation too. The amount eaten is more informative than the number displayed on the front of the packet.

On a trial outing, record the amount, time and other foods consumed nearby. You can then review intake and comfort instead of deciding that an entire product category works or does not work. A product may be practical during a break on an ultra but harder to eat on an intense section.

If your planned intake becomes difficult to sustain, consider which parts of the experience changed: portion size, taste, chewing, fullness or digestive symptoms. These observations ask different questions. Recording them separately helps you make a more useful comparison on the next trial rather than changing every part of your food selection.

Worked example: two servings of a bar

Fictional teaching example: a serving contains 6 g of protein. Two servings supply 6 × 2 = 12 g. If you eat them in the same hour, intake for that hour is 12 g/h. If they are spread across two hours, the total remains 12 g with a different distribution.

This number describes intake. It cannot predict tolerance or show that an approach is suitable without considering carbohydrate, other foods and the exercise context.

Frequently asked questions

Do you need protein every hour?

Not systematically. Its usefulness depends on duration and everything you eat. Guidance proposed for an ultramarathon should not become a requirement for a short outing or for every product you carry.

Can a protein bar suit a race?

It may have a place depending on composition, serving size, pace and your tolerance. Check its carbohydrate contribution and other ingredients. Test it in the situation where you plan to eat it instead of relying on its category alone.

Is there a universal maximum amount?

Research does not provide a g/h limit that predicts discomfort in every athlete. A dose studied in hot conditions is not a general threshold. Training trials help you understand your tolerance and the combined intake from different foods.

How Glyk considers this topic

Glyk considers protein among the factors in its estimated digestibility assessment. This concerns an amount of product in an exercise context, rather than the quality of your everyday diet.

The application’s rubric is an interpretation for comparing products. Its reference points are not clinical thresholds or predictions of symptoms. Available information and your own training experience remain complementary.

Understand the Glyk method

Scientific sources

These references inform the guide. Published recommendations, study findings and their interpretation in Glyk remain distinct. They do not validate the application’s scoring rubric.