Fat during exercise: energy and digestive comfort

Key takeaways

Using stored fat and eating fat during exercise are different things.
A gel without fat is not incomplete for that reason.
During very long events, foods containing fat can contribute energy and variety.

How the body uses fat

During endurance exercise, the body uses a mixture of fuels. The contribution of carbohydrate and fat changes with factors including intensity and duration. A study by Romijn and colleagues described these changes in trained participants. It examined the body’s use of stored and circulating fuels, not an amount of fat that should be eaten every hour.^[1]

Fat stored in the body provides substantial energy reserves. Using those reserves does not require immediately putting the same amount of fat into your stomach. This distinction matters when reading a label: a food can contain no fat while your body continues to use fat during the activity.

An outing also rarely stays at one perfectly constant intensity. A climb, acceleration or easier section can change the demands. It is therefore unhelpful to infer a complete eating strategy from ‘I run slowly, so I should eat fatty foods’. Your chosen products also need to support your intake and let you continue comfortably.

Do you need to eat fat during every activity?

A product used during a race does not need to resemble a balanced everyday meal. Its role may be to provide carbohydrate in a convenient format. The absence of fat in a gel is therefore not a drawback in itself. It also does not mean an athlete’s entire diet should be low in fat.

The useful question concerns the contribution you want. Are you looking for a quick intake, something more substantial during a break or a change of flavour on a long outing? A product that serves one purpose well may be less practical for another. Its fat content is information to place within that situation.

Equally, the presence of fat does not automatically make a bar unsuitable. A small, well-tolerated serving may fit your eating during exercise. Avoid turning one number into a universal ranking of gels, bars and solid foods. Look at serving size, carbohydrate supplied and the point when you can realistically eat it.

Its place in very long events

As exercise continues, food variety can become important. A flavour that is welcome at the start may be harder to accept after many servings. Foods containing fat can add energy and contribute variety. Their usefulness then depends on everything you eat and what you can keep consuming.

The ISSN position stand on single-stage ultramarathons considers energy intake from several macronutrients and highlights palatability and tolerance. Its context is very long events. It does not make fat consumption compulsory on every endurance outing or establish one food choice as the right solution for every participant.^[2]

At an aid station, consider what you have already eaten and what you can still manage. Food chosen to break monotony does not need to meet every intake goal alone. Consider variety, carbohydrate contribution and comfort separately, then check that their combination remains practical for the rest of the event.

Digestion, amount and context

Foods high in fat can take longer to digest. Large amounts may contribute to heaviness or nausea in some people. That does not mean any fat content causes a problem. Serving size and the complete food also matter. Your experience should be considered in the context of what you actually consumed.

Exercise-related symptoms have several possible causes, including heat, intensity, intake volume, food and individual sensitivity. The 2025 SDA–USSF position statement emphasises this diversity. Findings concerning low-carbohydrate, high-fat diets must not be treated as the effects of eating a single bar containing fat.^[3]

Describe a trial accurately. Did you eat a large portion at once? Was the product new? Were you on an intense section? Had you changed your drink or added other foods? These details do not prove a cause, but they make it easier to plan a more informative comparison next time.

No universal fat quantity in g/h guarantees comfort or predicts symptoms. A limit selected in an assessment tool remains an interpretive reference. It does not describe how every body will respond. Repeated experience with a known serving is more useful than drawing a broad conclusion from one isolated episode.

Choose and test your foods

Start with the serving you actually plan to eat. If values are given per 100 g, adjust them to the weight consumed. Then add repeated servings. Keep absent data unknown: the appearance of a bar or its name cannot reconstruct its nutritional contribution. The packet may also contain more than one declared serving.

Read carbohydrate alongside fat. Two servings with different carbohydrate amounts may not fulfil the same role during exercise. For a useful comparison, first decide what contribution you want, then examine the fat supplied with the amount needed. This avoids automatically preferring one format without considering what it provides.

Test practical aspects too: opening, chewing, taste after repeated servings and whether you can drink comfortably alongside it. Food that is pleasant while stationary may become difficult to eat in motion. Choosing a point when your pace allows comfortable eating can matter as much as nutritional composition.

After a trial, record consumption and how it felt. Distinguish fullness, flavour fatigue, difficulty chewing and digestive symptoms. Change an identifiable element for the next trial. The aim is to find foods that help you keep eating throughout the activity without imposing the same recipe on everyone.

Consider the full planned outing when packing. A convenient food in the first hour may be less appealing later, while food reserved for a break serves a different purpose from something eaten on the move. Keeping those uses clear helps you assess each product against a real situation instead of an abstract idea of the best endurance food.

Worked example: repeated servings

Fictional teaching example: one serving contains 4 g of fat. Three servings supply 4 × 3 = 12 g in total. Spread over three hours, the average is 4 g/h. If the servings are grouped together, the timing is different.

The total and average describe consumption, not a digestive limit. Also note carbohydrate and other foods consumed in the same period to understand the product’s full contribution.

Frequently asked questions

Is a gel without fat incomplete?

Not for that reason. A gel can fulfil its role by supplying carbohydrate during exercise. Each intake does not need to reproduce an everyday meal’s composition, and your body already has fat reserves available.

Is fat always difficult to digest?

No. The amount, complete product, context and your tolerance matter. A small serving can be well accepted. Discomfort after a fatty food does not establish that fat caused every symptom.

What place can it have on an ultra?

Foods containing fat may contribute energy and variety during very long exercise. Check that they remain easy to eat alongside your other intake. There is no universal hourly fat target to apply to every athlete.

How Glyk considers this topic

Glyk includes fat in its estimated digestibility assessment for exercise. This interpretation helps compare a food contribution within a context; it does not judge the value of fat in your everyday diet.

The rubric’s reference points are not scientific symptom thresholds. The score does not replace training trials or know every food consumed during your race. The method explains the assessment’s role and limits.

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.