Hello Runner,

Welcome back to your weekly moment of endurance nutrition!

Happy Weekend!
How was your week?

I had a good week.
This time, I ran three times during the week (as outlined in my training plan); before, I used to swap one day of running for a bike ride.
I also worked hard and really focused on my studies.
This week I realized that there are almost 12 weeks left until my second half marathon. The training cycle for another race is about to begin. This time, my goal is to improve my time. 

Not long ago, I talked about excessive protein intake, which isn’t beneficial for runners.

Today, the focus is on fiber. It’s a nutrient that’s extremely important for gut health and directly impacts your training, as it can either hinder or enhance your performance.

It’s 2026, and many runners still think that fiber is only related to “making a nice, firm poop”, but it goes far beyond that.

Let me explain.

Fiber is the part of plant foods that our digestive system cannot completely break down.

Some fibers pass through the digestive tract largely intact, helping provide bulk and support regular bowel movements. Others are fermented by our gut bacteria. This fermentation produces compounds called short-chain fatty acids (SCFA), which are involved in maintaining the health of the intestinal environment and influence metabolism + immune function.

For runners, dietary fiber is important for the gut microbiome, metabolic and cardiovascular health, bowel regularity, and overall dietary quality.

The most common complaints I hear in my practice are:

1- Constipation

2- Difficulty having a bowel movement before workouts

3- Urge to have a bowel movement when starting a workout

4- GI discomfort from carbohydrates during workouts

5- Low tolerance for carbohydrate intake during workouts

The common root causes → chronic dietary habits. Such as:

In most cases, the key is to adjust the amount of carbohydrates, hydration, and fiber in the diet.

The focus will always be on including high-fiber foods, since these foods provide much more than fiber. They also supply vitamins, minerals, antioxidants, polyphenols, carbohydrates, healthy fats, and, in some cases, plant protein.

However, some runners lead very busy lives, and in some cases, we may consider fiber supplements to help.

If you are prone to GI problems, the best options are less-fermentable or slowly fermentable fibers that are easier to tolerate.

Examples include:

Benefits → improve gut health without increasing gas; support beneficial microbiota (Bifidobacterium, Lactobacillus); stimulate SCFA production; strengthen the intestinal barrier; improve GI tolerance and performance.

✅ Foods that are not highly fermentable/low in gas (good for runners): oats, flaxseed, chia, oat bran, brown rice, walnuts, almonds, vegetable peels.

By contrast, large amounts of highly fermentable fibers such as inulin, FOS, GOS and resistant starch can produce more gas in sensitive people.

⚠️ Avoid: chicory root, artichoke, garlic, onion, leeks, legumes, cooled potatoes, cooled rice, green bananas, some specially processed starch products.

In practical terms, about 25–38 g/day is a useful range for many adult runners. However, your ideal intake depends on your energy needs and individual tolerance.

As a general rule, try to have a substantial fiber-rich meal about 3–4 hours before a hard or long training.

→ If you have a sensitive stomach, I'd say 5-4 hours before your long training.

If you are eating only 1–2 hours before running, consider choosing lower-fiber, easy-to-digest foods such as toast, a bagel, rice cakes, low-fiber cereal, applesauce, or a banana, depending on what you know your stomach tolerates.

→ During long runs, your priority is usually carbohydrate, fluids, and electrolytes rather than fiber.

If you like energy bars, be careful: many of them are high in fiber. Always read the nutrition facts label.

Whenever I see a runner in my practice with any of the complaints I mentioned above, most are afraid to eat fiber because they think they’ll have to go to the bathroom in the middle of their training. And that fear causes them to eat less than half the daily recommended amount.

In addition to adjusting quantities, I also adjust the timing of food intake. And that’s what will really help.

Sometimes, we don’t get it right the first time. But during my consultations, we make adjustments and test things out to see what works best.

If the topic of fiber still makes you feel uncomfortable or confused, I can help. Schedule a call, and let’s talk.

Scientific References

Any questions or something you would like to share? Drop me an email

Wishing you a great weekend and week ahead!

Here’s to health and good runs⚡️

Ana Paula Alonso