Key takeaways
Fibre is the main food for gut bacteria, which turn it into short-chain fatty acids such as butyrate
Health benefits in large studies were greatest at about 25 to 29 g of fibre a day, which many athletes undershoot
Eat plenty of fibre in training but cut back for a day or two before races and in the pre-race meal to reduce gut trouble
Endurance athletes are told two things that seem to clash: eat plenty of fibre for your gut and your long-term health, and avoid fibre before races. Both are right. The trick is timing. This guide covers what fibre does for your gut microbes, how much to aim for, and how to periodise it around training and racing.
What fibre does in your gut
Humans cannot digest most fibre, but gut bacteria can. A review by Makki et al. (2018) in Cell Host & Microbe explains how dietary fibres interact directly with gut microbes, which ferment them into short-chain fatty acids such as acetate, propionate and butyrate. These compounds feed the cells of the colon, help maintain the gut barrier, and influence immunity and metabolism.
The type, quality and origin of your food shape which microbes thrive. That is why diet is the strongest known driver of the microbiome, stronger than exercise. It also explains why studies of athletes' guts (see gut microbiome and athletic performance) struggle to separate training from what athletes eat.
How much fibre?
A large Lancet series by Reynolds et al. (2019) combined 185 prospective studies and 58 clinical trials. Comparing the highest with the lowest fibre intakes, observational data suggested a 15 to 30% lower risk of death from all causes and cardiovascular disease, and of coronary heart disease, stroke, type 2 diabetes and colorectal cancer. The benefit was greatest at 25 to 29 g of fibre a day, with hints that more could be better. Those outcomes overlap heavily with the ones fitness protects against, as covered in VO2max and longevity and VO2max and diabetes.
Many people, including athletes who lean on refined carbohydrate and sports products, fall short of that.
Fibre, fermented foods and microbial diversity
Does eating more fibre make your microbiome more diverse? Not necessarily, at least in the short term. In a 17-week randomised trial, Wastyk et al. (2021) assigned healthy adults to a high-fibre or a high-fermented-food diet:
- The high-fibre group's microbes gained more fibre-degrading enzymes, but overall diversity stayed stable.
- The fermented-food group (yoghurt, kefir, kimchi, sauerkraut and similar) saw diversity steadily increase and inflammatory markers fall.
It was a small study and did not look at performance, but it suggests variety, including fermented foods, matters. Like most microbiome research, these findings are early.
Periodising fibre around training and races
| Phase | Fibre approach | Why |
|---|---|---|
| Base training (zone 2 volume) | Generous, 25 to 30 g a day or more, from varied plants | Long-term health and gut microbes |
| Heavy training blocks | Keep fibre, add easily digested carbs | Meet high energy and carbohydrate needs |
| 1 to 2 days before a long race | Reduce fibre and fat | Less bowel content and gas |
| Pre-race meal (2 to 4 h before) | Low fibre, low fat, moderate protein, high carb | Faster gastric emptying |
| During long events | Simple carbs, drinks, gels | Fast absorption |
| After the race | Return to normal diet | Restore usual intake |
What sports nutrition guidance says
The joint position statement of the Academy of Nutrition and Dietetics, Dietitians of Canada and the American College of Sports Medicine (Thomas et al., 2016) sets carbohydrate targets by training load and advises that pre-event meals be relatively low in fat and fibre, high in carbohydrate, and familiar, to reduce gut problems. The statement also recommends that athletes get a personalised plan from a registered sports dietitian.
Carbohydrate itself trains the gut. Jeukendrup (2017) reviewed evidence that a high-carbohydrate diet increases the number and activity of intestinal glucose transporters, allowing more carbohydrate to be absorbed during exercise. So the low-fibre phase before a race should still be high in carbohydrate.
Practical tips
- Build fibre gradually. Sudden increases cause gas and bloating; your microbes need weeks to adapt.
- Pick varied sources. Oats, beans, lentils, whole grains, vegetables, fruit, nuts and seeds feed different bacteria.
- Add fermented foods if you enjoy them. Probiotic supplements are a different question; see our probiotics guide.
- Balance fibre with energy. In heavy training, very high fibre can blunt appetite and leave you short on carbohydrate and protein. Under-fuelling hurts adaptation; see GLP-1, appetite and fitness for why energy intake matters.
- Go low-fibre before key races, and rehearse it before long training runs. If symptoms persist, a short low-FODMAP trial is covered in runner's stomach.
- Hydrate. Fibre works best with enough fluid, and long sessions need sodium too; see our electrolytes guide.
- Exercise regularly. Training itself shifts the microbiome; see exercise and gut health and gut microbiome and VO2max.
Fibre supports health; training builds fitness. For the engine, see how to improve VO2max and track your progress with the VO2max calculator.
If you have persistent bloating, pain, a change in bowel habit, blood in your stool or unexplained weight loss, see a doctor before changing your diet drastically. People with IBS or inflammatory bowel disease may need a tailored plan.
Frequently asked questions
How much fibre should an endurance athlete eat?
In general, aim for at least 25 to 30 g a day from varied plant foods, which is where large analyses saw the greatest health benefit. Athletes with high energy needs often eat more without problems.
Should I eat low fibre before a race?
Many athletes reduce fibre for one to two days before a long race and choose a low-fibre, low-fat pre-race meal, which sports nutrition guidance recommends to lower the risk of gut problems.
Do prebiotics or fermented foods help athletes?
Fermented foods increased gut microbial diversity in a 17-week trial in healthy adults, but there is little direct evidence for performance. They are a reasonable part of a varied diet.
Can a high-fibre diet hurt my training?
Rarely, if you adapt gradually. Very high fibre can be filling and make it hard to eat enough energy and carbohydrate during heavy training, so balance it with easily digested carbs.
Sources
- Makki K, Deehan EC, Walter J, Bäckhed F. The impact of dietary fiber on gut microbiota in host health and disease. Cell Host Microbe, 2018.
- Reynolds A, et al. Carbohydrate quality and human health: a series of systematic reviews and meta-analyses. Lancet, 2019.
- Wastyk HC, et al. Gut-microbiota-targeted diets modulate human immune status. Cell, 2021.
- Thomas DT, Erdman KA, Burke LM. Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: nutrition and athletic performance. J Acad Nutr Diet, 2016.
- Jeukendrup AE. Training the gut for athletes. Sports Med, 2017.