Fiber in an Athlete's Diet

Fiber is a food component that athletes rarely think about: it provides no energy for training and does not build muscle. Yet it is largely fiber that determines satiety on a diet, gut health and even comfort during a run. The editorial team explains how much fiber an athlete needs and when it is worth temporarily limiting it.

What fiber is and what its types are

Dietary fibers are carbohydrates of plant origin that are not digested by the enzymes of the human small intestine. They reach the large intestine, where they are partly fermented by the microbiota to form short-chain fatty acids, and partly excreted unchanged.

Traditionally fiber is divided into soluble and insoluble. Soluble fibers (beta-glucans of oats and barley, pectins of fruit, gums of legumes) form a gel, slow gastric emptying and the absorption of glucose, and also lower LDL cholesterol levels. Insoluble ones (cellulose, hemicelluloses of bran) increase the volume of fecal masses and speed up transit.

Separately, fermentable fibers and resistant starch are distinguished — 'food' for beneficial bacteria. It is precisely they that most affect the composition of the microbiota, but at the same time they most often cause gas formation and bloating, especially with a sharp increase in their amount.

Health benefits and consumption norms

The largest review to date is the series of systematic reviews and meta-analyses by Reynolds and colleagues (2019) in The Lancet. People with the highest fiber consumption had lower all-cause mortality and a lower risk of coronary heart disease, stroke, type 2 diabetes and colorectal cancer compared with those who consumed the least.

The authors estimated that the greatest benefit is observed at a consumption of approximately 25–29 g of fiber a day, and still higher levels may provide additional protection. The EFSA (2010) considers 25 g a day sufficient for normal bowel function in adults.

Actual consumption in many European countries is below the recommended level. Athletes who focus on 'clean' protein food — chicken, fish, eggs, protein powder — risk getting even less fiber, especially if they limit fruit and grains on a diet.

Fiber consumption is also linked to the composition of the microbiota. In a study by Clarke and colleagues (2014), professional rugby players had greater gut microbiota diversity than a control group, which the authors attributed both to physical activity and to features of the diet.

Fiber and athletic form

On a weight-loss diet, fiber is one of the most useful allies. Bulky products with a large amount of fiber — vegetables, legumes, whole grains — give a feeling of fullness at a lower calorie intake. This makes it easier to maintain a deficit without constant hunger.

Fiber slows the absorption of carbohydrates and lowers the glycemic response to a meal. In ordinary meals this is an advantage: energy levels are more stable, with fewer fluctuations in appetite. But it is precisely this effect that becomes a drawback when you need to get energy quickly before a start.

During gaining mass, an excess of fiber can become a problem of a different kind: bulky food fills you up quickly, and reaching the required calorie intake becomes hard. In that case it is appropriate to get part of the carbohydrates from less bulky sources — white rice, bread, juices — while keeping fiber at about the recommended norm.

Products rich in fiber usually also contain potassium, magnesium, folate, polyphenols and other beneficial compounds. So it is better to 'get' fiber from whole food, and to consider supplements such as psyllium an auxiliary means.

When fiber should be limited

Gastrointestinal symptoms are one of the most frequent complaints of athletes in endurance disciplines, especially runners. The review by de Oliveira, Burini and Jeukendrup (2014) names a high content of fiber, fats and protein in food shortly before exertion among the dietary risk factors. Therefore, the day before a start and on competition day, fiber is usually reduced.

For athletes with chronic gastrointestinal problems during exertion, a diet low in FODMAPs — fermentable oligosaccharides, disaccharides, monosaccharides and polyols — is being studied. In a pilot study by Lis and colleagues (2018), a short-term reduction of FODMAPs reduced symptoms in some runners. Such a diet should be temporary and, preferably, under the supervision of a specialist.

A sharp increase in fiber also causes discomfort: bloating, gas, cramps. The gut and microbiota need time to adapt. Consumption should be increased gradually, over several weeks, while simultaneously increasing the amount of water drunk.

• add one new source of fiber per week;
• drink enough water, especially with psyllium supplements or bran;
• 1–2 days before an important start, switch to lighter, less 'fibrous' products;
• don't try new legume or vegetable dishes before competitions.

Editorial conclusions

Fiber does not add energy for training, but it substantially affects health, satiety and gut function. The guideline for adults is at least 25 g a day, and the optimal benefit is observed at approximately the range of 25–29 g and above.

On a diet, fiber helps to control appetite, and during gaining mass it is advisable to keep it at the norm level, without overloading the diet with bulky food.

Before starts and long efforts, fiber is temporarily reduced to avoid gastrointestinal discomfort.

We also recommend our materials on nutrition on competition day, on calculating macros for weight loss and on vegetarianism in strength sports.

References

1. Reynolds A, Mann J, Cummings J, et al. Carbohydrate quality and human health: a series of systematic reviews and meta-analyses. Lancet. 2019;393(10170):434–445.
2. EFSA Panel on Dietetic Products, Nutrition, and Allergies. Scientific opinion on dietary reference values for carbohydrates and dietary fibre. EFSA J. 2010;8(3):1462.
3. de Oliveira EP, Burini RC, Jeukendrup A. Gastrointestinal complaints during exercise: prevalence, etiology, and nutritional recommendations. Sports Med. 2014;44(Suppl 1):S79–S85.
4. Lis DM, Stellingwerff T, Kitic CM, et al. Low FODMAP: a preliminary strategy to reduce gastrointestinal distress in athletes. Med Sci Sports Exerc. 2018;50(1):116–123.
5. Clarke SF, Murphy EF, O'Sullivan O, et al. Exercise and associated dietary extremes impact on gut microbial diversity. Gut. 2014;63(12):1913–1920.
6. Thomas DT, Erdman KA, Burke LM. American College of Sports Medicine joint position statement: nutrition and athletic performance. Med Sci Sports Exerc. 2016;48(3):543–568.