What is fibre?
Many people associate “high fibre” with healthier food. Fibre consists mainly of plant carbohydrates that human digestive enzymes cannot fully break down. Even if you perform like Messi, Bolt or Phelps, that basic physiology still applies!
A useful broad distinction is soluble and insoluble fibre, although fermentability and viscosity also matter. Neither category is inherently better.
Some soluble fibres form viscous gels that slow gastric emptying and affect nutrient absorption. Many are fermented by gut microbes.
Insoluble fibre can retain water and increase stool bulk, supporting bowel movements and normal intestinal transit. An appropriate transit time matters for bowel function; faster is not always better.
Which foods provide fibre?
Soluble fibres occur in fruit, nuts, pulses, oats and some seaweeds.
Insoluble fibres occur in wholegrains, pulses, peppers, courgettes and leafy vegetables such as spinach, lamb’s lettuce and rocket. Most plant foods contain a mixture.
Check labels: in EU nutrition claims, “source of fibre” requires at least 3 g per 100 g or 1.5 g per 100 kcal. “High fibre” requires at least 6 g per 100 g or 3 g per 100 kcal. These thresholds describe a claim, not the complete nutritional quality of a product.
What is the gut microbiota, and does it matter?
The gut microbiota is the community of microorganisms living in the digestive tract, especially the colon. Fibre reaches the large intestine without being fully digested by our enzymes, and microbes ferment varying amounts. The original estimate that over 50% is degraded is not a fixed figure for every fibre or person.
The microbiota has important roles in digestion and health. Changes are associated with many conditions, but it is too simplistic to blame rising food intolerances on industrial chemicals or microbiota changes alone.
Fermentation of suitable fibres produces gases, including hydrogen, methane and carbon dioxide, which can contribute to bloating. It also produces short-chain fatty acids (SCFAs), involved in several beneficial processes:
- Supporting bowel function, with effects depending on the fibre and individual
- Influencing the intestinal environment and the metabolism of compounds such as bile acids
- Supporting some beneficial microbial populations
- Helping colonic sodium and water absorption
- Some viscous fibres can help reduce LDL cholesterol; this is not an effect of every fibre equally.
- Supporting normal stool formation and transit
- Some fibres act as prebiotics: substrates selectively used by microorganisms that confer a health benefit. They differ from probiotics, which are live microorganisms with demonstrated benefits when supplied appropriately. Not every fibre is a prebiotic.
- Higher-fibre dietary patterns are associated with lower risk of several chronic diseases, including cardiovascular disease and type 2 diabetes; they do not guarantee prevention.
Is everyone’s gut microbiota the same?
No. The historical figures in the source—100 trillion microorganisms, 1000 species, over 3 million genes, 150 times the human gene count and up to 2 kg—are broad older estimates, not precise measurements for every person. The microbiota differs considerably between individuals and changes with diet, environment and health; even the source’s “two-thirds unique” figure is not a fixed biological rule.
When might athletes adjust fibre intake?
Large amounts of fibre close to exercise can cause discomfort in some athletes, particularly during running or intense sessions. Adjust portions of vegetables, wholegrains, fruit skins and nuts to your tolerance and timing rather than removing them from the everyday diet.
Pulses before a race may cause gas for some people, but not everyone. Soaking, rinsing and discarding the soaking water can reduce some fermentable compounds. Follow proper cooking guidance, especially for dried kidney beans; reducing fibre is not the same as making beans safe.
Increase fibre gradually and drink enough fluid for your needs. SCFAs influence colonic water and sodium absorption, but this is not a substitute for replacing significant sweat losses appropriately.
Juice or whole fruit before training?
Juicing removes much of the fibre, unlike blending whole fruit. Juice can be a convenient carbohydrate source in some sporting situations, while whole fruit generally provides more fibre and fullness. Neither produces a universal immediate glucose surge or guarantees hunger soon afterwards. Choose according to portion, timing and tolerance.
Does fibre cause weight loss?
Some viscous fibres influence nutrient absorption and fullness. They can contribute to stomach distension and help a meal feel satisfying. Fibre-rich foods may support weight management, but fibre does not cause fat loss independently of the overall diet.
Are fibre-containing foods always wholegrain?
No. Wholegrains contain the grain’s 3 main components: bran, germ and endosperm. Refining removes much of the bran and germ, reducing fibre and some nutrients, although refined flour still contains protein and other constituents as well as starch.
The original article’s blanket statement about Spanish wholemeal bread labelling is outdated. Current bread standards distinguish fully wholemeal bread from bread made with a stated percentage of wholemeal flour. Check the percentage and the ingredient list; wholemeal flour may come from wheat or other grains. “Multigrain” means different grains, not necessarily wholegrains.
How much fibre is recommended for someone who exercises daily?
Needs depend on age, diet and tolerance. Common reference examples include 14 g per 1000 kcal, or around 25 g for adult women and 38 g for adult men in some guidelines. The source’s alternative of 0.5 g/kg and a 3:1 soluble-to-insoluble ratio should not be treated as a universal athlete prescription; a varied diet matters more than calculating a rigid ratio.
Example: 14 g per 1000 kcal gives around 35 g for someone eating 2500 kcal daily. A 70 kg person would also reach 35 g using the source’s weight-based example. The historical Spanish intake figure of below 20 g daily illustrates the gap described at publication, rather than a newly verified national average.
Fibre-rich foods are useful, but increase them gradually if your usual intake is low. Sudden changes can cause digestive discomfort. Replace some refined foods with wholegrains and gradually add fruit, vegetables, pulses and nuts.
Would a fibre supplement help?
A supplement can be useful for a specific indication, but replacing fibre-rich foods entirely would miss their vitamins, minerals and other nutrients. Choose according to needs and professional advice.
For help designing your personalised diet, explore our sports nutritionist service and prices.
