Dietary fiber
Also known as: Fiber
Dietary fiber consists mainly of carbohydrate compounds that resist digestion and absorption in the small intestine and are partly or fully fermented in the large intestine. It affects bowel function, satiety, the gut microbiota, and how meals are digested. Dietary fiber is unrelated to the contractile fibers that make up skeletal muscle.
Fiber is not one substance. Components differ in viscosity, solubility, fermentability, and water-holding capacity, so the familiar "soluble versus insoluble" split is useful but incomplete. Whole grains, legumes, vegetables, fruit, nuts, and seeds provide different mixtures. A food's fiber content and structure can influence its glycemic response, but fiber does not cancel every effect of the rest of the meal.
The DGE/ÖGE adult guideline is at least 30 grams per day. EFSA concludes that 25 grams per day is adequate for normal bowel function in adults in its opinion on carbohydrates and dietary fiber. These numbers arise from different reference frameworks and are population guidance, not a requirement to hit one exact figure every day. Individual tolerance and medical circumstances can alter practical advice.
Increase fiber gradually if current intake is low. A sudden large increase can cause bloating, gas, cramping, or changes in bowel habits, particularly when fluid intake is insufficient. Adequate hydration matters, and people with gastrointestinal disease or a prescribed low-fiber diet should follow individualized clinical guidance.
Timing can matter for comfort. A very large high-fiber meal immediately before a demanding session may feel heavy or trigger gastrointestinal symptoms, whereas the same food may be well tolerated earlier in the day. Reducing fiber around an event can be a short-term strategy for susceptible endurance athletes; it is not a reason to avoid fiber chronically.
In a training context, do not confuse dietary fiber with slow-twitch muscle fibers or fast-twitch muscle fibers. The former is a nutritional category; the latter are types of muscle cells involved in force production. Logging one provides no direct information about the other.