Dietary fibre belongs to non-starch polysaccharides, that are very resistant to enzymatic breakdown in the human gut. This means that only a small portion can be broken down to saccharides and subsequently absorbed into the bloodstream. Dietary fibre is an important component of a balanced diet. It is currently recommended that people ingest at least 30g of dietary fibre per day. Since fibre has a very low energy yield, its inclusion in the diet reduces its caloric density. They also improve digestion and reduce gastric emptying. Higher intake of fibre is linked to better health outcomes and has been associated with improved blood cholesterol levels, lower risk of cardiovascular disease, and improved gut microbiota, etc. All well and good, but is more really better? Do athletes whose daily energy intake is very high benefit from very high amounts of fibre as well? Here, we present you some thoughts on why athletes should be careful with their fibre intake.
Athletes, especially endurance athletes, require a higher energy intake because their energy expenditure is increased by training. Long training sessions must be fuelled properly with a sufficient amount of carbohydrates. Large amounts of food and especially carbohydrates, are very often combined with a lot of fibre. whole grain pasta, brown rice, oats are all high in fibre. This means that athletes can end up eating loads of fibre. And because they contribute to the feelings of satiation – one would feel full quicker and consequently feel less hungry for longer. This could then present a risk for inadequate food intake – also known as low energy availability (LEA), where an athlete does not consume enough energy to cover the needs of energy expenditure. The consequences of LEA or REDs have already been presented previously.
Second, foods high in fibre have lower energy density. Energy density represents the amount of energy per amount of food. For example, oil or peanut butter has a very high energy density, while lettuce has a very low energy density. By adding lots of vegetables to meals, you can reduce energy density. Experimental studies have shown that less energy-dense diet limits energy intake . In addition, one study showed that female athletes suffering from menstrual irregularities ate significantly less energy-dense meals compared to normally menstruating athletes. The same study, they reported that subjects with functional hypothalamic amenorrhea had 32% higher fibre intake than their eumenorrheic counterparts .
How fibre intake affects reproductive function and hormones is not yet clear. But the BioCycleStudy linked high fibre intake to lower oestrogen levels as well as lower LH, FSH, both hormones secreted by the pituitary gland, and progesterone. Interestingly, when the consumption of fibre was above dietary reference intake women experienced 22% anovulatory cycles compared to 7% in case of lower fibre intake. There could be many explanations for the mechanisms of why high fibre intake causes a reduction in circulating reproductive hormones. From decreased oestrogen reabsorption from the colon to a possible influence on the hypothalamus and LH and FSH secretion .
Gastrointestinal (GI) distress
Gastrointestinal problems, such as stomach pain/cramps, intestinal discomfort, urge to defecate and flatulence, are very common in endurance athletes. Gastrointestinal problems were more likely to occur after the ingestion of fat, protein and fibre. Firstly, these foods cause a delay in gastric emptying and secondly, they can cause the passage of fluids into the intestine and thus higher likelihood of the GI discomfort . Foods high in soluble fibre (oats, beans, apples etc.) are more prone to cause gas production through fermentation and consequently bloating. Therefore, limiting high fibre foods before the exercise/race could reduce the aforementioned symptoms.
Pre-race nutritional strategy often includes carbohydrate-rich foods to achieve adequate glycogen loading. Glycogen loading in itself leads to weight gain, as its synthesis binds the water. However, grains often contain higher amounts of fibre, especially unprocessed grains such as brown rice, whole-wheat pasta, oats, etc. During digestion in the intestines, the fibre binds the water, which further increases body weight and may later cause GI distress during the race. Ideally, your pre-race nutrition strategy should include low-fibre carbohydrate sources. Opt for gummy bears.
Higher fibre intake is associated with better health outcomes. However, in athletes, higher energy intake is more difficult to achieve when the diet is rich in fibre. Due to the greater amount of food required to meet the energy needs of athletes, the recommended fibre intake is usually exceeded. Additional fibre intake would probably not provide further benefits and should not be recommended in some cases (e.g., low energy availability). Higher fibre intake has been associated with lower oestrogen levels and consequently menstrual dysfunction, which is linked to many comorbidities and has a negative impact on performance. Therefore, it should be reconsidered if high fibre foods are truly beneficial for athlete health and performance.
1. Rolls, B.J., The relationship between dietary energy density and energy intake. Physiol Behav, 2009. 97(5): p. 609-15.
2. Melin, A., et al., Low-energy density and high fiber intake are dietary concerns in female endurance athletes. Scandinavian Journal of Medicine and Science in Sports, 2015. 26.
3. Gaskins, A.J., et al., Effect of daily fiber intake on reproductive function: the BioCycle Study. The American journal of clinical nutrition, 2009. 90(4): p. 1061-1069.
4. de Oliveira, E.P., R.C. Burini, and A. Jeukendrup, Gastrointestinal complaints during exercise: prevalence, etiology, and nutritional recommendations. Sports medicine (Auckland, N.Z.), 2014. 44 Suppl 1(Suppl 1): p. S79-S85.