How much is too much fibre?

The Link Between High-Fibre Diets and RED-S

As athletes looking to get the most out of our training, we have a tendency to prioritise the quality of the food we put into our bodies. We want premium, high-octane fuel; superfoods; whole foods; organic. And to some extent, we should! Training is inherently breaking the body down, in hopes that – provided with rest and the right nourishment from our diet – we can build back stronger. 

The tricky part is, as we ramp up our energy expenditure, we also ramp up our energy demand, and sometimes, focusing on “clean” nutrient-dense foods can actually stand in the way of optimising our training and nutrition. 

Yes – fruits, vegetables, and whole grains are important for overall health and well-being, but when trying to match the energy demands of an athletic lifestyle, overdoing it with these foods can set the stage for RED-S. 

How? There are a couple of avenues, but they intersect at fibre-content. 

Fibres are a type of carbohydrate that the body is unable to break down, and they are abundant in foods sourced from plants. There are a number of important roles that fibre plays in the body, including regulation of blood sugar and cholesterol in the bloodstream, feeding the good bacteria found in your gut to help maintain a healthy microbiome, and adding bulk and weight to your stool to help keep you regular! Meeting the recommendation for daily fibre intake may also lower your risk of chronic illnesses such as diabetes, heart disease, and colon cancer.  

Like the general population, athletes definitely need fibre in our diets. However, when we’re eating significantly larger amounts of food than our sedentary counterparts, we also have to be mindful of keeping the amount of fibre we consume in check. It might seem counter-intuitive, and is DEFINITELY counter to diet culture narratives, but oftentimes, swapping foods glorified for their health benefits for ones lower in fibre will help an athlete reach a more optimal energy balance, and consequently better health. 

It’s an example of “too much of a good thing”. As athletes, we might need to shift our thinking toward: everything in moderation, including superfoods.

Sure, foods like leafy greens, chia seeds, quinoa, and avocado can be incorporated into a healthy athlete’s diet, but it might be worth pairing these meals with some easy-to-digest simple carbohydrates to ensure you’re fueling your body for the demands of your sport or else you run the risk of energy deficiency and RED-S. There’s nothing wrong with a good old peanut butter and jelly on white bread. 

A 2021 review paper considering nonpharmacological strategies to treat RED-S presented a  variety of causes for low energy availability (LEA) relating to fibre, including the following:

Decreased energy intake: Fibre-rich foods are typically high-volume, but low in calories. By filling our plates and bellies with veggies, fruits, and whole grains, we can unintentionally fall behind on meeting our overall energy needs. 

In addition to taking up a large amount of space in our stomachs, high-fibre foods are difficult for the body to digest and take longer to pass through the body. This can leave an athlete feeling full long after a meal was consumed, increasing the likelihood of an energy deficit continuing throughout the day.

Foods that are slower to digest may also increase the likelihood of digestive upset during training, causing bloating, cramping, and possibly an emergency stop behind a bush. Low-fibre foods and simple sugars are generally recommended before and during training, which may help both improve overall energy intake and reduce the risk of an unhappy belly. Additionally, reducing bloating and digestive upset might make it easier and more comfortable to eat later on. 

Oestrogen Interference and Bone Health: A variety of studies featured in the review paper suggest that fibre intake can affect the body’s ability to maintain optimal oestrogen levels in females. There is currently insufficient research in how this manifests in males, who also produce oestrogen in lower quantities.

Increased dietary fibre has been linked to decreased oestrogen levels in healthy females independent of physical activity, which can also disrupt the production of other hormones and cause menstrual cycle irregularity. This may be caused by a mechanical interference where fibre reduces the body’s ability to reabsorb oestrogen circulating in the bloodstream, but it can compound the effects of RED-S, which already leads to a decrease in female sex hormones. 

A 2016 study looked at the connection between fibre intake, bone mineral density, and menstrual cycle irregularity in female endurance athletes. In contrasting females with irregular cycles to athletes with healthy cycles and a non-athlete control group, it was found that athletes with irregular cycles also had significantly higher intake of dietary fibre. These athletes also had the lowest bone mineral density. 

This research suggested that fibre can bind with minerals like calcium, magnesium, and iron, making them unavailable for use in the body. This lack of key nutrients, combined with decreased levels of bone-supporting oestrogen, sets the stage for poor bone health, which is perhaps one of the most severe and long-lasting consequences of RED-S.

In short, chronic energy deficits are bad, but chronic energy deficits combined with a high-fibre diet might be worse. 

For athletes struggling with RED-S, increasing energy intake alone might not be enough. For those who consume a high-fibre diet, working with a dietitian to make purposeful changes to reduce fibre intake and increase overall energy availability might be an important step in the recovery process. 

We’re here for you! Check out our list of specialists to connect with a dietitian with experience in sports nutrition and RED-S.