Fibre: The Athlete’s Nutritional Secret Weapon
When you think of nutrition for athletes, you probably think of calories, protein, carbs, and sports supplements.
The mention of fibre, fruits, and vegetables often gets relegated to the realm of a doctor, parent, or some other authoritative figure trying to encourage you to eat more.
You might see them as something you know is "good for you".
But you might not fully appreciate what that means and the impact it can have.
Because of this lack of clarity, you may not end up putting as much focus on eating sufficient fibre as you would on other elements of nutrition, like hitting your calorie and macronutrient targets.
The aim of this article is to give you that clarity (and hopefully encouragement) by providing a concrete idea of what fibre is, why it can help you as an athlete, and how to go about improving your intake.
What is Fibre?
Fibre refers to the part of plant foods that are technically indigestible by us as humans.
Because it's indigestible, it passes through most of your digestive system.
Then it reaches your large intestine where the majority of the gut microbiome resides.
The word microbiome here refers to the community of living microorganisms (e.g. bacteria) that reside in our gut.
As mentioned, we can't digest the fibre, but the microorganisms here can.
Technically, they ferment the fibre, but we can easily think of it as them consuming the fibre, using it as energy, and excreting waste products.
What are waste products to them are extremely beneficial to us.
These waste products are known as short-chain fatty acids.
These short-chain fatty acids are associated with a range of health improvements in the human body, which we'll get to later.
The type of fibre I've spoken about so far - the type that we can't digest, but the gut bacteria can - is known as soluble fibre.
Soluble here refers to the fact that if you put it in water, it would dissolve, break down, or mix with the water.
The other type of fibre, insoluble fibre, doesn't dissolve in water.
This type of fibre can't be digested even by the gut microbiota.
So whilst it breaks down somewhat through chewing and passing through the gut, the fibre structure itself remains solid.
This fibre then mixes with the rest of the food we eat, bulking it up, and passes completely through the digestive system.
This isn't a bad thing.
For one, it slows down the digestion process, which is beneficial at times, albeit not at others, which we'll get to later.
It also forms the bulk of our faeces, so without sufficient amounts of it, we don't have the necessary bulk to signal the need to use the toilet, which leads to potential problems that we'll also discuss later.
Why is Fibre Important for Athletes?
So we've established that fibre feeds the gut bacteria, whilst also regulating the passing of food through the digestive system.
So let's look at how that can practically improve the outcomes we care about as athletes.
1. Better Recovery and Training Adaptations
Intense training creates short-term inflammation in the body.
This is completely natural and part of what signals the body to adapt to the training.
But if inflammation is high on an ongoing basis, it impairs the body's ability to recover and adapt.
The short-chain fatty acids produced when the gut bacteria ferment fibre have direct anti-inflammatory effects throughout the body.
They also improve the integrity of the gut lining, which means fewer inflammatory compounds (like undigested food particles, bacterial endotoxins, pathogenic bacteria) get into the bloodstream.
Both these effects mean more of the body's resources can go towards recovery and other processes involved in training adaptations, rather than towards managing unnecessary inflammation.
Practically speaking, that could mean faster recovery, getting greater training benefit from each session, and being able to handle higher training volumes over time.
2. Reduced Cravings and Hunger
Many athletes struggle with hunger, particularly during high training volumes or when trying to reduce body fat.
High-fibre foods increase satiety through several mechanisms.
Firstly, they take longer to eat and to pass through the stomach, allowing more time for satiety hormones to be released.
They also fill up the stomach more, triggering an increase in the release of satiety hormones.
As well as that, the short-chain fatty acids produced from fibre fermentation can have a longer-term effect of directly influencing appetite centres in the brain, reducing cravings and impulsive eating.
They do all of this whilst also generally being lower in caloric content than other foods, which can mean being able to eat more food without adding additional calories.
3. Increased Nutrient Absorption
I already mentioned how fibre can help slow down the transit of food through the digestive system.
When taken to the extreme, this can mean constipation and the negative effects of that, namely the discomfort and potential for toxin reabsorption that can come along with that.
However, this generally only occurs when you increase fibre too quickly relative to your baseline.
When done gradually, however, it slows the transit of food down to a speed that allows the body sufficient time to break down and absorb the nutrients in the food.
As well as that, the short-chain fatty acids nourish the cells in the gut that are responsible for absorption.
As well as that, the gut bacteria themselves can produce vitamins like K2 and B-vitamins as part of the fibre fermentation process.
All of this means that by consuming sufficient fibre, you increase the absorption and potential benefits you get from the foods and supplements you are consuming in pursuit of your athletic goals.
4. More Stable Energy Levels
Your blood glucose (or blood sugar) levels fluctuate naturally throughout the day.
You might want to increase them during athletic activities to increase the glucose supply to the muscles.
But you might not want a big spike in blood sugar during the workday, where it can be followed by a dip, leading to what you might feel as an energy crash.
You might also experience a dip in blood glucose as a result of having spent too long without eating.
As well as that, physical activity itself can also have an effect.
Large fluctuations in blood sugar can lead to unpredictable energy levels, both in everyday life and in training.
Sufficient fibre intake can help reduce these fluctuations.
Firstly, fibre slows carbohydrate absorption, resulting in the glucose gradually entering the blood rather than quickly.
This can help avoid a rapid spike and the potential subsequent "crash".
Fibre also has the longer-term effect of increasing insulin sensitivity, which means that when blood glucose increases, the body is able to move it into the muscles more quickly.
For athletes, all of this means more efficient distribution of the carbohydrates you're consuming in preparation for training/competition, and stable energy levels throughout the day and in training/competition.
5. Improved Digestive Function
As I've already mentioned, one of fibre's most fundamental roles is maintaining healthy bowel movements.
Insoluble fibre adds bulk to stool and helps food move through the digestive tract at an optimal pace, preventing constipation and ensuring waste products are eliminated efficiently.
For athletes, this matters more than you might think.
For example, constipation can cause abdominal discomfort, bloating, and a general feeling of sluggishness that interferes with training quality.
You may have experienced this if you've ever gone out to train with a dodgy stomach.
As well as that, when waste products remain in the digestive system for too long, there's potential for reabsorption of toxins and metabolic waste that should be eliminated.
Regular, comfortable bowel movements - typically once or twice daily - indicate a well-functioning digestive system.
Adequate fibre intake (combined with proper hydration) is the most reliable way to maintain this regularity.
6. Less Time Getting Sick
Exercise enhances immunity over the long-term, which is great news for athletes.
However, athletes also have to contend with the fact that intense training can temporarily suppress immune function.
You also have the issue in team sports when there is a lot of close contact with other athletes, as well as shared spaces like changing rooms, and sometimes objects like water bottles, gym equipment, and towels.
Fibre intake modulates this immune challenge through several mechanisms.
The short-chain fatty acids produced from fibre fermentation strengthen the gut lining/barrier, which prevents pathogens from getting into the bloodstream.
Along with that, 70-80% of the body's immune cells are created in the gut, and the short-chain fatty acids that come from the fermentation of fibre are critical to the production of these immune cells.
Research backs this up, where athletes with higher fibre intakes experience fewer upper respiratory infections, and recover more quickly when illness does occur.
This is a significant performance advantage given how easily training can be disrupted by even minor illnesses.
"Your best ability is availability" is a phrase worth keeping in mind.
7. Decreased Long-Term Risk of Disease
Whilst immediate performance benefits are important, athletes should also consider their long-term health beyond their competitive years.
High fibre intake is one of the most consistently protective dietary factors across numerous health outcomes.
Higher fibre consumption is associated with reduced risk of cardiovascular disease, type 2 diabetes, certain cancers (particularly colorectal cancer), and all-cause mortality.
For athletes investing significant time and effort into their physical development, ensuring this investment translates into long-term health - not just short-term performance - should be a priority.
The mechanisms behind these protective effects include improved metabolic health, reduced chronic inflammation, better weight management, and enhanced gut barrier function preventing the passing of harmful compounds into circulation.
Optimising Your Fibre Intake as an Athlete
If you've now been convinced of the value of fibre, you'll be wanting to know how to go about optimising your intake.
How Much Fibre per Day is Optimal for Athletes?
Current general population recommendations suggest 25-40g of fibre per day, whilst average actual intakes range from 15-25g per day.
This tells you that most people are under-consuming fibre.
There's also a good argument to say that athletes require even more than general populations, given the increased food intake requirements and the increased inflammation from training.
But given that even the general population levels are above what most people usually consume, my recommendation for most athletes is to simply start consuming more than you currently do.
In terms of targets, I usually recommend building that up towards that 40g target.
I say "building up" because increasing fibre intake too quickly can lead to unwanted consequences like gas, bloating, and digestive discomfort.
We almost have to "train the gut" to handle more fibre.
From What Sources?
Fibre generally comes from fruits, vegetables, nuts, seeds, legumes, and whole grains.
These are food sources athletes will often neglect in favour of higher protein and higher carbohydrate sources.
However, hopefully you've been convinced to start including more in, now knowing the benefits of doing so.
If that is the case, here are a list of some fibre sources worth starting to emphasise within your diet:
Legumes: Lentils (15g per cup), chickpeas (12g per cup), black beans (15g per cup), kidney beans (13g per cup)
Whole grains: Oats (8g per cup cooked), quinoa (5g per cup cooked), brown rice (3.5g per cup cooked), whole wheat pasta (6g per cup cooked)
Vegetables: Broccoli (5g per cup), Brussels sprouts (4g per cup), carrots (4g per cup), sweet potatoes (4g per medium potato)
Fruits: Raspberries (8g per cup), pears (5.5g per medium fruit), apples with skin (4.5g per medium fruit), bananas (3g per medium fruit)
Nuts and seeds: Chia seeds (10g per 2 tablespoons), flaxseeds (8g per 2 tablespoons), almonds (3.5g per 28g serving)
Other: Avocados (10g per medium avocado), dark leafy greens (5g per cooked cup of spinach or kale)
Strategic food choices make getting in adequate fibre manageable within typical athletic caloric needs.
For example, 200g of lentils (15g fibre), 125g of raspberries (8g fibre), and half a head of broccoli (7g fibre) provides 30g of fibre for only about 450 calories.
A Final Note on Timing
On a final note, I want to save you some (potentially disastrous) trial and error by talking about timing your fibre intake around training.
You'll recognise the issue with fibre intake if you've ever gone out to training immediately after a high-fibre meal.
Bloating, stomach cramping, and urgent bowel movements can follow.
At best, this means suboptimal performance.
At worst, it means running off the field halfway through a training session trying to make it to the toilet on time.
The solution obviously isn't to avoid fibre, but to be strategic about the timing of it.
That means pre-training choosing lower-fibre, easily digestible carbohydrate sources like white rice, potatoes, and white bread-based foods, with moderate protein, and minimal fats and fibre.
Post-training can be a great time to get in some fibre, knowing it will be a while till your next session, albeit you might not feel like eating large volumes of food after a training session.
In that case emphasising your fibre intake in your meals furthest away from training, and particularly on non-training days, is going to allow you to get in the optimal amounts, whilst avoiding the downsides of high-fibre intakes around the training window.
Article Written By Conor O’Neill
P.s. You can find out more about Performance Nutrition Coaching HERE.