How Changing Fibre Intake Can Improve A Cyclist's Power-To-Weight Ratio

Weight loss isn’t always performance-enhancing for endurance athletes, as poor weight-loss practices can negatively impact their ability to produce power.

However, having an optimised power-to-weight ratio is a key aspect of performance, particularly when riding involves fighting gravity (e.g., hill climbs). A better power-to-weight ratio than a competitor can give you a competitive advantage.

In the long term (i.e., weeks and months), several strategies can be implemented to enhance the power-to-weight ratio, from fat loss to maximising training gains in power.

A select number of acute (days and hours) dietary strategies can also be implemented in the days leading up to a key competition that can provide meaningful improvements in power-to-weight ratio without restricting energy intake and the negatives associated with this.  

Fibre is a major macronutrient within our diet; it is an indigestible form of carbohydrate that provides almost no energy but is key for health, preventing disease, and supporting our gut health. It also helps make our stools softer and bulkier, helping with the transit of food through our digestive system.

As such, fibre adds bulk to our digestive system and weight. Therefore, manipulating fibre in our diet (by reducing our fibre intake) over short periods can allow us to reduce this bulk and, as a result, our scale weight. Potentially providing a small but significant improvement to our power-to-weight ratio.

How much weight can we lose by reducing our fibre intake? It’s more than you might think. A recent study from Liverpool John Moores University (See here) looked at exactly this in a controlled manner. 

They took 19 active males and monitored them through a period of habitual diet and training before prescribing them a personalised low-fibre approach to follow over 4 days. Training was replicated so it was the same during both the habitual diet phase and the low fibre phase.

The diets consumed in both the habitual and low fibre phase where matched in terms of energy, carbohydrate, fat, protein, sodium and fluid on a meal by meal basis.

A low fibre intake is typically categorised as below 10 grams of fibre per day. During the low-fibre phase of the study, participants consumed around 8 grams of fibre, compared to around 30 grams during their habitual diet, which is in line with current fibre intake recommendations. So, around 22 grams less fibre per day was consumed during the 4 day low-fibre phase.

 The results showed a few key things..

Reduced body weight—As anticipated, eating less than 10 grams of fibre per day resulted in a significant drop in scale weight in the region of 600 grams in 4 days. This is not an insignificant amount of weight given the relative ease of implementing this intervention.

Increased appetite – Fibre plays a role in helping us feel full, so it’s no surprise that less fibre results in more hunger.

Less visits to the toilet – Having less fibre resulted in a slower transit of food through the digestive system, resulting in less bowel movements and harder stools.

Given that it can take up to two days for food to fully transit through the gut and up to three days of low fibre intake to see an effect, it’s no surprise that the lowest weights were recorded on the fourth day of the low-fibre trials.

There was a lot of variability between individuals in terms of how many days it took to see the reduction in weight, which suggests that individuals need to experiment with this strategy in training before putting into into action ahead of competition.

 The study also found this intervention was well tolerated by participants, which means it’s a relatively easy strategy to implement.

As such it’s no surprise that given the potential for weight loss, with few downsides in the short term, this is a dietary strategy that many professional road cyclists will utilise particularly during hilly races where small improvements in power to weight ration, can provide a competitive edge.

Long-term, a low-fibre diet certainly isn’t recommended or optimal for health given the key role that fibre plays in digestive health; however, short-term periods of low fibre intake are unlikely to have any long-term negative effects.

Aside from the performance aspect, changes in fibre in the diet are another reason why your weight on the scales can fluctuate significantly on a day-to-day basis, often independent of your total energy intake. 

• Coach Ben