Blog Post

Does Citrulline Supplementation Help Endurance Performance?

Citrulline for Endurance Blog Featured Image

Citrulline supplementation has gained attention for its potential benefits in improving endurance performance.

There are a bunch of proposed mechanisms for how it can help endurance athletes achieve better race times.

Although some mechanisms make sense, the biggest thing to look at is outcomes. Does citrulline supplementation improve performance?

While anecdotal data has merit, particularly when we lack research, in this article we will look directly at what the research we have available has found.   

Mechanisms

Before jumping into the research, it is worth looking at the mechanisms to understand how citrulline could help theoretically.

Improved Blood Flow

Vasodilation

The most compelling mechanism is that citrulline is converted to arginine, a precursor for nitric oxide. Increased nitric oxide leads to vasodilation, improving blood flow, oxygen delivery, and nutrient transport.

We also see other options like beetroot juice showing benefits partly due to this effect.

For context, citrulline supplementation increases arginine levels even more than directly supplementing arginine.

Ammonia Clearance

During prolonged exercise, ammonia accumulates in muscles, leading to fatigue. Citrulline helps in the urea cycle to convert ammonia into urea, which can be excreted. This theoretically reduces fatigue and improves endurance.

Improved ATP Production (If Combined with Malic Acid)

Citrulline is often combined with malic acid to form citrulline malate. This can help with ATP production and help reduce lactic acid production. This mechanism would be far less relevant for longer endurance events than for shorter ones.

Performance Outcomes Seen in Research

As of 2023, 9 relevant studies were looking at this topic.

Four of them were time-to-exhaustion (TTE) studies and 5 were time-to-completion.

As a general note: It makes sense to pay less attention to TTE studies than time trial or race-style studies. When you look at sports nutrition, you will see protocols that lead to impressive TTE results that do not translate as well to time-to-completion results. Considering time-to-completion is what we normally care about, it makes sense to focus on that.

Since we only have 9 total studies, it does make sense to factor in the TTE studies though.

The short and sweet summary of these studies is that on average citrulline did NOT improve performance.

Caveats to Be Aware of When Interpreting This Research

If citrulline does help performance, it cannot be a big benefit, otherwise it would have shown up in the research. But there are still a few gaps worth addressing:

  • Ideally, we want larger sample sizes and more studies in general.
  • While multiple dosing protocols have been tested, it would be good to see this doubled down on. For example, there is speculation that taking citrulline daily for an extended period might have more benefits than just taking it acutely. I am not suggesting that I believe it will. I am just using that as an example of areas where more research is warranted on top of the research that has already been done looking at that concept.
  • None of these studies were done exclusively on women. There is not anything that makes me believe that the results will be different, but as with other areas in sports nutrition, there is still a huge gap that needs to be addressed.

Other Angles to Consider

Unless future research provides a compelling case for the alternative, it does not look like citrulline significantly improves endurance performance.

There are two other angles I want to focus on though.

Citrulline Likely Reduces Muscle Soreness

Most people training hard will experience muscle soreness. At minimum this is uncomfortable, but in some cases, it could also impact training.

If somebody is looking to reduce this soreness, citrulline has been consistently shown to help.

There is substantially more research looking at this topic than endurance performance, with positive findings on average.

Citrulline Might Be More Effective for Shorter Bursts

Citrulline has a bit of evidence it can help with strength and power performance.

A review by Eric Trexler looking at this found small beneficial effects in this space. I would not read too much into this because the effects were bordering on trivial, but it is still worth being aware of.

It is a stretch, but there could be indirect benefits of citrulline related to stuff like this.

Summary

While the mechanisms make sense at face value, the research has not been promising for citrulline and endurance performance.

It is not at a stage where it is safe to say confidently that there is no benefit. But at most, if there is a benefit, it would have to be small, otherwise it would have shown up in the research already.

By Aidan Muir

Aidan is a Brisbane based dietitian who prides himself on staying up-to-date with evidence-based approaches to dietetic intervention. He has long been interested in all things nutrition, particularly the effects of different dietary approaches on body composition and sports performance. Due to this passion, he has built up an extensive knowledge base and experience in multiple areas of nutrition and is able to help clients with a variety of conditions. One of Aidan’s main strengths is his ability to adapt plans based on the client's desires. By having such a thorough understanding of optimal nutrition for different situations he is able to develop detailed meal plans and guidance for clients that can contribute to improving the clients overall quality of life and performance. He offers services both in-person and online.