Should Triathletes Go Low-Carb for Ironman? An Evidence-Based Perspective
Few nutrition topics divide triathletes quite like carbohydrates
On one side are athletes experimenting with low-carb or ketogenic approaches, claiming improved endurance and freedom from constant fueling. On the other are athletes pushing carbohydrate intake higher than ever, both in training and on race day, in pursuit of marginal gains.
Ironman racing sits at the centre of this debate.
It’s long enough that energy supply matters enormously, but still intense enough that performance depends on how efficiently that energy can be used.
Understanding where low-carb strategies fit — and where they fall short — requires looking beyond anecdotes and into physiology, research, and real-world practice.
This article explores why low-carb approaches appeal to triathletes, what actually happens to performance when carbs are restricted, what the science tells us, and how most athletes can apply a more balanced, practical approach.
At first glance, the logic behind low-carb nutrition for Ironman seems compelling.
Even a relatively lean athlete stores only around 2,000–2,500 calories of carbohydrate in the form of muscle and liver glycogen.
In contrast, that same athlete will carry tens of thousands of calories of fat — more than enough energy to complete several Ironman races.
The idea is simple: if you can train your body to rely more heavily on fat, you reduce the risk of running out of fuel. Low-carb diets are said to increase fat-burning enzymes and mitochondrial adaptations, a process often described as becoming “fat-adapted.”
Research does support part of this story.
After several weeks on a very low-carbohydrate diet, athletes can significantly increase fat oxidation at given exercise intensities.
From a purely metabolic standpoint, the body becomes better at burning fat.
The problem is that Ironman performance is not decided by fat oxidation alone.
The Performance Trade-Off Most Triathletes Miss
There are two key problems with the low carb approach that triathletes misunderstand.
First, is that most triathletes will be burning a significant amount of carbohydrates at their intended Ironman race pace.
While individual physiology varies, you can expect that carbohydrate will account for about 70% of the energy that you use, with the remaining 30% coming from fat.
That means that even in these long distance events, carbohydrate and glycogen stores are vital for top performance.
The other key consequences of prolonged carbohydrate restriction is a reduced ability to use carbohydrates efficiently when they are available.
This matters because higher-intensity efforts — those brief but decisive moments in a race — are heavily dependent on carbohydrate metabolism.
Examples include:
The start of the swim
Surges to respond to terrain or other athletes
Riding into headwinds or up climbs
Late-race efforts when fatigue is already high
Fat cannot supply energy at the same rate as carbohydrates.
When carbohydrate availability and utilisation are compromised, these efforts feel disproportionately difficult, even if overall endurance appears preserved.
In practice, many athletes notice that while they can “keep going,” they struggle to produce power, pace, or repeated efforts at intensities that matter for racing well.
What the Research Actually Shows
The most robust evidence examining low-carb diets and endurance performance comes from the SUPERNOVA studies led by Dr Louise Burke and colleagues in Australia.
While fat oxidation increased substantially in the low-carb group, race performance declined.
Athletes became less efficient, meaning they required more oxygen and energy to maintain the same pace.
This loss of efficiency is critical in Ironman triathlon.
Success is not about burning the most fat; it is about moving as fast as possible while expending the least energy. Increased fat use did not translate into improved performance when it mattered.
While these studies were conducted in race walkers rather than triathletes, the physiological principles apply across endurance sports.
Individual Responses: Why Some Athletes Still Swear by Low-Carb
Despite the broader evidence, some triathletes report positive experiences with lower-carb approaches.
This doesn’t mean they are imagining it, but the reasons often extend beyond carbohydrate restriction itself.
Common factors include:
Improved diet quality: Moving from a highly processed diet to a whole-food-based low-carb diet often leads to better energy, body composition, and consistency, regardless of macronutrient ratios.
Gastrointestinal tolerance: Some athletes struggle with gut symptoms when fueling aggressively with carbohydrates and would feel better with a more conservative approach.
Training intensity: Athletes training predominantly at low intensities may feel fewer performance drawbacks from reduced carbohydrate availability.
Individual metabolic differences: Genetics, training history, and metabolic flexibility vary widely between athletes.
In practice, however, for every athlete who thrives on low-carb, many experience stalled progress, reduced training quality, slower recovery, or an inability to hit previous power and pace targets.
In more extreme cases, prolonged under-fueling can contribute to low energy availability and health issues.
If you want to avoid issues with underfueling your training, in my free fuelling guide for triathletes, I walk you through how to eat before, during and after exercise. This guide will help you get the most out of your training and take your performance to the next level.
What Elite Ironman Athletes Are Actually Doing
If low-carb strategies consistently produced superior results, they would be widely adopted at the top of the sport, but that simply hasn’t happened.
Instead, elite Ironman athletes are pushing carbohydrate intake higher, not lower.
Many are experimenting with intakes well above traditional guidelines, sometimes exceeding 120g of carbohydrate per hour during racing.
These decisions are guided by performance data, extensive testing, and support from experienced sports nutrition professionals.
The consistent outcome is clear: carbohydrates support sustained power, speed, and decision-making deep into an Ironman race.
Finding a Practical Middle Ground
This doesn’t mean triathletes need to eat high-carb at all times or ignore individual preferences.
A flexible, context-driven approach is often most effective.
For most athletes, practical principles include:
Prioritise food quality daily
Build meals around vegetables, fruits, lean proteins, and healthy fats. Carbohydrates should complement this foundation, not replace it.
Match carbs to training demands
Harder, longer sessions require more carbohydrates. Easier or shorter days may require less, but complete avoidance is rarely necessary.Experiment cautiously
Any major dietary change should be tested during lower-priority training phases, not close to key races.
Monitor health and recovery
Persistent fatigue, declining performance, or mood changes can be signs of inadequate fueling rather than poor fitness.
The Bigger Picture for Ironman Performance
The idea that low-carb athletes will dominate Ironman racing is not supported by current evidence.
While some individuals may perform well with reduced carbohydrate intake, carbohydrates remain a central fuel for sustained, high-quality endurance performance.
Ironman success depends on consistency, efficiency, and the ability to fuel both training and racing appropriately over months and years—not short-term dietary experiments.
For most triathletes, a balanced approach that respects both physiology and long-term health will outperform any extreme strategy.
If you’d like to learn the most effective nutrition habits for improving your performance, check out this video on YouTube where I run you through the strategies that will actually make a difference to your training.
