How To Burn More Fat As A Triathlete — Nutritionist Gives The Truth
The internet is full of advice about becoming 'fat adapted' — train fasted, avoid carbs, try keto. But so many of those miss the point entirely, and are actually putting your training and health at risk.
In this article, we’ll go through a deep dive on fat burning… And the science might surprise you.
I have had so many comments on my YouTube channel about this over time. Some just curious questions, and some people telling me how my advice on pushing carbohydrates is wrong
But here’s the truth: I’m not just an “eat carbs all the time” kind guy. I’m an eat the right carbs at the right time person.
My role as a registered nutritionist is ultimately to keep my athletes safe and improve their performance, and if there are strategies that help me do both of those things, then I’m game.
I am super keen on this topic because it does have potential.
Before we get into that, though, let’s start with what we actually know about fat adaptation.
The science of fat metabolism
Your body is constantly using a mix of both fat and carbohydrates for fuel. Think of it as a hybrid car that can switch between electric and gas. The proportion of each fuel source depends on various factors, like your diet and how hard you’re exercising.
During lower-intensity exercise, you burn fat primarily. As intensity increases, you use more carbohydrates. This is simply because fat metabolism is slower and requires more oxygen than carbohydrates to create energy. Fat is great for long, steady efforts but cannot keep up when you need quick energy.
Here's where it gets interesting.
Your body can improve its ability to burn fat—this is what we call metabolic flexibility. And yes, training in a fasted state or with low carbohydrate availability can improve this capacity.
This is actually well-documented in the research. Studies show that when you train with low carbohydrate availability, you increase various enzymes and proteins involved in fat metabolism. In theory, this sounds great.
But there’s a catch — and this is where most people get it wrong.
While you might improve your ability to burn fat, you simultaneously decrease your ability to use carbohydrates effectively. It's like making your hybrid car more efficient with electricity but less powerful with the gas engine.
This becomes a problem because carbohydrates are still a crucial fuel source even in an Ironman triathlon. In fact, we know that the fastest long-distance triathletes aren't the ones who burn the most fat — they're the ones who can process and use the most carbohydrates.
Even at Ironman pace, you're still working at an intensity where carbohydrates provide a significant portion of your energy. And because you only have enough carbohydrate stored after a proper carb loading protocol for about 90 minutes of hard, steady exercise, this can be a significant limiting factor to race performance.
This is why the pros push as many carbs as possible during races, not trying to minimize them.
Before we move onto the next section, if you’re interested in improving your nutrition as a triathlete then check out my free fuelling guide for triathletes. It’ll help you to know what to eat before, during and after training, and turn nutrition into a strength!
The truth about training strategies
You might have heard about strategies like "train high, sleep low" — a protocol that's gotten a lot of attention in the endurance world, and it is a fascinating idea.
The basic idea is to manipulate your carbohydrate availability across different training sessions with the aim of harnessing your body’s natural ability to more effectively burn fat or carbs.
You start with an evening session where you're fully fueled with carbohydrates — that's the "train high" part. This would be a high-intensity interval session where you want maximum performance.
Then, instead of replenishing your carbohydrate stores after that session, you deliberately keep them low. You might have a low-carb dinner or even skip the carbs entirely. This creates a situation where your muscle glycogen is depleted and is the “sleep low” part.
The next morning, you train in this glycogen-depleted state. This session is performed at a lower intensity, like an easy run or ride.
The theory is that this combination creates a stronger stimulus for adaptation, particularly for fat metabolism and mitochondrial development.
I've used this approach myself and with some athletes I work with.
And yes, there is some evidence supporting it. A few studies have shown improvements in performance and fat oxidation rates.
But what people don't talk about enough is that the research is mixed. Some studies show benefits, other research doesn’t show any benefit.
More importantly, it's incredibly easy to get wrong. If you're not careful about your overall nutrition, you can end up with chronically low carbohydrate intake, which is not the point of the protocol, and this can devastate your training quality and recovery.
Think about it: You're potentially compromising two training sessions instead of one. The evening session might suffer because you've restricted carbs too heavily, and the morning session is obviously going to be challenging with low glycogen stores.
While we discuss nutritional strategies for fat adaptation, we must address the elephant in the room: the ketogenic diet.
This is probably the most extreme approach to improving fat metabolism, and I get questions about it all the time. The ketogenic diet involves drastically reducing carbohydrates—usually to less than 20 grams per day—while significantly increasing fat intake. This forces your body to primarily use fat and ketones for fuel.
And here's the thing. It absolutely works for increasing fat metabolism. The research is crystal clear on this: Following a ketogenic diet properly makes your body much better at burning fat for fuel.
The trade-off for burning more fat
But remember what we talked about earlier — there's always a trade-off.
Like our other strategies, as you enhance fat metabolism, you simultaneously downregulate your body's ability to use carbohydrates effectively. Your body literally reduces the enzymes needed for carbohydrate metabolism.
This becomes a real problem for triathletes because you need carbohydrates for high-intensity performance, which is also undisputed in the research. These higher-intensity efforts rely heavily on carbohydrate metabolism, whether it's those crucial intervals in training, responding to surges in a race, or pushing through the final miles of a run.
So while you might become a great fat burner, you're essentially putting a governor on your high-end performance, which will impair overall training and racing potential for most people.
Now, in the interest of being completely transparent, I should mention that there are some athletes who've had success with low-carb approaches. Particularly in ultra-endurance events, there are examples of athletes performing well on ketogenic diets. And some athletes report feeling better overall - more stable energy levels, fewer stomach issues, better mental clarity.
These experiences shouldn't be dismissed. They're valid, and they show that nutrition isn't always one-size-fits-all. However, what’s crucial to understand is that these are the minority, and this is shown in the research.
When we look at studies investigating this, most athletes get worse when they follow ketogenic diets. Their performance decreases and their race times suffer as a result.
But it wouldn't be fair to say that this happens with everyone. Even in the same study, you get a few athletes who actually improve their performance.
Now one of the things which people routinely talk about within this fat metabolism debate is the timing and duration of nutritional protocols like the keto diet.
While dietary changes like going keto can increase fat oxidation in just a few days, the adaptations we truly want from training take much longer - we're talking weeks or months of consistent work.
And the research here is particularly interesting. Studies show that well-trained endurance athletes naturally have higher fat oxidation rates than untrained individuals, regardless of their diet. This happens through consistent training over time, not through dietary manipulation.
But one of the things I commonly hear is that If you just stick with low-carb long enough, your body will fully adapt. While there is some truth to long-term adaptations, the research consistently shows that high-intensity performance capabilities remain compromised.
Even in studies lasting several months, athletes on ketogenic diets still showed reduced ability to perform high-intensity efforts and that’s because you just can’t overcome the natural physiology in that carbohydrate metabolism is always going to be more efficient than fat, and fat always requires oxygen to metabolise, meaning it can’t be used for those times when you’re really working hard.
The real solution
So what's the actual answer? How do you become better at burning fat while maintaining high performance?
The solution is actually much simpler — and it's something the best long-distance triathletes have known for years:
Consistent training over time.
And this is what I focus on with the triathletes I work with. Instead of forcing their body into fat adaptation through restrictive eating patterns, we prioritise building consistent training volume over time, getting their easy sessions truly easy and fueling appropriately for different training intensities.
And what’s fascinating is that when you do this right, you naturally become better at burning fat without compromising your ability to use carbohydrates. It's like upgrading your hybrid car's electric and gas systems.
Practical application
So how do you apply this? Here's a framework I use with triathletes:
For easy sessions (Zone 2), you can get away with lower carb intake (around 30g per hour) because you're naturally using more fat for fuel
For moderate to high-intensity sessions, fuel properly with carbohydrates to support quality training
Focus on building your aerobic base through consistent training, rather than trying to force adaptation through nutrition
Ensure your daily diet supports training and recovery - this means adequate carbohydrates and calories
The ability to burn fat efficiently is important for endurance performance, but the path to getting there isn't through restrictive eating or fancy protocols.
It's through consistent training, proper fueling, and patience.
This approach might not be as exciting as the latest fat-burning hack you saw on YouTube, but it's what actually works… and more importantly, it's sustainable.
