Burning Fat As Fuel
As a competitive age-group triathlete training during Ramadan, I’ve been thinking a lot about two things recently:
the incredible power of low-intensity training, and
our body’s ability to burn fat as fuel.
With my coach Luke, I’ve naturally dialed back my training during this month, focusing mainly on low-intensity work. It’s been a timely reminder of some principles that apply not just to competitive athletes, but to the active, health-conscious office worker too - which perhaps you are.
Whether you want to lose body fat, improve your 5K time, live longer or simply have more energy each day, slowing down might be the best move you can make.
The “harder is better” trap
When life gets busy, we instinctively feel the need to “make the most” of limited exercise time. Thirty minutes feels precious, so “making the most” usually means going hard: a fast 5K before work, a quick HIIT workout at lunch or a brutal spin class before dinner. We finish drenched in sweat, heart pounding, reassured that we’ve done something worthwhile.
But here’s the catch: when we squeeze intense workouts into already stressful days, we might not be getting the health, performance or physique benefits we are hoping for. Why? When we push ourselves hard physically, we are putting our body under stress. At the same time, when we have upcoming deadlines at work, an overflowing inbox and a pile of life admin at home - this also puts the body under stress. It all adds up.
When the body is in this “fight or flight”, stressed state, metabolism shifts toward energy preservation. If we shoe horn intense exercise into an already stressful day, we may blunt many of the metabolic benefits we’re seeking.
From a performance perspective, if we want to get fitter, faster, stronger, when the body is constantly stressed, it struggles to adapt to training - which means that those fitness gains you are working so hard towards would be blunted.
“But if I’m not working hard, I don’t feel like I’m getting any benefits.” It is something I often hear and something I used to think myself. To which my response is to gently nudge people towards the science of training zones and metabolic flexibility.
Training zones & metabolic flexibility
Now bear with me here, because this concept is important to understand for every health conscious person, from dog walkers to professional athletes. Training can be divided into “zones”:
Zone 1 is the lowest intensity - breathing is calm and conversation is effortless, think walking, light jogging, very easy cycling or any form of light movement.
Zone 2 is slightly harder - you can still talk, but someone on the phone would notice you’re exercising.
Zone 3 is moderate - conversation is possible but only in short phrases.
Zone 4 is hard - talking would be limited to a few words and the effort feels demanding.
Zone 5 is maximal - all-out, “I might be sick” kind of intensity that can only be sustained briefly.
For many time-crunched office workers with a “harder is better” mindset, most training ends up in Zones 3–5. But if we only train at higher intensities, we miss out on huge potential metabolic and performance benefits.
The power of “easy” exercise
Metabolically, at these lower intensities (zones 1-2), the body burns fat as its primary fuel source (in zones 3 and above the body shifts to carbohydrate as the primary fuel). When we spend time training in these low zones our body then becomes more efficient at burning fat as fuel throughout the day, both while we are exercising and when we are sitting at our desk.
Incorporate low zone training into your programme and you will begin to burn more fat and have steadier energy throughout the day. You won’t be relying on sugary snacks to give you energy because your body is better able to burn the available fat stores for energy. Of which there is plenty - even lean athletes carry tens of thousands of calories in fat.
And so, spending time in zones 1–2 trains the body to become more metabolically flexible - less reliant on sugar and more capable of burning fat. This then has the positive downstream effect of less sugar cravings which typically nudge you towards making those food choices you later regret.
Low-intensity training also improves fitness as the heart becomes more efficient. Stroke volume - the amount of blood pumped with each heartbeat - increases and resting heart rate drops (a typical marker of overall fitness). These gradual adaptations, known as cardiac remodeling, occur through this consistent low-intensity work.
This is why myself and other endurance athletes spend 80% of our training in zones 1 and 2. Higher intensity sessions still matter, but they sit on top of a large aerobic base built through easier work. I like to picture it as a pyramid. The wider the base of low-intensity training, the higher the potential peak of performance.
And crucially, these “easier”: sessions do not push the body into the same “fight or flight” state that the intense sessions do. This allows the body to adapt and recover so that we can enjoy the metabolic and performance benefits we’re seeking.
Over the past few weeks, I’ve seen my base fitness improving even though the training has felt very easy. It’s a mindset shift and one that took me time to fully appreciate: harder is not always better, and just because a training session feels easy doesn’t mean it isn’t incredibly effective.
Three simple ways to start this week
If you’d like to experiment with slowing down, here are three simple steps:
1. Replace one hard workout with a zone 1 or 2 session
Swap one intense workout this week for a low zone session. Go for a walk, gentle run, or any form of light movement that you enjoy.
2. Use the conversation test
You don’t need a fancy heart-rate monitor or watch to know what zone you are in. Simply ask yourself: could I hold a conversation right now? If yes, you’re in zone 1 or 2. If speaking becomes difficult, you’ve drifted into the higher zones. Rein it in, tiger.
3. Look for opportunities to walk more
Start building your aerobic base outside of formal workouts. A 20-30 minute walk has amazing benefits - it all adds up and quietly trains your body to become a healthier, fitter, more efficient, metabolically flexible machine!
Deepen Your Learning
In case you missed it two weeks ago or this article inspired you to pick it up, re-sharing this excellent podcast episode with Alan Couzens all about the magic and beauty of low zone training.
Alongside movement, nutrition is the other key lever to improve our metabolic flexibility. Here’s a great, actionable article from the Glucose Goddess on how what we eat for breakfast shapes our energy and metabolism for the rest of the day.
