“I’ll ride before breakfast because I’ll burn more fat.” “I’ll stay in the fat-burning zone.” “I won’t bother with that third gel; that’s another 100 calories saved.” Or perhaps the most common question: “I ride eight or ten hours a week, so why am I not losing weight?” 

All of these thoughts seem perfectly logical. If the objective is to reduce body fat, surely burning more fat during exercise, consuming fewer calories while riding and increasing the amount of cycling you do should all help. The problem is that they confuse two related, but very different, physiological processes: using fat as a fuel and losing body fat. 

There is genuine physiology behind what is commonly called the “fat-burning zone”, sometimes associated with Zone 2 exercise. Our muscles need a continuous supply of ATP to produce power, and during endurance exercise that ATP is predominantly generated from a mixture of carbohydrate and fat. At lower intensities, fat can provide a relatively large proportion of that energy. As intensity increases and ATP needs to be produced more rapidly, carbohydrate becomes progressively more important. Somewhere between very easy and harder exercise there is therefore an intensity at which absolute fat oxidation reaches its highest rate. Research in trained men by Achten and Jeukendrup (2003) measured maximal fat oxidation at an average of 0.52g/min at approximately 63% VO₂max, although there was considerable variation between individuals.  

Importantly, however, riding fasted doesn't suddenly switch the body from carbohydrate to fat. You still have substantial carbohydrate available after an overnight fast, particularly as glycogen stored within your muscles. Van Loon et al. (2003) demonstrated this particularly well in endurance-trained men completing two hours of cycling at 60% VO₂max following an overnight fast. Across the exercise period, approximately 43% of energy was supplied by muscle glycogen and 12% by blood glucose, while plasma free fatty acids supplied approximately 28% and muscle-plus lipoprotein-derived triglycerides approximately 15%. In other words, even during a two-hour, moderate-intensity ride performed after an overnight fast, around 55% of the energy was still coming from carbohydrate.  

That is probably quite different from the picture many cyclists have of a fasted ride. Fasting increases our reliance on fat, but it doesn't turn the body into a fat-burning-only engine. Carbohydrate and fat are being used simultaneously, with their relative contributions changing according to intensity, duration, training status and nutritional state. Earlier work by Romijn et al. (1993) demonstrated the same principle across different exercise intensities: as intensity increased, muscle glycogen and blood glucose became progressively more important, while lipid metabolism followed a more complex pattern.  

So what actually happens when we say we're “burning fat”? It's easy to imagine the fat we can see around the stomach or waist simply being released and transported to our legs. In reality, body fat is predominantly stored as triglycerides within adipose tissue throughout the body. Through a process called lipolysis, triglycerides are broken down and fatty acids released into the circulation. Working muscles can take up those fatty acids and ultimately transport them into the mitochondria, where beta-oxidation breaks them down and they contribute to the production of ATP. 

But the fat being oxidised doesn't all come from the body fat we can see. Skeletal muscle itself contains stores of triglyceride known as intramuscular triglycerides (IMTG). These provide a local source of fatty acids for the working muscle and can make a meaningful contribution during endurance exercise. Indeed, the Van Loon study concluded that intramuscular triglycerides represented an important fuel source during moderate-intensity exercise in endurance-trained men.  

We can even put some numbers on this. Using the average maximal fat-oxidation rate measured by Achten and Jeukendrup of 0.52 g/min, two hours of riding would theoretically equate to approximately 62 g of fat oxidised, or roughly 560 kcal of energy from fat. Some trained individuals will be considerably above or below this because fat-oxidation rates vary enormously. That's real fat being used as fuel, but it doesn't mean you've just lost 62 g of body fat. 

Fat is continually moving in and out of storage. Fatty acids can be released from adipose tissue and oxidised, but they can also be released and subsequently re-stored. Some of the fat oxidised during cycling came from triglycerides already stored within the muscle rather than adipose tissue. Once you finish the ride and start eating, substrate utilisation changes again. The amount of fat oxidised during two hours of cycling therefore isn't a measurement of how much body fat you've lost. 

This is why two cyclists can complete the same ride but use very different fuels. One rider is fasted and relies more heavily on fat; the other eats breakfast and consumes carbohydrate, increasing carbohydrate oxidation. The fasted cyclist may therefore oxidise more fat during the ride, but that doesn't mean they will lose more body fat. Body composition is determined over much longer periods than the few hours spent on the bike. 

The same principle applies to fuelling. Three hours into a long ride, leaving a 100-calorie gel in your pocket may seem an easy way to increase your deficit. But during a demanding four-hour ride, that carbohydrate also supports power output and the training you're trying to complete. Deliberately under-fuelling to save calories or maximise fat oxidation can therefore work against the purpose of the session. 

This also helps explain why someone can cycle eight or ten hours a week yet struggle to lose body fat. Exercise expenditure doesn't translate neatly into predicted weight loss. Riou et al. (2015), reviewing 61 studies involving 928 participants, found an average energy compensation of 18%, but with enormous individual variation. Some comes from increased food intake, but expenditure can change too: fatigue may reduce activity and even small amounts of spontaneous movement; walking, standing and fidgeting can fall. Consequently, adding the calories shown on your Garmin to your normal daily expenditure doesn't necessarily predict the deficit, or fat loss, that follows. 

And this is where cycling makes nutrition harder, rather than easier. Someone who wants to become leaner while riding regularly has several nutritional objectives occurring simultaneously. They need sufficient carbohydrate for demanding training, glycogen restoration between sessions, enough protein to repair and preserve muscle, and enough overall nutrition to recover properly, while their total intake still needs to be compatible with a gradual reduction in body fat. 

This is why performance nutrition can sometimes appear to contradict weight-loss nutrition. On the morning of a demanding ride, the advice might be to eat a carbohydrate-rich breakfast, consume carbohydrate while riding and then eat carbohydrate and protein afterwards. At the same time, the rider is trying to reduce the amount of energy stored on their body. It is perfectly reasonable to wonder why they're being told to eat more precisely when they're expending the most energy. 

The answer is that nutrition around training isn't solely being used to manipulate body weight. It also must support the training itself and the adaptation that follows it. A four-hour training day therefore shouldn't necessarily have the same nutritional strategy as a rest day. There may be sessions where relatively low carbohydrate availability is entirely appropriate and others where deliberately restricting carbohydrate simply makes it harder to achieve the purpose of the training. 

This is also why doing lots of cycling doesn't necessarily make nutrition less important. In many respects, it makes it more complicated. The cyclist who wants to become leaner while continuing to improve performance isn't simply trying to lose weight. Ideally, they want to reduce adipose tissue while preserving muscle, training quality, recovery and the ability to adapt to the work they're doing. Those objectives must coexist. 

So, does the fat-burning zone work? If by “work” we mean that there are exercise intensities at which the body derives a substantial amount of its energy from fat, then absolutely. What it cannot tell you is how much body fat you will ultimately lose. Nor can a fasted ride, the number of calories displayed on your Garmin, or the number of gels you managed not to eat. Those things tell us something about what happened during the ride. Body composition requires us to understand everything happening around it. 

For cyclists who want to become leaner without compromising their riding, getting that balance right can be surprisingly complicated. Nutrition has to provide sufficient energy at the appropriate times to support training and recovery, while the overall pattern of intake still needs to be compatible with gradual fat loss. Neither simply eating less nor deliberately maximising fat oxidation during every ride adequately addresses both sides of that problem. 

And that is why burning fat isn't the same as losing fat. 

References: 

Achten, J. & Jeukendrup, A.E. (2003). Maximal fat oxidation during exercise in trained men. International Journal of Sports Medicine, 24(8), 603–608.  

Romijn, J.A., Coyle, E.F., Sidossis, L.S., Gastaldelli, A., Horowitz, J.F., Endert, E. & Wolfe, R.R. (1993). Regulation of endogenous fat and carbohydrate metabolism in relation to exercise intensity and duration. American Journal of Physiology, 265, E380–E391.  

van Loon, L.J.C., Koopman, R., Stegen, J.H.C.H., Wagenmakers, A.J.M., Keizer, H.A. & Saris, W.H.M. (2003).Intramyocellular lipids form an important substrate source during moderate intensity exercise in endurance-trained males in a fasted state. The Journal of Physiology, 553(2), 611–625.  

Riou, M-È., Jomphe-Tremblay, S., Lamothe, G., Stacey, D., Szczotka, A. & Doucet, É. (2015). Predictors of Energy Compensation during Exercise Interventions: A Systematic Review. Nutrients, 7, 3677–3704.  

If you feel uncertain about whether your daily nutrition is truly supporting your health, performance, and long-term wellbeing, G2 Nutrition offers a highly personalised diet analysis with a bespoke 6-week optimisation programme, available for £250. 

This is a tailored, results-driven service designed to elevate your approach to nutrition, refining not only what you eat, but how your body performs, recovers, and thrives day to day. 

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