Does ‘Fasted’ Training Really Burn More Fat?

Eat before you go for a run, or after? Can mealtime magic propel you into the mythical ‘fat-burning zone’? All will be revealed…

Prof. Ross Tucker |

The idea behind training before your breakfast is that you start your run in a carbohydrate-depleted state. Your liver glycogen (which provides the glucose that goes into the blood) has been almost entirely used up overnight, so when you start your run the next morning, you’re forced to rely on fats as a source of energy for your contracting muscles. 

The physiological principle is ‘You burn what you have available’; and also that over time, your body adapts to burning that particular fuel. Think of it as the metabolic equivalent of ‘Practice makes perfect’. 

Indeed, one of our powerful adaptations to endurance training (those long runs we do) is that we become ‘fat-burning machines’. The run-before-breakfast concept is meant to amplify that benefit further.

Scientists have found that even a small meal 15 minutes before a training session shifts our metabolism towards burning carbohydrates, so our adherence to the principle is quite sensitive. 

Trying to take advantage of this is where things become a little trickier. Our metabolic machinery is under the control of so many hormones and self-regulated feedback loops, and these all interact with training in complicated ways – so it’s not a simple truth that running before you eat is guaranteed to make you burn more fat overall, or to make you a better runner. 

Where there does appear to be a long-lasting benefit is that training before we eat causes molecular changes that include better sensitivity to insulin, and healthier patterns of fat storage inside our muscles. These health benefits were described in a recent review in the Journal of Physiology, which concluded that “restricting carbohydrate intake before and during exercise can enhance some health benefits of exercise”.

So far so good

But what about the performance and fat-burning benefits? This paints a slightly more complex picture. 

A study of 14 well-trained cyclists found that when they trained for just three weeks on a low-glycogen diet, their use of muscle triglycerides increased, while their muscle glycogen use went down. This is the good news – another demonstration of the principle that our bodies adapt to the energy source that is available.

Now for the bad news. Despite becoming more effective fat-burners, the cyclists on the low glycogen diet did not improve their performance; in fact, many got worse. Why? Because their training intensities were inferior, compared to when they trained with high energy and high carbohydrate availability. 

In other words, compromising fuel supply does cause metabolic adaptations – but the advantages may be cancelled out by the compromised training we do while attaining them! 

My practical advice, then, is to be very careful when you try to shortcut fat burning. Do it only when the intensity of your run doesn’t matter. Long, slow runs lend themselves to this ‘trick’, but harder sessions do not.


Maintaining balance

Other studies further highlight how difficult it can be to ‘trick’ the body. 

When athletes have either trained before breakfast, or skipped breakfast entirely, it’s been found that they have lower metabolic rates for up to two days after the session. They also burn less fat and more carbohydrates throughout the day – an effect that is the opposite to what they achieve during the actual training session. 

When provided with free access to an all-you-can-eat buffet, those who skip breakfast will eat more for lunch, but then less at dinner. Compensation, followed by compensation. 

All of this suggests that skipping breakfast, or training without eating, might increase your fat use during the session; but over time, your body attempts to restore balance by overcompensating in the other direction.

Practically, what this brings home is that we should probably not focus disproportionately on our one hour of exercise, and on one meal, when the totality of exercise and diet happens 24 hours a day, seven days a week, all year round. 

That’s not a call to become obsessive about our diets all the time, mind you. Rather, it should be a reminder of the complexity of our metabolisms, making us realise that it’s a general pattern of good behaviour that matters, not the hacks and short-cuts we often try to take.

In summary, training before we eat breakfast can certainly help us shift our metabolic systems towards fat use, and away from carbohydrate use. This may be performance-enhancing; and it probably has some health benefits, because of how it affects blood sugar, insulin sensitivity and fat storage. 

However, it comes with some potential downsides, and should not be done at the expense of training intensity, or of hunger. We also need to be mindful about what we do and eat for the 23 hours after we train. 

Ultimately, it’s about listening to your body, and understanding what it’s telling you – and avoiding the traps that may undermine your training.


South African-based Professor Ross Tucker is one of the world’s top sports scientists, and is the co-host of the Real Science of Sport podcast with RW editor Mike Finch. Follow him on @scienceofsport, and the pod on @sportsscipod.


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