What’s the point of an fasted workout?

By Guillaume Judas – Photos: Mizuno / Brooks / PEDaLED / Orbea / Hoka

Running, cycling, or swimming on an empty stomach means doing your session in the morning shortly after waking up and before breakfast. But in reality, you can consider yourself fasted 6 hours after your last meal. An evening session just before dinner, provided you haven’t consumed any calories after lunch, offers the same benefits. And the same drawbacks.

Beyond the benefits related to organizing a busy day, an fasted workout would allow for faster weight loss or leaning out (approaching race weight). One explanation for this: after a night’s sleep and a last meal several hours ago, glycogen reserves (fuel for effort) in the muscles and liver are diminished. After a few tens of minutes of effort, the body is then forced to primarily draw on fatty acids for energy. Hence, an additional use of this type of fuel during this session compared to an outing performed in the middle of the day. So theoretically, a more favorable ratio between time spent and fat utilization. But it’s not that simple.

Accelerated weight loss

One kilogram of excess fat is 9000 calories. And to lose weight, you need to burn more calories than you consume. It’s mathematical.

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When running, you burn around 500 to 700 calories per hour, depending on your weight and pace. On a bike, it’s more like 500, and in swimming, around 600. It would therefore take between 15 and 20 hours of one of these so-called endurance sports without changing your usual caloric intake to lose one excess kilogram. That’s a lot, especially since the preferential consumption of fat reserves implies adopting a relatively slow pace, which means less caloric expenditure.

While a slow fasted session allows for preferential consumption of fat reserves, its low intensity requires little recovery time.

While a slow fasted session allows for preferential consumption of fat reserves, its low intensity requires little recovery time. Indeed, after exertion, the body’s basal metabolism increases to meet its needs. It therefore consumes more calories, even at rest, during the recovery phase. If a long and/or intense session performed during the day primarily consumes glycogen reserves, the recovery phase significantly increases the basal metabolism, even more than a moderate-paced session of average duration. The benefits of fasted sessions for weight loss are therefore not so obvious in purely accounting terms.

On the other hand, one should not overlook the digestive comfort associated with such a session, and the very pleasant sensations that come with it. An asset appreciated by many cyclists and triathletes, who nevertheless take care to bring the necessary carbohydrate fuel to limit the risk of a sudden drop in performance during the session.

A method to improve endurance

Exercising on an empty stomach with limited glycogen reserves implies the implementation of adaptation systems, as the body then faces a difficult situation. Deprived of sugars or almost, it must function with fatty acids as fuel, which are less efficient and therefore yield less performance. A state roughly equivalent to what it encounters at the end of long-distance events, such as the marathon and its famous “wall” at the 30 km mark, when glycogen reserves are depleted, but it is still necessary to maintain a certain intensity of effort to finish without seeing performance drop dramatically.

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Fasted sessions therefore allow the body to be confronted with this phenomenon, and to re-establish the adaptive processes it has assimilated during events. More surprisingly, exercising on an empty stomach accustoms the body to resynthesize low glycogen reserves in the muscles, and thus to increase their capacity to store it. And when reserves are full, this additional storage capacity ensures a few extra minutes at full performance.

Disadvantages and precautions

You should not conduct a fasted session in the same way as a daytime session. First, in terms of intensity. Because your glycogen reserves are limited, you risk failure. But first and foremost, you must start by drinking, at least a large glass of water. Water is essential for the body’s functioning, and the risks of dehydration are real after a night of fasting. With the possibility of joint or tendon injuries as a result.

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Next, you adjust your pace, avoiding elevation changes. Lacking sugars, the body goes into reserve mode, and to burn fat more quickly, it’s better to aim for an intensity that corresponds to the preferential consumption of fat, so around 60 to 70% of maximum capacity, no more. A pace that is too fast, or worse, a session of interval training, not only quickly depletes the low glycogen reserves but also risks using proteins as a substitute substrate, which form the very structure of muscles. Proteins are only made available for energy production as a last resort. Cellular damage is major, and we then speak of “tapping into muscle,” which is, of course, completely unproductive in addition to causing very severe muscle soreness.

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Finally, the duration of the effort should be between 40 and 60 minutes when running or swimming, and up to a maximum of 2 hours when cycling (because cycling is a supported sport and there can be moments of coasting). This is enough to trigger the desired adaptation processes, and not too much to avoid excessively fatiguing the body. Carrying an energy gel or a fruit paste can provide some assurance in case of a real bonk. You should drink if possible (not easy when running or swimming), but more surely upon return, before the real breakfast or dinner, which should not be skipped in order to replenish reserves.

Primarily reserved for experienced athletes for a simple matter of effort management, fasted sessions should not be repeated more than three times a week. This is for your general physical condition and the need to maintain qualitative sessions in addition to progress. It is therefore a relatively easy training method to implement, but one from which no miracles should be expected.

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