Fasting and Exercise: When Fasted Training Is Smart and When It Backfires

Maybe you know this kind of morning.

If you recognize yourself in this scene, you're in good company. Few topics are debated as passionately in gyms and fasting forums. Some swear by it, others warn against it.

Both sides are partly right. I'm writing this as a physician who values exercise and fasting and takes studies seriously. Three questions interest me: What happens in your metabolism when you train fasted? When does the effect backfire in hard sessions? And why might the female body do the math differently from the male body?

By fasted, I mean training after a night without food, usually in the morning before breakfast. I only touch briefly on fasting for several days. You can read what exactly interrupts a fast in the article What breaks a fast?

Do you burn more fat when you train fasted?

In short: Yes, during the session. A meta-analysis of 27 studies found on average around 3 grams more fat burned per endurance session in the fasted state. Over several weeks, however, women in a randomized trial lost the same amount of body fat, whether they trained fasted or after eating. Burning fat during training is not the same as losing fat from your body.

Maybe fasted training feels especially honest to you. Empty stomach, clear head, your body has to tap into its reserves. The image is understandable. But it's only half right.

After a night without food, your insulin level is low. Insulin is something like the bouncer at the door of your fat cells. When there's a lot of it in your blood, the door tends to stay shut, and fat is stored. When there's little of it, fatty acids are allowed out and can travel to your muscles as fuel.

But what counts for your body in the end? The hour on the bike or the whole week? A team in New York asked exactly this question.

A similar picture emerged with interval training. In women with overweight, body fat percentage fell by the same amount after six weeks, fasted or fed. More on that in the next section.

Things get interesting when you look not only at the training hour, but at the whole day. Two studies did exactly that, one with women and one with men.

What does this mean for you? If fasted training comes easily to you in the morning, there's little reason not to schedule easy sessions that way. If it's a struggle, the studies suggest you're missing out on little when it comes to fat loss. Instead, pay attention to how you eat for the rest of the day. Do cravings hit in the afternoon, or do things stay calm? You can read more in the article Cravings during intermittent fasting.

And now you know why three grams of fat in the morning are not a shortcut, but a small building block.

What fasted training can change in your mitochondria

In short: In a training study, young men trained for six weeks either fasted or with carbohydrates. Only in the fasted state did two key mitochondrial enzymes increase, by 47 and 34 percent. Endurance and performance, by contrast, improved similarly in both groups. In men with overweight, insulin sensitivity rose only with training before eating. In women with overweight, interval training showed no difference.

Maybe you know days when your body seems to have only one gear. Without breakfast you get shaky, after lunch you feel heavy as lead. Experts would ask how flexible your metabolism is right now.

Imagine your metabolism as a hybrid engine. It can run on sugar, fast and powerful. It can run on fat, slower and with more stamina. A flexible engine switches silently, depending on what is available and what is needed.

Goodpaster and Sparks describe metabolic flexibility as the ability to adapt to changing metabolic demands. In overweight and type 2 diabetes, this switching is often impaired. The power plants that handle the switching in your muscle are called mitochondria.

And in type 2 diabetes? A review from the University of Bath has so far found no advantage of fasted training there. In some cases, the long-term blood sugar marker HbA1c even fell more when people trained after eating. The authors suspect that an already inflexible metabolism narrows the difference between fasted and fed. You can read more in Fasting with type 2 diabetes and in Exercise after eating.

What does this mean for you? If you want to make your metabolism more flexible, easy fasted sessions can be one building block. The foundation remains regular exercise in the first place. You can read how exercise can challenge the power plants of your cells in the article Strengthening your mitochondria.

And now you know why fasted training rebuilds the engine rather than emptying the tank.

Easy or hard: why intensity changes everything

In short: Fat is the fuel for a calm to brisk pace. In a laboratory study, the highest fat burning occurred at about 64 percent of maximal oxygen uptake. Above around 89 percent, fat hardly played a role anymore. Without breakfast, strength athletes managed fewer repetitions. With depleted stores, interval performance dropped. And a meal beforehand improved performance in long endurance efforts.

Sound familiar? The easy morning run feels effortless. But as soon as you pick up the pace, every step feels like running through sand.

Your body has two fuels with very different characters. Fat is like a big log in the fireplace: lots of energy, but it burns slowly. Carbohydrates are like kindling: a smaller supply, but they flare up immediately. The faster you need energy, the more your muscle reaches for the kindling.

Your body stores this kindling as glycogen in the liver and muscles. A review from Bath and Maastricht describes carbohydrates as the main fuel at moderate to high intensity. Because the stores are relatively small, their availability often limits performance during long efforts.

And what does your stress system do? A team at the University of North Carolina had twelve active, moderately trained men exercise for 30 minutes each at different intensities.

With easy training, cortisol stayed almost the same. From moderate intensity upward it rose markedly, by 83 percent at 80 percent. Being fasted is another stimulus for your body. Whether the two stimuli add up has hardly been studied in humans so far.

This comparison summarizes the studies in this article. It is not an individual training recommendation.

What does this mean for you? A rough guide in everyday life is the talk test. If you can still speak in full sentences while training, you're usually in the calm to brisk range. There, fasted training is quite doable for many people. For intervals, race pace and heavy strength sets, energy in the tank tends to be worth it. You can read more about calm training in the article Understanding Zone 2 training.

And now you know why the same fasted hour can be a gain for one person and a performance slump for another.

Muscle and protein: does fasted training break down muscle?

In short: After a night without food, the risk to your muscles seems to be low. A meta-analysis of four studies found similar effects of fasted and fed strength training on fat-free mass, muscle growth and strength. With deliberately depleted carbohydrate stores, however, protein breakdown during exercise rose markedly. What seems to matter is energy and protein across the whole day.

Maybe you worry that your body will go after your muscles when you train fasted. You've worked hard for them. That worry deserves an honest answer.

For your body, protein is mainly building material and hardly fuel. Imagine heating your home in winter with the timber of your own roof. Your body only does that on a larger scale when other supplies run low.

Things look different with several days without food. In the Oslo study, strength was preserved after a week of fasting, but fat-free mass fell by 4.6 kilograms. That includes water and glycogen, not just muscle.

What does this mean for you? After a fasted session, plan a proper meal with protein, for example with eggs, yogurt, quark, tofu or legumes. In my view, it's worth not putting it off longer than necessary. You can read how you can protect your muscles while fasting in the article Muscle loss during fasting.

And now you know why your muscles have less to fear from an empty morning than from an empty day.

Women and fasted training: why your body may do the math differently

In short: Relative to fat-free mass, women on average already burn more fat during exercise than men do. In studies with women, fasted training brought no added benefit for fat loss. At the same time, the female hormonal axis seems to react sensitively to energy deficiency. In one study, the LH rhythm fell out of step below 30 kilocalories per kilogram of fat-free mass per day.

Maybe you've already heard contradictory advice. Some say women should never train fasted. Others call that scaremongering. I can well understand if this leaves you feeling unsure.

Both positions fall short. The female metabolism is not fragile porcelain. But it seems to keep a closer count of how much energy is available right now. Imagine a household that, on a tight budget, cuts the big investments first. Ovulation and bone building are among those investments.

The difference shows up even at rest. At Washington University in St. Louis, six women and six men with the same body fat percentage were compared. After 14 hours without food, the women released more fat from their stores. As fasting continued, the men caught up, and after 22 hours both were level. A review from Odense and Wageningen also describes that women reach higher ketone levels during fasting. You can read how to recognize ketosis in the article Recognizing and measuring ketosis during fasting.

Energy availability is the energy that remains for everything else once training is subtracted: brain, immune defense, hormones, bones. With 50 kilograms of fat-free mass, 30 kilocalories per kilogram would be 1,500 kilocalories left over after training. If you train a lot and eat little, you can slip below that faster than you think.

The same experiment showed something else. Markers of bone formation already fell at all three reduced levels, that is, already at 30 kilocalories. Bone breakdown only increased with the strongest restriction, to 10 kilocalories. The authors warn that young women might, as a result, not reach their peak bone mass.

This comparison summarizes individual studies from this article. The studies are small and mostly do not compare women and men directly.

Important for putting this in context: a single fast does not automatically throw your cycle off. At the National Institutes of Health in Bethesda, normal-weight women who did little exercise fasted for three days in the first half of their cycle. LH pulses became less frequent on the last day of fasting, but ovulation and cycle length remained unchanged. What seems critical above all is the combination of a lot of training and an ongoing lack of energy.

And does the cycle phase matter? At the University of Jyväskylä in Finland, fat burning was measured in 34 women with a natural cycle, fasted, in both halves of the cycle. Peak values were 0.40 and 0.41 grams per minute, practically the same. The intensity with the highest fat burning did not shift either. For fat burning, then, the phase seems less important than often assumed. You can read how fasting can feel across the cycle in the article Fasting for women and the cycle. How training and cycle hormones interact is described in Exercise, hormones and the cycle.

The International Olympic Committee brings all of this together under the term REDs, Relative Energy Deficiency in Sport. The 2023 consensus describes a syndrome of health and performance consequences. It can affect women and men when energy is persistently too low relative to training. The consensus also emphasizes the growing role of low carbohydrate availability and the interplay with mental health.

What does this mean for you? If you're a woman and want to train fasted, that can work well for easy sessions. Make sure you eat enough over the day and don't stay in a deficit for many hours at a stretch. Watch your cycle like a weather report. If it changes, that's a good reason to have a doctor take a look. During menopause, the rules shift once again, more on that in the article Fasting during menopause.

And now you know why the female body is not weaker when it comes to fasted training, but simply does the math more carefully.

Train low, Sleep low and Ramadan: what sport learns from them

In short: In competitive sport, fasted training is dosed deliberately rather than used across the board. In a study with 21 triathletes, the Sleep low strategy cut their time over ten kilometers by 2.9 percent after three weeks. In male elite endurance athletes, a similar approach brought no added benefit. During Ramadan, it was mainly sprints and short maximal efforts that suffered, while endurance and strength remained largely stable.

Maybe you're wondering how the pros do it. Do they all train fasted? Or do they eat energy bars all the time?

The answer lies somewhere in between. Sports scientists speak of periodization. Carbohydrates are used like a tool, according to need. A team led by Impey and Morton at Liverpool John Moores University calls the principle Fuel for the work required: fuel matched to the work that is coming up.

The Liverpool review sums it up: in around three quarters of the Train low studies, signals, gene activity and enzymes in the muscle increased. Performance, however, improved in only 37 percent of eleven studies, and in 63 percent it stayed the same. The authors stress that absolute training intensity should not suffer in the process.

This overview summarizes the studies from this section. It is not a recommendation for any particular strategy.

If you fast and train during Ramadan, you'll find further pointers in the article Fasting during Ramadan.

What does this mean for you? If you train ambitiously, ask yourself before every session: is today about quality or about calm volume? For quality, it's fine to have energy in the tank. For calm volume, training fasted can be an extra stimulus. In my view, strategies like Sleep low belong in experienced hands, for example with guidance from sports medicine or exercise science.

And now you know why competitive sport doesn't ask whether fasted training is good, but when.

When you're better off not training fasted

In short: Talk to your doctor before training fasted if you take insulin or other medications that can cause low blood sugar. In my view, the same applies if your cycle is irregular or absent and if you have an eating disorder. Likewise during pregnancy and before long or very hard efforts. In a classic study, blood sugar during hours of cycling without carbohydrates fell to 2.5 millimoles per liter, shortly before exhaustion set in.

Maybe you've experienced suddenly feeling faint while training fasted. Your hands tremble, your sweat turns cold, your legs go weak. That is a signal you should take seriously.

Your body normally keeps blood sugar remarkably stable. The liver releases sugar from its stores and makes new sugar. A team at Yale University measured this directly in the liver during a 68-hour fast. Even in the first 22 hours, around 64 percent of sugar production came from newly made sugar.

During long, demanding efforts, this system can reach its limits.

One detail from the Leuven study is interesting. After six weeks of fasted training, blood sugar no longer dropped during a two-hour fasted effort, whereas it did in the carbohydrate group. So the body seems to be able to adapt to fasted training.

With type 1 diabetes, training is an art of its own. An international consensus describes blood sugar targets for safe training as well as adjustments to nutrition and insulin dose. According to this consensus, fear of low blood sugar is one of the hurdles that can keep people with type 1 diabetes from exercising. Whether you train fasted is not something you decide alone here, but together with your diabetes team.

You can read how fasting can influence blood sugar and insulin in the article Fasting, blood sugar and insulin resistance.

And now you know why a piece of banana at the right moment is sometimes smarter than any amount of discipline.

Fasted training through the KPNI lens: four systems in conversation

Back to your morning run. Why does the same hour feel easy on some days and like a struggle on others? This question leads right into Clinical Psychoneuroimmunology, KPNI for short.

KPNI looks at the nervous system, immune system, metabolism and hormonal system not separately, but as one conversation. In fasted training, all four have a say.

You can read how the stress axis and fasting can interact in the article Fasting with chronic fatigue.

I see a fifth level as well: your attitude. For Nonviolent Communication, Marshall Rosenberg coined a principle that fits well here: observing is something different from evaluating. “I was slower than usual in my intervals today” is an observation. “I'm undisciplined” is an evaluation. Behind the observation there is often a need, for energy, sleep or recovery.

And now you know why the same fasted hour is training on a good day and an additional burden on a bad one.

Three levers if you'd like to train fasted

You don't have to train fasted. If you'd like to try it, these three directions can give you some orientation. They are not a training plan.

If you'd rather not assess your situation on your own: below this article, you'll find the option to book an appointment.

And now you know why the smartest question is not whether you train fasted, but when and how hard.

Frequently asked questions about fasted training and fasting

Where this topic connects with other areas

Fasted training touches on endurance, intensity, cellular energy, hormones and recovery. If you want to dig deeper, it's worth taking a look at these neighboring topics from the Sport Guide.