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Is the body just as strong after seven days of fasting? The findings surprised researchers

The research project is the first to examine how muscles are affected after an extended period of fasting.

Louise Skak has been running all her life and was interested in knowing how her body would react to fasting and exercise.
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“For me, it's important to be mentally strong and have the ability to persevere during training. That was one of the reasons I wanted to participate and commit to this," says Louise Skak.

She is an ultra-runner, and was one of the 13 participants who volunteered to fast for seven days in a research project at the Norwegian School of Sport Sciences (NIH). 

Ran to and from work

To better understand how the body adapts to hunger, the participants were to fast for an entire week. There were secen men and six women. 

Water was the only thing they consumed. 

Professor Jørgen Jensen researches and teaches metabolism at the Norwegian School of Sport Sciences.

Before the fasting period, they underwent thorough strength tests. They also tested their maximum oxygen uptake.

During the fasting period, they lived normal lives with school and work. Everyone tried to exercise as much as possible.

“I tried to maintain my training, including running to and from work. It went well, but slower than usual. I was also colder and more tired than usual,” says Skak. 

On day six of fasting, they tested strength and endurance again.

It gave two completely different answers.

First time leg strength has been tested after fasting

“This is incredibly exciting. Our findings actually show that you can maintain the strength of your leg muscles during a week of fasting. Endurance, on the other hand, drops significantly,” says Professor Jørgen Jensen.

He is one of the researchers behind the study. 

The study is the most comprehensive investigation into how a week of fasting affects physical performance.

Others have examined grip strength, but this is also the first time leg strength has been tested after seven days of fasting.

During the seven-day fasting period, participants lost an average of 5.7 kilograms and 1.4 kilograms of fat.

Despite the loss of body mass, participants managed to maintain maximum muscle strength in their legs after fasting.

Sugar stores in the muscles were halved, while the proteins that help cells produce energy did not change.

“This means the body has a mechanism that allows us to conserve carbohydrates. And if you conserve sugar stores, you also conserve the muscle,” explains Jensen.

The brain needs sugar

Every morning, participants in the project visited NIH for blood tests and weighing.

They wore glucose monitors continuously throughout the week and actually collected all their urine to give to the researchers.

This made it possible for researchers to calculate how much protein was being broken down in the body.

The only way the body can make carbohydrates is by breaking down protein. Normally, we have a store of 500 grams of carbohydrates in the body.

That is quickly used up when no food is consumed. In one day, we use around 300 grams of carbohydrates, and the brain uses 120 of those.

“In the study, we found a mechanism whereby the muscles don't use carbohydrates during exercise. The brain needs sugar, and it's important that the muscles don't use the carbohydrates that the brain needs,” says Jensen.

Drop in maximum oxygen uptake

However, a certain enzyme that suppresses carbohydrates increased by as much as 13 times. This led to the participants burning fewer carbohydrates and experiencing poorer endurance during high-intensity training.

Participants underwent tests of maximum oxygen uptake before the fasting period and again on day six of fasting. The results showed a marked decline.

Danish Louise Skak is an experienced ultrarunner who has competed in long-distance races for many years.

The participants' maximum oxygen uptake decreased by 13 per cent.

Participants also reported that it was very difficult to complete high-intensity sessions. The effect of these also diminished.

It showed that the major bodily changes first occurred after three days without food, indicating that shorter fasting periods do not have the same effects.

The body manages to maintain blood sugar

Fasting did not affect the activity of another enzyme previously thought to be important for muscle breakdown.

The findings of preserved strength during fasting stand in stark contrast to a week of bed rest. In that case, a 5-10 per cent decline in strength has been observed, even without significant loss of lean body mass.

Lean body mass includes bones, muscles, internal organs, water, and other things that are not fat.

“From a survival perspective, this is very exciting and new. It shows that the body can manage to maintain blood sugar and conserve carbohydrates. We also saw that participants recovered quickly after fasting,” says Jensen.

The findings showed that participants retained almost 50 per cent of the glycogen in their muscles.

More days of fasting are needed

Jensen emphasises that these findings cannot be related to intermittent fasting, where one does not eat during a time window of, for example, eight to ten hours, or for two days a week.

“It's entirely different. You don't get the increase in ketone bodies in the blood that fasting individuals do. For them, significant adaptations occurred in the body throughout the week,” he says. 

Ketone bodies

Ketone bodies are incomplete breakdown products of fat. They are normally formed in the liver during the oxidation of fatty acids and are used as an energy source in muscles and other organs.

(Source: Great Norwegian Encyclopedia) 

Strength training and low intensity can be combined with fasting

But what can a regular exerciser who is curious about whether fasting might be beneficial take away from this research?

Professor Jensen says it is absolutely safe for healthy individuals to undergo such a period. But you should probably skip the high-intensity sessions.

Skak did light training and actually ran a marathon two days after the fasting period.

“It gave me mental strength and showed me that I can achieve what I want. The body tolerates more than I think. After day four, the feeling of hunger disappeared and was replaced by a sense of lightness in the body,” says Skak.

Participants also reported that it was possible to perform gentle training and strength training during the fasting period.

All participants in the project were all in normal physical condition and already used to exercising beforehand.

To summarise: The participants retained muscle strength and energy-producing enzymes during fasting, but their ability to burn carbohydrates and perform at high intensity was reduced.

Reference:

Kolnes et al. Effects of seven days’ fasting on physical performance and metabolic adaptation during exercise in humans, Nature Communications, vol. 16, 2025. DOI: 10.1038/s41467-024-55418-0

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Read the Norwegian version of this article on forskning.no

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