Health influencers and nutrition experts have championed a wide range of weight-loss approaches over the years — from the ketogenic diet and liquid-only cleanses to restrictive calorie intake — yet few have achieved the level of mainstream popularity that intermittent fasting enjoys today.
Praised by some as a simple and effective health strategy, the discussion around the practice is never-ending, with one of the most asked questions being: how exactly does it work?
Highlights
- A simulation video shows the body changing during a 36-hour fast, including transitions from ketosis to autophagy.
- Experts say evidence for major long-term benefits of intermittent fasting in humans remains limited.
- Potential risks, meanwhile, include fatigue, nutrient deficiencies, and possible links to disordered eating.
As curiosity around fasting continues to rise, a simulation video from the YouTube channel Wellness Wise takes viewers through the physiological changes that unfold inside the body when it goes 36 hours without food.
The animation offers a detailed look at how the body adapts as it shifts through different metabolic stages before reaching a point where it relies more heavily on stored fat for energy, a process often associated with fat loss.
A simulation video broke down the metabolic shifts that occur when the body is deprived of food for 36 hours
Image credits: pvproductions/Freepik (Not the actual image)
Wellness Wise has 198K subscribers on YouTube. Their simulation video showcasing the workings of intermittent fasting runs for 56 minutes.
It was posted a year ago and has amassed 28M views as of this writing.
The visuals in the footage are graphic, depicting highly realistic human organs and other internal structures.
“What happens to your body when you fast for 36 hours?” the narrator of the video asks before following up with detailed information.
The video claims that four hours after eating, digestion slows, insulin levels drop, and the body begins using stored energy to function.
Image credits: Wellness Wise/YouTube
Around 8 hours in, it shifts to glycogen (carbohydrate) stores for fuel.
By 12 hours, fat burning begins as the body moves toward ketosis, a natural metabolic state in which it uses stored fat for energy.
After 16 hours, a natural cellular cleanup process called autophagy begins. At 24 hours, repair processes and insulin sensitivity improve. By 30 hours, growth hormone rises, helping preserve muscle.
Ultimately, at 36 hours, autophagy peaks, supporting cell renewal and metabolic reset.
Image credits: olenchic/Pixabay (Not the actual image)
Rishi Sunak, the former UK prime minister, was known for undertaking a 36-hour fast, with a source close to him telling the Sunday Times in 2024 that he consumed only water, tea, or black coffee from 5 pm on a Sunday to 5 am on a Tuesday.
Adam Collins, associate professor of nutrition at the University of Surrey, corroborated that fasting for 36 hours offers significant weight-loss benefits and strengthens “metabolic flexibility” — the body's ability to switch between fuels.
Intermittent fasting, however, is not 100% safe.
A US-based diet and mental health service provider, The Emily Program, outlined several side effects of the practice.
The severity of the side effects may vary and may not apply to all who try intermittent fasting
Image credits: drobotdean/Freepik (Not the actual image)
Excessive hunger pangs, fatigue, low energy, headaches, dizziness, digestive issues, irritability, mood changes, bad breath, and sleep disturbances are among the most commonly reported side effects of prolonged periods without food.
More concerning potential risks include impacts on reproductive health, as well as nutrient deficiencies and malnutrition.
Image credits: qimono/Pixabay (Not the actual image)
There also isn’t an extensive amount of data regarding intermittent fasting.
The Emily Program states: “Current human research is very limited in size and duration, in part because this diet fad is not sustainable.
James Betts, a professor of metabolic physiology at the University of Bath, echoed this sentiment, despite the growing number of supporters who endorse it.
Image credits: Wellness Wise/YouTube
Speaking to The Guardian, he said, “There are a lot of proposed benefits to running on fats. But a lot of the research hasn’t really been borne out in human beings, so we don’t see dramatic benefits, certainly not in the short term.”
Intermittent fasting may, in some cases, contribute to the onset of eating disorders
Image credits: Wellness Wise/YouTube
A December 2022 study by Kyle T. Ganson, assistant professor at the University of Toronto’s Factor-Inwentash Faculty of Social Work, and his associates explored the association between intermittent fasting and disordered eating among 2,762 Canadian adolescents and young adults.
They surveyed participants on their use of intermittent fasting over the previous 12 months.
Per the study, women showed the strongest associations between intermittent fasting and disordered eating, whereas men showed a significantly weaker correlation.
Image credits: Maksim Goncharenok/Pexels (Not the actual image)
Intermittent fasters, according to The Emily Program, are hard on themselves for breaking their fast too early or eating too late.
Depriving oneself of food for an extended period of time can also lead a person to feel out of control when the eating window arrives, the organization states.
Not achieving expected results within a short time frame can further push a person to eat less while promoting low self-esteem and body image issues.
"Moderation is the best choice", netizens conclude about weight loss
