A simulation has shown how the human body responds to going without food for three days.
As the global focus on weight loss intensifies, an increasing number of people are turning to fasting as a way to shed pounds and boost their health.
While going without food for a day or two may seem extreme to some, many claim it offers significant benefits. But what actually happens inside the body during an extended fast?
A new simulation by the YouTube channel Gravity Transformation walks viewers through a 72-hour fast hour by hour, illustrating the changes the body undergoes—from the first missed meal to the stage where nutrient deficiencies may begin to pose a risk.
The First 6–10 Hours: Burning What’s Left
In the initial hours of fasting, the body begins to deplete its remaining carbohydrate reserves from previous meals.
During this period, the pancreas releases glucagon, which triggers the release of stored glucose from the liver and muscles to maintain energy levels.
As blood sugar levels drop, hunger hormones like ghrelin surge, intensifying feelings of hunger—though many individuals push through this phase despite the discomfort.
10–16 Hours: Growth Hormones and Fat Burning
10 Hours In: Hormonal Shifts Begin
By the 10-hour mark, the pituitary gland increases production of human growth hormone, which plays a key role in preserving muscle by slowing its breakdown.
16 Hours: Fat Burning and Cellular Cleanup
Around hour 16, the body begins relying more heavily on fat for energy. This is also when autophagy—a cellular process that clears out damaged cells, bacteria, and junk proteins—starts to activate.
24–32 Hours: Full Fat Reliance and Muscle Preservation
After a full day without food, glycogen stores are depleted, and the body becomes almost entirely dependent on fat for fuel. As a result, protein breakdown slows down, helping to preserve muscle mass.
48–72 Hours: Mental Clarity and Medical Risk
By day two, many people report improved mood, heightened alertness, and a sense of tranquility. However, by the third day, the risk of nutrient deficiencies increases significantly. Experts advise against continuing a fast beyond this point without medical supervision.
While simulations provide insight into short-term fasting, real-world cases reveal that human survival without food can exceed expectations—especially in extreme environments.
For example, in 2012, Swedish man Peter Skyllberg survived an estimated 60 days in a snow-covered car, reportedly sustained only by eating snow. Other extraordinary cases include illusionist David Blaine, who fasted for 44 days inside a suspended glass box; Mahatma Gandhi, who survived 21 days on water alone during a hunger strike; and a Japanese hiker who endured 24 days without food or water after being stranded in the mountains.
Professor Stephen Joseph from the University of Nottingham explains that certain factors—such as isolation, cold temperatures, and reduced physical activity—can actually extend a person’s survival time. Psychological strength also plays a crucial role.
“In extreme situations, people can enter a fight-or-flight state, with adrenaline surging through their bodies,” he said. “These adrenaline responses can be critical to whether or not someone survives.”
However, Joseph emphasized that not everyone reacts to life-threatening situations in the same way. “Personality matters in disasters. Some individuals may panic and lose hope, while others remain optimistic and find ways to adapt,” he noted.
“Hope is likely one of the most important factors,” he added.
