A viral video is giving viewers a clear look at what the body does during a 24-hour fast — and it suggests the first changes can kick in earlier than many people realize.
With Ramadan nearing its conclusion for Muslims worldwide and Eid al-Fitr expected to begin next Thursday, people observing Islam are well acquainted with fasting and the discipline it requires.
During Islam’s holiest month, Muslims fast from dawn to sunset for worship, self-discipline, to remember the Quran’s revelation, and to build empathy, gratitude, and spiritual reflection. Outside of religious practice, others choose fasting mainly for perceived health benefits.
That’s helped 24-hour fasting resurface as a recurring online wellness trend, particularly while intermittent fasting remains popular among those aiming to lose weight, support metabolic health, or better manage eating routines.
One widely shared simulation lays out the process hour by hour, showing the body shifting between energy sources and making internal adjustments as time passes without food.
At its core, the concept is straightforward: when you stop eating, your body continues operating by pulling from energy already stored. However, medical experts stress that fasting isn’t a quick fix — and it also isn’t appropriate for everyone without careful consideration.
Cleveland Clinic says intermittent fasting may help some people with weight loss, blood pressure, and insulin resistance, but it is not a cure-all and still depends on overall nutrition.
Meanwhile, Johns Hopkins Medicine also notes that intermittent fasting is more about when you eat than what you eat, and any benefits still sit alongside the bigger picture of diet and lifestyle.
In the GrowFit Health YouTube simulation, the first stage begins quickly. Within around four hours, you may start feeling hungry as digestion winds down and insulin levels drop.
Soon after, the body begins leaning more heavily on stored sugar for energy as it transitions through the early phases of fasting.
By roughly the eight-hour mark, blood sugar can start running lower, prompting the body to tap into glycogen stores to keep energy levels up.
Even though this is only about a third of the way through a full 24 hours, the simulation suggests this point is when many people notice a shift in how they feel.
At 12 hours, the video describes the body entering “mini-ketosis,” meaning it starts producing and using ketone bodies — essentially moving toward using fat as fuel.
While intense hunger may have been more noticeable earlier, Healthline explains that what’s happening around 12 hours can lead to decreased appetite and weight loss, though some people may also experience fatigue.
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At around 16 hours, the simulation claims the process becomes more significant as autophagy begins, with the body recycling components by breaking down damaged cells and toxins.
Such activity provides plenty of health benefits, like getting rid of nonfunctional cell parts, and even removing viruses and bacteria, according to Cleveland Clinic.
Reaching 24 hours without food is described as a “deep repair” phase in the simulation, where fat becomes the primary fuel source.
The video also suggests inflammation may decrease and insulin sensitivity may improve, changes often linked to possible longer-term health benefits.