A simulation breaking down the effects of a 36-hour fast on the human body has captivated millions online.
Fasting, the practice of abstaining from food for a set period, has surged in popularity due to its benefits, which range from weight loss and improved metabolism to enhanced cognitive function and disease prevention.
Many adopt it for religious reasons, others for fitness goals, and some for perceived mental clarity. But despite its widespread appeal, health professionals are urging caution.
The simulation, posted by Wellness Wise, uses simplified graphics and commentary from popular practitioner Dr. Kiltz to illustrate what happens in your body hour by hour during a prolonged fast.
So, what happens during a 36-hour fast?
4 Hours: Your body enters the catabolic phase, where it stops digesting new food and begins breaking down stored energy.
8 Hours: Blood sugar levels drop, and your body taps into glycogen stores for energy. This marks the beginning of your body operating independently from fresh fuel.
12 Hours: Glycogen is mostly depleted, prompting the liver to break down fat into ketones - energy molecules that take over as your primary fuel source. This process, known as "metabolic switching," is what many believe drives weight loss during fasting.
16 Hours: A crucial cellular process known as autophagy kicks in. According to the Cleveland Clinic, this mechanism helps clean up and recycle damaged cell components, essentially renewing your body at the cellular level.
24 Hours: At this point, you’re deep into fat-burning territory. Inflammation may decrease, and insulin sensitivity improves. The body is believed to enter full repair mode.
30 Hours: Human growth hormone (HGH) surges, helping preserve muscle mass and promote further fat loss.
36 Hours: Maximum autophagy is reached. According to the simulation, autophagy increases by 300%, putting your body in a full clean-up and regenerative mode.
Metabolism boosts, damaged cells are purged, and the body experiences what’s referred to as "deep healing."
What do researchers and experts say?
Not everyone agrees that these benefits are guaranteed. James Betts, professor of metabolic physiology at the University of Bath, cautioned: “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 health benefits, certainly in the short term,” he told The Guardian.
Meanwhile, long-term fasting has also been linked to potential risks. According to News In Health, fasting beyond 16 to 18 hours daily may increase the risk of gallstones. Common side effects can include irritability, intense cravings, digestive issues, and low mood.
However, other experts believe the upsides are worth it. “Many things happen during intermittent fasting that can protect organs against chronic diseases like type 2 diabetes, heart disease, age-related neurodegenerative disorders, even inflammatory bowel disease, and many cancers,” says Mark Mattson, a Johns Hopkins neuroscientist.
According to Well Defined, fasting can reduce inflammation, lower insulin levels, support cellular regeneration, and encourage fat loss without risking muscle depletion - especially if the fast doesn’t extend beyond 72 hours.
Health professionals recommend starting slow with intermittent fasting methods like the 16/8 split - fasting for 16 hours and eating during an 8-hour window - before attempting extended fasts like the 24- or 36-hour versions.
Staying hydrated with water or zero-calorie drinks is essential, and how you break your fast matters. Overeating immediately afterward can lead to discomfort and negate potential benefits.
Experts have stated that fasting is not just about skipping meals - it's about “unlocking hidden strengths within ourselves” and creating space for both physical and emotional healing.