In the pursuit of weight loss and better health, people often explore a wide range of strategies—from calorie-restrictive diets to the latest supplements, detoxes, and fitness regimes. One method that has recently gained traction among health-conscious individuals, athletes, and even celebrities is intermittent fasting.
This dietary pattern doesn’t focus on what you eat but rather when you eat. Among its many variations, the 36-hour fast stands out as one of the most intense. A recent viral video simulation sparked widespread discussion by visually breaking down what happens in the body hour by hour during a 36-hour fast.
But what actually occurs in the body during this extended fasting window? Are there real benefits, or are the risks greater than advertised? Let’s explore the scientific breakdown, potential advantages, and the health dangers of going without food for 36 hours straight.
What Is a 36-Hour Fast?
A 36-hour fast is a form of extended intermittent fasting where an individual abstains from consuming any calories for a full day and a half. For example, you might finish dinner at 8 p.m. on Monday and not eat again until 8 a.m. on Wednesday.
Unlike the more common 16:8 fasting (16 hours fasting, 8-hour eating window) or OMAD (One Meal A Day), the 36-hour fast is more intense and is considered by some experts as alternate-day fasting taken to the extreme.
This method is said to promote weight loss, cellular repair, and metabolic flexibility, but it also presents clear risks, especially for beginners or people with specific health conditions.
Timeline: What Happens in Your Body During a 36-Hour Fast?
A 55-second animated simulation shared by a popular health YouTube channel breaks down the physiological changes the body undergoes during a fast of this length. Here’s what the science says:
0–4 Hours: Digestion Mode
After your last meal, your body is still digesting food and absorbing nutrients. Glucose from carbohydrates is still being used as the primary fuel source.
4–8 Hours: Transition Begins
As glucose levels drop, the body starts burning stored glycogen (a form of sugar stored in the liver and muscles). At this stage, insulin levels decrease, which may help in controlling fat storage.
8–12 Hours: Glycogen Depletion
Your blood sugar begins to stabilize at a lower level. Glycogen stores start running low, especially if you are active. The body prepares to switch energy sources.
12–16 Hours: Fat Burning Initiates
At this point, ketogenesis begins—your liver starts converting stored fat into ketones, which serve as an alternative energy source. This process, often referred to as metabolic switching, marks the early stages of fat loss.
16–24 Hours: Autophagy Activation
Autophagy is a natural process where your body cleans out damaged cells and regenerates new ones. This is believed to support healthy aging, immune defense, and cellular repair.
24–30 Hours: Deep Fat-Burning State
Now the body enters full fat-burning mode, and insulin sensitivity increases. Fatty acids become the dominant energy source, and inflammation markers may begin to decline.
30–36 Hours: Peak Autophagy
By 36 hours, autophagy reaches its highest levels. This is also the stage associated with potential regeneration of certain tissues and the activation of longevity pathways. Some research suggests metabolic health can improve significantly here, though more evidence is needed.
Potential Benefits of Extended Fasting
While the 36-hour fast is not suitable for everyone, it does have some scientifically supported benefits, particularly when practiced occasionally under supervision.
1. Fat Loss
Extended fasting depletes glycogen reserves and forces the body to rely on fat stores for energy. This can promote sustained fat loss, especially visceral fat, which is linked to metabolic diseases.
2. Reduced Inflammation
Some studies have shown that fasting reduces inflammatory cytokines, potentially lowering the risk of chronic diseases like heart disease and type 2 diabetes.
3. Improved Insulin Sensitivity
Extended fasting improves the body’s ability to use insulin effectively, which can stabilize blood sugar and reduce the risk of insulin resistance.
4. Cellular Repair and Longevity
Autophagy is believed to play a key role in preventing certain types of cellular damage, possibly reducing the risk of neurodegenerative conditions and certain cancers.
5. Mental Clarity
Many people report enhanced mental focus and energy levels during prolonged fasts, likely due to ketone production and stabilized blood sugar levels.
The Risks: Why a 36-Hour Fast May Be Dangerous
Despite the promising benefits, a 36-hour fast can pose serious health risks—especially when done unsupervised or without proper hydration and nutrient support.
1. Nutrient Deficiencies
If prolonged fasting is done regularly without attention to nutrient intake, it can lead to vitamin and mineral deficiencies, including magnesium, potassium, and B vitamins.
2. Dehydration and Electrolyte Imbalance
Without proper fluid and electrolyte intake, fasting may cause dehydration, headaches, fatigue, and heart palpitations.
3. Mood Swings and Irritability
The drop in blood sugar can cause mood disturbances, including anxiety and irritability, especially for those prone to emotional eating.
4. Dizziness and Lightheadedness
As glycogen stores drop, so does water retention, which can contribute to lightheadedness and low blood pressure.
5. Disordered Eating Risks
According to The Emily Program, extended fasting may trigger or worsen disordered eating behaviors such as bingeing, restricting, and obsession with food in vulnerable individuals.
6. Bad Breath and Digestive Disruption
The production of ketones may lead to halitosis (bad breath). Additionally, prolonged fasting may disrupt normal digestive function, leading to constipation or acid reflux.
Who Should NOT Try a 36-Hour Fast?
Before attempting any prolonged fast, it’s important to consult with a healthcare provider—especially if you fall into any of the following categories:
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Pregnant or breastfeeding women
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Individuals with diabetes or blood sugar disorders
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Anyone with a history of eating disorders
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Children and adolescents
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Individuals taking medications that must be taken with food
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People with chronic conditions like heart or kidney disease
Expert Recommendations
If you are interested in trying extended fasting, consider these best practices from nutritionists and medical professionals:
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Start slowly: Begin with 12- or 16-hour fasts to observe how your body reacts.
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Stay hydrated: Drink plenty of water and consider electrolyte supplementation.
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Break the fast gently: Reintroduce food slowly with light, nutrient-dense meals.
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Listen to your body: If you feel weak, dizzy, or mentally impaired, break the fast.
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Cycle your fasts: Extended fasting should not be done every day. Many experts recommend no more than once a week or once every two weeks.
Conclusion: Is a 36-Hour Fast Worth It?
Fasting for 36 hours is a powerful metabolic intervention, but it is not without risks. While the benefits—such as autophagy, fat loss, and insulin sensitivity—are scientifically supported, these results vary widely by individual and depend on overall health, hydration, and diet quality.
Extended fasting should be approached cautiously and under medical supervision, especially for those new to fasting or dealing with underlying health conditions. For most people, shorter fasting windows like 14:10 or 16:8 may offer health benefits without the added stress or risk of a full-day fast.
As with any health strategy, personalization and balance are key. Always consult your healthcare provider before making drastic dietary changes.
Sources
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Harvard T.H. Chan School of Public Health – Intermittent Fasting
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National Institute on Aging – Caloric Restriction and Fasting Diets