By Charlotte Webster-
Fasting has long fascinated scientists, nutritionists, and spiritual practitioners alike. Emerging research now suggests that abstaining from food for 72 hours is not merely a dietary or spiritual exercise; it may trigger profound biological processes that cleanse the body, support cellular repair, and improve overall metabolic health.
Experts explain that during this three-day fast, the human body engages in mechanisms that could enhance everything from insulin sensitivity to cardiovascular function.
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“The body is remarkably adaptive,” says Dr. Valter Longo, a leading researcher in longevity and fasting. “When nutrients are withheld for a few days, the body enters a protective state that may promote cellular regeneration and repair.”
When a person stops eating, the metabolism shifts in a remarkable way. Initially, the body relies on stored glucose in the liver to provide energy, but after roughly 24 to 36 hours, glycogen stores become depleted.
At this point, the body transitions into a metabolic state known as ketosis, where fat is converted into ketones to supply energy. Ketones serve not only as fuel but also as signalling molecules, triggering processes such as autophagy, a cellular self-cleaning mechanism that breaks down damaged proteins, clears toxins, and recycles cellular components for energy.
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Research indicates that autophagy peaks during fasts lasting between 48 and 72 hours, which is why this duration has attracted particular scientific attention. During this process, defective mitochondria, misfolded proteins, and even pre-cancerous cells may be removed, allowing the body to repair itself at a cellular level.
While human studies are ongoing, evidence from animal research and preliminary human trials suggests that this cellular cleanup can improve metabolic health, reduce inflammation, and enhance resilience at the cellular level.
Beyond autophagy, fasting dramatically influences hormone levels. Insulin levels drop, improving insulin sensitivity and potentially reducing the risk of type 2 diabetes.
Growth hormone rises, supporting fat metabolism and preserving lean muscle tissue. Collectively, these metabolic and hormonal shifts allow the body to focus on repair rather than growth or storage, effectively creating a biological reset.
A 2024 article in the Times of India described the effects of a 72-hour fast, noting that the body shifts from relying on sugar to burning fat, inflammation decreases, and repair processes are activated, supporting both cardiovascular and metabolic health.
While fasting for three days is not suitable for everyone, research indicates that extended fasts are associated with significantly lower circulating insulin and a deeper metabolic shift toward ketosis, as the body moves from glucose to fat metabolism and promotes enhanced fat oxidation.
Studies show that circulating glucose and insulin levels drop as fasting continues, which supports reduced fasting insulin and improved metabolic function.
Nutritionists also observe psychological benefits related to eating behaviours, noting that mindful engagement with food and eating patterns which can be heightened after periods of intentional nutrient abstinence is linked with better control over food cravings and a more thoughtful relationship with eating.
Even programs unrelated to fasting, such as the Greggs Foundation’s breakfast clubs, demonstrate how structured interventions can improve health outcomes.
While Greggs provides children with free breakfasts to support metabolism and learning, a 72-hour fast triggers internal physiological processes that cleanse and repair the body. Both approaches illustrate how structured methods, whether external or internal, can enhance health and resilience.
The idea of fasting “eating up toxins” refers to the activation of autophagy and cellular recycling. During a prolonged fast, damaged organelles and proteins are broken down and repurposed for energy, while potentially harmful molecules are removed.
Although it is technically an oversimplification to say that the body literally “eats toxins,” this metaphor captures the essence of what happens during extended fasting.
Dr. Longo cautions that while fasting can activate protective pathways, individuals must approach extended fasts carefully, ideally under medical supervision. He warns that dehydration, electrolyte imbalances, and preexisting health conditions can make unsupervised fasting risky.
Research shows that extended fasting acts as a kind of metabolic “reset,” prompting the body to shift from glucose to fat metabolism, improve insulin sensitivity, and lower chronic inflammation changes linked with better overall health outcomes and reduced disease risk.
Longer fasts also activate cellular repair pathways, such as autophagy, that help clear damaged components and reduce oxidative stress, processes thought to contribute to improved metabolic resilience.
Animal studies suggest that periodic fasting can extend lifespan and enhance physiological functions tied to ageing and chronic disease, and early work indicates that fasting may also support brain health and protect against cognitive decline.
Extended fasting has been shown to significantly shift the body’s energy metabolism from glucose to fat and ketone use, leading to markedly elevated circulating ketones as the primary fuel source during prolonged nutrient abstinence.
Research on long‑term fasting indicates ketosis is reliably detectable in most participants after several days without food, suggesting a metabolic shift that aligns with claims of increased ketone production and energy adaptation.
Metabolic studies also describe how the transition to ketone fuel and reduced insulin levels during extended fasts represent a fundamental shift in the body’s energy use, which can influence inflammation and cellular stress markers.
Although evidence on systemic inflammatory marker changes in prolonged fasts remains mixed, some fasting protocols have been associated with shifts in glycemic control, insulin sensitivity, and inflammatory profiles.
Research further shows that elevated ketone bodies, which become a key energy substrate when glucose is unavailable, support brain function and metabolic effects beyond simple energy supply. Ketones are known to influence cellular homeostasis, oxidative stress responses, and pathways linked to cognitive processes.
Experts stress that prolonged fasting should not be attempted casually. Ideal candidates are healthy adults without chronic conditions such as diabetes, cardiovascular disease, or kidney disorders. First-time fasters are advised to gradually build up their tolerance through intermittent fasting, and proper hydration and electrolyte balance are essential.
Breaking the fast requires care, with foods reintroduced slowly to avoid digestive distress. With those seeking structured guidance, programs like Dr. Longo’s Fasting Mimicking Diet provide a controlled approach that replicates the benefits of fasting while minimizing risk.
Fasting is not a new concept. Cultures and religions around the world have long employed fasting for spiritual, mental, and physical benefits, from Ramadan in Islam to retreats in Buddhism.
Modern science is now revealing the biological mechanisms behind these long-standing practices, showing that periods of nutrient abstinence can activate natural repair systems that improve long-term health.
While calling fasting “the best medicine on earth” may be an overstatement, the biological evidence for its restorative effects is compelling. During a 72-hour fast, the body shifts metabolism, triggers autophagy, reduces inflammation, and enhances insulin sensitivity, collectively supporting better overall health.
Dr. Longo notes, “Fasting is a tool, not a miracle. But if used responsibly, it offers a way to engage the body’s natural repair systems in a way that modern lifestyles often inhibit.”
With increasing interest in longevity, metabolic health, and mindful living, fasting for 72 hours may no longer seem extreme. Instead, it could represent a scientifically grounded method to reset the body, mind, and metabolism, unlocking benefits that extend far beyond simple calorie restriction.
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