Intermittent fasting has become a widely discussed approach to health, energy regulation, and longevity. While eating patterns vary, one of the most common methods involves eating within an eight-hour window and fasting for the remaining 16 hours of the day. During that fasting period, the body undergoes a significant metabolic shift—one that distinguishes fasting from a typical eating pattern with regular meals and snacks.
When we eat, the body stores some of that energy in the liver in the form of glycogen. Glycogen acts as a readily available fuel source, especially when the body needs quick energy. However, after roughly 10 to 12 hours without food, glycogen reserves begin to run low. This is one reason people may feel more irritable, tired, or emotionally reactive when they have not eaten for a while—a state often described as being “hangry.”
Once glycogen stores become depleted, the body begins to rely more heavily on fat for fuel. Fat cells release fatty acids into the bloodstream, which travel to the liver. There, they are converted into usable energy for the body and brain. In this state, the body is quite literally burning fat to sustain itself.
This metabolic shift is one of the central benefits associated with intermittent fasting. Blood samples from people who fasted for 12 to 24 hours have shown a substantial increase in energy derived from fat, with the most notable changes occurring after about 18 hours. This process helps move the body into a state known as ketosis.
Ketosis occurs when the body produces chemicals called ketones as it burns fat. These ketones serve as an alternative energy source, particularly for the brain. In the brain, ketones may also stimulate the release of brain-derived neurotrophic factor, or BDNF, a molecule that supports the growth and strengthening of neurons and neural connections. These connections are especially important in areas of the brain involved in learning and memory.
This may help explain why increased ketone production has been linked to improvements in memory, including in people showing early signs of cognitive decline. Ketone production is also used in medical contexts, such as in dietary approaches for some patients with severe epilepsy.
Fasting is not the only way to increase ketone levels. A ketogenic diet, which emphasizes higher fat intake while sharply reducing carbohydrates, can produce a similar effect. In one example, people who followed this approach for three months experienced weight loss, reduced body fat, lower blood pressure, and changes in a hormone associated with aging and disease.
However, fasting appears to raise ketone levels more dramatically than diet alone. Ketogenic diets may increase ketones several times over, while fasting has been shown to increase ketone production by much larger amounts. This suggests that fasting may have a stronger effect on the body’s metabolic state and may offer broader health benefits.
For many people who eat three meals a day with snacks in between, the body may rarely enter ketosis. Without extended periods between meals, glycogen stores remain available, and the body continues to rely primarily on glucose rather than fat-derived ketones for energy.
Fasting and ketosis are not new biological processes. They have played an important role in human survival throughout history, helping our ancestors endure periods when food was scarce. Today, these same mechanisms are being reconsidered not as signs of deprivation, but as potential tools for supporting long-term physical and mental health.
Intermittent fasting is ultimately about more than simply skipping meals. It reflects the body’s ability to adapt, conserve energy, and shift fuel sources when food is unavailable. As research continues, fasting and ketosis may become increasingly important in conversations about health, aging, brain function, and disease prevention.