What Happens in the Body During Fasting?
When we stop eating, our bodies gradually change how they obtain energy. Insulin levels decline, stored glucose becomes available, and the body increasingly relies on fat.
These changes help explain why fasting has attracted interest among researchers studying metabolic health, diabetes, and aging.
Insulin and Blood Glucose
After eating, insulin helps move glucose into cells and encourages energy storage.
During fasting, insulin levels generally decline, allowing stored fat to become more available for energy.
Meanwhile, the liver releases glucose stored as glycogen to maintain blood sugar levels.
As glycogen supplies decline, the liver produces additional glucose through a process called gluconeogenesis.
These changes are normal adaptations that help the body function between meals.
Fat Burning and Ketones
Our bodies continuously use a mixture of glucose and fat for energy.
During fasting, the balance gradually shifts toward greater fat burning.
The liver also produces ketones from fat. Ketones provide an alternative energy source, including for the brain.
This ability to switch between fuels is called metabolic flexibility.
People with insulin resistance may have difficulty regulating energy use efficiently.
However, producing more ketones does not necessarily mean better long-term health.
Autophagy: Cellular Recycling
Autophagy is a natural process through which cells break down and recycle damaged proteins and worn-out cellular components.
It plays an important role in cellular maintenance and healthy functioning.
Animal studies suggest that fasting can stimulate processes associated with autophagy.
But measuring autophagy in humans is difficult.
We do not know whether fasting for 16, 24, or 48 hours produces clinically meaningful increases in human autophagy.
Claims that autophagy begins at a specific fasting hour are not supported by reliable human evidence.
Is Longer Fasting Better?
Longer fasts generally produce greater changes in insulin levels, fat metabolism, and ketone production.
However, greater metabolic changes do not necessarily produce better health outcomes.
Extended fasting can increase the risk of dehydration, dizziness, inadequate nutrition, and loss of muscle.
These concerns become particularly important as we age.
There is no established fasting duration that provides the greatest health benefits for everyone.
The Bottom Line
Fasting produces genuine changes in metabolism, including lower insulin levels, increased fat use, and greater ketone production.
Autophagy is an intriguing possibility, but its relationship to fasting and human longevity remains uncertain.
For older adults, maintaining muscle strength and adequate nutrition may be more important than extending fasting periods.
Learn More
Explains how fasting shifts energy use from glucose toward fat and ketones.
Mark Mattson and Rafael de Cabo review fasting, metabolism, and cellular adaptation.
Health Curious provides educational information, not individualized medical advice. People taking medications that can cause low blood glucose, or those who are underweight or frail, should consult their healthcare professional before attempting prolonged fasting.