The Biggest Lie About Fasting & Low Blood Sugar
In a Nutshell
Fasting does not cause hypoglycemia because the liver produces glucose via gluconeogenesis from fat once glycogen stores (400 calories) are depleted within 24 hours, while blood sugar remains stable between 70-80. The body’s primary fuel system is fat burning and ketosis, not glucose, and muscle glycogen is preserved for immediate use only. Chronic high-carb eating prevents glycogen depletion and creates insulin resistance, which is why people feel shaky without snacks; fasting 12-24 hours activates fat burning, autophagy, and ketone production without blood sugar crashes.
These notes were generated by AI and may contain inaccuracies.
What really happens to your blood sugar when you fast for 24 hours? The common belief is that you will experience hypoglycemia, which is why people push snacks to avoid shakiness, dizziness, and fogginess. This is one of the biggest lies around. Fat burning is presented as the backup fuel when you run out of sugar, but this is exactly the opposite of how the body works.
Fat burning or ketosis is actually the primary operating system in the body for fuel. Throughout human history, people did not have consistent meals or snacks. They hunted and foraged, sometimes eating in the afternoon or not for days. For thousands of generations, the human body was designed to function, be sharp, and hunt without a constant glucose supply problem. The body used fat as the primary fuel system.
The body has stored sugar in the liver called glycogen, with only 400 calories of stored glucose in the liver. Even a lean person has 120,000 stored calories in fat. Glycogen is only for quick energy needed for very small amounts of time. Within 8 hours, 50% of the liver's glycogen is gone. By 12 hours it decreases more, and at 24 hours, 90% of the glycogen is depleted.
The average person never really depletes their liver of glycogen because they eat three plus meals per day heavy in carbohydrates plus carbohydrate snacks, so the glycogen never empties.
In 1973, researchers took 19 healthy volunteers and fasted them, then performed needle biopsies of the liver. At 8 hours of fasting, more than half the glycogen in the liver was completely gone. By 12 hours, levels were minimal. At 24 hours, 90% of glycogen was depleted.
If glycogen is virtually depleted after 24 hours, why does blood sugar not crash? Blood glucose comes down just slightly but barely moves at all. After 24 hours, blood sugar will probably be between 70 or 80. The liver does not just store glucose - it makes it through a process called gluconeogenesis (glucose neo new genesis). The liver makes sugar out of fat.
Most of the body can very easily run on fat fuel, and only a small portion of the body really needs glucose primarily.
Muscles also store glycogen, but that glycogen in muscles can only be used by the muscles. No other part of the body can use it. This is by design because when running away from danger, the initial fuel used is not fat. When fasting, the body does not eat muscle because muscles have fuel sitting in them called glycogen. There is also a spike of growth hormone that prevents the breakdown of muscle. The body preserves muscle until all fat reserves are tapped out.
There are at least 120,000 calories of fat on the body even if not overweight.
The body switches to fat burning usually between 12 and 24 hours. When eating stops, the body flips a switch. When switching over to fat burning and ketosis, fat burning starts, things help make ketones, and then something turns on the full fat burning effect.
When not eating, the body cleans up damaged tissue or damaged proteins through a process called autophagy, becoming efficient and recycling that into fuel. Autophagy only occurs if fasting. Ketones have a natural anti-inflammatory effect.
When someone chronically consumes glucose, they develop insulin resistance where the cell no longer can take in as much glucose as a protective mechanism, and it starts to starve.
Fasting blood glucose is higher in the morning despite eating no carbohydrates through the night because gluconeogenesis is out of control. The machine that controls it is broken to some degree - called insulin resistance, seen in prediabetes and diabetes. The solution is going low-carb and doing intermittent fasting for longer periods until waking blood glucose normalizes. One simple solution is going for a walk to burn off extra glucose.
Hypoglycemia is not a blood glucose problem. People who get dizzy, faint, or irritable when not eating have been told they have a blood glucose problem, and doctors recommend having candy or glucose pills. But when not eating or fasting, three main things happen: low insulin (which lowers blood glucose), glucagon (which mobilizes blood glucose to raise it), and adrenaline and cortisol (which also mobilize blood glucose to raise it).
In 2001, researchers fasted 33 healthy people for 72 hours. There were zero cases of hypoglycemia. People who cannot go 3 hours without getting shaky or irritable are on a high-carb diet, and the carbs caused the problem. The Whitehall study showed oversecretion of insulin occurs 3 to 4 years before type 2 diabetes. Eating every 3 hours is the worst advice because the body was designed to fast for long periods of time.
After 24 hours of fasting, blood sugar does not crash, and the fat-burning machine is the primary operating system.
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