Why does blood sugar remain stable during fasting even though you’re not eating?
The Short Answer
Your body maintains blood sugar during fasting through multiple mechanisms: the liver releases stored glucose (glycogenolysis), the liver produces new glucose from non-carbohydrate sources (gluconeogenesis), and hormones like glucagon, cortisol, and adrenaline regulate the process. Your brain needs glucose, and your body has reliable systems to provide it without eating. Blood sugar doesn’t crash during fasting: it’s carefully maintained. This is why people fasting for 16, 24, or even 72 hours generally remain functional.
Something to Sit With
Have you feared blood sugar crashes during fasting?
Knowing the body has multiple systems to maintain blood sugar, does that change your perspective?
What might you try if you trusted your body’s ability to regulate blood sugar without constant eating?
Learn More
The Fear vs. The Reality.
The fear: “If I don’t eat, my blood sugar will crash. I’ll get shaky, weak, unable to think. I need to eat regularly to maintain blood sugar.”
The reality: Healthy individuals maintain stable blood sugar for extended periods without eating. The body has redundant systems for it. And the crash people fear is better documented after a meal than during a fast: after a high-glycemic meal, blood sugar rises quickly and falls to its lowest point about four hours later, with hunger rising alongside it.[1]
Understanding this mechanism can remove unnecessary fear about fasting.
Mechanism 1: Glycogenolysis.
Your liver stores glucose in a form called glycogen, essentially a quick-access glucose reserve.
When you stop eating:
- Blood glucose begins to drop slightly
- The pancreas releases glucagon (insulin’s counterpart)
- Glucagon signals the liver to break down glycogen
- Glucose is released into the bloodstream
- Blood sugar remains stable
The liver holds about 80 grams of glycogen on average, though the normal range runs from zero to 160.[2] That lasts roughly 24 hours, depending on activity level.[3] This is your first line of defense.
Mechanism 2: Gluconeogenesis.
“Gluconeogenesis” means “making new glucose.” Once glycogen runs low, the liver (and to a lesser extent, the kidneys) can produce glucose from non-carbohydrate sources:
- Amino acids: Some protein can be converted to glucose
- Glycerol: The backbone of fat molecules can become glucose
- Lactate: Recycled from muscle activity
This process ensures glucose availability even when glycogen is depleted. The brain consumes roughly 120 to 130 grams of glucose a day under normal conditions[4], and gluconeogenesis can provide much of that during extended fasting.
Mechanism 3: Ketone Adaptation.
As fasting extends beyond 24-48 hours, a third mechanism kicks in:
- Fat is increasingly broken down into fatty acids
- The liver converts fatty acids into ketone bodies
- Ketones cross the blood-brain barrier
- The brain begins drawing on ketones alongside glucose[3]
- Further in, ketones become its predominant fuel, though the founding measurements were made in three obese patients at five to six weeks of therapeutic starvation, so the far end of that shift lies a long way past two days[4]
At that end of the range this dramatically reduces the glucose the brain requires, further easing the demand on gluconeogenesis. The body becomes more efficient at maintaining blood sugar because it needs less of it.
Hormonal Regulation.
Several hormones work together to maintain blood sugar:
Glucagon: Released when blood sugar drops; signals glucose release from liver.
Cortisol: The stress hormone also raises blood sugar by promoting gluconeogenesis. Morning cortisol helps maintain blood sugar after overnight fasting.
Adrenaline: Stimulates glycogen breakdown and gluconeogenesis. Noradrenaline rises during a fast, more than doubling by the third or fourth day.[5] The same study, though, found that catecholamine levels do not track metabolic rate, and that the metabolic rise itself peaks around 36 hours, is no longer significantly above baseline by 72, and did not change significantly in men across the whole protocol.[5] The hormone is not a tidy explanation for how a fast feels, and neither is the rate.
Growth hormone: Rises during fasting, and in the six men measured over a five-day fast, the rise had already happened by the first fasted day rather than building over the long one[6]. On Fung’s account, growth hormone maintains muscle mass and lean tissue in a fast’s protein-conservation phase, after five days.[3] That is his account of the mechanism rather than a measured result.
This hormonal orchestra keeps blood sugar in a normal range without external food input.
What “Low Blood Sugar” Actually Means.
True hypoglycemia, dangerously low blood sugar, is rare in healthy individuals who are fasting. When people say “my blood sugar is low,” they often mean:
- “I feel hungry” (not the same as low blood sugar)
- “I’m used to eating now and I’m not” (habit, not physiology)
- “I feel slightly uncomfortable” (which usually passes)
Actual symptoms of genuine hypoglycemia include confusion, visual disturbances, difficulty speaking, loss of coordination, and in severe cases, loss of consciousness. These are rare in healthy people during reasonable fasts.
If you do experience severe symptoms, it warrants medical attention, but normal hunger during fasting is not dangerous hypoglycemia.
The Irony of Frequent Eating.
Paradoxically, frequent eating can make blood sugar less stable:
- Eating raises blood sugar
- Insulin is released to lower it
- Sometimes insulin overshoots
- Blood sugar drops below baseline
- You feel hungry and shaky
- You eat again to “fix” it
- The cycle repeats
This roller-coaster is documented after high-glycemic meals, where the four-hour trough comes with greater hunger and heightened activity in a reward region of the brain.[1] Many people find that once they adapt to fasting, they experience steadier energy than when eating frequently.
Further reading:
- Cahill GF Jr, “Fuel metabolism in starvation”[7] — Annual Review of Nutrition, 2006
- Fung J & Moore J, The Complete Guide to Fasting[8] (2016) — Chapters on the physiology of fasting
- Jensen TL et al., “Fasting of mice: a review”[9] — Laboratory Animals, 2013
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