Have the benefits of intermittent fasting been too overhyped?

Thirty-six hours without food sounds brutal, but your body already has a plan for it, and that plan flips on like a switch somewhere around hour twelve.

Quick Take

A 36-hour fast shifts the body from burning carbs to burning stored fat, a change confirmed in controlled human studies.
Ketone levels can jump more than four-fold, giving the brain and heart an alternative fuel source.
Autophagy, the cell-cleanup process often credited to fasting, shows only modest effects in human muscle at 36 hours.
Leptin, the hormone that controls hunger signals, drops noticeably during extended fasts.
Claims about fasting benefits often outrun what the human data actually shows.

The Fuel Switch Is Real and Measurable

Dr. Eric Berg’s popular video walks viewers through a 36-hour fast hour by hour, describing glycogen depletion, rising ketones, and organ-level repair. The core claim holds up. A human exercise study on 36-hour fasting found it “significantly altered substrate utilization at rest and throughout exercise to exhaustion,” meaning the body truly does trade carbs for fat as its main fuel source once glycogen runs low.

That switch does not happen instantly. For the first twelve hours or so, the body still runs mostly on glucose left over from the last meal. As insulin drops, the liver starts releasing stored glycogen. Once that runs dry, usually somewhere in the 18 to 24-hour range, fat breakdown ramps up to cover the gap, setting the stage for ketone production later in the fast.

Ketones Rise, But Not Every Organ Reacts the Same Way

Ketone bodies are the byproduct of fat burning, and they climb sharply during extended fasts. One twice-weekly 36-hour fasting study recorded a 4.4-fold increase in beta-hydroxybutyrate, the main ketone measured in blood. That is a real, sizable metabolic shift, not a marketing exaggeration, and it explains why some fasters report steadier energy or mental clarity once ketones take over as a fuel source.

Where the picture gets murkier is autophagy, the cellular cleanup process often billed as the fast’s biggest reward. A study on skeletal muscle found autophagy was “only modestly affected in humans by 36 h of fasting,” and effects varied depending on whether someone was already trained or untrained. The Cleveland Clinic notes that most autophagy timing evidence comes from animal studies, and researchers still do not know the ideal fasting window to trigger it in people.

Hormones Shift Too, Reducing Hunger Over Time

Fasting does not just change fuel sources, it changes hormone signals tied to appetite. Leptin, the hormone that tells the brain the body has enough stored energy, drops noticeably after 36 hours of fasting, based on measurements taken during controlled fasting periods. That decline helps explain why extended fasts, once past the roughest early hours, often come with a genuine drop in hunger rather than an increase.

A British expert quoted on fasting habits described the mechanism plainly, saying the body is “transitioning from utilizing primary carbohydrates for energy to relying on fat for fuel” across that 36-hour window. That framing matches the lab findings closely and gives a simple, accurate way to think about what is happening internally, without needing a biology degree to follow along.

Why the Bigger Claims Deserve a Second Look

Fasting content online tends to package every metabolic finding into one tidy story: glycogen out, ketones in, cells repair, done. The actual research is messier. Reporting on intermittent fasting broadly has noted that mechanistic lab findings often get presented as settled facts even when human studies vary widely by age, fitness level, and fasting method. That gap between headline and data is worth keeping in mind before treating any single video as the full picture.

None of this undercuts the basic physiology. Substrate use does shift, ketones do rise, and hunger hormones do change during a 36-hour fast. The caution applies to how far those facts get stretched, especially around autophagy and organ-by-organ “repair” claims that outpace what controlled human trials have actually confirmed so far.

For readers weighing whether to try an extended fast, the honest takeaway is straightforward. The body’s fuel-switching response is well documented and predictable. The deeper cellular repair claims are plausible but still being studied, and anyone with existing health conditions should talk to a doctor before pushing past a day without food.