Intermittent Fasting: Why You Can't Last 16 Hours Anymore

Why you can't handle 16-hour fasts like in 2017

Why does someone who worked twelve hours without eating in 2017-2018 now feel dizzy by mid-morning just from skipping breakfast? The question comes from those who once ate only a salad for breakfast or fasted from 22:00 until 14:00 the next day, yet today, despite three months of healthy eating, still experience weakness, brain fog, and irritability when their stomach growls.

The dominant metabolic explanation is that the body has become accustomed to relying on hepatic and muscular glycogen—the quick route—and easy carbohydrates. When these are missing, fatigue and apathy set in. The proposed solution is training the slow pathway: adapting the organism to burn fat and produce ketone bodies. However, critics warn that this shift **does not happen in three days or weeks**, but over several months.

What is ketosis and why it suppresses hunger

Ketosis is the state where the body, having depleted liver glycogen, begins producing ketones from fat. Here lies the most repeated fact: **ketones suppress appetite**. This explains why some report that after twenty hours, hunger vanishes abruptly, as if the stomach "shuts down."

The time needed to enter ketosis depends on three factors: previous food intake, physical activity, and existing reserves. An active person low in carbs may enter sooner; one coming from sandwiches and pasta, later. The unmistakable symptom—besides acetone-scented breath and sweat—is precisely that hunger disappears.

Ghrelin: Why hunger strikes before mealtime

A mechanism explains the initial suffering: ghrelin, the hunger hormone, is secreted **about twenty or thirty minutes before** your usual mealtime. The body is a clock. If you persist, it eventually stops expecting food at those times. It is compared to sleepiness: fifteen or twenty minutes of drowsiness, and if you resist, you wake up.

Therefore, many agree that the easiest step is **eliminating breakfast** first, and only then, once the habit is established, skipping dinner to reach twenty or twenty-two hour windows. Some eat once daily, while others eat before feeling hungry—at noon—to avoid bingeing on junk food.

Skeptics' warning: Hypoglycemia and months of adaptation

Against the enthusiasm, one current argues that jumping from sandwiches and macaroni to fasting is reckless. The argument: insulin is inflexible, what goes up must come down, and the metabolic shift to use fat as primary fuel **takes many months**. Skipping meals without training this pathway, they warn, leads to hypoglycemia and impaired fat metabolism.

They also question the narrative of digestive rest and cellular repair, claiming repair processes are triggered by melatonin (sleep), and that nutrient scarcity puts the body in conservation mode rather than repair. They also warn of rebound effects: upon resuming eating, the body stores energy for the next shortage.

How long autophagy lasts and why 16/8 falls short

Autophagy—the recycling of damaged cellular components—requires more hours than many believe. It is claimed that **the process starts after twenty or twenty-two hours**, meaning the 16/8 method barely reaches the threshold. Hence the defense of One Meal A Day (OMAD) or, at minimum, a 20/4 window.

For deep fasts, veterans recommend **48 to 72 hours, two or three times a year**, reintroducing food with proteins. A warning: during the fast, only water and salt; herbal teas or coffee would break the process according to this thesis. Black coffee, however, does not.

Payés bread and the trap of accidental calorie restriction

The ironic detail: the asker boasts a healthy diet—eggs, fish, avocado, no pasta—but admits to a slice of payés bread soaked in oil. Alarms ring: the insulin spikes caused by flour are incompatible with fasting without prior metabolic adaptation.

Another silent risk is accidentally trinc a **hypocaloric diet**, compensating too little during the eating window. Intermittent fasting is not about eating less, but eating the same amount in fewer hours. Confusing the two has consequences.

In the end, there is no universal answer: some tolerate twelve hours, others get dizzy after four. The difference, they say, lies in metabolic flexibility. Or simply being born with a more tolerant stomach.