The last entry covered how to protect muscle while eating less or fasting. This one is about the reason many people are willing to go a few more hours without food: autophagy.
Autophagy is the cell's built-in recycling system. It wraps damaged proteins and worn-out mitochondria in a membrane, sends them off to be broken down, and reuses the parts. It is directly tied to aging: a 2025 review in Cell lists "disabled autophagy" as one of 14 hallmarks of aging. To count as a hallmark, a change has to clear three bars: it gets worse with age; making it worse in experiments speeds up aging; and intervening on it slows or even reverses aging. So keeping autophagy at a good level is a sensible goal. The question for this entry is whether going a few more hours without food achieves it.
Plenty of fasting apps and popular-science posts tell a similar story: after 12 hours without food you start burning fat; at hour 16, "autophagy" kicks in and your cells start "taking out the trash." That is often how the "16" in 16:8 gets explained. Some people stretch their fasts to 18 or 20 hours as a result, or go days without food for "deep autophagy."
Does a 16-hour fast really switch on autophagy? Should you lengthen your fasts to get it?
Where Does "16 Hours" Come From?
We found two relevant sources.
The first is a 2010 mouse study. Researchers fasted mice for 24 or 48 hours, sliced their livers and brains, and counted autophagosomes under the microscope (small vesicles that wrap material for breakdown; more of them means more active autophagy). After 24 hours of fasting, autophagosomes in liver cells and neurons rose markedly, and more so at 48 hours 1. The timing was 24–48 hours, and the subjects were mice.
The second is a 2018 review. It proposed a "metabolic switch": about 12 hours after you stop eating, the glycogen stored in the liver gradually runs out and the body shifts to mobilizing fat and making ketones 2. The review is about the fuel switch from glucose to fat. Where it mentions autophagy, it cites animal studies; it presents no human measurements of autophagy and never assigns autophagy a time point.
Neither source measured autophagy in people.
Nor can the mouse figure of 24–48 hours be transferred hour for hour to humans. The hormonal and blood-glucose changes after two days without food in a mouse resemble those after about five days in a person 3 (as we noted in the first entry, a mouse's day is not a human's day). For a mouse, going one or two days without food is already close to a human multi-day fast.
"Autophagy switches on at 16 hours" has no clear source in human research. It looks more like a memorable number left behind when mouse data, the "12-hour fuel switch," and the popular 16:8 schedule were blended together (that is our inference; the exact origin of the popular claim is hard to trace).
Autophagy Runs All the Time; There Is No Moment It "Switches On"
Autophagy is the cell's recycling system: it wraps damaged proteins and worn-out mitochondria in a membrane, sends them to be broken down, and reuses the parts. In our overview of hormesis, it was one of the pathways driven by energy stress: when a cell senses it is short of energy, autophagy is turned up.
"Turned up" is the accurate way to say it. Autophagy does not wait until you are hungry to appear. In the mouse study above, normally fed mice already had autophagosomes in their neurons; fasting made them more numerous and larger 1. People are the same: in blood drawn from healthy adults eating normally, autophagy in blood cells runs steadily at a certain rate 4.
Autophagy has a level, not an on-off switch. Beyond the fact that the number 16 has no source, the claim "autophagy switches on at hour 16" assumes a switching-on moment that does not exist. The right question is: after a certain length of fast, how much higher than usual is autophagy?
There Is No Way Yet to Measure "Autophagy in Progress" in People
To answer "how much higher," you first have to be able to measure it. In living people, autophagy is hard to measure.
The usual method in research is to measure the amount of a few marker proteins (such as LC3-II). But an amount is only a snapshot of the "stock" at one moment. A high reading could mean autophagy has sped up—or that the downstream breakdown step has slowed and material is piling up. To learn the real rate, you have to add a drug to living cells that blocks the breakdown step and watch how fast the marker protein accumulates. That rate is called "autophagic flux."
The two methods can give opposite answers. One study took muscle from people after exercise: measured as stock, LC3-II fell by 24%, which looks like less autophagy; measured as flux, it did not fall—judging by the effect size, it actually rose 5.
Measuring flux requires living cells, so human data come mainly from blood cells drawn from a vein, plus a few muscle biopsies. We found no in-vivo data on how autophagy changes during fasting in the human liver or brain.
For ordinary people, there is no "autophagy" line on a hospital lab report, and wearables and apps can't measure it either.
Here is what researchers have actually measured in people, as far as we could find:
| Protocol | What was measured | Result |
|---|---|---|
| 36-hour fast | Muscle biopsy, marker-protein stock | Fell only in people who did not normally train; the authors concluded the effect was slight 6 |
| Starvation of up to 4 days | Blood cells, autophagic flux | Rose only in neutrophils (one type of white blood cell), not in other white blood cells 7 |
| Eating only between 8 a.m. and 2 p.m. (an 18-hour fast) for 4 days | Blood cells, gene expression | Expression of one autophagy-related gene rose; flux was not measured 8 |
| Three days a week, eating only between 8 a.m. and noon and only 30% of usual calories (followed by a 20-hour fast), for 6 months 9 | Blood cells, autophagic flux | Higher than the control group at 6 months, but not higher than this group's own starting point 10 |
| Fasting-mimicking diet (several days of a low-calorie, low-protein formula), 6 days | Blood cells, autophagic flux | Differed from the control group on day 6, but not at every time point 11 |
| Protein halved (from 20% to 10% of calories, total calories unchanged) for 4 weeks | Blood cells, autophagic flux | No change 4 |
Every fasting protocol in the table ran longer than 16 hours. The one with an 18-hour daily fast looked only at gene expression and did not measure flux. The 36-hour and 4-day studies found changes that were small or confined to one cell type.
Even fasting 20 hours three days a week for half a year bought only a weak signal in blood cells. "Sixteen hours" has no matching result in human studies.
Fasting Longer Doesn't Buy a Proven Benefit Either
"A little more hunger can't hurt" is a natural thought. It has two problems.
First, there is no evidence yet of what benefit people get when autophagy is turned up. The evidence that "boosting autophagy extends life" comes from genetically modified mice: mice engineered to overexpress an autophagy gene (Atg5) throughout the body lived 17.2% longer at the median 12; another genetically modified mouse with more active baseline autophagy lived longer in both sexes 13. Those are the results of changing genes. Whether fasting can do the same thing in people is a separate question. We found no published human data on whether people with higher autophagy get sick less or live longer. In other words: a mechanism, but no proof.
Second, longer fasts have their own costs. In a medically supervised water-only fasting study, 20 middle-aged people fasted for about 10 days on average; inflammatory markers such as C-reactive protein rose in their blood, and pathways related to platelet activation and clotting were upregulated 14. Going days without food changes the state of the whole body, not just how much "trash" gets cleared.
Going hungry longer for a potential mechanism that can't be measured and hasn't been shown to benefit people isn't worth it.
What Prime Lab Recommends: Don't Lengthen Your Fasts for Autophagy
1️⃣ Don't lengthen your fasts for autophagy. None of 16, 18, or 20 hours corresponds to "more autophagy" in people. If you already do 16:8, there is no need to grit your teeth through the last few hours to "reach autophagy"; if you don't, there is no need to start for autophagy's sake.
2️⃣ Judge your fasting length by three things only: whether it helps you eat less, sleep well, and not feel hungry. If you need to eat less and narrowing your mealtimes naturally brings your daily total down, use it. If your weight and markers are normal, eating dinner a bit earlier and not eating in the 3–4 hours before bed is enough; that gives you a natural overnight fast of 12–14 hours. If you want to try a different fasting length, test it for two to four weeks and watch your weight and waist, whether you fall asleep easily or wake up hungry at night, and whether you feel shaky or overeat to compensate during the day. If things improve, keep it; if they get worse, shorten it.
3️⃣ If autophagy matters to you, get enough exercise. In Rethinking Exercise Through the Lens of Hormesis, we recommended exercise as a way to promote autophagy because it is more convenient and easier to build into daily life. That advice stands: 150–450 minutes of moderate-intensity exercise a week, plus two to three strength sessions. That article cited mouse research (30 minutes after exercise, autophagy rose in skeletal muscle, heart muscle, liver, and pancreas); in people, the muscle-biopsy study above suggests, by effect size, that autophagic flux rises after exercise and can stay elevated for up to 24 hours 5. The health benefits of exercise itself rest on a large body of evidence and do not depend on autophagy as the explanation.
4️⃣ Don't cut protein for autophagy. While eating less or fasting, eat protein in the amount from the last entry (1.6–2.0 g per kg of body weight per day) and keep up your resistance training.
5️⃣ These people especially should not extend fasts for autophagy: anyone taking insulin or sulfonylurea glucose-lowering drugs (talk to your doctor before any kind of fasting); pregnant or breastfeeding women; anyone with a history of eating disorders; anyone with a BMI under 18.5; and people over 65 who already have low muscle mass. Don't fast on your own—only under medical supervision.
The next entry looks at 16:8 itself: is it the timing that works, or simply eating less?
References15
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