Fasting is one of the longevity movement’s favourite rituals but the main reason people give for it does not hold up well in humans.
The promise is autophagy. Skip food for long enough and the body, short of incoming fuel, starts breaking down and recycling its own worn-out parts. That much is real. The catch is the timing. In humans, a 24-hour fast produces little to no autophagy. It may take three full days of not eating to begin switching it on. Thirty minutes of aerobic exercise, however, starts the same process.
This is the case Tommy Wood, a physician, neuroscientist and author of The Stimulated Mind, who studies metabolism and brain health, makes against fasting’s reputation. Not that it is useless. That it has been oversold on the back of work done in mice.
“There are now these really prestigious papers from really prestigious professors that say fasting upregulates autophagy, mitochondrial biogenesis, all these amazing things,” he tells Unfiltered. “And they happen in mice, but they don’t happen in humans.”
Fasting’s main benefit: just eating less
Wood does not think fasting does nothing. He thinks it does one significant thing, and that the one thing has a duller name.
“The main benefit of fasting is the fact that you decrease caloric intake,” he says. Most people in the developed world eat in chronic excess, and pulling that back has real metabolic payoffs. But the fast is only one way to get there. Counting calories does it. Eating less calorie-dense food does it. “All of that seems to work in the same way,” Wood says. “There really is nothing magic about fasting.”
Does a 24-hour fast trigger autophagy in humans?
Barely. This is the part that gives fasting its mystique, the idea that it is more than a diet, and it is where the human evidence thins out.
“If you look at activation of autophagy after, say, a 24-hour fast, there isn’t really any in humans at all,” Wood says. The threshold he gives is steep. “It probably is going to take three or more days of fasting to start to see an upregulation of these sort of cleaning processes.”
Three days. Not the sixteen-hour eating window. Not the occasional full day off food. The intermittent fasting most people actually practise sits well below the point where, on Wood’s reading, autophagy even starts to stir in human tissue.
Does exercise trigger autophagy faster than fasting?
Yes, and that is the useful part. The cleanup fasting struggles to produce in people, exercise produces quickly.
“You can upregulate those processes almost immediately with aerobic exercise,” Wood says. “Just 30 minutes of aerobic exercise starts to activate autophagy. In humans, they’ve shown it in muscle tissue.” In rodents, exercise that triggers autophagy in muscle also triggers it in other organs, which hints at a whole-body effect from a single session rather than a multi-day fast.
The nutrition scientist Bill Lagakos has a line for this that Wood likes: exercise is “fasting fast-forwarded.” If the goal is the cellular state fasting is supposed to deliver, half an hour of intensity gets there sooner.
Why fasting and autophagy studies don’t translate from mice to humans
The popular claims overstate the case because they were built on the wrong animal. Wood points to a recent meta-analysis, by a researcher he counts as a friend, that sets fasting responses in humans against those in rodents. The mismatch is the entire point. The dramatic effects on autophagy and mitochondrial biogenesis are robust in mice and largely missing in people.
“This is now, I think, showing pretty clearly that fasts, even up to three days, don’t activate mitochondrial biogenesis, don’t particularly activate autophagy,” Wood says, “and actually you will instead activate those processes through exercise.”
A mouse is not a small person. It runs a faster metabolism in a far smaller body, and what lights up its cells on a short fast does not scale to a human one. Nobody faked the studies. They simply got read across a gap between species that the biology does not cross.
Fasting and muscle loss: why you still need to lift
There is a caveat Wood keeps, and it cuts towards exercise rather than away from it. He is not anti-fasting. “If fasting is something that works well for you, it allows you to eat in a way that you enjoy, it helps support physical health or mental health, great, you should do it.”
But long fasts threaten the tissue that matters most with age, so they come with a condition. “If you’re going to fast for extended periods of time, you should do resistance training during that fast, because that will help you preserve muscle mass.” Even a hard caloric deficit does not have to cost muscle, as long as something keeps loading it. Which means a serious fast needs training attached to it. And training was already the thing doing the cellular work.
So the choice narrows to two options. Either fasting does something for a human body that exercise cannot, or it is the longer, hungrier route to a place exercise reaches in half an hour, with a muscle bill that comes due in the gym regardless. The human data Wood lays out has not found the first one yet.
Photography Pablo Merchan Montes
