For those who can’t function without breakfast, the idea of fasting on purpose might be repellent. But going without food for more than a few hours between meals may be the key to safeguarding brain function over the long term.
Scientists are just starting to make sense of the complex mechanisms which underscore how the brain relates to what we put into — or leave out — of our bodies. But what we do know about the brain and undergoing prolonged periods without food is tantalizing for future research to dig into.
Here’s the background — There may be an evolutionary reason why fasting may benefit the brain. Mark Mattson is a professor of neuroscience at John’s Hopkins School of Medicine and has researched fasting and the brain extensively. He tells Inverse that humans originally evolved to go without food for extended periods of time.
“Individual brains had to function very well in a food-deprived state. Otherwise, they're not going to be successful in acquiring food,” Mattson says.
“They have to have that to be alert, their cognition has to be really good because they have to figure out how to locate the food.”
Fasting triggers a shift in the resources your body uses for energy. Your metabolism moves from using glucose to ketones to power the body. Ketones are a type of acid produced by the liver from fat — interestingly, ketogenic diets rely on this shift, too.
But it’s not just fat-burning — the increased use of ketones during fasting periods and the switch back to use of glucose following eating is known as “metabolic switching.” In turn, this triggers a biological cascade in the body which scientists believe may build the brain’s resilience and productivity, and boost its support system.
“It is thought cells go into a survival and repair mode during the fasts, followed by growth and regeneration during the refeeding phase,” Matthew Phillips explains to Inverse. Phillips is a neurologist at Waikato Hospital in New Zealand and studies fasting and ketogenic diets and the brain. In his work, he sometimes uses these “metabolic strategies” as part of treatments for his patients.
Ultimately, evidence suggests four brain health effects linked to fasting:
- Brain cell generation
- Cognitive and psychological benefits
- Resilience to neurological conditions
- Slowing the effects of aging
These are four of the potential brain benefits of fasting, ranked in order of the strength of the evidence backing them:
4. Fasting may help create more baby brain cells — Whether for set windows throughout the day, every other day, or longer, there’s reason to believe that fasting for extended periods may help generate new brain cells.
In one study published in 2019, groups of mice that were deprived of food every other day for windows of between 12 and 16 hours had higher levels of specific protein markers compared to mice that were not deprived. These markers indicate new brain cells were being made, according to the paper, suggesting that the fasting mice may have been making new brain cells more efficiently and at a faster rate than the control mice.
Ghrelin, the hormone which prompts us to eat, dials up during periods without food (to no one’s surprise). There is evidence to suggest this increase also spurs the creation of new brain cells. In a 2015 study, mice that ate every other 24 hours had higher levels of ghrelin than mice that ate when they wanted to. In turn, the researchers found the fasting mice had more markers of new brain cells in an area of their brain called the hippocampus than did their non-fasting counterparts.
The researchers went further in this study. They found that mice without any ghrelin in their bodies that also fasted every other day did not see an increase in new brain cells. Together, the findings suggest fasting-induced boosts in ghrelin play a crucial role in brain health. Mattson suspects the same processes might be at play in human brains, too, but there isn’t enough evidence so far to know this for sure.
3. Fasting may improve cognition and mood — One has only to scan the subreddits and other forums of the web to find plentiful, anecdotal evidence of fasting’s head-clearing, intelligence-boosting effects.
“I truly function better when I'm in a fasted state. I feel more energetic, better able to handle complex work issues, and generally happier,” writes one Reddit user, u/bstephens00.
“I also felt a little quicker mentally, it was easier to make decisions and I had more willpower. It's like I had less brain fog,” writes another, u/supervexed.
But it’s not just speculation — some of the science backing fasting’s protection against cognitive decline may also correspond with enhanced mental states.
“It's pretty clear the brain functions quite well in a fasted state once a person is adapted to intermittent fasting,” Mattson says.
“Fasted individuals often report improved cognition, energy, sleep, mood, and self-confidence,” Phillips agrees, referring specifically to healthy individuals who fast.
On the animal side, in a 2005 study, lions in captivity went from a consistent, daily diet to a more random “gorge and fast” regimen. They became more active, engaged in more instinctive behavior like sniffing and stalking, and paced less. In a separate 2013 study, mice that fasted every other day performed better than those that did not on two tests designed to measure their memory, learning, and brain function.
In humans, some fasting studies looked specifically at people who fast as a part of their religious practice. A review of studies conducted in people who fasted for Ramadan found mixed results on fasting and cognition, although many of the studies included in the review did not take sleep schedules — a critical component of functioning — into account. However, one study involving 13 people that did control for sleep found that, after a week of fasting, participants’ sleep, concentration, and emotional balance improved. It may be no coincidence then, that fasting in spiritual practice is related to an altered state of mind — a feeling of being purified and recharged.
2. Fasting could help treat brain conditions — Aside from brain diseases associated with old age, fasting could help manage other brain and nervous-system conditions.
“One thing we found pretty recently, that may explain the ability of intermittent fasting to reduce levels of anxiety and also protect against a number of neurological disorders, is that intermittent fasting will enhance the ability of nerve cell networks to control their activities and electrochemical activity,” Mattson says.
Fasting was prescribed as a treatment for people with epilepsy more than 2,000 years ago. The ancient technique is still used today to treat people with the condition, as are ketogenic diets, which echo some of the effects of fasting on the body.
A 2013 study involving three children with epilepsy followed the children as they undertook both intermittent fasting and keto. Following the regimen, the children showed small improvements in seizure control, and further mouse studies suggest that the ingestion of ketones, which increase in fasting, may delay or reduce seizures.
A rat study from 2008 also found that just one day of fasting after a serious brain injury helped save brain tissue. In yet another mouse study from 2014, mice on an intermittent fasting diet also showed lower than expected cell death following a stroke. It’s thought that an increase in specific ketones like BHB, considered more “efficient,” in the metabolism, may also play a role in this seeming protective effect on the brain.
1. Fasting could slow a decline in brain function — Intermittent fasting may also be a way to protect against conditions marked by a decline in brain function - like Alzheimer’s, Parkinsons’, and Huntingtons’ diseases — although the evidence here, too, stems mainly from mouse studies.
In one study, mice altered to have Alzheimers’-related genes were put on an intermittent dieting regimen. In old age, the mice showed less cognitive decline and damage, and performed better than their free-eating peers in a task designed to test cognitive ability .
Taken together, the evidence suggests the switch between eating and fasting periods, which acts like a hard reset on the body, may be the key, Mattson says.
“The nerve cells in the brain go into a stress-resistance, conserve-resources mode. And then when the animal eats, the cells go into a growth and plasticity mode,” he says.
In humans, the effects of fasting on neurodegeneration are not well-studied, but Phillips says it’s a promising avenue for future research.
“Evidence in animals is probably strongest for neurodegenerative disorders, which shows fasting improves cognition, stalls age-related cognitive decline, and usually slows neurodegeneration,” he says.
There are tentative signs people with Parkinsons may see some benefits from following a keto diet, including decreased pain, sleepiness, and cognitive impairment. The ketone BHB may play a leading role in this protective effect. BHB generates a signal that regulates neuron function and resistance to injury — processes essential to maintaining brain health into old age.
The Inverse analysis — Most of what we know about fasting and the brain stems from research done in animal models. How far this research applies to humans is not known, but data collected from mice and other animals in the lab has laid the groundwork for breakthroughs in human biology before. If these preliminary experiments hold up in future research done in people, then fasting may significantly affect cognition, alertness, and learning.