The Brain Without Food: Why We Fast

(Editor’s note: This article from the Spring 2016 issue of Brain World magazineIf you enjoy this article, consider a print or digital subscription!)

Despite representing only 2 percent of the body mass, the brain consumes anywhere from a third to half of the energy we get from our food. It’s the last part of you to suffer when you don’t eat enough. The body is very clever about keeping the brain alive and healthy at the expense of every other organ when resources are short. That might give fasting — abstaining from food or drink for health or spiritual reasons — a special place among the behaviors that can affect brain health.

As a cultural or religious practice, fasting has a long enough history that some anthropologists are tempted to think it has a biological or evolutionary basis. Maybe periods of food scarcity in our hunter-gatherer past gave us a healthy respect for starvation, and we institutionalized this aspect of the human condition much like we did the seasons in the agricultural era, considering such whims of nature to be the work of gods.

The Bahá’í faith observes fasting between sunrise and sunset during the month of ‘Alá’ (March). Buddhist monks and nuns commonly don’t eat after the noon meal. The Old Testament’s Book of Isaiah describes fasting as being beneficial, and the many Christian denominations prescribe a myriad of abstinence practices, like giving something up for the 40 days of Lent. Hinduism is full of forms and systems of fasting, it’s a form of repentance for sin in Judaism, and fasting is one of the Five Pillars of Islam, notably celebrated during the holy month of Ramadan.

Fasting in religious practice, redemption, or enlightenment, might have unwittingly given us valuable research about the workings of the brain if we just took notice. In medieval Europe, novice nuns were put on enforced fasts to allow the Holy Spirit to enter them, and when they saw angels, demons, or Christ, they were probably just hallucinating as they gradually starved.


Under normal conditions, humans can live for roughly five minutes without oxygen, five days without water, and five weeks without food (magician David Blaine held his breath underwater for 17 minutes, and, in 1920, Irish hunger striker Terence MacSwiney lived without food for 74 days, protesting British occupation).

Depending on what you’ve been eating and how long it takes to digest, your body turns to the muscles and liver to convert stored glycogen into energy about six hours after eating. After around 24 hours without food it starts converting fat and other tissues. Fat itself can’t pass the blood-brain barrier, so the liver metabolizes it to produce ketones — units of energy — leading to a state called “ketosis.” For the next three days or so, your body will be doing what’s called “autophagy” (literally “self eating”), metabolizing itself to produce energy, and among the tissues the body breaks down to do so are certain neurons that the brain considers nonessential for short-term survival.

Researchers at France’s National Centre for Scientific Research selectively starved flies to the point where their brains had to either switch off energy-intensive memory formation or let the insect die (they did the latter). As the journal Cell Metabolism reported, such self-cannibalizing of the brain also tends to turn hunger signals up, making the search for food an even more urgent priority. Scarier still, your brain starts to literally shrink. In 2010, the International Journal of Eating Disorders reported reduced brain volume in anorexia nervosa sufferers (it’s reversible if you start eating again in time).


A quick search online would make you think fasting can cure everything from dandruff to cancer. It has a lot of supporters (though scientists are more skeptical), and those who love it tend to use a more spiritual than biological approach. One website lists benefits that include anti-aging, better attitude, clearer skin, drug detoxification, more energy, new ideas, rest for digestive organs, spiritual awareness, and weight loss.

But before you dismiss such proponents as cranks, consider that the ancient Greeks cured cases of epilepsy with starvation. Today we know ketone bodies can stabilize neurons and protect against runaway brain activity that can manifest as a seizure. Since so many of the upsides listed above — even unscientific ones like “better attitude” — are ultimately based in the brain, maybe we shouldn’t dismiss the effect that ketosis can have. There’s a ketogenic diet in the nutrition world that’s indeed used to treat epilepsy to this day.

Of course, some of the reasons put forward for fasting are scientifically dubious at best, and in some cases the jury’s simply still out. One of the most popular diet trends is detoxing — regimens of cutting all but a select few foodstuffs from our diet for extended periods. Steve Jobs ate only fruit for long periods, but when actor Ashton Kutcher (who played him in the 2011 film “Jobs”) tried the same thing, he ended up hospitalized in agony with pancreatic problems.

Still, many detox devotees will tell you they go through a period of intense exhaustion, depression, lack of mental focus or sickness during the initial “purge,” followed by greater health, vitality, and mental clarity than ever. Could we have gone through periods of detox enforced by resource scarcity during our evolution, with the positive response to such activity today being simply the expression of a healthy behavior we’ve forgotten?

So far, science tends not to agree. As one nutrition science department chair told the website
LiveScience this year, our liver and kidneys do a remarkable job of cleansing our bodies of
toxins — we don’t have to do it for them. “Increasing fruit and vegetable intake, whole grain intake and drinking more water over sweetened beverages would go a lot farther to improve someone’s health over the long-term than a cleanse.” An emeritus professor of complementary medicine at Exeter University called detoxing “a word being hijacked by entrepreneurs, quacks, and charlatans to sell a bogus treatment that allegedly detoxifies your body of toxins you’re supposed to have accumulated.”

But it might not even be that simple. Our oversweetened, oversalted, preservative-filled Western diets today are vastly different from the raw and dirty (in a good way) plant and protein matter we hunted and pulled out of the ground when we developed our oversized brains.

Our diets have changed way too fast for evolution to catch up — maybe we do get gummed up with toxins our natural cleansing systems can’t easily dislodge. Another common claim is that fasting can free up energy usually used in digestion to rejuvenate other bodily systems. That seems to imply that diverting energy units between bodily systems after they’ve been digested and synthesized is as easy as sending an email.

It’s actually more the case (though no less simplistic) that the digestive system extracts certain resources that are different from those of the endocrine system, the blood, etc. One neurologist we spoke to had what he called a “hard time” believing the theory.

Finally, there’s one of the most popular reasons to fast — losing weight. There are too many factors involved to provide a simple answer (you won’t take in as many calories, sure, but without food your body will also slow your metabolism so it retains as much fat as it can), but it really comes down to simple math — calories in versus calories out. If there aren’t enough calories going in to fuel your brain from food, your body will start to break itself down to provide them.


It’s not rocket science that we can’t last forever without food. Even though religion is behind some of our most dearly held fasting observances, it’s also responsible for a wildly dangerous darker side.

Inedia, or breatharianism, claims humans can live solely on prana (the Hindu life force from sunlight) without consuming food or drink of any kind, including water. It sounds like any number of harmless New Age scams (the Breatharian Institute of America offers its “Immortality Initiation Workshop” for only $1 billion), but up until 2012, four deaths have been linked to the publications of an Australian breatharian. Still, before you scoff at the idea altogether, in 2013 an Israeli breatharian spent eight days without food or drink under constant video surveillance, emerging in good spirits and with no change throughout daily blood tests, baffling the attendant cardiologist.

Most of what we know from the brain’s response to starving comes from the notorious Minnesota Starvation Experiment. Carried out on volunteers for a year, beginning in November 1944, the study revealed how prolonged nutritional stress results in depression, hysteria, hypochondriasis, “severe” emotional distress, irritability, bouts of self-mutilation, apathy, lethargy, diminished sexual interest, social withdrawal, and declines in concentration, comprehension, and judgment.

According to more recent research, the limbic system — which is associated with emotional response, memory, and attachment — is directly affected by starvation. When the frontal lobes start to break down, the process causes deficiencies in insight and impulse control, and brain imaging has revealed failures between the circuits that regulate our reward systems, emotions, the ability to plan ahead, and our understanding of consequences. A 2008 review of nutritional therapies talked about depression in particular, highlighting its connection to neurotransmitters like serotonin, dopamine, and noradrenaline, pointing out that amino acids essential to the human diet are helpful in treating mood disorders like depression.

Eventually, the body will simply run out of options to maintain the brain. With no more glucose, fat, tissue, or muscle mass to spare, two common diseases of starvation are likely — marasmus and kwashiorkor. Marasmus is the state of weighing less than 60 percent of the normal weight for your age and height, a condition that can lead to higher incidence of infection. Kwashiorkor is a response to protein deficiency (rather than simply energy deficiency), which can result in fluidic inflammation — an enlarged fatty liver causing the distended belly, giving the illusion of being well-fed even while the sufferer is severely malnourished.


Assuming there even is one, what’s the perfect fast for brain health? We’re lucky to live in an age where science has given us very clear and specific clues about unhealthy behaviors and how to correct them, so the answer to whether fasting is good or bad for the brain depends on everything from the length of a fast to the reasons you’re doing it, keeping in mind details like your age and your initial state of health.

According to the evidence, a fasting regime of eight to 16 hours followed by physical activity one to three times a week has exhibited neuroprotective effects. But the scientist who gave Brain World that advice added a very important caveat — “to some extent.” It’s hardly cause to shout about fasting’s positive effects on the brain from the rooftops.

As always, don’t try anything too radical without qualified medical advice, and put a little faith in your own internal messaging system. We’ve all experienced busy days and missed meals, so when you finally sit down to eat you’re sometimes hungrier than usual. Our bodies are remarkably clever at telling us what they need if we take the time to listen — whether it’s sleep, exercise, sex, food … or no food at all.

(Editor’s note: This article from the Spring 2016 issue of Brain World magazineIf you enjoy this article, consider a print or digital subscription!)

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