Jay Richards, author of Eat, Fast, Feast: Heal Your Body While Feeding Your Soul―A Christian Guide to Fasting, found that almost the first benefit of fasting, after a day or so, was mental clarity. Why would that be? Surely, fasting would make us dizzy, unfocused, constantly obsessed with visions of food. But then again, maybe not. He suggests,

Assume you wanted to design the human metabolism for survival over the long haul of human history. Isn’t this how you’d do it? After all, our brain is our most important survival organ. What if the brains of early humans slowed down and their thoughts scattered to the winds whenever they missed a meal or two—just when they most needed to hunt and forage for food? This trait would have gotten weeded out of the gene Eat, Fast, Feast pool after a few centuries of erratic food supply. Since we’re still here, we can assume that our ancestors enjoyed some metabolic way to survive the frequent bouts of scarcity. And we have inherited the same metabolic system, even if we rarely need to use it in the twenty-first century.

Jay Richards, Eat, Fast, Feast: Heal Your Body While Feeding Your Soul―A Christian Guide to Fasting, Harper One, 2020, p. 166

But why does fasting clarify the mind? The likely answer is ketosis (your body is burning stored fat for energy instead of blood sugar):

We don’t yet have the full story about how fasting boosts brain function, but we can piece together threads of evidence to form a working theory. One important thread is ketones themselves, which seem to be a high-octane brain fuel. Ketones, unlike regular fatty acids, can cross the blood-brain barrier. And ketosis, whether achieved by diet, fasting, or both, is certainly associated with mental clarity. Many people report that the sensation is really strong when blood ketone levels reach the threshold of “nutritional ketosis”—about one mmol/L (millimoles per liter).

Jay Richards, Eat, Fast, Feast: Heal Your Body While Feeding Your Soul―A Christian Guide to Fasting, Harper One, 2020, p. 168

Like most subjects in health and medicine, the benefits of fasting are research in progress but there is a general consensus as to what is happening and why:

In recent years, researchers have been working to pin down the mechanics of how fasting affects the brain, using lab mice given treadmill exercises during a fasting regimen:

In lab animals, fasting, as well as exercise, stimulates the production of a protein in nerve cells called brain-derived neurotrophic factor, or BDNF. This protein plays critical roles in learning, memory, and the generation of new nerve cells in the hippocampus. BDNF also makes neurons more resistant to stress. Fasting also triggers a process called autophagy, where cells remove damaged molecules and dysfunctional mitochondria, and turns off cell growth. So neurons are in a kind of “resource conservation and stress resistance” mode during fasting. When the animal, and by extrapolation probably the human, eats after fasting, neurons shift to a “growth” mode — they make lots of proteins, grow, and form new synapses. We think these cycles of metabolic challenge, whether it’s exercise or fasting, and then a recovery period may optimize neuroplasticity, learning, memory, and the resistance of the brain to stress.

Alexis Wnuk, “How Does Fasting Affect the Brain?” at BrainFacts.org (July 13, 2018)

Note: The medical research paper, published in FASEB J, is open access (free to the public).

Neuroplasticity (the brain’s self-healing properties) is very important for recovery from brain injuries.

But could fasting just add to the stress in our lives? A writer and researcher on fasting asks us to distinguish between types of stress:

First we have to talk about how stress can be both bad and good. Good stress, meaning stress that doesn’t last for very long or become chronic, actually sharpens biological functions. It’s a primal response to challenges and dangers we face in our environment. So under good stress, your brain increases the production of certain molecules, such as glucocorticoids, catecholamines, and glucose levels, which prepare your tissues for responsiveness to challenges or danger. One of the effects of this stress response is sharpened senses and heightened awareness.

Jordan Rosenfeld, “Seeking Mental Clarity? Try Fasting” at Medium (December 1, 2018)

There is reason to believe that fasting can provide some protection against serious late-life health disorders, cognitive disorders, including Alzheimer syndrome, says a diet specialist:

Certainly fasting may have significant benefits in reducing weight, type 2 diabetes along with its complications – eye damage, kidney disease, nerve damage, heart attacks, strokes, cancer. However, the possibility also exists that it may prevent the development of Alzheimer’s disease as well.

The method of protection may also have to do with autophagy – a cellular self cleansing process that may help removed damaged proteins from the body and brain. Since AD may result from the abnormal accumulation of Tau protein or amyloid protein, fasting may provide a unique opportunity to rid the body of these abnormal proteins.

Jason Fung, MD, “How does fasting affect your brain?” at Diet Doctor.com ( October 16 2016 )

Richards himself is determined to stay the course:

Once I really shifted to the fasting lifestyle, and quit grazing on sugar, my mental function improved enough that it created a massive disincentive ever to go back. I now fast, if possible, before I have a stressful presentation, media interview, or debate. And I’ve written a lot of this book while fasting. This mental clarity would likely happen to many people who just never reach the stage where they experience it. It only takes about two weeks to break a sugar addiction. But if you imagine misery for as far as the eye can see when you cut sugar from your diet, then you’re not likely to try it even for two weeks.

Jay Richards, Eat, Fast, Feast: Heal Your Body While Feeding Your Soul―A Christian Guide to Fasting, Harper One, 2020, p. 167

Fasters are usually advised to let their health care advisers know that they are fasting, in case any health changes they experience (significant weight loss, for example) are not the promised benefits but need further evaluation.

Further reading on brain health and happiness:

Aging brains need exercise, not sofas for neurons Neuroscientist Yuri Danilov reassures seniors, we do not lose neurons as we age.

Can fitter brains help us fight depression? Philosopher J. P. Moreland continues his account of working his way through a devastating anxiety disorder.

Up! Up! Up! Keep those hippocampi up! The lighter side of neurobabble about our brains and ourselves. (Michael Egnor)

and

How the brain heals itself: Our amazing neuroplasticity