How does fasting affect your brain?


Despite popular concerns to the contrary, fasting has potentially incredible benefits to various brain functions. Perhaps the most amazing benefit may stem from the activation of autophagy, a cellular cleansing process. Recently, one of the pioneers of research into autophagy was awarded the 2016 Nobel Prize for Medicine in growing recognition of this major pathway of disease. Fasting also has known anti-seizure effects.

From an evolutionary standpoint, mammals respond to severe caloric deprivation with a reduction in the size of all organs with two prominent exceptions – the brain and the male testicles. The preservation of the size of the testicles is also a significant advantage in trying to pass on our genes to the next generation.

The preservation of cognitive function makes quite a lot of sense for the survival of the species. Suppose we are caveman, and it’s winter and food is scarce. If your brain started to slow down, well, the mental fog would make it that much harder to find food. Our brainpower, one of the main advantages we have in the natural world, would be squandered. Each day without food would slowly erode our mental functioning until we are slobbering idiots, incapable of basic bladder function let alone going out to hunt for food. During starvation, higher cognitive function is maintained or even boosted.

This has been known throughout history. In Ancient Greece, the great thinkers would fast for days on end, not because they needed to lose weight, but because they believed that fasting would increase their mental agility. In stories of Japanese prisoners of war in World War II (Unbroken by Laura Hillenbrand), many have described the amazing clarity of thought that often accompanies starvation. In this book, the main character describes a prisoner who would read entire books from memory, and another who learned the Norwegian language in a few weeks. Incredibly, these feats were so commonplace that prisoners simply accepted it as a fact of life that starvation increases cognitive ability.

Mental sharpness increases during fasting

In mammals, mental activity increases when hungry and decreases with satiation. We have all experienced this as ‘food coma’. Think about that large Thanksgiving turkey and pumpkin pie. After that huge meal, are we mentally sharp as a tack? Or dull as a concrete block? How about the opposite? Think about a time that you were really hungry. Were you tired and slothful? I doubt it. Your senses were probably hyper-alert and you were mentally sharp as a needle. The idea that food make you concentrate better is entirely incorrect. There is a large survival advantage to animals that are cognitively sharp, as well as physically agile during times of food scarcity.

foodcomaStudies have also proven that mental acuity does not decrease with fasting. One study compared cognitive tasks at baseline and after a 24 hour fast. None of the tasks – including sustained attention, attentional focus, simple reaction time or immediate memory were found to be impaired. Another double-blinded study of a 2-day ‘almost total’ caloric deprivation found no detrimental effect even after repeatedly testing cognitive performance, activity, sleep and mood.

When we say we are ‘hungry’ for something (hungry for power, hungry for attention), does it mean we are slothful and dull? No, it means that we are hyper-vigilant and energetic. So, fasting and hunger clearly activate us towards our goal. People always worry that fasting will dull their senses, but in fact, it has the opposite, energizing effect.

These sorts of tests are easy to see in animal studies. Aging rats were started on intermittent fasting regimens and markedly improved their scores of motor coordination and cognitive tests. Learning and memory scores also improved after IF. Interestingly, there was increased brain connectivity and new neuron growth from stem cells. This is believed to be mediated in part by BDNF (Brain Derived Neurotrophic Factor). In animal models, both exercise and fasting significantly increase BDNF expression in several parts of the brain. BDNF signaling also plays a role in appetite, activity, glucose metabolism and autonomic control of the cardiovascular and gastrointestinal systems.

Fasting and neuro-degenerative diseases

crimprovesmemory-300x260There are also very interesting mouse models of neuro-degenerative diseases. Mice maintained on IF, compared to normal mice, showed less age-related deterioration of neurons and less symptoms in models of Alzheimers disease, Parksinon’s and Huntington’s disease.

In humans, the benefits to the brain can be found both during fasting and during caloric restriction (CR). During exercise and CR, there is increased synaptic and electrical activity in the brain. In a study of 50 normal elderly subjects, memory test improved significantly with 3 months of CR (30% reduction in calories).

memory-and-insulin-300x277Neurogenesis is the process where neural stem cells differentiate into neurons that are able to grow and form synapses with other neurons. Both exercise and CR seem to increase neurogenesis via pathways including BDNF.

Even more interestingly, the level of fasting insulin seems to have a direct inverse correlation to memory as well. That is, the lower you are able to drive down fasting insulin, the more improvement on memory score that is seen.

Increased body fat (as measured by BMI) has also been linked to decline in mental abilities. Using detailed measurements of blood flow to the brain, researchers linked a higher BMI to decreased blood flow to those areas of the brain involved in attention, reasoning, and higher function.

Intermittent fasting provides one method of decreasing insulin, while also decreasing caloric intake.

Could fasting help prevent Alzheimer’s disease?

Alzheimer’s disease (AD) is characterized by the abnormal accumulation of proteins. There are two main classes – amyloid plaques and neurofibrillary tangles (tau protein). The symptoms of AD correlate closely with the accumulation of these plaques and tangles. These abnormal proteins may destroy the synaptic connections in the memory and cognition areas of the brain.

Certain proteins (HSP-70) act to prevent damage and misfolding of the tau and amyloid proteins. In mouse models, alternate daily fasting increased the levels of HSP-70. Autophagy removes these tau and amyloid proteins when they are damaged beyond repair. This process, too, is stimulated by fasting.

dementiaThere is substantial evidence that risk of AD is related to obesity. A recent population based twin study demonstrated that weight gain in middle age predisposes to AD.

Taken together, this suggests a fascinating possibility in the prevention of Alzheimer’s disease. Over 5 million American have AD and this number will likely increase rapidly due to the aging population. AD creates significant burdens upon families that are forced to care for their afflicted members.

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


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