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Benefits of fasting for the brain: truth or myth? 

By Lottie Booth, Brain Injury Linkworker for the North 

 

What do we mean by fasting?  

Fasting refers to voluntarily abstaining from food or drink for a set period. There are several types. Intermittent fasting (IF) involves eating within a restricted window (e.g. eight hours) and fasting for the remaining time (e.g. 16 hours). The fasting-mimicking diet (FMD) is a low-calorie diet designed to simulate the effects of fasting without complete food abstinence [1, 2]. Prolonged fasting (PF) typically lasts two to five days, and prolonged water fasting (PWF) involves consuming little or no calories for several days [3]. 

 

Fasting and brain function 

The brain primarily relies on glucose for energy. After 12-36 hours of fasting, the body begins to produce ketone bodies, which are produced in the liver from fatty acids as an alternative energy source [3]. This metabolic shift may offer neuroprotective benefits, helping to maintain stable brain functioning when food is scarce. In one study, healthy participants underwent prolonged water fasting for 72 hours, Magnetic resonance imaging (MRI) revealed no signs of brain damage or dysfunction, and brain chemistry remained stable [4]. Some research suggests that intermittent fasting may enhance brain energy efficiency and protect against neurodegeneration (the death of neurons), which in turn may protect against age-related cognitive decline and the negative effects of brain injury [5]. 

 

Mechanisms behind the effects of fasting 

Several biological mechanisms may explain the impact of fasting on brain health. Ketosis is when the body produces ketones, providing an alternative energy source for the brain [6-8]. Autophagy is a cell recycling process that removes damaged cells and promotes regeneration. Intermittent fasting has been shown to induce adaptive autophagy which may support brain repair, especially following injury [9-10]. Fasting may also increase brain-derived neurotropic factor (BDNF), which is a protein essential for learning and memory. Animal studies show increased BDNF levels and reduced stress following intermittent fasting [11]. Additionally, intermittent fasting may reduce inflammation and oxidative stress, protecting neurons from damage [12-14]. Repeated fasting may also stimulate immune cell regeneration. This process could enhance these brain repair processes [15]. 

 

Fasting and neurological conditions 

Evidence, mostly from animal studies, suggests that fasting could help after acquired brain injury (ABI), including traumatic brain injury (TBI), and with neurodegenerative diseases.  

ABI often impairs glucose metabolism. In such cases, intermittent fasting and prolonged fasting may be able to provide ketones as an alternative energy source, supporting brain function and recovery [16]. Ketones may also reduce inflammation and oxidative stress in the brain following a TBI, aiding post-injury healing [17].  

After intermittent fasting, rats with a TBI showed reduced brain inflammation and increased neurogenesis (the generation of new neurons) in the hippocampus, a region of the brain critical for memory and learning. And a review of animal studies found that intermittent fasting may promote recovery after TBI by improving mitochondrial function (the process of how cells produce energy), and increasing the survival of brain cells, helping them to stop dying [18]. Additionally, intermittent fasting in rodents has been shown to reduce brain damage after ABI, specifically stroke, while oxidative stress was lowered, and neurotrophic signalling was enhanced, improving neuron survival and repair [19].  

Benefits after fasting were also seen in the context of neurodegenerative diseases. In humans, intermittent fasting has been found to improve episodic memory, executive function, attention and overall cognition in elderly individuals with mild cognitive impairment and Alzheimer’s disease over a three-year study [20]. In addition, mice with Alzheimer’s disease showed enhanced working memory, short-term memory, long-term memory and learning after intermittent fasting and fast-mimicking diet. These improvements were associated with increased BDNF and reduced neuroinflammation [2, 21]. 

Fast-mimicking diets may also reduce risk factors for diabetes and cardiovascular conditions in humans [2, 14]. People with both conditions are at a significantly higher risk of developing dementia, particularly vascular dementia [22]. And further evidence from studies with humans undergoing intermittent fasting show that this may protect against neurodegeneration, including for those with Alzheimer’s and Parkinson’s disease [7]. Intermittent fasting may also improve vascular health and insulin sensitivity, reducing the risk for vascular dementia [23] Beyond the cognitive benefits shown to date, this type of fasting (IF) has also been found to improve mood and reduce tension, with participants reporting less anger and irritability [24].  

 

Risks of fasting 

It is important to note that fasting is not universally safe and can impose health risks. During prolonged water fasting, some people have reported mild side effects like insomnia, fatigue, dizziness, headaches and nutrient deficiencies [3]. Intermittent fasting tends to be generally better tolerated, though a minority of people do report hunger, irritability or low energy [24]. Excessive prolonged fasting may overstimulate autophagy, harming cognition from neurodegeneration and negative effects on memory; particularly in older adults, where autophagy is already less efficient [10]. 

Fasting may be unsuitable for certain groups of people. Those with diabetes, or other health conditions, should seek medical advice before fasting [3]. Fasting should always be tailored to each person to ensure safety and effectiveness [7, 12, 19].  

 

Conclusion: truth or myth?  

Fasting shows promising potential for brain health however the current evidence remains inconclusive. Most research relies on animal models, and there are limited data from studies with humans, making it hard to generalise much of the findings. There are also insufficient long-term studies on the benefits and risks of fasting for those with neurological conditions, further research is needed to confirm these effects [9]. 

Fasting may support brain health through mechanisms like ketosis, autophagy, and inflammation reduction. Intermittent fasting and fast mimicking diet appear most practical for everyday life to support cognitive function and perhaps brain repair. However, fasting is not a one-size-fits-all solution. It can pose risks, especially for older adults and those with underlying health conditions. People should always consult with a healthcare professional before fasting.   

 

References  

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