Fasting is considered to be generally safe
The majority of the benefits result from regulating energy-balance
Fasting results in increased breakdown of dysfunctional and damaged tissue (autophagy, mitophagy, apoptosis)
Fasting also improves ‘fat adaptation’ (the use of fat for fuel)
Fasting increases cortisol and stress hormone levels but these return to normal with adaptation to fasting
Fasting is at least as effective for weight/fat loss as calorie restriction but is more effective for reducing IGF-1
Fasting benefits asthma, autoimmune conditions, reduces side-effects from chemotherapy, reduces blood pressure, and improves mood disorders and skin health
Fasting has recently re-entered the mainstream consciousness as a diet and lifestyle practice that is perceived as an easy and effective way to improve health. Fasting though is nothing new and has been a dietary strategy employed for mental, physical, and spiritual health for millennia. A ‘natural’ free-living human, throughout much of our existence as a species, would have gone through frequent periods of fasting, both day-to-day and periodically. Only very recently have we had an abundance of food which has caused fasting to be seen as outside of the norm. The adaptations that we have developed to respond to nutrient and energy scarcity allowed us to survive in varied environments and through times of scarcity. As it turns out, many of these adaptations are also beneficial to health and to our ability to not just survive but to thrive.
What is fasting?
Fasting is the act of abstaining from all or some kinds of food or drink. Fasting has traditionally been associated with spiritual and religious practices rather than the primarily health-focus we see today. Thus, most definitions qualify fasting with ‘especially as a religious observance’.
There is no quantified definition for fasting though, and it could be considered to be any period of abstention from food or restriction of calories that is outside the ‘norm’ of dietary patterns. So, it can vary from missing one or more meals per day (intermittent fasting) through to missing entire days of eating (as in alternate-day fasting) or having a couple of days a week in which nothing is eaten, or calories are strictly controlled (such as in 5:2 fasting).
Religious fasting is still common and is reflected in the dietary habits of millions of believers and the health impact of religious fasting has been documented among Buddhists, Hindus, Christians, and Muslims.1 The most commonly observed religious fast carried out nowadays is the Ramadan fast of Islam. All adherents who are not ill, travelling, elderly, pregnant, diabetic, or menstruating, are expected to fast completely (without foods or liquids) between sunrise and sunset. Because the majority of the nearly 2 billion Muslims fast during this time, it’s unsurprising that much of the research on fasting has been carried out during Ramadan.
Overall, the reviews of studies conducted so far show that fasting provides a range of health benefits,2, 3 and that much of the benefit results from energy-restriction. By reducing the ‘feeding window’ available, people tend towards eating less overall because even if they ‘overeat’ during their non-fasting times, they do not eat more than they would have if not fasting. It has been observed in randomised controlled trials that fasting does not result in increased calorie intake during the non-fasting times.2
So, fasting is an effective way to achieve calorie-restriction, but it might also offer additional benefits by encouraging secondary processes such as reduced insulin-like growth factor (IGF-1), an anabolic chemical which, while beneficial to growth and repair, is also implicated in the development of cancer when produced in excess.
Fasting also increases autophagy and apoptosis (self-destruction and immune destruction of dysfunctional cells and tissues),4 reduces monocyte-driven inflammation and might offer additional benefits over and above the restriction of energy intake.5 Medically supervised fasts of 200-500 calories per day for 7-21 days have been shown to be beneficial for inflammatory disorders, reduce pain and inflammation, and improve mood and cognition, and are thought to work by also encouraging increased brain availability of serotonin, opioids, cannabinoids, and neurotrophic factors, along with neuroendocrine activation and by providing a mild stress response (which also results in the aforementioned reductions in monocyte driven inflammation and increased autophagy).6
Along with energy-restriction, fasting results in increased proportions of fat being used as fuel (‘fat-adaptation’) and the combination of fasting , fat-adaptation, and calorie restriction also improve glucose homeostasis (blood sugar control), blood lipids, and increase the availability of neuroprotective chemicals such as brain-derived neurotrophic factor (BDNF) which is critical to the repair and maintenance of the brain and central nervous system.2
In the studies performed to date (many of them on the Ramadan fast), fasting has been shown to be safe and with few, if any adverse effects. Water-only fasting equal to, or greater than two days has similarly been shown to be incredibly safe overall.7