Intermittent fasting is beneficial for many health conditions, including obesity, diabetes, cardiovascular disease, cancers, and neurodegenerative brain disorders

In December 2019 researchers from the USA published their review on the effects of intermittent fasting. The suggestion that calorie restriction had a beneficial effect on aging and life span was first proposed in 1997. However, at the time it was not recognised that calorie restriction, with calories often being consumed within a few hours, produced a daily fasting period of up to 20 hours. Since then, studies involving controlled intermittent fasting have been undertaken which have shown metabolic switching, ie when the metabolism changes from liver-derived glucose to fatty cell–derived ketones which occurs either daily or on several days each week. After meals, glucose is used for energy, and fat is stored in adipose tissue as triglycerides. During periods of fasting, triglycerides are broken down to fatty acids and glycerol, which are used for energy. The liver converts fatty acids to ketone bodies, which provide a major source of energy for many tissues, especially the brain, during fasting. In the fed state, blood levels of ketone bodies are low, although they rise within 8 to 12 hours after the onset of fasting.

Alternate-day fasting, 5:2 intermittent fasting (fasting 2 days each week), and daily time-restricted feeding are the three most widely studied forms of intermittent fasting.

The practice of long-term fasting (from many days to weeks) was not covered in this review.

Studies have consistently shown that intermittent fasting can have an effect on a wide range of chronic disorders, including obesity, diabetes, cardiovascular disease, cancers, and neurodegenerative brain diseases. More specifically it can improve glucose regulation and insulin resistance, blood pressure and heart rate, abdominal fat loss, cholesterol levels and inflammation. It has also been seen that calorie restriction in overweight adults can lead to improvements in verbal and working memory, and cognition.

Intermittent fasting has been shown to improve many cardiometabolic risk factors in non-obese individuals. Intermittent fasting can also be as effective for weight loss as standard diets. In addition, intermittent fasting reduces markers of inflammation and oxidative stress that are associated with atherosclerosis. Improvements in cardiovascular health indicators typically become evident within 2 to 4 weeks after the start of alternate-day fasting and then disappear over a period of several weeks after resumption of a normal diet. However, the potential benefits of intermittent fasting in patients with myocardial infarction remain to be tested.

Similarly, intermittent fasting is thought to reduce energy metabolism in cancer cells, preventing their growth and leaving them susceptible to treatments. It is thought that intermittent fasting may provide protection against cancer while improving the stress resistance of normal cells. Several case studies involving patients with glioblastoma suggest that intermittent fasting can stop tumor growth and extend survival. However, no studies have yet determined whether intermittent fasting has any impact on cancer recurrence in humans.

Some studies have suggested that excessive energy intake, particularly in midlife, increases the risks of stroke, Alzheimer’s disease and Parkinson’s disease. However, information from studies on intermittent fasting in persons at risk or affected by a neurodegenerative disorder are lacking.

Weight loss is known to reduce the symptoms of asthma in obese patients. A reduction in symptoms has been shown to be associated with significant reductions in markers of inflammation and oxidative stress. Multiple sclerosis is an autoimmune disorder and two recent studies have shown that patients with multiple sclerosis who adhere to intermittent-fasting regimens have reduced symptoms in as short a period as 2 months. As it reduces inflammation, intermittent fasting would also be expected to be beneficial in rheumatoid arthritis, and indeed, there is evidence supporting its use in patients with arthritis.

Intermittent-fasting regimens reduce tissue damage and improve functional outcomes of traumatic and ischemic tissue injury. One study showed that 2 weeks of pre-operative daily energy restriction improved outcomes in patients undergoing gastric-bypass surgery. Such findings suggest that preoperative intermittent fasting can be a safe and effective method of improving surgical outcomes.

Physical fitness has also improved with intermittent fasting, eg young men who fasted daily for 16 hours lost fat while maintaining muscle mass during 2 months of resistance training.

Emerging evidence has also suggested that intermittent fasting may prove to be a practical approach for reducing the morbidity and mortality associated with traumatic brain and spinal cord injuries in athletes.

de Cabo R, Mattson MP. Effects of Intermittent Fasting on Health, Aging, and Disease. N Engl J Med. 2019 Dec 26;381(26):2541-2551.

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