Fasting

Water only fasting
As the name implies, this is a type of fasting where only water may be consumed – no other food or beverage is permitted.

Liquid fasting
Some fasts include abstaining from solid foods but consuming calorie-containing liquids, such as vegetables juices or broth.

Intermittent fasting
In this form of fasting, a person alternates between periods of fasting and non-fasting, with a defined schedule. Intermittent fasting is of interest to researchers for its potential effects on insulin sensitivity and other aspects of health. Popular types of intermittent fasting include: 16:8, 16-hr fast with a 8-hr eating window, 18:6, 18-hr fast with a 6-hr eating window, 20:4, 20-hr fast with a 4-hr eating window. The alternate day fast is a 36 hour fast followed by a 12-hr eating window.

Caloric restriction
A calorie restricted (CR) diet reduces calorie intake without malnutrition. It is chronic and does not include a refeeding phase. Most clinical trials of CR aim for 25-30% total calorie reduction. The 5:2 diet is a calorically restricted diet in which 2 days per week one consumes 500-600 calories, and eats normally the remaining 5 week days.

Physiological effects
A fast with duration sufficient to deplete the body's glycogen stores, at least 18 hours, increases the blood ketone β-hydroxybutyrate allowing for nutritional ketosis.

Fasting increases autophagy in mice.

The US National Institute on Aging and the National Institute of Diabetes and Digestive and Kidney Diseases conducted the CALERIE (Comprehensive Assessment of Long-term Effects of Reducing Intake of Energy) study using a 2-year CR diet (25% reduction) in non-obese adults (n = 143) compared to an ad libitum group (n =75). The CR group reported improvements in mood and improved sleep quality without adverse effects or immune compromise. There was also improvement in cardiovascular risk factors including C-reactive protein.

Health effects
There is no evidence on the benefits or harms of fasting for ME and CFS patients. However, there is a growing body of evidence suggesting possible health benefits of water-only fasting to the microbiome, mitochondria and the immune system, and as a cancer adjuvant.

Immune system
Caloric restriction significantly reduced the amount of circulating lipopolysaccharide-binding protein. In vitro, short-term (19 hour) fasting reduced monocyte metabolic and inflammatory activity and drastically reduced the number of circulating monocytes. This effect is thought to be due to redistribution of these cells to the bone marrow. Caloric restriction and intermittent fasting strongly reduced the accumulation of pathogenic monocytes in the central nervous system, reduced monocyte pro-inflammatory activity, and improved disease outcome in a mouse model of multiple sclerosis and in a preclinical model in multiple sclerosis patients.

Chronic fatigue syndrome
Dr. Courtney Craig has proposed the use of fasting, caloric restriction and a ketogenic diet in the treatment of mitochondrial damage in ME/CFS.

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 * 2016, Fasting Diets Are Gaining Acceptance