Fasting

Fasting is the deliberate act of refraining from eating, and in some cases drinking, for a defined period of time. While commonly associated with religious observance or cultural practice, fasting also has important physiological, medical, political, and social dimensions. In scientific terms, fasting refers to the metabolic state that arises when the body has completed digestion and absorption of nutrients from a previous meal, typically several hours after eating. This state triggers a series of metabolic adaptations that influence energy use, hormone regulation, and organ function.

Physiological and Metabolic Context

From a physiological perspective, fasting begins after the absorption of nutrients from a meal, usually around three to five hours post-consumption. At this stage, circulating glucose levels decline, insulin secretion decreases, and the body transitions from using externally derived glucose to endogenous energy stores. Initially, liver glycogen is broken down to maintain blood glucose levels, particularly for glucose-dependent tissues such as the brain and red blood cells.
As fasting continues, gluconeogenesis becomes more prominent, producing glucose from non-carbohydrate sources such as amino acids and glycerol. With prolonged fasting, the body increasingly relies on fatty acid oxidation, leading to the production of ketone bodies. These ketones partially replace glucose as the primary fuel for the central nervous system, representing a significant metabolic shift known as ketosis.

Types of Fasting

Fasting occurs in several distinct forms, each defined by duration, purpose, and pattern:

• Overnight or Short-Term Fasting: This is the most common form and occurs naturally between the evening meal and breakfast. It is routinely required before certain blood tests to establish baseline metabolic measurements.
• Diagnostic Fasting: Conducted under medical supervision, diagnostic fasting may last from several hours to over a day, depending on age and clinical indication. It is often used to investigate disorders such as hypoglycaemia.
• Intermittent Fasting: This dietary approach alternates periods of eating and fasting on a regular schedule, such as daily time-restricted eating or alternate-day fasting. It is commonly adopted for weight management or metabolic health.
• Prolonged Fasting: Also known as extended or water fasting, this involves fasting for periods longer than 24 hours, sometimes extending to several days. This form carries greater physiological stress and medical risk.
• Religious and Cultural Fasting: Many religions prescribe fasting at specific times, often with defined rules regarding food, drink, and behaviour.
• Political or Protest Fasting: Hunger strikes are a form of fasting used to express dissent, draw attention to injustice, or exert moral pressure for social or political change.

Medical Applications

Fasting is routinely practised in medical settings, particularly prior to surgery or procedures requiring general anaesthesia. This precaution reduces the risk of pulmonary aspiration, a potentially life-threatening complication caused by vomiting and inhaling gastric contents during anaesthesia. Fasting is also required before certain diagnostic tests, including lipid panels and glucose tolerance assessments, to ensure accurate baseline results.
In some cases, supervised fasting is used diagnostically to observe metabolic responses or to provoke symptoms under controlled conditions. Such practices are carefully monitored due to the risks associated with prolonged nutrient deprivation.

Health Effects and Benefits

Research indicates that certain forms of fasting may produce beneficial health effects, particularly in individuals who are overweight or have metabolic disorders. Alternate-day fasting, which involves alternating between a day of significant caloric restriction and a day of normal intake, has been shown to improve cardiovascular and metabolic biomarkers. Studies have reported reductions in body weight, body mass index, and cardiometabolic risk factors over periods of several months.
Intermittent fasting has also been associated with improvements in insulin sensitivity and lipid profiles, although evidence for its effectiveness in achieving sustained weight loss, especially with fasting periods shorter than 24 hours, remains limited. The health benefits of fasting appear to overlap significantly with those achieved through overall calorie restriction.

Mental Health and Cognitive Effects

Some evidence suggests that fasting may positively influence mental health and cognitive function. Reviews have reported improvements in alertness, mood, and subjective feelings of wellbeing during fasting periods. These effects may be mediated by changes in neurotransmitter activity, ketone production, and reductions in oxidative stress. However, responses vary considerably between individuals, and fasting may exacerbate anxiety, irritability, or low mood in some populations.

Weight Loss and Body Composition

Weight loss is a common motivation for fasting, particularly intermittent and prolonged fasting. During the early stages of fasting, weight loss is rapid and largely reflects depletion of glycogen stores and associated water loss. In the initial gluconeogenic phase, a substantial proportion of weight lost may consist of lean muscle mass rather than fat.
As fasting continues and ketosis becomes established, fat oxidation increases, and the proportion of fat lost relative to lean tissue improves. Typical observed weight loss during prolonged fasting averages approximately one kilogram per day in the first week, decreasing thereafter. Despite these effects, prolonged fasting carries significant risks and is not generally recommended without medical supervision.

Adverse Effects and Risks

Fasting is not without potential harm, particularly when extended or unsupervised. One of the most serious complications is refeeding syndrome, a metabolic disturbance that occurs when nutrition is reintroduced after a period of starvation or severe malnutrition. The sudden shift from catabolism to anabolism can cause dangerous electrolyte imbalances, including low levels of potassium, magnesium, and phosphate, leading to neurological, cardiac, and respiratory complications.
Fasting may also increase the risk of gallstone formation, particularly during rapid weight loss. Reduced gallbladder activity and increased cholesterol secretion into bile contribute to stone development. Additional risks include dehydration, hypotension, nutrient deficiencies, and impaired immune function.
There is currently no evidence-based medical consensus that fasting promotes longevity in humans, despite ongoing interest in this area.

Political and Social Applications

Fasting has long been used as a tool of political protest and moral persuasion. Hunger strikes represent a form of nonviolent resistance in which individuals refuse food to highlight perceived injustice or to pressure authorities into action. Historical examples include the fasts undertaken by Mahatma Gandhi during the Indian independence movement and the 1981 Irish hunger strike, in which prisoners protested for political recognition and improved conditions.
In the United States, civil rights leader César Chávez used spiritual fasting to promote nonviolence and social justice, framing fasting as a personal and collective moral act. Such practices have influenced subsequent activist movements worldwide.

Religious Perspectives

Fasting holds significant importance across many religious traditions, though practices vary widely in duration, strictness, and purpose. In Judaism, multiple fast days are observed throughout the year, ranging from dawn-to-nightfall fasts to full 25-hour fasts during Yom Kippur and Tisha B’Av, during which both food and drink are prohibited.
Christian fasting practices are particularly prominent during Lent. In the Catholic Church, fasting and abstinence are regulated by canon law, with specific obligations on Ash Wednesday and Good Friday, as well as abstinence from meat on Fridays. Eastern Orthodox Christianity observes numerous fasting periods, including weekly fasts and extended seasonal fasts.
Members of the Church of Jesus Christ of Latter-day Saints practise a monthly fast, abstaining from food and drink for a 24-hour period and donating the equivalent cost to charitable causes. Across religions, fasting is commonly associated with repentance, spiritual discipline, solidarity with the poor, and moral reflection.