The OneYearFast Fasting Dictionary

What is AMPK?

AMPK, or adenosine monophosphate-activated protein kinase, is an enzyme that helps cells respond to low energy availability.

AMPK acts as an important cellular energy sensor. When cellular energy is low, AMPK becomes more active and helps the body conserve energy while increasing processes that generate energy.

Why does AMPK matter during fasting?

During fasting, nutrients and circulating insulin levels generally fall while the body's energy demands continue. This creates conditions in which AMPK signaling can become more important.

AMPK is associated with increased fatty-acid oxidation, glucose conservation and cellular processes involved in energy management.

AMPK is also frequently discussed alongside autophagy and mTOR, two important pathways involved in the body's response to nutrient availability.

What is autophagy?

Autophagy is a natural cellular process in which cells break down and recycle damaged, worn-out or unnecessary cellular components.

The word comes from Greek and essentially means "self-eating." Despite the somewhat alarming name, autophagy is a normal part of cellular maintenance.

What does fasting have to do with autophagy?

Nutrient availability influences the cellular pathways that regulate autophagy. During periods of fasting, lower nutrient and insulin signaling can favor cellular recycling pathways, including autophagy.

However, there is no scientifically established universal point at which human autophagy suddenly "switches on." Claims that autophagy begins precisely after a particular number of fasting hours should therefore be treated cautiously.

Why is autophagy interesting?

Autophagy is involved in:

• Cellular maintenance
• Recycling of cellular components
• Removal of damaged proteins and organelles
• Adaptation to nutrient scarcity
• Cellular stress responses

What is a circadian rhythm?

A circadian rhythm is an approximately 24-hour biological cycle that helps regulate many processes in the body. Circadian rhythms influence:

• Sleep and wakefulness
• Hormone production
• Body temperature
• Appetite
• Metabolism
• Blood pressure
• Digestive activity

How does fasting interact with circadian rhythms?

Food is an important signal for the body's biological clocks. The timing of eating can therefore influence metabolic rhythms. This is one reason researchers are interested in time-restricted eating, in which food consumption is confined to a particular part of the day.

Fasting is not only about how long someone goes without food. It may also matter when the fasting and eating periods occur.

What is DEXA?

DEXA, or dual-energy X-ray absorptiometry, is a medical imaging technology commonly used to measure bone mineral density and body composition. A DEXA scan can provide information about:

• Bone mineral density
• Bone mineral content
• Fat mass
• Lean mass
• Regional body composition

Why is DEXA relevant to fasting?

Weight alone does not tell us what has happened to the body. A person can lose weight through changes in fat, water, glycogen or lean tissue.

DEXA can provide a much more detailed picture of body composition than a conventional bathroom scale.

What are electrolytes?

Electrolytes are minerals that carry an electrical charge when dissolved in body fluids. Important electrolytes include:

• Sodium
• Potassium
• Magnesium
• Calcium
• Chloride

Why do electrolytes matter during fasting?

When food intake stops, the body changes how it handles water and minerals. Changes in insulin and glycogen stores can influence fluid and sodium handling, particularly during longer fasts.

This is one reason electrolyte status becomes increasingly relevant during prolonged fasting.

Electrolytes are not the same thing as calories. A drink containing electrolytes may or may not contain meaningful amounts of energy depending on its ingredients.

What is fat oxidation?

Fat oxidation is the process by which the body breaks down fatty acids and uses them to produce energy.

The body continuously uses a mixture of carbohydrate and fat for energy. The proportion changes according to factors such as food intake, activity, hormones and energy availability.

What happens during fasting?

As fasting continues, insulin generally decreases and stored carbohydrate becomes less available. The body increasingly relies on stored fat and fatty acids as an energy source.

Is fat oxidation the same as fat loss?

No. Fat oxidation means that fat is being used as fuel. Net fat loss occurs when the body ultimately uses more stored energy than it replaces.

What is glucagon?

Glucagon is a hormone produced primarily by the pancreas. Its role is broadly opposite to insulin when it comes to maintaining blood glucose.

When blood glucose is falling, glucagon helps signal the body to make more glucose available.

What happens to glucagon during fasting?

As food intake stops and insulin levels decline, glucagon becomes increasingly important in maintaining blood glucose. It supports processes including:

• Glycogen breakdown
• Glucose production
• Mobilization of stored energy

What is glycogen?

Glycogen is the body's stored form of glucose. It is stored primarily in the liver and in skeletal muscle.

Liver glycogen helps maintain blood glucose between meals and during fasting. Muscle glycogen is primarily used by the muscles themselves.

What happens to glycogen during fasting?

When food stops arriving, the body initially relies substantially on stored glycogen. As fasting continues, glycogen stores become progressively depleted and the body increases its reliance on fat and other fuel sources.

What is glycogen depletion?

Glycogen depletion occurs when stored glycogen is progressively used faster than it is replaced. This can happen during:

• Fasting
• Prolonged exercise
• Low-carbohydrate diets
• Energy restriction

Why does glycogen depletion matter during fasting?

As glycogen availability decreases, the body increasingly relies on other energy sources, particularly fat. The liver also increases its production of glucose through gluconeogenesis.

Why can body weight fall quickly at the beginning of a fast?

Glycogen is stored together with water. Consequently, depletion of glycogen can cause a relatively rapid reduction in body weight that is partly water rather than body fat.

What is growth hormone?

Growth hormone, or GH, is a hormone produced by the pituitary gland. It plays roles in:

• Growth
• Tissue maintenance
• Protein metabolism
• Fat metabolism

What happens to growth hormone during fasting?

Fasting can increase pulsatile growth hormone secretion. This does not mean that fasting simply produces muscle growth. Growth hormone interacts with many other hormones and metabolic pathways.

What is insulin?

Insulin is a hormone produced by the pancreas. Its central role is to help regulate blood glucose and nutrient storage.

When we eat, particularly when carbohydrate is consumed, blood glucose generally rises and insulin secretion increases.

What happens to insulin during fasting?

During fasting, insulin levels generally fall. Lower insulin signaling is associated with increased access to stored energy, including stored fat.

Insulin is one of the major hormones connecting food availability with energy storage and energy mobilization.

What is insulin resistance?

Insulin resistance occurs when cells respond less effectively to insulin. The body may compensate by producing more insulin to achieve the same metabolic effect.

Insulin resistance is strongly associated with metabolic disorders, including type 2 diabetes.

How does fasting relate to insulin resistance?

Periods without food can lower insulin exposure and alter glucose metabolism. Research into fasting and time-restricted eating has therefore examined whether these approaches can improve insulin sensitivity and other markers of metabolic health.

Insulin resistance should not be confused with the temporary metabolic changes that occur naturally during fasting.

What is insulin sensitivity?

Insulin sensitivity describes how effectively cells respond to insulin. Greater insulin sensitivity generally means that the body can achieve a given metabolic response with less insulin.

Why is insulin sensitivity important during fasting?

Fasting changes the hormonal environment of the body, including insulin levels. Research into fasting and time-restricted eating frequently examines whether these changes can improve insulin sensitivity over time.

What is ketogenesis?

Ketogenesis is the metabolic process through which the liver converts fatty acids into ketone bodies. The three primary ketone bodies are:

• Beta-hydroxybutyrate (BHB)
• Acetoacetate
• Acetone

When does ketogenesis occur?

Ketogenesis becomes more active when carbohydrate availability and insulin signaling are relatively low and fatty-acid availability is increased. This can occur during fasting, prolonged exercise, carbohydrate restriction and certain metabolic conditions.

What are ketones?

Ketones are water-soluble molecules produced primarily by the liver when the body is using significant amounts of fat for energy. The main ketone bodies are beta-hydroxybutyrate, acetoacetate and acetone.

Why do ketones increase during fasting?

As fasting continues, insulin generally falls, stored fat becomes more available and the liver increases ketone production. Ketones can then provide an alternative fuel source for several tissues, including the brain.

How can ketones be measured?

Ketones can be measured in blood, breath or urine. Blood beta-hydroxybutyrate measurements generally provide a more direct measurement of circulating ketones than urine testing.

What is ketosis?

Ketosis is a metabolic state in which ketone production and availability are elevated because the body is relying more heavily on fat-derived energy.

Fasting is one way to enter ketosis. Other routes include carbohydrate restriction and certain medical diets.

Is ketosis the same as ketoacidosis?

No. Nutritional ketosis and fasting ketosis are normal metabolic states.

Diabetic ketoacidosis is a serious medical condition involving abnormally high ketones, usually in the context of insulin deficiency and uncontrolled diabetes.

What is lean mass?

Lean mass is the portion of body weight that is not classified as fat mass. Depending on the measurement method, lean mass includes:

• Muscle
• Organs
• Body water
• Connective tissue
• Other non-fat tissues

Why does lean mass matter during fasting?

When evaluating weight loss, it is important to distinguish changes in fat mass from changes in lean mass. A reduction in body weight does not automatically mean that all of the weight lost came from fat.

What is lipolysis?

Lipolysis is the process of breaking stored triglycerides in fat cells into fatty acids and glycerol. The released fatty acids can then be transported to tissues and used for energy.

What happens during fasting?

Lower insulin levels and other hormonal changes during fasting can promote access to stored fat. As lipolysis increases, more fatty acids become available for energy production and ketone formation.

What is metabolic flexibility?

Metabolic flexibility is the body's ability to switch between different energy sources according to availability and physiological demands.

For example, after eating, the body may rely more heavily on glucose. During fasting, it can progressively increase its use of stored fat and ketones.

Why is metabolic flexibility relevant to fasting?

Fasting creates a natural transition between fed and fasted metabolism. This makes fasting an interesting model for understanding how effectively the body switches between fuels.

What are mitochondria?

Mitochondria are structures inside cells that play a central role in producing usable cellular energy. They are often described as the cell's "powerhouses," although their functions are considerably more complex than that phrase suggests.

Why are mitochondria relevant to fasting?

Fasting changes cellular energy availability and metabolic signaling. Research has investigated relationships between fasting, mitochondrial function, oxidative metabolism and cellular stress responses.

What is mTOR?

mTOR stands for mechanistic target of rapamycin. It is a major cellular signaling pathway involved in nutrient sensing, growth, protein synthesis and cellular metabolism.

What happens to mTOR during fasting?

Nutrient availability influences mTOR signaling. During periods of nutrient abundance, mTOR activity generally supports anabolic processes such as growth and protein synthesis.

During fasting, reduced nutrient signaling is associated with reduced mTOR activity, which is one reason mTOR is often discussed alongside autophagy.

What is NAD+?

NAD+, or nicotinamide adenine dinucleotide, is a molecule found in cells that plays an important role in energy metabolism and cellular reactions. It exists in different chemical forms and participates in processes involving the transfer of electrons.

Why is NAD+ relevant to fasting?

NAD+ is connected to cellular energy metabolism and enzymes involved in cellular regulation. Because fasting changes energy metabolism, researchers are interested in how nutrient availability may influence NAD+ metabolism.

What is a photoperiod?

A photoperiod is the amount of light and darkness an organism experiences during a 24-hour period. In nature, photoperiod changes with the seasons.

Why might photoperiod matter to fasting?

Light is one of the major signals that helps regulate circadian rhythms. Because circadian rhythms influence metabolism, sleep and hormone patterns, seasonal changes in daylight may indirectly influence eating behavior and metabolic timing.

That creates an interesting research chain: light, then circadian rhythm, then eating timing, then metabolism, then fasting.

What is resting metabolic rate?

Resting metabolic rate, or RMR, is the amount of energy the body uses while at rest to maintain essential physiological functions. These include:

• Breathing
• Circulation
• Temperature regulation
• Cellular activity
• Organ function

Why does RMR matter during fasting?

When people fast or lose weight, they often wonder whether their metabolism has slowed. Measuring RMR can provide more useful information than simply looking at body weight.

RMR and basal metabolic rate (BMR) are closely related but are measured under different conditions.

What is visceral fat?

Visceral fat is fat stored inside the abdominal cavity around internal organs. It is different from subcutaneous fat, which is stored underneath the skin.

Why does visceral fat matter?

Excess visceral fat is associated with metabolic dysfunction and increased risk of several chronic diseases.

Because fasting can alter energy balance, insulin signaling and fat metabolism, researchers have studied whether fasting and related dietary approaches can reduce visceral fat.

What is VO₂ max?

VO₂ max is the maximum rate at which the body can take in, transport and use oxygen during intense exercise. It is widely used as a measure of cardiorespiratory fitness.

Why does VO₂ max matter to fasting?

Weight and metabolic health are only part of the picture when assessing the effects of a long-term fasting experiment. VO₂ max provides a measure of physical fitness and aerobic capacity.

Tracking VO₂ max over time can therefore help investigate whether a fasting routine is associated with changes in physical performance.