Many people wonder whether fasting increases the number of calories their body burns. Whilst fasting influences how your body uses energy stores, it does not substantially increase calorie expenditure beyond normal resting levels. During fasting periods, your body continues burning calories at a rate determined primarily by your basal metabolic rate (BMR)—the energy required for essential functions such as breathing, circulation, and temperature regulation. Weight loss from fasting occurs mainly through consuming fewer calories overall, rather than through enhanced calorie burning. Understanding the metabolic changes during fasting helps clarify realistic expectations for this dietary approach.
Quick Answer: Fasting does not substantially increase calorie expenditure; your body continues burning calories at approximately your basal metabolic rate (typically 1,200–2,000 kcal daily), with weight loss occurring primarily through reduced calorie intake rather than enhanced calorie burning.
Fasting does not directly 'burn' additional calories in the way that physical activity does. Rather, it influences how your body utilises energy stores and maintains its basal metabolic rate (BMR). Your BMR represents the calories your body requires to sustain essential physiological functions such as breathing, circulation, cellular repair, and temperature regulation, typically accounting for 60–75% of total daily energy expenditure.
During fasting periods, your body continues to burn calories (kilocalories/kcal) at a rate determined primarily by your BMR, which varies according to factors including age, sex, body composition, and genetics. For most adults, BMR ranges from approximately 1,200 to 2,000 kcal per day, though this varies considerably with body size and composition. When you abstain from food, your body initially uses readily available glucose from the bloodstream and glycogen stores in the liver. Hepatic (liver) glycogen typically depletes over 12–24 hours, while muscle glycogen is largely preserved unless you're exercising.
Once liver glycogen reserves deplete, the body transitions to alternative fuel sources, predominantly fat stores, through a process called lipolysis. The thermic effect of food (TEF)—the energy required to digest and process nutrients—is absent during fasting, reducing total daily energy expenditure by approximately 10% compared to feeding days.
It is important to understand that fasting itself does not create a 'metabolic advantage' that substantially increases calorie expenditure beyond normal levels. The total calories burned during a fasting period remain largely consistent with what your body would burn at rest during fed states. Weight loss associated with fasting occurs primarily through calorie restriction—consuming fewer calories overall—rather than through enhanced calorie burning. Some research suggests short-term fasting may temporarily preserve or slightly increase metabolic rate through hormonal adaptations, but prolonged severe calorie restriction can conversely reduce BMR as the body adapts to conserve energy.
Multiple physiological and lifestyle factors determine how many calories your body expends during fasting periods. Body composition plays a crucial role: lean muscle tissue is metabolically active and burns more calories at rest than adipose (fat) tissue. Individuals with greater muscle mass therefore maintain higher calorie expenditure during fasting. Age also influences metabolic rate, with BMR typically declining by approximately 2–3% per decade after age 30, primarily due to gradual loss of muscle mass.
Physical activity levels significantly affect total daily energy expenditure during fasting. Even when not eating, movement—whether structured exercise or non-exercise activity thermogenesis (NEAT) such as walking, fidgeting, or household tasks—increases calorie burn substantially. However, some individuals experience reduced energy levels during fasting, potentially decreasing spontaneous physical activity and thereby lowering overall calorie expenditure.
Hormonal responses to fasting vary between individuals and influence metabolic rate. Short-term fasting (24-48 hours) triggers increased secretion of noradrenaline (norepinephrine), which may temporarily elevate metabolic rate. Growth hormone levels may also rise, helping preserve muscle mass. Conversely, extended fasting can reduce thyroid hormone production (particularly T3), which may slow metabolism. Metabolic flexibility—the body's ability to switch efficiently between glucose and fat as fuel sources—varies considerably between individuals and affects how readily someone adapts to fasting.
Genetic factors, sex (males typically have higher BMR than females due to greater muscle mass), and pre-existing medical conditions such as thyroid disorders or diabetes also substantially influence calorie expenditure during fasting periods.
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Start HereFasting initiates a coordinated series of metabolic adaptations that alter how your body generates and utilises energy. During the initial 12–24 hours of fasting, your body depletes hepatic glycogen stores whilst blood glucose levels gradually decline. Insulin secretion decreases markedly, whilst counter-regulatory hormones including glucagon, cortisol, and growth hormone increase to maintain blood glucose homeostasis through gluconeogenesis—the synthesis of glucose from non-carbohydrate sources such as amino acids and glycerol.
As fasting extends beyond 24 hours, the body increasingly relies on lipolysis and ketogenesis. Adipose tissue releases fatty acids into the bloodstream, which most tissues can oxidise directly for energy. The liver converts some fatty acids into ketone bodies (primarily beta-hydroxybutyrate and acetoacetate), which serve as alternative fuel sources, particularly for the brain. This metabolic state, termed nutritional ketosis, typically develops after 24–72 hours of complete fasting, depending on prior glycogen status and individual factors.
Protein metabolism also changes during fasting. Initially, the body may break down some muscle protein for gluconeogenesis, but adaptive mechanisms—including increased growth hormone and decreased insulin—help preserve lean tissue during prolonged fasting. The body becomes progressively more efficient at utilising fat stores whilst sparing protein.
These metabolic shifts represent evolutionary adaptations that enabled human survival during periods of food scarcity. However, the extent and speed of these changes vary considerably between individuals. People with insulin resistance or metabolic syndrome may experience different metabolic responses compared to metabolically healthy individuals. The thermic effect of food (TEF)—the energy required to digest, absorb, and process nutrients—is absent during fasting, which means total daily energy expenditure is slightly lower than on feeding days (by approximately 10% of energy intake).
When comparing calorie expenditure between fasting and regular eating patterns, the differences are more nuanced than commonly assumed. During normal eating, your body burns calories through three primary components: basal metabolic rate (BMR), physical activity, and the thermic effect of food (TEF). TEF accounts for approximately 10% of total energy expenditure and represents the calories burned during digestion and nutrient processing. This component is absent during fasting periods.
However, the total daily energy expenditure may not differ substantially between fasting and non-fasting days when calorie intake is matched. Research comparing intermittent fasting protocols (such as 16:8 time-restricted eating or alternate-day fasting) with continuous calorie restriction shows similar weight loss outcomes when total weekly calorie intake remains equivalent. This suggests that the primary mechanism of weight loss is the calorie deficit rather than unique metabolic effects of fasting itself.
Some studies indicate that short-term fasting (24–48 hours) may temporarily increase metabolic rate due to increased noradrenaline secretion, potentially offsetting the loss of TEF. This contrasts with prolonged severe calorie restriction, which typically reduces metabolic rate as an adaptive response. Meal frequency appears less important for metabolic rate than previously thought; whether you consume your daily calories in three meals or within a restricted eating window has minimal impact on total energy expenditure, provided total intake and macronutrient composition remain constant.
Intermittent fasting may offer practical advantages for some individuals in achieving calorie restriction, as limiting eating windows can naturally reduce overall food intake. However, current evidence does not show clear superiority of intermittent fasting over continuous energy restriction when energy intake is matched. Individual adherence and sustainability remain the most critical factors for long-term weight management success.
Whilst fasting can be safe for many healthy adults, it is not appropriate for everyone and carries potential risks that require careful consideration. Certain populations should avoid fasting or only undertake it under medical supervision, including:
Pregnant or breastfeeding women
Children and adolescents (under 18 years)
Individuals with a history of eating disorders
People with type 1 diabetes or those taking medications that affect blood glucose (particularly insulin or sulfonylureas)
Those with a history of hypoglycaemia
Individuals who are underweight (BMI <18.5 kg/m²)
People with chronic kidney disease or liver disease
Common adverse effects of fasting include hunger, irritability, difficulty concentrating, headaches, fatigue, and dizziness, particularly during initial adaptation periods. These symptoms typically resolve as the body adjusts, but persistent or severe symptoms warrant medical review. Dehydration is a significant risk, as people often obtain substantial fluid from food; adequate water intake is essential during fasting. The NHS recommends 6-8 cups or glasses of fluid daily (approximately 1.2-1.5 litres), with increased intake during hot weather or physical activity.
You should contact your GP or seek medical advice if you experience:
Severe dizziness, fainting, or persistent lightheadedness
Extreme weakness or inability to perform daily activities
Persistent nausea or vomiting
Signs of hypoglycaemia (confusion, sweating, trembling) if diabetic
Obsessive thoughts about food or weight
Amenorrhoea (loss of menstrual periods) in women
Call 999 or go to A&E immediately if you experience:
Chest pain or palpitations
Severe confusion
Fainting
Difficulty breathing
If you take regular medications, particularly for diabetes, hypertension, or cardiovascular conditions, consult your GP before starting any fasting regimen, as medication timing and dosing may require adjustment. NICE guidance emphasises individualised approaches to weight management, and fasting may not be suitable or necessary for everyone. A balanced, sustainable eating pattern that creates a modest calorie deficit, combined with regular physical activity, remains the evidence-based foundation for healthy weight management. Any dietary intervention should be discussed with healthcare professionals to ensure it aligns with your individual health status and medical needs.
If you think a medicine or medical device has caused a side effect, report it to the MHRA Yellow Card scheme (yellowcard.mhra.gov.uk or the Yellow Card app).
Short-term fasting (24–48 hours) may temporarily maintain or slightly increase metabolic rate through hormonal changes such as increased noradrenaline secretion. However, prolonged severe calorie restriction can reduce metabolic rate as the body adapts to conserve energy.
Your body typically begins transitioning to fat burning after 12–24 hours of fasting, once liver glycogen stores deplete. Nutritional ketosis, where the liver produces ketone bodies from fat, usually develops after 24–72 hours depending on individual factors and prior glycogen status.
Fasting is not appropriate for pregnant or breastfeeding women, children under 18, individuals with eating disorder history, people with type 1 diabetes or taking glucose-lowering medications, those who are underweight, or people with chronic kidney or liver disease. Always consult your GP before starting any fasting regimen if you have medical conditions or take regular medications.
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