Yes, humans burn calories naturally every day through a process called metabolism. Metabolism refers to all the chemical processes that occur in the body to keep us alive and functioning. These processes require energy, which is provided by the calories from the foods and drinks we consume. Even when a person is at rest, their body still needs energy for breathing, blood circulation, nutrient breakdown and assimilation, and hundreds of other bodily functions. The number of calories needed at rest is known as the basal metabolic rate (BMR).
How many calories do humans burn naturally?
The number of calories a person burns each day depends on several factors like age, sex, body size and composition. On average, the estimated daily calorie burn for adult men is between 2,000-3,000 calories per day, and for adult women is between 1,600-2,400 calories per day. This varies from person to person based on their unique attributes.
Here is a breakdown of estimated daily calorie burn for different groups of people:
| Group | Daily Calorie Burn |
|---|---|
| Sedentary adult men | 2,000-2,600 calories |
| Active adult men | 2,400-3,000 calories |
| Sedentary adult women | 1,600-2,000 calories |
| Active adult women | 2,000-2,400 calories |
As shown, active people who exercise regularly tend to burn more calories daily than sedentary people. Male adults generally burn more calories per day than female adults due to having greater amounts of muscle mass.
What factors influence calorie burn?
Some of the main factors that impact how many calories a person burns each day include:
- Body size and composition: People who have more body weight and muscle mass burn more calories because it takes more energy to sustain larger bodies and metabolically active muscle tissue.
- Age and sex: Younger adults and males typically burn more calories than older adults and females due to differences in hormonal regulation and muscle mass percentages.
- Physical activity level: The more active someone is, the more calories they burn through exercise and extra movement throughout the day.
- Diet induced thermogenesis: Digesting, absorbing and metabolizing foods increases calorie burn, with protein causing the largest rise followed by carbohydrates and fats.
- Genetics: Some people inherit a faster or more efficient metabolism that can burn more calories naturally.
- Environmental temperature: Colder environments lead to increased calorie burn from shivering and heat production.
How do BMR and TEF contribute to calorie burn?
Two major components that factor into daily calorie expenditure are basal metabolic rate (BMR) and thermic effect of food (TEF).
Basal metabolic rate refers to the calories burned at rest by vital organ function and tissue maintenance each day. It accounts for 60-70% of total daily calorie expenditure. BMR is influenced by muscle mass, sex, age, and genetics.
Thermic effect of food or diet induced thermogenesis is the energy cost of digesting, absorbing and processing the nutrients in the food we eat. It makes up about 10% of total calorie burn. TEF is influenced by the size of a meal and the macronutrient composition, with protein causing the greatest rise in TEF.
Together, BMR and TEF account for 70-80% of the total calories we expend each day. The remaining 20-30% comes from physical activity and movement. Understanding how BMR and TEF contribute to energy expenditure can help people make dietary choices that support their natural calorie burn.
How to determine your BMR?
Basal metabolic rate can be estimated using equations that take into account individual factors like age, sex and body composition. Some examples of BMR equations include:
- Mifflin St Jeor Equation: For men: BMR = 10W + 6.25H – 5A + 5. For women: BMR = 10W + 6.25H – 5A – 161
- Harris Benedict Equation: For men: BMR = 66 + (13.75 x W) + (5 x H) – (6.8 x A). For women: BMR = 655 + (9.6 x W) + (1.8 x H) – (4.7 x A)
Where W is body weight in kg, H is height in cm, and A is age. These equations provide an estimated basal metabolic rate within about 5-10% accuracy for most people.
There are also online BMR calculators that simplify the process and calculate BMR based on inputs for age, sex, weight and height. Getting assessed at a doctor’s office or dietitian’s clinic can also provide a highly accurate BMR measurement using indirect calorimetry testing.
How can you boost your metabolism and calorie burn?
While a person’s basal metabolic rate and total daily energy expenditure are largely influenced by fixed factors like genetics, there are some strategies that can help give your metabolism a boost:
- Build muscle through strength training – muscle tissue burns more calories than fat
- Do HIIT workouts – intense bursts of exercise spikes calorie burn
- Drink green tea – compounds like EGCG may slightly speed metabolism
- Eat spicy foods – capsaicin can temporarily increase calorie burn
- Get enough sleep – lack of sleep may slow metabolism
- Stay hydrated – dehydration hampers metabolic function
While these strategies may provide a small uptick in energy expenditure, the overall calorie burn effects are modest for most people. More significant metabolism changes come from loss of body fat and gain of lean mass over time.
Does metabolism slow down with age?
It is well documented that metabolism does slow down naturally as we age. Total daily energy expenditure decreases by about 1-2% per decade after age 20. The exact reasons are still unclear, but contributing factors likely include:
- Loss of muscle mass and strength (sarcopenia)
- Decreased physical activity
- Changes in hormone levels
- Reduced rate of cellular turnover
- Drop in Na/K-ATPase activity needed for thermogenesis
The age-related decline in metabolism helps explain why maintaining a healthy body weight often becomes more difficult later in life. To counteract the slowdown, older adults can build muscle through exercise, eat adequate protein, stay active, and minimize muscle loss through injury or illness.
Do metabolic disorders affect calorie burn?
Yes, certain metabolic disorders can directly impact the body’s ability to burn calories effectively. Two common metabolic conditions associated with reduced metabolic rates and energy expenditure are hypothyroidism and Cushing’s syndrome.
Hypothyroidism – Occurs when the thyroid gland does not produce enough thyroid hormone. Since thyroid hormone regulates metabolism, reduced levels slow down metabolic processes and calorie burning.
Cushing’s syndrome – Caused by high levels of cortisol from the adrenal glands. Excess cortisol promotes protein breakdown, increased fat storage, and insulin resistance, which diminishes calorie expenditure.
Treating the underlying metabolic disorder can help restore normal metabolic function and energy expenditure. Medications, supplements, diet, or surgery may be part of treatment depending on the cause.
Can certain medications affect metabolism?
Yes, some types of medications can impact resting metabolic rate and daily calorie burn. Examples include:
- Steroids – May increase lean tissue and metabolism initially, but long-term use suppresses metabolism.
- Antihistamines – Older generation antihistamines like diphenhydramine are linked to decreased metabolic rate.
- Beta blockers – Used for heart conditions. Can reduce metabolic rate and lipolysis (fat breakdown).
- Antipsychotics – Used for mental illnesses. Associated with metabolic dysregulation and weight gain.
- Antidepressants – Variable effects, but some increase weight and lower RMR.
The impacts can vary widely based on the individual, drug class, and dosage. If a medication seems to be slowing metabolism, dosage adjustment or alternate therapy may be considered in consultation with a doctor.
Can digestive issues slow metabolism?
Gastrointestinal conditions that impair digestion and nutrient absorption can lower the thermic effect of food (TEF), reducing overall calorie expenditure. Examples include:
- Celiac disease – Damages small intestine and prevents nutrient absorption.
- Crohn’s disease – Chronic inflammation in the digestive tract.
- Gastritis – Inflammation of the stomach lining.
- Gastroparesis – Delayed emptying of the stomach.
These conditions allow fewer nutrients to be used for energy production and decrease the energy spent digesting food. Treatment of GI issues through diet changes, supplements, or medication can help restore normal digestive function and metabolic rate.
Can calorie intake affect metabolism?
Yes, the amount of calories consumed can impact resting and total daily energy expenditure to some degree. Very low calorie diets substantially lower metabolism, whereas overfeeding increases metabolism:
- Calorie restriction – Cutting calories below 20% of maintenance slows metabolic adaptation.
- Starvation – Severe calorie restriction causes up to 40% drop in metabolic rate.
- Overfeeding – Consuming excess calories requires energy to process and increases TEF.
However, these effects tend to be temporary. Metabolic rate eventually normalizes when previous calorie levels are resumed. Smaller calorie deficits or surpluses have minor metabolic impacts for most people.
What is the starvation response?
The starvation response, also called metabolic adaptation, is a defense mechanism to conserve energy in response to prolonged, very low calorie intake. It involves the following metabolic changes:
- Lowered BMR and reduced TEF.
- Shift to using stored fat for fuel through lipolysis.
- Production of glucose through gluconeogenesis.
- Decreased leptin levels and increased ghrelin levels.
- Suppressed thyroid hormone production.
This coordinated physiological response maximizes the use of stored and dietary calories. It helped humans survive periodic famines throughout history. During short term dieting, the starvation response is modest. Severe calorie deficits for extended periods can trigger a substantial drop in metabolism.
Does exercise boost metabolism long-term?
Exercise causes an acute spike in calorie burn during and immediately after working out. It also promotes the building of calorie-burning muscle mass with strength training. However, the long-term metabolic boost from exercise is more modest than commonly believed:
- Aerobic exercise increases BMR for 1-72 hours after exercise.
- Adding muscle through strength training increases BMR more substantially.
- HIIT workouts may cause greater metabolic perturbation.
- Metabolic adaptations restore baseline metabolic rate between exercise bouts.
While exercise has immense health and fitness benefits, it should not be viewed as a massive long-term calorie burner for weight loss or maintenance. Healthy eating habits remain pivotal for managing energy balance.
Can specific diets boost metabolism?
Some diets and eating patterns claim to boost metabolism, but the effects tend to be mild or transient. For example:
- Ketogenic diets – Raise BMR slightly due to gluconeogenesis.
- Intermittent fasting – No effect on total 24-hour energy expenditure.
- Low carb diets – Increase resting metabolism only briefly.
- High protein diets – Higher TEF, but no major BMR effect.
For most people, sticking to a calorie deficit consistently remains far more important for weight management than banking on small metabolic advantages from different diets.
Do thermogenic supplements work?
Thermogenic supplements claim to increase calorie burning by revving up metabolism. Some popular options include:
- Caffeine
- Green tea extract
- Capsaicin and chili peppers
- Protein powders
- Ephedra
Research shows that several of these can temporarily raise resting energy expenditure slightly. However, the effects are often fleeting as the body adapts. More research is needed on long-term effectiveness and safety.
Can calorie burn differ between individuals?
Yes, the rate at which calories are burned at rest and during activity can vary substantially from person to person. Some key factors behind these individual differences include:
- Body size and composition – Muscle burns more calories than fat.
- Genetics – Some inherit faster/more efficient metabolisms.
- Age and sex – Metabolic slowdown with age, females burn less.
- Hormones – Thyroid hormones regulate metabolic rate.
- Environment – Colder climates require more energy for heat.
Two people of the same age/sex/weight can still have very different metabolic rates and total energy expenditures. Tracking individual calorie needs through metabolic testing or careful calorie titration helps determine personalized energy balance.
Do calories burned decrease with weight loss?
Typically, yes – the number of calories a person burns at rest and during activity decreases along with body weight. This occurs partly due to having less metabolically active tissue after losing weight. Changes include:
- Lower BMR from reduced body mass, especially muscle.
- Reduced TEF due to eating less food.
- Decreased calories burned through exercise.
- Possible adaptive thermogenesis (metabolic slowdown).
This metabolic “slowdown” is not due to damage or changes in metabolism itself. Smaller bodies simply need less energy, so weight loss without adjusting food intake leads to burning fewer calories.
Should calorie intake decrease with weight loss?
In most cases, yes – calorie intake needs to be reduced as body weight decreases in order to keep losing weight or maintain the lower weight. As the body shrinks, it burns fewer calories at rest and during activity. Failing to adjust calorie intake accordingly can stall weight loss and lead to weight regain.
A common guideline is to reduce calorie intake by 10% after losing 10-20 lbs. Tracking weight and adjusting portions and calories as needed is key. Some eventually reach an energy equilibrium where calorie needs stabilize at the new lower body weight.
Conclusion
Humans do burn calories naturally each day through internal functions like breathing, blood circulation, nutrient processing, and tissue renewal and repair. The number of calories burned at rest and during activity is determined by a complex interplay of factors like age, sex, body size, muscle mass, genetics, and health status. While basal metabolic rate and total energy expenditure decrease with age and weight loss, there are habits like eating whole foods, staying active, and maintaining muscle mass that can support calorie burning.
