Understanding Metabolism: How many calories does my body burn naturally?

October 30, 2025 •

4 min read

Up to 75% of your daily calorie burn comes from just existing, performing basic functions like breathing and circulating blood. So, how many calories does my body burn naturally? The answer is unique to every individual and depends on multiple physiological factors.

Quick Summary

The number of calories your body burns naturally is determined by your basal metabolic rate (BMR). This is influenced by age, sex, weight, and muscle mass, and can be estimated using calculation formulas. Non-exercise activities and food digestion also contribute significantly to your daily energy expenditure.

Key Points

Basal Metabolic Rate (BMR): BMR represents the majority of your daily calorie expenditure, covering essential bodily functions at rest.
BMR vs. RMR: BMR measures energy at strict rest, while RMR is slightly higher and accounts for low-effort daily activities.
Influencing Factors: Your natural calorie burn is influenced by age, sex, body size, muscle mass, and genetics.
Calculation Tools: Formulas like Mifflin-St Jeor can help estimate your BMR, which can then be used to determine your total daily energy needs based on activity levels.
Boost Calorie Burn: Strategies like strength training to increase muscle mass, boosting NEAT (Non-Exercise Activity Thermogenesis), and eating more protein can help increase your metabolism.

The Science of Your Natural Calorie Burn

Your body is constantly burning calories, even when at rest. This fundamental process is known as your metabolism. The total number of calories you burn daily, or your total daily energy expenditure (TDEE), is composed of three main components:

Basal Metabolic Rate (BMR): The energy your body uses to perform basic, life-sustaining functions like breathing, circulating blood, and regulating body temperature. For most people, BMR accounts for 60-75% of total calorie burn.
Thermic Effect of Food (TEF): The calories burned while your body digests, absorbs, transports, and stores the food you eat. TEF typically represents about 10% of your total daily calorie expenditure.
Activity Energy Expenditure (AEE): The energy burned during physical activity. This includes both planned exercise and non-exercise activity thermogenesis (NEAT), which covers all other daily movement, from walking to fidgeting.

BMR vs. RMR

While the terms Basal Metabolic Rate (BMR) and Resting Metabolic Rate (RMR) are often used interchangeably, there's a subtle distinction. BMR is the lowest rate of energy expenditure measured under strict, controlled conditions (e.g., after fasting and waking from sleep). RMR, while similar, is the energy expended while at rest but under less restrictive conditions and is often about 10% higher than BMR. For practical purposes outside of a lab, the Mifflin-St Jeor equation is used to estimate RMR accurately.

Key Factors Influencing Your Metabolism

Many variables beyond your control contribute to your natural calorie burn. Understanding these can help you better manage your nutritional needs and fitness goals.

Age: Metabolism tends to decrease with age, primarily due to a gradual loss of muscle mass. A significant study found that metabolism remains relatively stable until around age 60, after which it declines by about 0.7% annually.
Sex: Men generally have a higher BMR than women due to having more lean muscle mass and higher testosterone levels on average.
Body Composition: Muscle tissue is more metabolically active than fat tissue, meaning it burns more calories at rest. The more lean muscle mass you have, the higher your BMR.
Body Size: Larger individuals require more energy to perform basic bodily functions, thus they have a higher BMR than smaller individuals.
Genetics: Your genetic makeup influences your natural metabolic rate and how your body responds to diet and exercise. Some people are genetically predisposed to a faster or slower metabolism.

Calculating Your Estimated BMR

To get a clearer picture of your natural calorie burn, you can use a formula to estimate your BMR. The Mifflin-St Jeor equation is widely considered one of the most accurate estimation tools.

Mifflin-St Jeor Equation

Men: BMR = (10 x weight in kg) + (6.25 x height in cm) – (5 x age in years) + 5
Women: BMR = (10 x weight in kg) + (6.25 x height in cm) – (5 x age in years) – 161

Once you have your BMR, you can calculate your TDEE by multiplying it by an activity factor based on your lifestyle:

Sedentary (little or no exercise): BMR x 1.2
Lightly Active (light exercise 1–3 days/week): BMR x 1.375
Moderately Active (moderate exercise 3–5 days/week): BMR x 1.55
Very Active (hard exercise 6–7 days/week): BMR x 1.725
Extra Active (very hard exercise/physical job): BMR x 1.9

Strategies to Increase Your Natural Calorie Burn

While you can't drastically alter your baseline metabolism, you can influence the other components of your daily energy expenditure. Here are some strategies:

Build Lean Muscle Mass: As muscle burns more calories at rest than fat, incorporating strength training into your routine is an effective way to boost your BMR. Resistance training exercises like lifting weights, using resistance bands, or bodyweight exercises can help increase muscle mass over time.
Increase Non-Exercise Activity Thermogenesis (NEAT): Fidgeting, standing more, taking the stairs, and walking around while on the phone all contribute to NEAT. Simple daily movements can significantly increase your total calorie burn without structured exercise.
Optimize the Thermic Effect of Food (TEF): Not all foods require the same amount of energy to digest. Protein has the highest thermic effect, meaning your body burns more calories processing it compared to carbohydrates or fats. A diet rich in lean proteins can help slightly boost your overall calorie expenditure..
Stay Hydrated: Drinking plenty of water is essential for optimal metabolic function. Some research suggests that drinking cold water may cause your body to expend a small amount of energy to heat it up, further contributing to your calorie burn.
Get Enough Sleep: A lack of sleep can negatively affect your metabolism and hormonal balance. Insufficient sleep is associated with a lower metabolic rate, as your body tries to conserve energy. Prioritizing adequate rest is crucial for metabolic health.

BMR Calculation Formula Comparison


Feature	Mifflin-St Jeor Equation	Harris-Benedict Equation (Revised)
Accuracy	Generally considered more accurate for estimating RMR in both normal weight and obese individuals.	Was the standard for many years, but the revised version is less accurate than Mifflin-St Jeor.
Development Year	1990	1919, with revisions in 1984
Formula for Men	BMR = (10 x W) + (6.25 x H) – (5 x A) + 5	BMR = 88.362 + (13.397 x W) + (4.799 x H) – (5.677 x A)
Formula for Women	BMR = (10 x W) + (6.25 x H) – (5 x A) – 161	BMR = 447.593 + (9.247 x W) + (3.098 x H) – (4.330 x A)

Note: W = weight in kilograms, H = height in centimeters, A = age in years.

Conclusion

While a significant portion of your daily calorie burn is naturally determined by your basal metabolic rate, it is not an entirely fixed number. Factors like age, sex, and genetics play a role, but your lifestyle has a powerful influence on your overall metabolic health. By incorporating strength training to build muscle, increasing everyday non-exercise activity, and making informed nutritional choices, you can effectively boost your natural calorie burn. Understanding your BMR is the first step toward tailoring a nutrition and fitness plan that aligns with your body's specific energy needs for effective weight management.

For more information on adding movement into your daily routine, you can visit the MedlinePlus guide to burning more calories.

Frequently Asked Questions

The primary factor is your Basal Metabolic Rate (BMR), which is the number of calories your body requires to perform basic life-sustaining functions while at complete rest.

Yes, metabolism typically slows down with age. This is largely due to a natural loss of lean muscle mass and hormonal changes, although research suggests the decline is more gradual than often believed, accelerating after age 60.

While your genetic BMR is hard to change significantly, you can increase your overall calorie burn. Strategies include building muscle mass through strength training, increasing your NEAT (everyday movement), and eating enough protein.

Muscle tissue is more metabolically active than fat tissue. This means it burns more calories even when your body is at rest. The more lean muscle mass you have, the higher your basal metabolic rate.

NEAT stands for Non-Exercise Activity Thermogenesis. It includes all the energy you expend on daily movements that are not planned exercise, like standing, walking, and fidgeting. NEAT can account for a large portion of your daily calorie burn and can be a powerful tool for weight management.

Severely restricting calories can slow your metabolism as your body enters 'starvation mode' to conserve energy. This can lead to fatigue, muscle loss, and make long-term weight loss more difficult. A small, sustainable calorie deficit is healthier and more effective.

Yes, this is known as the Thermic Effect of Food (TEF). Protein has the highest thermic effect, meaning your body uses more energy to digest it compared to carbohydrates or fats. Diets rich in lean protein can modestly increase your calorie burn.