Understanding Why People Need Different Amounts of Energy

December 28, 2025 •

4 min read

Your basal metabolic rate (BMR), the minimum number of calories your body needs to function at rest, accounts for 60% to 70% of the total energy you use daily. This inherent physiological difference is the primary reason why people need different amounts of energy, even when performing similar activities.

Quick Summary

Daily energy requirements vary widely based on individual metabolism, physical activity level, and body composition. Influential factors include age, sex, genetics, and overall health status, all contributing to different caloric needs among individuals.

Key Points

BMR Dominates: Your Basal Metabolic Rate accounts for most of your daily energy expenditure, covering essential functions like breathing and circulation.
Activity Varies Most: Physical activity is the most variable component of your total energy needs, ranging from a small to a large percentage depending on your lifestyle.
Age and Sex Influence: Males generally have higher energy needs than females, and energy requirements decrease with age due to changes in body composition.
Muscle Mass Matters: Individuals with higher lean muscle mass have a higher BMR, as muscle tissue is more metabolically active than fat tissue.
Health and Environment Play a Role: Illness, pregnancy, lactation, and even temperature can temporarily or long-term affect your body's energy demands.
Genetics Pre-determines a Baseline: While lifestyle is key, your genetic makeup also contributes to your baseline metabolic rate.

The Foundations of Individual Energy Needs

At the core of an individual's energy requirement lies their total daily energy expenditure (TEE), which is comprised of three main components: the basal metabolic rate (BMR), the thermic effect of food (TEF), and physical activity level (PAL). Each component is influenced by a unique set of factors, creating a complex and personalized energy equation.

Basal Metabolic Rate (BMR)

Your BMR is the energy your body expends on vital functions at complete rest, including breathing, circulation, and cell production. It's the most significant portion of your TEE and is influenced by several inherent traits:

Body Size and Composition: Larger individuals with greater body surface area generally have a higher BMR because they have more tissue to sustain. Additionally, muscle tissue requires more energy to maintain than fat tissue. This is a key reason why two people of the same weight can have different BMRs if one has a higher percentage of lean muscle mass.
Sex: Males typically have a higher BMR than females, largely due to a generally higher proportion of muscle mass and a larger body size. Hormonal differences, such as higher testosterone levels, also contribute.
Age: Your BMR naturally decreases with age, primarily because of a gradual loss of muscle mass. Hormonal and neurological changes further influence this decline. Children and teenagers, by contrast, have a high BMR to fuel rapid growth and development.
Genetics: An individual’s genetic makeup plays a significant role in determining their BMR. While statistics show that most people's metabolic rates fall within a certain range, some individuals are genetically predisposed to having a faster or slower metabolism.

Physical Activity Level (PAL)

Beyond the energy needed for basic survival, physical activity is the most variable component of daily energy expenditure. It includes everything from planned exercise to everyday movements like walking, fidgeting, and maintaining posture. For sedentary individuals, physical activity may account for as little as 15% of their total energy needs, while for very active individuals, it can be as much as 50%. The intensity, duration, and type of activity all dictate the amount of energy expended. For instance, high-intensity interval training burns more calories in a shorter period than a long, low-intensity walk. Furthermore, a person's fitness level can also influence their movement economy; a highly trained individual might burn fewer calories for the same task than an untrained person.

The Thermic Effect of Food (TEF)

The TEF is the energy required to digest, absorb, and metabolize the food you eat. It accounts for approximately 10% of total energy expenditure and varies based on the meal's composition. For example, metabolizing protein requires more energy than metabolizing carbohydrates or fats. This is a minor, but consistent, factor in the overall energy balance.

Other Influential Factors

Several other temporary or long-term conditions can also affect an individual's energy needs:

Pregnancy and Lactation: A pregnant or lactating woman's body requires significantly more energy to support the growth of the fetus or to produce milk. This results in a higher BMR and total energy expenditure.
Illness and Injury: The body's energy demands increase dramatically during illness or injury to fight infection and repair damaged tissues. Fever, for example, can notably increase metabolic rate.
Environmental Temperature: Both very cold and very hot temperatures increase the body's energy expenditure as it works harder to maintain a stable core temperature through shivering or sweating.
Hormonal Imbalances: Conditions affecting hormones, such as hyperthyroidism (overactive thyroid) or hypothyroidism (underactive thyroid), can cause a significant increase or decrease in metabolic rate, respectively.

Comparison of Energy Needs by Lifestyle

Different combinations of these factors can lead to a wide variance in daily caloric requirements. This table illustrates how factors like activity level and sex create distinct energy profiles. These are general estimates and actual individual needs can vary.


Activity Level	Sedentary Female	Active Female	Sedentary Male	Active Male
Energy Need (Approx. Kcal/Day)	1,800 - 2,000	2,400	2,000 - 2,600	2,800 - 3,200
Key Factors	Lower BMR due to smaller size, lower muscle mass, and minimal physical activity.	Higher physical activity expenditure, potentially higher muscle mass, increasing BMR.	Higher BMR due to larger body size and higher muscle mass, minimal physical activity.	Highest energy expenditure due to a combination of higher BMR and significant physical activity.

Conclusion

In conclusion, the question of why people need different amounts of energy is answered by the interplay of several factors, including an individual's BMR, PAL, and TEF. The most significant drivers are body composition, age, and sex, which are largely determined by genetics and life stage. While the variations in BMR between individuals of similar size and age are often smaller than people assume, the cumulative effect of physical activity over the course of a day can account for very large differences in total energy expenditure. Understanding these principles allows for a more personalized approach to nutrition and health. Rather than comparing your caloric intake to someone else's, it's more effective to focus on your own unique needs based on your body and lifestyle to maintain a healthy energy balance. For more detailed information on energy requirements, the National Institutes of Health provides extensive resources on dietary reference intakes.

Frequently Asked Questions

Yes, metabolism does slow down with age, but contrary to popular belief, the decline isn't as drastic as often assumed during middle adulthood. The primary cause is typically a loss of lean muscle mass, not a major slowdown of the metabolic rate itself. Changes in lifestyle and activity levels are often more significant contributors to weight gain with age.

Men typically require more calories than women because they generally have a larger body size and a higher percentage of lean muscle mass. Since muscle tissue is more metabolically active than fat, it requires more energy to sustain, leading to a higher basal metabolic rate.

Physical activity is the most variable factor in your total energy expenditure. For a sedentary person, it might account for as little as 15% of energy needs, whereas for a very active person, it can account for up to 50%. Increasing your activity level is one of the most direct ways to increase your daily energy expenditure.

The thermic effect of food (TEF) is the energy your body uses to digest, absorb, and process the nutrients from your meals. It accounts for about 10% of your total energy expenditure and is slightly higher for protein-rich meals compared to meals high in carbohydrates or fat.

While genetics do influence your metabolic rate, the variation between most individuals is not drastically large—most fall within a 200-300 kcal daily range of each other. Statistically, extreme genetic variance is rare, and lifestyle factors like diet and exercise are more significant drivers of overall energy balance.

Health conditions can significantly impact energy needs. An underactive thyroid (hypothyroidism) can slow metabolism, leading to lower energy requirements and potential weight gain. Conversely, an overactive thyroid (hyperthyroidism) can speed up metabolism, increasing energy needs and causing weight loss.

Yes, pregnancy and lactation substantially increase a woman's energy requirements. The body needs additional calories to support the growth of the fetus during pregnancy and for milk production during breastfeeding. The extra energy demand can be anywhere from 500 to over 800 kcal per day.