2 Increase Your BMR
Build What Burns at Rest
TL;DR
Basal metabolic rate (BMR) forms the foundation of your daily energy expenditure. Age, genetics, medical conditions, body size, and body composition all influence it. Lean muscle contributes to resting energy expenditure, but adding muscle does not dramatically “rev up” metabolism. The increase in resting calorie expenditure is real, but modest.
The larger reason to preserve and build lean muscle is its role in strength, function, glucose disposal, body composition, resilience, and independence. This week, you begin protecting that tissue through resistance-based movement and adequate protein intake, building durable capacity rather than chasing a metabolic shortcut.
Introduction
In the previous chapter, you made movement visible. You learned that active calories are the most responsive part of your metabolism—the portion you can influence hour by hour through simple choices. But movement is only one part of the energy equation. Beneath every step you take and every workout you complete lies a quieter, constant force: your basal metabolic rate.
Your Basal Metabolic Rate (BMR) represents the energy your body burns simply to stay alive. Even at rest—while sleeping, reading, or sitting quietly—your body is working continuously: breathing, circulating blood, maintaining cellular integrity and ion gradients, repairing tissue, regulating temperature, and sustaining organ function. This background demand does not fluctuate dramatically from moment to moment, yet it accounts for the largest share of your total daily energy expenditure.
Unlike active calories, BMR is not immediately adjustable. It shifts gradually over time, influenced by age, genetics, medical conditions, body size, and body composition. Lean muscle contributes to resting energy expenditure, but its effect should be kept in perspective. Building muscle will not transform resting metabolism overnight. Its greater value lies in the many other benefits that come with preserving and developing metabolically and functionally important tissue.
This chapter is therefore not about trying to “hack” your metabolism. It is about understanding the foundation beneath daily movement and beginning to protect the tissue—especially skeletal muscle—that later strength chapters will develop much more fully. If tracking active calories restored your sense of agency in motion, this chapter extends that awareness into stillness and helps you understand the metabolic structure supporting long-term health and function.
In This Chapter
Basal Metabolism: The Foundation Beneath Movement
Basal metabolic rate represents the energy your body expends at rest to sustain life. It includes the continuous work of breathing, circulating blood, maintaining ion gradients, repairing tissue, regulating hormones, and supporting organ function. Even when you appear still, your physiology is active.
For most individuals, BMR accounts for the largest share of total daily energy expenditure. Several factors influence it:
- Age: BMR generally declines gradually over time.
- Sex: Men generally have a higher BMR, largely because they tend to have greater lean mass.
- Height and body size: Larger individuals typically have higher absolute metabolic rates.
- Genetics: Inherited traits influence baseline metabolic rate and are not directly modifiable.
Medical conditions can also influence metabolic rate. Hyperthyroidism, severe burns, and some other physiologic stress states may increase resting energy expenditure, while hypothyroidism and certain chronic illnesses may reduce it.
While many of these determinants are not easily changed, one stands apart as meaningfully modifiable: body composition. Different tissues have different metabolic demands. Muscle tissue has a higher metabolic rate than fat tissue by approximately 8.5 kcal/kg/day (13 − 4.5). Because muscle is more metabolically active than fat tissue, greater lean mass can modestly increase resting energy needs, although total calorie requirements are influenced much more broadly by body size, activity level, and overall energy expenditure.
Although organs such as the liver, heart, brain, and kidneys are highly metabolically active, their total mass is relatively small. Skeletal muscle, by contrast, represents a large and modifiable reservoir of metabolically active tissue. Muscle tissue uses more energy at rest than fat tissue, but the absolute difference is modest. This is an important distinction: adding muscle can help preserve or modestly increase resting energy expenditure, but the larger health return comes from what muscle allows you to do and how it supports strength, glucose handling, function, and resilience.
| Organ / tissue | Approx. mass (kg) | Approx. kcal/kg/day | Approx. resting kcal/day |
|---|---|---|---|
| Skeletal muscle | 28.0 | 13 | 364 |
| Liver | 1.8 | 200 | 360 |
| Brain | 1.4 | 240 | 336 |
| Heart | 0.33 | 440 | 145 |
| Kidneys | 0.31 | 440 | 136 |
| Adipose tissue | 15.0 | 4.5 | 68 |
| Residual tissues* | 23.16 | 12 | 278 |
Illustrative values based on a 70-kg reference adult. Estimated resting contribution is calculated as tissue mass × resting metabolic rate. Individual organ and tissue masses vary substantially by body size, sex, age, and body composition. *Residual tissues include bone, blood, skin, connective tissue, gastrointestinal tract, lungs, and other smaller tissues.6
The table helps put the physiology into perspective. In this representative example, roughly 28 kilograms of skeletal muscle contributes about 364 resting calories per day, while a liver weighing only about 1.8 kilograms contributes roughly 360. The brain, heart, and kidneys are similarly expensive tissues for their size. Muscle matters because it is abundant and modifiable, not because it has an exceptionally high metabolic rate per kilogram.
For that reason, improving body composition should not be framed as a way to dramatically increase calorie burn at rest. The stronger argument is that maintaining or building lean mass helps preserve resting metabolic capacity while improving physical strength, stability, glucose disposal, mobility, and long-term functional independence. Muscle is not merely aesthetic tissue. It is metabolic and functional infrastructure.
Estimating, Tracking, and Contextualizing Your BMR
Accurately measuring basal metabolic rate requires calorimetry. Direct calorimetry measures heat production in a controlled chamber and is highly precise, but rarely practical outside research environments. Indirect calorimetry estimates energy expenditure by measuring oxygen consumption and carbon dioxide production. It is more accessible in clinical settings and is often paired with assessment of VO₂ max, an important marker of cardiorespiratory fitness and longevity.
A true BMR assessment requires strict conditions: an 8–12 hour fast, at least 30 minutes of rest, a thermoneutral environment, an empty bladder, and specialized equipment. For most individuals, this level of precision is unnecessary. What matters more is understanding relative magnitude and tracking trends over time.
Instead of direct measurement, BMR is typically estimated using validated equations based on age, height, weight, and sex. Commonly used formulas include the Mifflin–St Jeor equation, the revised Harris–Benedict equation, and the Katch–McArdle formula, which incorporates lean body mass.
Because this formula accounts for lean mass rather than total body weight, it may provide a more individualized estimate when accurate body-composition data are available. As discussed earlier, not all body weight is metabolically equivalent; lean tissue contributes more to resting energy expenditure than fat tissue. For that reason, formulas incorporating lean mass may be particularly useful when body composition has been measured reliably, such as with DEXA. Even so, all such formulas remain estimates, and small differences between them are usually less important than consistent use and trend awareness.
To illustrate how these formulas compare, I calculated my own estimated BMR using each method:
- Mifflin–St Jeor: 1,643 calories per day
- Revised Harris–Benedict: 1,697 calories per day
- Katch–McArdle (DEXA lean mass 62.5 kg): 1,720 calories per day
Despite using different equations and inputs, the results fall within a relatively narrow range. This illustrates an important point: absolute precision is less important than consistency and trend awareness. Most individuals will find that their estimated BMR values cluster similarly across methods.
Wearable devices typically report Resting Energy Expenditure (REE) rather than true BMR. REE includes minimal postural activity and sometimes an estimated thermic effect of food. In general, BMR is approximately 90% of REE. Although wearable-derived values are estimates, they can be useful for observing trends over months and years. For example, using my Apple Watch REE averaged over one year (1,888 calories per day), multiplying by 0.90 yields an estimated BMR of approximately 1,699 calories per day—again closely aligned with the calculated formulas.
The BMR: Active Calories Ratio
The relationship between BMR and active calories depends largely on activity level. As physical activity increases, active calories make up a larger share of total daily energy expenditure. This does not make BMR and active calories interchangeable; it simply illustrates how strongly movement can alter total daily energy output even when resting metabolism changes slowly.
| Activity level | BMR : Active Calories | Relative comparison |
|---|---|---|
| Sedentary (no regular exercise) | 1 : 0.2–0.3 | BMR is roughly 3–5× greater |
| Lightly active (exercise 1–3 days/week) | 1 : 0.4 | BMR is roughly 2.5× greater |
| Moderately active (exercise 3–5 days/week) | 1 : 0.5–0.6 | BMR is roughly 1.7–2× greater |
| Very active (exercise 6–7 days/week) | 1 : 0.7–0.8 | BMR is roughly 1.25–1.4× greater |
| Extreme activity | 1 : 0.9–1.0 | Active Calories approach BMR |
As activity level increases, active calorie expenditure can approach basal metabolic rate. BMR changes gradually, while daily movement can shift total energy expenditure substantially from one day to the next.
Active Calories fluctuate from day to day, while BMR shifts gradually over time. Together, they account for most of total daily energy expenditure, but they are not the whole equation. The thermic effect of food contributes an additional, smaller component. Movement is responsive and behavior-driven; muscle is structural and changes more slowly. Understanding these major components allows you to interpret changes in weight, performance, and energy with greater clarity. The goal is not perfect measurement, but informed awareness. When you understand how the major components interact, energy balance becomes less mysterious and more manageable.
The Thermic Effect of Food: A Supporting Player
The Thermic Effect of Food (TEF) refers to the energy your body expends to digest, absorb, transport, and store nutrients after a meal. While often overlooked, TEF represents a measurable component of total daily energy expenditure, typically accounting for approximately 5–15%.
Unlike BMR, which reflects energy required for survival at rest, and active calories, which reflect movement, TEF reflects the metabolic cost of fueling. Importantly, TEF varies by macronutrient composition, with protein requiring the greatest thermic expenditure and fat the least.
| Nutrient | Approx. thermic effect | Calories per gram |
|---|---|---|
| Protein | 20–30% | 4.1 |
| Carbohydrates | 5–10% | 4.1 |
| Fat | 0–3% | 9.9 |
| Alcohol | 15–20% | 7.1 |
Approximate thermic effect of macronutrients expressed as the percentage of caloric content expended during digestion and metabolism. Protein has the highest thermic effect, while fat has the lowest.
Protein has the highest thermic effect, meaning a substantial portion of its calories are expended during digestion and metabolic processing. In addition to this thermic advantage, protein is more satiating than carbohydrates or fats when total calories are equal.1,2 Carbohydrates occupy a middle range; simple sugars are digested rapidly and tend toward the lower end of the thermic spectrum, whereas complex carbohydrates—such as whole grains and fibrous vegetables—require more processing and trend toward the higher end. Fats have the lowest thermic cost, meaning most of their caloric content is retained.
Here is a simple way to put those percentages into perspective. If you consume 100 calories from protein, roughly 20–30 of those calories may be used in digesting, absorbing, and processing it. For 100 calories of carbohydrate, the cost is closer to 5–10 calories, while fat may require only about 0–3 calories. Fat is also more energy dense, providing about 9 calories per gram compared with about 4 calories per gram for protein and carbohydrate.
Other physiologic factors can also influence TEF. Obesity and insulin resistance are associated with a reduced thermic response to meals, potentially related to differences in glucose handling and sympathetic activation.3,4 Capsaicin and caffeine may produce small, transient increases in energy expenditure.5
Perspective, however, is essential. TEF contributes to overall “energy out,” but its effect is modest compared with the much larger components of BMR and physical activity. TEF optimization should therefore be viewed as complementary rather than primary within a broader metabolic strategy.
Calories
Begin stimulating and supporting lean muscle mass with two sessions of resistance-based movement and include a meaningful source of protein at each meal.
Last week, you made movement visible. This week, you begin reinforcing the structure beneath it. For the next seven days, introduce two simple behaviors:
Complete at least two sessions of resistance-based movement.
Include a meaningful source of protein at each meal.
During your resistance sessions, do not worry about finding the perfect exercise. Just begin. Choose movements that ask your muscles to work against resistance, whether that comes from your body weight, machines, resistance bands, or free weights. The exact movements matter less this week than simply getting started and establishing the habit. We will get much more specific about strength training, exercise selection, progression, and programming in later chapters.
Protein provides the substrate necessary for muscle protein synthesis and carries the highest thermic effect of all macronutrients, reinforcing the metabolic principles introduced in this chapter.
Do not overcomplicate this week. The goal is not optimal programming; it is awareness and initiation. You are introducing a stimulus—not designing a periodized training plan. More detailed guidance on strength training structure, progression, recovery, and precise protein targets will follow in upcoming chapters.
Resistance provides the stimulus. Protein provides the building blocks. Consistency drives adaptation. You are not chasing a metabolic spike—you are building durable capacity.
In the previous chapter, you made movement visible. You learned that active calories respond immediately to behavior and can be influenced hour by hour. In this chapter, you moved deeper—into the foundation beneath that movement.
Basal metabolic rate represents the energy required to sustain life at rest. It changes slowly and is influenced by age, genetics, health status, and body composition. The thermic effect of food contributes modestly to daily expenditure and is shaped by macronutrient composition. Together, these components form the structure of your energy balance.
Active calories are responsive. Lean mass is structural. Fueling is supportive.
Lean muscle is not a metabolic shortcut. Adding it will not radically increase the number of calories you burn at rest. Its value is broader and more durable: it helps preserve resting metabolic capacity while supporting strength, glucose disposal, body composition, resilience, and independence.
This chapter was not about chasing higher numbers, but about understanding the architecture beneath them. As you move forward, we will build on this foundation—first by continuing to move more, then by developing strength and other forms of physical capacity, with each habit layering upon the last.
Metabolism is not fixed. It is something you can understand, support, and shape over time. One week. One habit. One healthier you.
For educational purposes only. This information is not a substitute for individualized medical advice. Read the Health Disclaimer.
References
- Leidy HJ, Todd CB, Zino AZ, et al. Consuming High-Protein Soy Snacks Affects Appetite Control, Satiety, and Diet Quality in Young People and Influences Select Aspects of Mood and Cognition. J Nutr. 2015;145(7).
- Moon J, Koh G. Clinical Evidence and Mechanisms of High-Protein Diet-Induced Weight Loss. J Obes Metab Syndr. 2020;29(3).
- Poher AL, Altirriba J, Veyrat-Durebex C, Rohner-Jeanrenaud F. Brown adipose tissue activity as a target for the treatment of obesity/insulin resistance. Front Physiol. 2015;6:4.
- Tataranni PA, Larson DE, Snitker S, Ravussin E. Thermic effect of food in humans: methods and results from use of a respiratory chamber. Am J Clin Nutr. 1995;61(5).
- Schubert MM, Irwin C, Seay RF, Clarke HE, Allegro D, Desbrow B. Caffeine, coffee, and appetite control: a review. Int J Food Sci Nutr. 2017;68(8).
- Gallagher D, Belmonte D, Deurenberg P, et al. Organ-tissue mass measurement allows modeling of resting energy expenditure and metabolically active tissue mass. Am J Physiol Endocrinol Metab. 1998;275(2):E249–E258.