By Fabio Comana

Mention the term ‘metabolism’ and it captures attention given how obsessed many of us have become over calories. As a certified personal trainer, this is a topic you may get asked about from time to time.

To the general public, boosting your metabolism is associated with the transformation from ‘fat-to-fit’ or ‘flab-to-fab’. This is an outcome of enhanced caloric expenditure, increased lean body mass, greater fat utilization and overall weight loss.

Our resting metabolic rate (RMR) plays a significant role in this, and it’s understandable why it garners so much attention. But before we dive into RMR, let’s take a look at the components of total daily energy expenditure (TDEE).

In this article:

TOTAL DAILY ENERGY EXPENDITURE

Scientific references to metabolism refer to the bodily processes needed to maintain life. But for most of us, it refers to total daily energy expenditure and how it influences our energy in versus energy out equation.

Our TDEE is essentially comprised of three components:

  • Resting metabolic rate (RMR): the energy required to keep your body functioning at rest
  • The thermic effect of food (TEF): the energy cost of chewing, swallowing, digesting, absorbing and storing food
  • The thermic effect of physical activity (TEPA): the energy of activity (e.g., exercise, physical activity) and non-exercise activity thermogenesis (NEAT).*

* Non-exercise Activity Thermogenesis: Energy expended for everything you do that does not include sleeping, eating, physical activity or exercise – ranges from simple standing to fidgeting and moving about.

Can we calculate our RMR? Can we influence it or is it predetermined? Let’s examine these questions more closely.

WHAT IS RESTING METABOLIC RATE?

Resting metabolic rate is the total number of calories burned when your body is completely at rest. RMR supports breathing, circulating blood, organ functions, and basic neurological functions. It is proportional to lean body mass and decreases approximately 0.01 kcal/min for each 1% increase in body fatness.

CALCULATING RMR

CALORIMETRY

Direct calorimetry measures the amount of heat produced by a subject enclosed within a small chamber to calculate energy expenditure. Indirect calorimetry measures oxygen utilization rates via gas analysis to calculate energy expenditure.

Although direct and indirect calorimetry provide accurate estimates of RMR, these techniques are expensive, time consuming and difficult to access.

As a result, more accessible and affordable techniques that estimate RMR have been developed over the past 100 years. They measure this value with varying degrees of accuracy.

Perhaps the most common methods utilized today are mathematical formulas. You can access these with calorie calculators on the Internet, an app or through wearable devices.

Here’s an example of one online calculator you can use to find out your RMR:

RMR Calculator – Resting Metabolic Rate

HARRIS AND BENEDICT EQUATION

The Harris and Benedict (H&B) equation created in 1918 and amended in 1984 remains widely used today (1-2).

While it was intended to measure basal metabolic rate (BMR) or basal energy expenditure (BEE), they are used interchangeably with RMR.

Technically, BMR measures energy expenditure in a darkened room (reclining position) after eight hours of sleep and following a 12-hour fast whereas RMR measurements are less restrictive and reflect the body’s resting energy expenditure after an overnight fast.

The revised H&B equations for males and females (2) are:

  • Men: 88.362 + (13.397 × weight in kg) + (4.799 × height in cm) – (5.677 × age in years)
  • Women: 447.593 + (9.247 × weight in kg) + (3.098 × height in cm) – (4.330 × age in years)

As an example, for a 38-year old female, who stands 5’6” (167.6 cm) and weighs 145 pounds (65.9 kg), her BMR or RMR would equal approximately 1,411 calories.

This is the energy needed daily to maintain normal physiological function.

MIFFLIN-ST JEOR FORMULA

The Mifflin-St Jeor formula, created in the 1990s, provided an alternative and more valid estimate of RMR (3).

The equations for males and females are:

  • Men: (10 × weight in kg) + (6.25 × height in cm) – (5 × age in years) + 5
  • Women: (10 × weight in kg) + (6.25 × height in cm) – (5 × age in years) – 161

Using that same example and this equation, the female’s RMR would equal approximately 1,356 calories.

POTENTIAL ERRORS

At first, this error appears small (i.e., 55 calories), but extrapolated over a one-year period, it amounts to almost six pounds of energy or body weight.

Furthermore, these formulas also imply that all individuals of the same gender, age, height and weight have the same RMR, a fact that is certainly not accurate. Your lean body mass will significantly influence RMR and should always be considered.

Although the Katch-McArdle and Cunningham formulas are derived from lean body mass rather than total body weight, they rely upon an accurate measurement on lean body mass.

The errors of these formulas can be quite significant – studies have demonstrated accuracy within 10% of true RMR (Mifflin St Jeor) to as high as 36% error in obese individuals (H&B) (4-5).

Although newer equations continue to emerge (e.g., Oxford equations), they are still subject to varying degrees of error.

UNCONTROLLABLE RMR FACTORS

Age, genetics and even biological adaptations are just a few non-controllable events.

AGE

For example, age-related reductions in our resting metabolic rate (RMR) can decrease by approximately 2% per decade after peak growth is attained (late teens for females, early 20’s for men) (6).

Considering how RMR contributes around 60-to-75% of TDEE, in practical terms this equals approximately 25-to-30 calories per day for the average adult or 2½-to-3 pounds (1.1-1.4 Kg) per year.

GENETICS AND EPIGENETICS

Genetics and epigenetics can also play a significant role. Over 100 different genes have been identified by scientists that are related to obesity.

The fat mass and obesity-associated gene, the FTO gene, can cause people to overeat due to low satiety (7).

Eating behaviors associated with a low satiety include eating larger portions, preferring calorie-dense foods high in fat and sugar, enjoying palatable foods like appetizers and snacks, and snacking more frequently.

This FTO gene can also alter RMR by up to 160 calories per day – this amounts to almost 17 pounds (7.5 Kg) over the span of a year.

Epigenetics is the field of study that examines inheritable changes within our genetic expression that occurs without change to our underlying DNA sequence.

It is both a regular and natural occurrence and is influenced by age, environment, diet, geographical location, lifestyle and disease.

Research continues to examine potential links between epigenetics and TDEE considering how it can influence food uptake and overall metabolism – possibly altering RMR by a few percentage points or 60-to-75 calories daily (8).

CONTROLLABLE RMR FACTORS

This list is potentially endless, but the reality is that most fitness professionals usually limit their strategies to exercise, macronutrients, calories and various stimulants.

STIMULANTS

For example, evidence supports a temporary thermogenic-boosting effect by 4-to-5% with edibles like caffeine and capsaicin that can amount to approximately 15-to-25 calories in a day (9).

LEAN BODY MASS

Building lean body mass is another effective method for boosting RMR. Peak muscle mass in humans usually occurs at ages 28-to-32 after which muscle losses begin to occur.

The ability to preserve muscle mass or even better, build muscle mass can help preserve our age-related losses. Even a small gain of 2-to-4 pounds of muscle mass can provide a 7-to-8% boost in metabolism, which can add approximately 90-to-110 kcal to TDEE per day or 9-to-11 lbs. per year.

SLEEP

It might interest you to know that even a lack of sleep (i.e., sleep debt) can negatively impact your RMR.

LOW CALORIC INTAKE

Thirty years of research demonstrates how the practice of eating very low caloric intakes (e.g., starvation, 800-calorie diets) can suppress RMR, a number that by some estimates can be as high as 20%.

Under this stress, sustained, elevated levels of cortisol can suppress thyroid stimulating hormone production which will ultimately impact thyroid hormones that regulate metabolism.

Furthermore, these starvation states can also waste away valuable muscle mass which in turn will also reduce RMR.

To put this into perspective, for a person with an RMR between 1,200 and 1,500 calories, a 20% suppression can amount to 240-to-300 kcal/day or approximately 25-to-31 pounds annually.

ASSESSING STARVATION STATES FOR OPTIMAL RMR

So, how do you gauge whether you are in starvation states where RMR might be negatively impacted?

FORMULAS AS GUIDELINES

Unless true RMR is known which can set a minimal threshold for daily caloric intake, you might just be guessing with mathematical formulas (even though the Mifflin St Jeor is probably the best to use).

An alternative to the BMR formulas is to simply follow the commonly suggested minimal numbers of 1,000-to-1,200 calories for women and 1,200-to-1,600 calories for men.

These numbers, however, provide estimates at best because the macronutrient composition of a diet (e.g., high protein, fiber), timing and even physical form of food (i.e., liquid versus solid) can all impact TEF, absorption and ultimately RMR.

HUNGER SCALE

The sensation of hunger is another viable option to use as a guide, but the sensation of hunger is considered plastic (i.e., modifiable) and for some, confusion exists in distinguishing hunger from appetite.

Regardless, the hunger scale can help you gain a sense of whether they are providing adequate food calories to their body to avoid starvation – in other words, the opportunity to listen to your body.

Hunger ScoreDescription
1Starving, weak, dizzy, headache, lack of concentration
2Irritable, cranky, very hungry, low energy, lots of stomach growling
3Strong urge to eat, stomach growls a little
4Feeling a little hungry – thinking about food
5Body feels fueled (starting to feel satisfied), neither hungry nor full
6Fully satisfied – little full, but pleasantly full
7A little uncomfortable, but could still eat additional item
8Feeling stuffed
9Feel very bloated – very uncomfortable, stomach hurts
10Feel sick from overeating

Ideally, you would spend their waking hours between hunger scores of 4-and-6.

In other words, when a ‘4’ is reached, eat something to prevent dropping to the ‘3’ where ravenous, binge eating is more likely, but learn to stop at a ‘6’ rather than a ‘7’ or higher as many individuals do.

HUNGER VS. APPETITE

Lastly, take the time to understand some basic differences between hunger and appetite which are outlined below:

HUNGER

It is considered a biological response to replenish the body’s energy reserves.

  • Protects us from starvation.
  • Usually triggered by an event occurring below the neckline:
    • Low blood sugar.
    • Empty (growling) stomach.
    • Hormone fluctuations.
    • Need to warm body (hypothermia).
  • Gradual onset, appearing after several hours without food and typically diminishes after eating.
  • It is generally satisfied by almost any food that provides energy (calories).

APPETITE

It is considered a desire or interest to eat a specific food.

  • Usually triggered by an event occurring above the neckline consciously or subconsciously:
    • Thoughts, emotions and moods.
    • Social (e.g., happy hour)
    • Cultural (e.g., family)
    • Environmental (e.g., walking into a bakery).
  • More rapid onset and often independent of hunger.
  • Not time-dependent and may persist after eating.
  • Usually only satisfied by a specific food (e.g., sweet, salty) which may then evoke emotions and thoughts afterwards (e.g., pleasure, guilt, shame).

While RMR is an important component of TDEE, an accurate measurement remains elusive for many. Subsequently, we resort to mathematical formulas, but considering their potential errors, the values determined should always be considered a general estimate rather than an accurate value. Given this, there may also be value in including other methods as a guide to avoiding starvation.

Lastly, while we need to acknowledge the fact that RMR is not entirely controllable, there are some influencing factors we can manipulate and should leverage every opportunity to exploit them.