What are the primary determinants of Basal Metabolic Rate?

BMR is primarily determined by the amount and metabolic rate of specific tissues, particularly high-metabolic-rate organs like the heart, liver, kidneys, and brain. These organs account for roughly half of total BMR despite being a small percentage of body mass.

How accurate are common BMR prediction equations?

Most BMR prediction equations, even good ones, can have significant errors, sometimes overestimating or underestimating BMR by 300-400 calories. Limits of agreement for 95% of errors typically span around 800 calories, meaning relatively large errors are common.

Why do BMR prediction equations have large errors?

Large errors often occur because organ mass, especially of high-metabolic-rate organs (heart, liver, kidneys, brain), varies significantly between individuals and is only weakly correlated with easily measurable factors like height, weight, age, sex, or total fat-free mass. This variability in organ size is not well captured by standard equations.

Do men and women have different BMRs?

Men generally have higher BMRs than women, a difference of about 150-200 calories, but this is primarily due to differences in total body size and composition (men tend to have more fat-free mass). When controlling for fat-free mass, BMRs are generally similar between sexes.

How does age affect Basal Metabolic Rate?

Research indicates that BMR, when adjusted for body mass and composition, remains fairly stable through most of adulthood and doesn't rapidly decrease until around 60 years old. Any declines before this age are usually due to muscle mass loss or increased sedentary behavior.

What is 'metabolic adaptation' during weight loss?

Metabolic adaptation refers to a disproportionate decrease in BMR during weight loss, beyond what would be expected from the loss of body mass. This 'extra' reduction is often around 5-10% of BMR and can be partially explained by losses in high-metabolic-rate organ mass, particularly liver and kidney mass.

How does weight gain affect BMR?

During the first few weeks of overfeeding or a bulking phase, BMR can temporarily increase by 5-10% more than predicted, for reasons not fully understood. However, this 'reverse metabolic adaptation' is not sustained long-term, and BMR eventually scales more typically with increased body weight and composition.

Do athletes have higher BMRs than non-athletes?

Yes, athletes tend to have higher BMRs than non-athletes, and this is primarily attributed to differences in organ mass, not just muscle mass. Athletes' high-metabolic-rate organs (heart, liver, kidneys) increase linearly with fat-free mass, unlike in the general population where their proportion tends to decrease with overall fat-free mass.

Is an athlete's higher BMR a training adaptation?

Evidence suggests that the higher BMR in athletes is largely a training adaptation. Studies show that BMR increases in both resistance and aerobic training interventions, surpassing what's explained by muscle gain alone. Longitudinal research also documents proportional increases in organ size with training over time.

Everything you Need to Know About Basal Metabolic Rate (Episode 143)

Stronger By Science

Sports4 min read91 min video

Oct 16, 2024|5,496 views|233|29

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Key Moments

TL;DR

Greg Knuckles discusses Basal Metabolic Rate (BMR), its determinants, prediction inaccuracies, sex differences, and impact of age and weight.

Key Insights

BMR (or RMR) is the energy your body burns at rest, with organs like the heart, liver, kidneys, and brain being major contributors.

BMR prediction equations have significant limitations and can be inaccurate, often with wide limits of agreement (around 800 calories).

Sex differences in BMR are largely explained by variations in body composition, particularly fat-free mass and essential fat.

Adult BMR remains relatively stable until around age 60, after which it begins to decline.

Weight loss leads to a disproportionate decrease in BMR (metabolic adaptation), partly due to loss of organ mass.

Athletes generally have higher BMRs than non-athletes, primarily due to proportionally larger organs, which appears to be a training adaptation.

ANNOUNCEMENT REGARDING THE PODCAST'S FUTURE

The podcast is announcing its indefinite hiatus after the next episode, marking the end of a five-year run. This decision stems from the host's personal preference for writing over speaking and the co-hosts' diverging career paths. While the podcast format has been valuable for introducing personalities and building connections, the host finds writing more conducive to detailed, source-linked scientific communication. The final episode will be an all-Q&A session, encouraging listeners to submit questions.

DEFINING BASAL METABOLIC RATE (BMR)

The episode clarifies terminology, distinguishing between Basal Metabolic Rate (BMR), Sleeping Metabolic Rate (SMR), and Resting Metabolic Rate (RMR). While BMR technically refers to energy expenditure upon waking, much of the research and discussion pertains to RMR, which is energy expenditure at rest for a short period. For practical purposes and audience familiarity, the term BMR will be used interchangeably with RMR throughout the discussion, acknowledging that RMR is typically slightly higher than BMR.

DETERMINANTS OF BMR AND PREDICTION UNRELIABILITY

Contrary to common assumptions, BMR is not solely determined by factors like height, weight, age, sex, or total fat-free mass. Instead, it's dictated by the specific masses and metabolic rates of individual tissues. High-metabolic organs—heart, liver, kidneys, and brain—constitute a small fraction of body mass but account for over half of BMR. Muscle and fat have significantly lower metabolic rates. This organ-centric determinant is crucial for understanding prediction inaccuracies.

THE INACCURACY OF BMR PREDICTION EQUATIONS

BMR prediction equations, often used in online calculators and even in some commercial body composition assessments (e.g., Bod Pod), are not as accurate as generally believed. They can underestimate BMR by 300-400 calories or more. Bland-Altman plots reveal wide limits of agreement, typically around 800 calories, meaning 95% of errors fall within this range. This significant variability is largely due to the unpredictable mass of high-metabolic organs relative to easily measured metrics like height or fat-free mass.

SEX DIFFERENCES AND ORGAN MASS

While men typically have higher BMRs than women, this difference is largely attributed to variations in total body size and composition, with men generally having more fat-free mass. Equations based purely on fat-free mass often show no sex difference. However, women tend to have a higher BMR per unit of fat-free mass because smaller individuals, and thus women more frequently, have a proportionally larger percentage of their fat-free mass composed of high-metabolic organs.

IMPACT OF AGE AND WEIGHT LOSS

BMR remains relatively stable throughout adulthood until around age 60, after which it begins to decline. Weight loss induces 'metabolic adaptation,' where BMR decreases disproportionately to the tissue lost. Research suggests that approximately half of this adaptation is due to a reduction in high-metabolic organ mass (especially liver and kidneys), with the other half potentially being true metabolic adaptation, though the effect is generally modest (5-10% decrease).

WEIGHT GAIN AND REVERSE METABOLIC ADAPTATION

While weight loss can lead to metabolic adaptation, the impact of weight gain is less pronounced. There might be a short-term, temporary increase in BMR (around 5-10%) during the initial weeks of overfeeding, which doesn't persist as weight is gained. This phenomenon, sometimes termed 'reverse metabolic adaptation,' is not well-understood and appears to resolve, with BMR eventually tracking closer to predictions based on increased body mass.

BMR IN ATHLETES AND TRAINING ADAPTATIONS

Athletes generally exhibit higher BMRs than non-athletes, not primarily due to higher muscle mass, but because their high-metabolic organ mass scales more linearly with fat-free mass. This suggests that BMR in athletes is more adaptable. Evidence indicates that the elevated BMR in athletes is a training adaptation, as BMR increases proportionally with exercise interventions and organ size, even in sub-elite populations, supporting the idea that starting an exercise program can increase BMR beyond just muscle gain.

Mentioned in This Episode

●Software & Apps

●Companies

●Organizations

●Studies Cited

●Concepts

●People Referenced

Common Questions

BMR (Basil Metabolic Rate) is the energy burned first thing in the morning after sleep. RMR (Resting Metabolic Rate) is the energy burned at rest most of the time, typically slightly higher than BMR. SMR (Sleeping Metabolic Rate) is the energy burned while sleeping, usually the lowest of the three.

Topics

Basal Metabolic Rate BMR Prediction RMR SMR Organ Mass Weight Gain Athletes BMR Human Metabolism

Mentioned in this video

softwareMacroFactor

An app that Greg Knuckles is working on, which uses BMR estimations to calculate total daily energy expenditure.

personMuller and colleagues

Researchers whose work illustrated that smaller individuals, or those with less fat-free mass, have a higher proportion of their fat-free mass comprised of metabolically active organs.

personMei and colleagues (2013)

Researchers of a study titled 'Organ Size Increases with Weight Gain in Power Tred Athletes,' which documented proportional increases in organ size with training.

personPonzer and colleagues (2022)

Researchers of the study 'Daily Energy Expenditure through the Human Life Course,' which found BMR remains stable through most of adulthood before declining around age 60.

personHarris and colleagues (2012)

Researchers who conducted an 8-week overfeeding study demonstrating a short-term BMR increase during weight gain.

conceptSMR (Sleeping Metabolic Rate)

The amount of energy the body burns during sleep, which is typically the lowest of the three metabolic rate measurements.

studyCunningham Equation (1980)

An older BMR prediction equation that performs well for athletes in some contexts.

conceptBMR (Basil Metabolic Rate)

Essentially how much energy the body burns at rest. While the podcast mainly discusses RMR, BMR is used as the common term.

companyMyo Adapt

An app company mentioned as being developed by co-hosts Pack and Milo.

studyMifflin-St Jeor Equation

A popular BMR prediction equation that takes height, weight, age, and sex into account, showing a constant difference between men and women.

personJames Steel

A researcher Stronger by Science is collaborating with to conduct a large meta-analysis on BMR research.

personEric Trexler

Former co-host and partner at Stronger by Science, whose introduction was the original reason for starting the podcast.

conceptBland-Altman plot

A statistical method used to quantify the accuracy of a prediction equation by comparing measured and predicted values, revealing average error and limits of agreement.

personSabuni (2013)

Lead author of a 2013 study that cataloged the numerous BMR prediction equations developed by various researchers.

conceptRMR (Resting Metabolic Rate)

The amount of energy the body burns at rest most of the time, typically slightly higher than BMR and the focus of most research due to easier assessment.

studyCunningham Equation (1991)

A BMR prediction equation frequently used, especially by body composition machines like Bod Pods, that predicts BMR from fat-free mass.

studyOrgan Size Increases with Weight Gain in Power Tred Athletes

A 2013 paper by Mei and colleagues on football players, showing longitudinal increases in heart, kidney, and liver mass proportional to fat-free mass gains.

organizationStronger by Science

The brand and business that hosts the podcast, article series, and various fitness resources mentioned.

personMolina and Deau

Researchers who conducted studies examining cadavers to understand the relationship between organ mass and body metrics.

personMcKenzie Shelters and colleagues (2020)

Researchers who conducted a meta-analysis analyzing changes in BMR after exercise interventions, showing increases beyond muscle mass changes.

personGreg Knuckles

Host of the Stronger by Science podcast, who is hosting this episode solo for the first time.

studyCatch McArdle Equation

Often erroneously referred to, this equation is generally similar to the 1991 Cunningham equation for BMR prediction from fat-free mass.

studyDaily Energy Expenditure through the Human Life Course

A 2022 study by Ponzer and colleagues demonstrating BMR stability in adulthood and decline post-60, widely reported in health media.

conceptMetabolic Adaptation

The disproportionate decrease in BMR during weight loss, beyond what would be expected from tissue loss, which is partially explained by organ mass reduction.

studyTen Half Equation

A BMR prediction equation that performs well for athletes.

toolTik Tok