BMR Calculator

What is Basal Metabolic Rate (BMR)?

Basal Metabolic Rate (BMR) is the rate of energy expenditure per unit time by endothermic animals at rest. It represents the minimum amount of energy your body needs to maintain basic, vital functions when you are in a complete state of rest. These essential processes include breathing, blood circulation, controlling body temperature, cell growth, brain and nerve function, and muscle contraction.

For an accurate BMR measurement, strict criteria must be met: the individual must be in a physically and psychologically undisturbed state, in a thermally neutral environment, and in a post-absorptive state (meaning not actively digesting food).

BMR vs. Resting Metabolic Rate (RMR)

While often used interchangeably, there's a subtle distinction between BMR and Resting Metabolic Rate (RMR). BMR is measured under very stringent, clinical conditions (complete rest, fasting, thermally neutral environment), which can be difficult to achieve outside of a laboratory. RMR is a more common measurement that uses less strict criteria and is typically measured after an overnight fast and a good night\'s sleep, but not necessarily in a complete resting state. For most practical purposes, the terms are used to describe the calories burned at rest.

How BMR Affects Weight & Influencing Factors

Your Basal Metabolic Rate accounts for approximately 70% of your daily calorie expenditure. It directly affects how many calories your body burns at rest, which in turn determines whether you maintain, gain, or lose weight.

Several factors influence BMR:

• Age: BMR typically declines by 1–2% per decade after age 20, mainly due to loss of fat-free mass.
• Sex: Men generally have higher BMRs than women due to a higher proportion of lean body mass.
• Body Composition: Increasing muscle mass has the effect of increasing BMR, as muscle tissue is more metabolically active than fat tissue.
• Height and Weight: Taller and heavier individuals generally have higher BMRs.
• Genetics: Individual variability in BMR can be high due to genetic predispositions.
• Illness and Stress: Illness (e.g., burns, fractures, infections, fevers) and increased stress levels can temporarily increase BMR.
• Environmental Temperature: Lower temperatures can result in higher BMRs as the body expends more energy to maintain core temperature.
• Menstrual Cycle (in females): BMR can vary to some extent with the phases of the menstrual cycle, often rising during the luteal phase due to increased progesterone.

BMR Estimation Formulas

While direct measurement of BMR is complex, several widely used equations can provide a reliable estimation based on your age, sex, height, and weight. In the formulas below, P is total heat production at complete rest (calories/day), m is mass (kg), h is height (cm), and a is age (years).

The Physiology of Metabolism

The body's generation of heat, known as thermogenesis, can be measured to determine energy expenditure. The hypothalamus, located in the diencephalon, plays a crucial role in regulating metabolism. It controls and integrates activities of the autonomic nervous system, which regulates vital functions like heart rate, digestion, and the secretions of metabolic glands such as the thyroid.

Metabolism involves two key processes:

• Catabolism: The breakdown of large molecules into smaller ones, associated with the release of energy. Example: breakdown of proteins into amino acids.
• Anabolism: The building up of larger molecules from smaller ones, which requires energy. Example: formation of proteins from amino acids.

Adenosine triphosphate (ATP) is the primary energy currency of the cell, driving various cellular processes, including muscle contraction. The breakdown of ATP through processes like the Krebs cycle provides the energy needed for these functions, which ultimately contribute to your BMR.

Energy Expenditure Breakdown by Organ

About 70% of a human's total energy expenditure is dedicated to the basal life processes taking place in the body's organs. Here's an approximate breakdown of how energy is consumed by different organs at rest:

Beyond BMR, about 20% of your energy expenditure comes from physical activity and another 10% from thermogenesis, or the digestion of food (postprandial thermogenesis), which increases BMR to different degrees depending on the food composition.

Research on Individual Differences in BMR

BMR can vary significantly between individuals. Studies have shown that while factors like fat-free mass, fat mass, and age explain a large portion of this variation, a considerable percentage remains unexplained. This highlights that BMR is a complex trait influenced by a multitude of factors, including genetics and lifestyle.

Interestingly, research suggests that BMR has decreased over the past few decades, even when adjusted for body composition and age, indicating a broader trend in reduced energy expenditure in populations.