- Schofield Equation - October 31, 2023
- Physical Activity Level (PAL) - October 31, 2023
- BMR vs TDEE - October 30, 2023
What is Basal Metabolic Rate (BMR)
Basal metabolic rate (BMR) refers to the rate at which energy is used for basal metabolism. Energy used for movement and digestion is specifically excluded. BMR is generally expressed as calories used per hour or per day.
Basal Metabolism
Basal Metabolism is the metabolic cell activity(5) involved in processes such as respiration, heart function, blood circulation, and temperature regulation.(3) This can be pictured by thinking of the energy you are using when waking in the morning, still lying in a rested and fasted state, with your cells using energy only for basic or vital functions and not for physical activity, digestion or food assimilation.
Basal metabolism is used at all times but much of the time humans are also using energy for physical activity and food digestion and assimilation. Basal metabolism accounts for 60-75% of daily calorie use.(1)
BMR Calculator
A BMR calculator is an automated program which uses a formula to estimate BMR (basal metabolic rate) based on the user’s age, weight, height, and sex.
How to Find Your BMR
You can find your BMR through measurement at a laboratory or by using a calculator.
Measurement of BMR in a Laboratory(1,6)
BMR testing in a laboratory takes approximately half an hour and involves the measurement of the individual’s exhaled breath while awake but lying down and inactive.
Protocols vary but BMR lab testing is generally performed after an overnight sleep. Prior inactivity and fasting are required to ensure that no energy is being used as a result of earlier activity or food intake. A ‘thermoneutral’ environment is also required so that energy isn’t used to keep warm or cool.
Resting Metabolic Rate
In practice, laboratories often measure Resting Metabolic Rate instead of BMR because the testing conditions are less stringent(1,6) and easier to apply.
Resting metabolic rate is approximately 10-20% higher than BMR(6).
BMR and RMR Testing Requirements Compared
| LAB TEST REQUIREMENTS | BASAL METABOLIC RATE | RESTING METABOLIC RATE |
| PRIOR FASTING | 8-12 hours fasting is required prior to testing | 3-4 hours prior fasting is required |
| SLEEP | Overnight sleep is required beforehand | Overnight sleep is not required prior to test |
| TIME OF TEST | Test is performed in the morning, after a restful night’s sleep | Test can be at any time of day |
Sleeping Metabolic Rate
Sleeping metabolic rate is slightly lower than RMR and BMR but the terms are often used interchangeably since the main reference is to energy expenditure without physical activity and without the energy costs of food processing and absorption(2)
The Sleeping Metabolic Rate is 5-10% lower than RMR(6)
Resting, Basal, and Sleeping Metabolic Rates Compared(6)
| RMR | Resting Metabolic Rate | 10-20% higher than BMR |
| BMR | Basal Metabolic Rate | Higher than SMR. Lower than RMR |
| SMR | Sleeping Metabolic Rate | 5-10% lower than BMR |
Fat-Free Mass: The Most Important Predictor of BMR
Fat-free mass is the most important predictor of an individual’s basal metabolic rate (BMR),(2,6) explaining much of the change in an individual’s BMR over time. It is also the explanation for a large percentage of the variability between people.
| FAT-FREE MASS (FFM) | |
| What is fat-free mass | Fat-free mass is basically everything in the body minus the fat (lipids).(8) It includes tissue such as that found in muscles, brain, liver, heart(6) and other internal organs(8) |
| Energy use of FFM and fat mass compared | Fat-free mass and fat mass both use energy, as they are each metabolically active, but fat-free mass uses substantially more energy than fat mass does(7) |
| Factors which are correlated with variations in fat-free mass | Age, Sex, Racial Group, Levels of Physical Training(6) |
| Relationship to BMR | The most important predictor of BMR |
| Lean body mass | Fat-free mass and lean body mass are often used interchangeably but there is a difference in meaning. Fat-free mass does not include any fat whereas lean body mass includes a small amount of fat: Lean body mass includes the non sex-specific essential fat that is common to everyone (regardless of sex), for example, fat in bone marrow and internal organs(8) |
Most explanations of Basal metabolic rate variation are associated to some degree with fat-free mass. For example, BMR is closely associated with age but the decreases in BMR that are commonly seen in ageing adults are due mainly to loss of fat-free mass as individuals age(1,5) and not as a direct result of ageing.
People vary in their levels of fat-free mass and also in their proportions of fat-free mass to total body weight.
Fat-free mass can be estimated by a few different methods including underwater weighing (traditional method) and, more recently, by air displacement plethysmography (ADP).(6)
Growth and BMR
Growth is another important predictor of basal metabolism with BMR being higher during times of growth.(5,6) During times of growth, energy is used to maintain existing tissue and also to deposit new tissue. Perhaps surprisingly, the energy needed for depositing tissue is relatively low compared to that needed to maintain existing tissue.(6)
A child at one month old uses slightly more than one third of daily energy for growth but this rate soon slows down so that by one year energy used for growth will be only 3% and will remain low until puberty when it increases slightly (to 4%).(6)
| AGE | PERCENTAGE OF DAILY ENERGY USED FOR GROWTH(6) |
| One month old | slightly more than one third |
| One year old | 3% |
| Puberty | 4% |
Inter-Individual Variation in BMR
BMR varies between individuals(5) and is associated with a number of factors.
| VARIABLES ASSOCIATED WITH BMR | EXPLANATION |
| SIZE | BMR is generally higher the larger a person is(1) |
| BODY COMPOSITION | People with more lean body mass are likely to have a higher BMR. This includes men in general and people who are physically fit(5) |
| HEIGHT | Height is related to the amount of skin surface and this affects how much energy is used for temperature regulation. Taller people tend to have a higher BMR(5) |
| SEX | Women generally have more fat tissue than men and this is associated with a lower BMR.(5) Hormones also affect BMR to some degree(5,6) |
| AGE | BMR reduces as people age which is related to decreasing lean body mass.(5) Even in people whose weight doesn’t change, FFM tends to decline over time for this reason(6) |
| FAMILY TRAITS/GENETICS | Genetic variations also affect BMR(5) |
| HEALTH STATUS | BMR is lower in people who are malnourished(5) and during fasting(5). BMR is also increased during fever and stress(5) |
| PREGNANCY AND LACTATION STATUS | see Intra-Individual Variation below |
Intra-Individual Variation in BMR
BMR changes during the course of a lifetime(5) and as a result of actions and lifestyle
| INDIVIDUAL FACTORS RELATED TO BMR | EXPLANATION |
| STAGE OF LIFE | For example a newborn baby has a much higher BMR than an adult does, and the newborn’s BMR rate is higher than that of a fetus still in the womb |
| MUSCLE MASS/FAT-FREE MASS | The amount of muscle mass and fat-free mass you have(5) |
| HEALTH STATUS | See Inter-Individual Variation table above |
| CAFFEINE AND NICOTINE | These both increase BMR(5) |
| PREGNANCY AND BREASTFEEDING(4) | BMR increases during pregnancy due to the uterus and fetus plus extra demands on the lungs and heart.(6) Lactation causes an increased BMR of 4-5%.(6) |
BMR During Pregnancy and Lactation
Basal metabolic rate increases during pregnancy. The degree of increase is not constant between women as increased BMR in pregnancy is related to nutritional status prior to pregnancy as well as to the rate of fetal growth during pregnancy. It has been reported that BMR change is up to eight times greater for some women than others.(4)
Women who are breastfeeding also have an increased BMR associated with the approximate 900 calories needed to produce each liter of breast milk although some of these calories come from maternal fat stores and don’t require the intake of additional calories.(4)
Hourly vs. Daily Basal Metabolic Rates
BMR is variously defined as an hourly rate or as a daily rate.
When given as an hourly rate, BMR is generally shown as number of calories per kilogram of body weight used per hour.
When given as a daily rate, BMR is simply shown as number of calories used per day. A daily rate lends itself to dietary planning since it equates to the number of calories needed to maintain current weight.
| HOURLY/DAILY | WHEN USED | HOW SHOWN | EXAMPLE |
| Hourly BMR Rate | Commonly used for results from laboratory tests. Traditional way of showing BMR | Shows number of kcalories per kilogram of weight per hour | 23 kcal/kg/hour |
| Daily BMR Rate | Used by calculators | Shows total number of calories used per day | 1600 calories per day |
Related Subjects
Humans use energy for basal metabolism, digestion and physical activity. When added together, the total energy used in a day is called Total Daily Energy Expenditure (TDEE).
| THE THREE RELATED ENERGY USES | Acronym Used | Explanation |
| Basal Metabolism | BMR | Basal Metabolism |
| Thermic Effect of Food | TEF | Energy used for Digestion and Assimilation of Food |
| Physical Activity | PA | Energy used for Physical Activity |
| Total Daily Energy Expenditure | TDEE | Total energy used per day: BMR+TEF+PA=TDEE |
Abbreviations and Definitions
Note: There is a degree of overlap and inconsistency in the terms used in literature related to metabolism. You should always check the meaning intended by the author of any material.
BMR – Basal Metabolic Rate – usually measured when awake but lying still with restrictions on prior food/activity
SMR – Sleeping Metabolic Rate
RMR – Resting Metabolic Rate- usually measured when awake but sitting or standing. Not as strict as BMR
REE – Resting Energy Expenditure – same as RMR
TEF- Thermic Effect of Food – The energy cost of processing and digesting food
Essential body fat – The small amount of fat that is common to everyone. Includes fats such as the fat in bone marrow
Fat-free mass – The total body mass minus ‘extractable lipids’, or more simply: total body mass minus fat
Lean body mass – Fat-free mass plus essential body fat
References
1. Barbara Bushman (Editor). Complete Guide to Fitness and Health (2nd edition) ISBN 9781492533672
2. Yan Y. Lam and Eric Ravussin. Analysis of Energy Metabolism in Humans: A Review of Methodologies. http://dx.doi.org/10.1016/j.molmet.2016.09.005
3. Raven, Wasserman, Squires, Murray
4. Susan Tucker Blackburn. Maternal, Fetal, and Neonatal Physiology: A Clinical Perspective (2nd Edition) 9780721680125
5. Debruyne Pinna, Nutrition for Health and Care (6th Edition) ISBN 9781305627963
6. A. Catherine Ross et al. Modern Nutrition in Health and Disease (11th Edition) ISBN 9781605474618
7. Christopher Pankey, Kyle Flack, Kelsey Ufholz, Luann Johnson, James Roemmich. Influence of fat‑free mass and resting metabolic rate on increased food reinforcement after exercise training. https://doi.org/10.1007/s11332-021-00876-y
8. William McArdle, Frank Katch, Victor Katch. Essentials of Exercise Physiology (8th Edition) ISBN 9781451191554