Introduction
Definition of basal metabolic rate (BMR)
Basal metabolic rate (BMR) refers to the minimum amount of energy a resting, awake, and fasting organism requires to maintain vital physiological functions such as breathing, circulation, and organ function. It is the amount of energy an individual expends to sustain life at complete rest in a thermoneutral environment, which means that the surrounding temperature is comfortable and requires no additional energy expenditure for temperature regulation. BMR is typically expressed as the number of calories an individual would burn in a day if they did nothing but rest. Factors affecting an individual’s BMR include age, sex, body composition, and genetics.
Importance of measuring BMR
Measuring BMR is important for several reasons:
- Weight management: Understanding an individual’s BMR can help them determine the number of calories they need to consume to maintain, gain, or lose weight. If an individual consumes fewer calories than their BMR, they may lose weight, while consuming more than their BMR may result in weight gain.
- Nutritional needs: BMR measurement can also help determine an individual’s daily nutritional needs. Individuals’ calorie intake should match their BMR to provide sufficient energy for their body’s basic functions.
- Diagnosis of health conditions: A low BMR can indicate an underlying medical condition, such as an underactive thyroid. At the same time, a high BMR can be a sign of hyperthyroidism or other metabolic disorders.
- Monitoring athletic performance: Athletes and fitness enthusiasts can use BMR measurement to track their progress and adjust their diets and training plans accordingly.
Explanation of the indirect calorimetry method for BMR measurement
Indirect calorimetry is a method used to measure an individual’s BMR by estimating the amount of oxygen consumed and carbon dioxide produced during respiration. The principle behind this method is that the amount of oxygen an individual consumes is directly proportional to the amount of energy they expend. During the measurement, the individual is required to rest in a quiet and comfortable room for 10 to 12 hours after fasting for at least 8 hours. They are also asked to avoid strenuous exercise and smoking before the test. Then, a mask is placed over the individual’s nose and mouth, connected to a machine that measures the volume of air the individual breathes in and out. The machine also measures the oxygen and carbon dioxide concentrations in the air.
The individual is asked to relax and breathe normally while the machine measures the oxygen and carbon dioxide levels. The data collected by the machine is then used to calculate the individual’s BMR. The calculation takes into account factors such as the individual’s age, sex, weight, and height and uses a formula to determine the amount of energy expended by the individual. Indirect calorimetry is considered the gold standard method for measuring BMR because it accurately estimates an individual’s energy expenditure. However, it can be time-consuming and requires specialized equipment and expertise.
Factors Affecting BMR
Age and gender
Age and gender can have significant effects on an individual’s BMR.
- Age: As an individual age, their BMR tends to decrease. It is due to the natural loss of muscle mass and bone density, which are more metabolically active tissues compared to fat. As a result, older individuals require fewer calories to maintain their bodily functions than younger individuals. The decrease in BMR with age is relatively small but may become more significant as individuals advance into their senior years.
- Gender: Men generally have a higher BMR than women due to a higher proportion of muscle mass, which is more metabolically active than fat. Testosterone also plays a role in the higher BMR in men, as it helps maintain and build muscle mass. Women have a higher proportion of body fat and a lower metabolic rate than muscle mass. However, other factors such as body composition, activity level, and genetics also affect an individual’s BMR.
Body composition (lean body mass vs. fat mass)
Body composition refers to the proportion of fat, muscle, bone, and other tissues that make up an individual’s body. Body composition can have a significant effect on an individual’s BMR.
- Muscle mass: Muscle tissue is more metabolically active than fat tissue, requiring more energy to maintain. Therefore, individuals with a higher proportion of muscle mass generally have a higher BMR than those with less muscle mass. It is because muscle tissue requires more energy to maintain its cellular structure compared to fat tissue.
- Body fat: Conversely, fat tissue has a lower metabolic rate than muscle tissue. Individuals with a higher proportion of body fat have a lower BMR than those with less. It is because fat tissue requires less energy to maintain its cellular structure compared to muscle tissue.
Thyroid function
Thyroid function can significantly impact basal metabolic rate (BMR). The thyroid gland produces hormones called triiodothyronine (T3) and thyroxine (T4), which play a key role in regulating metabolism. When thyroid function is normal, T3 and T4 hormones are released in the right amounts to help regulate the body’s metabolic rate. However, if there is a problem with the thyroid gland and it produces too little thyroid hormone (hypothyroidism), the body’s metabolic rate can slow down. It can lead to symptoms such as fatigue, weight gain, and cold intolerance.
Genetics
genetics can play a role in determining an individual’s basal metabolic rate (BMR). BMR is largely determined by the body’s muscle mass, organ size, and amount of fat tissue. Genetic factors influence these factors, which can affect an individual’s BMR.
Stress levels
Stress levels can influence basal metabolic rate (BMR). The body produces a hormone called cortisol when it is under stress, and it can raise BMR. The body’s “fight or flight” reaction, which primes the body to react to a perceived threat, includes increased BMR.
Preparing for BMR Measurement
Avoiding exercise and caffeine before the test
Avoiding exercise and caffeine before a basal metabolic rate (BMR), test is generally recommended to ensure accurate results. Exercise can temporarily increase your metabolism and heart rate, affecting the accuracy of the BMR measurement. It is recommended to avoid vigorous exercise for at least 12 hours before the test. Caffeine is a stimulant that can also affect your metabolism and heart rate—avoiding caffeine for at least 4 hours before the test is recommended, as it can also affect the accuracy of the BMR measurement.
Fasting for several hours before the test
Fasting for several hours before a basal metabolic rate (BMR) test is typically recommended to ensure accurate results. Fasting means abstaining from food and drink for a certain period. The exact length of the fast may vary depending on the testing facility or healthcare provider, but it is typically recommended to fast for at least 8 hours before the BMR test.
Choosing the right time of day for the test
Choosing the right time of day for a basal metabolic rate (BMR) test can also affect the accuracy of the measurement. It is generally recommended to schedule the test for the morning after fasting overnight. The reason for scheduling the test in the morning is that your metabolism is generally at its lowest point after a period of rest, such as sleeping. By testing in the morning, you can get a more accurate measurement of your resting metabolic rate without the influence of recent physical activity or food consumption.
Avoiding smoking and alcohol before the test
It is generally recommended to avoid smoking and alcohol before a basal metabolic rate (BMR) test to ensure accurate results. Smoking can temporarily increase your heart rate and metabolism, affecting the accuracy of the BMR measurement. It is recommended to avoid smoking for at least 2 hours before the test. Alcohol can also affect your metabolism and alter your body’s energy expenditure. It is recommended to avoid alcohol for at least 24 hours before the test.
Methods for Measuring BMR
The Harris-Benedict equation
The Harris-Benedict equation is one of the most commonly used methods for estimating basal metabolic rate (BMR), the amount of energy (measured in calories) your body needs to perform basic functions at rest. The Harris-Benedict equation uses your age, sex, height, and weight to estimate your BMR. The equation has two different versions: one for men and one for women. For men, the Harris-Benedict equation is:
BMR = 88.362 + (13.397 x weight in kg) + (4.799 x height in cm) – (5.677 x age in years)
For women, the Harris-Benedict equation is:
BMR = 447.593 + (9.247 x weight in kg) + (3.098 x height in cm) – (4.330 x age in years)
The Mifflin-St. Jeor equation
The Mifflin-St. Jeor equation is another commonly used method for estimating basal metabolic rate (BMR), which is the amount of energy (measured in calories) your body needs to perform basic functions at rest—the Mifflin-St. Jeor equation uses your age, sex, height, and weight to estimate your BMR. It is considered more accurate than the Harris-Benedict equation, particularly for overweight or obese individuals.
For men and women, the Mifflin-St. Jeor equation is:
BMR = (10 x weight in kg) + (6.25 x height in cm) – (5 x age in years) + s
Where “s” is a gender-specific constant:
For men, s = 5
For women, s = -161
Use 3D body scanner to test BMR（visbody）
A 3D body scanner, such as the Visbody scanner, uses advanced imaging technology to create a 3D model of a person’s body. This model can calculate a range of body composition metrics, such as body fat percentage, lean mass, and body volume. These body composition metrics can be combined with BMR measurements to calculate an individual’s total daily energy expenditure (TDEE), which is the total number of calories they burn daily. By accurately estimating TDEE, individuals can determine how many calories they should consume to maintain their current weight or to achieve their desired weight loss or weight gain goals.
Interpreting BMR Results
What the BMR numbers mean
BMR stands for Basal Metabolic Rate, the amount of energy (measured in calories) your body requires to sustain its basic functions while at rest, such as breathing, circulating blood, and maintaining organ function. The BMR number represents the minimum number of calories your body needs to consume to maintain these basic functions without considering any additional energy expenditure from physical activity, digestion of food, or other daily activities.
Knowing your BMR can help determine how many calories you must consume daily to maintain, lose, or gain weight. For example, suppose your BMR is 1500 calories daily, and you engage in moderate physical activity. In that case, you may need to consume 2000-2500 calories daily to maintain your current weight, depending on your age, sex, and other factors.
How to adjust caloric intake based on BMR.
To adjust your caloric intake based on your BMR, you must determine your daily caloric needs based on your activity level and weight goals. Here are the general steps:
- Determine your BMR using an online BMR calculator or consult a healthcare professional or registered dietitian.
- Determine your activity level using a physical activity level (PAL) calculator. It will give you an estimate of the calories you burn through physical activity.
- Multiply your BMR by your PAL to estimate your total daily energy expenditure (TDEE).
- To maintain your weight, consume the same number of calories as your TDEE.
- To lose weight, consume fewer calories than your TDEE (aim for a deficit of 500-1000 calories per day to lose 1-2 pounds per week).
- To gain weight, consume more calories than your TDEE (aim for a surplus of 250-500 calories per day to gain 0.5-1 pound per week).
Remember that these calculations are estimates and should be adjusted based on individual factors such as age, sex, height, weight, and activity level. Additionally, it’s important to prioritize nutrient-dense, whole foods to meet your caloric needs and support overall health. Consulting with a registered dietitian can also help determine an individualized plan for adjusting caloric intake based on BMR.
