1.7 Outline what basal metabolic rate (BMR) is

This guide will help you answer 1.7 Outline what basal metabolic rate (BMR) is.

Basal metabolic rate—often shortened to BMR—refers to the number of calories the body needs to perform its most basic functions. These are the life-sustaining tasks your body completes all the time, even at rest. BMR keeps your organs working and maintains basic homeostasis. In other words, it is the bare minimum of energy needed for your body just to keep working.

BMR does not take into account physical activity, digestion (thermic effect of food), or energy used for movement. Think of it as the energy your body uses while you are lying still, awake, and at a comfortable temperature. This basic energy is not something you can control but is important for everyone’s health and weight management.

Functions Included in BMR

Your BMR covers many important internal activities required for life. These do not include any voluntary movement or actions, such as walking, eating, or exercising. Key functions BMR includes are:

• Breathing
• Circulation of blood
• Cell production and repair
• Hormone production and release
• Body temperature control
• Brain and nerve function
• Organ function (liver, kidneys, heart, etc.)

Even when sleeping or resting, your body uses calories to keep you alive.

Why BMR Matters

Knowing about your BMR helps people make better decisions about nutrition, weight management, and health. BMR is a significant part of your total daily energy expenditure (TDEE), which is the combined total of:

• Basal metabolic rate (BMR)
• Physical activity
• Digestion of food

BMR accounts for the largest portion of TDEE—usually 60-75%. This means that most of the calories people use each day support basic bodily functions rather than movement or exercise.

If you want to adjust your weight, learning about BMR can help. Eating fewer calories than your BMR (without medical supervision) can be harmful. The body needs enough energy to keep working properly, or serious health issues can develop.

What Affects BMR?

BMR is not the same for everyone. Many factors influence how many calories a person’s body needs at rest. These include:

• Age: Younger people tend to have higher BMRs. As you age, BMR usually drops.
• Sex: Men often have a higher BMR than women.
• Body Size and Composition: More muscle means a higher BMR. Muscle burns more calories than fat.
• Genetics: People inherit their base metabolism rate from their family.
• Hormones: Thyroid hormones play a key role. If the thyroid is overactive (hyperthyroidism), BMR rises. If it is underactive (hypothyroidism), BMR drops.
• Health Conditions: Some illnesses or medical treatments can affect BMR.
• Body Temperature: Infections or fever can increase BMR, so the body uses more energy.
• Environmental Temperature: Cold environments can make BMR higher, as the body works to keep warm.

Here is a closer look at a few influencing factors:

Age and BMR

Children and teens need more energy for growth and development. Their BMR is higher than that of adults. As people get older, their muscle mass tends to decrease, and fat usually increases. That shift lowers BMR because muscle cells require more energy to sustain than fat cells.

Sex and BMR

Typically, men have larger bodies and more muscle, so their BMR is usually higher than women’s. Women have a higher percentage of body fat, which does not use as much energy as muscle. Hormonal differences between men and women also affect BMR.

Muscle Mass and BMR

Muscle is active tissue; it needs more energy than fat, even at rest. Someone who is very muscular will have a higher BMR than a person of the same size but with less muscle. This is why regular strength training can help raise BMR over time.

Thyroid Function and BMR

The thyroid is a small gland in the neck that controls how quickly the body uses energy. Overactive thyroid (hyperthyroidism) means the body uses energy faster and the BMR goes up. Underactive thyroid (hypothyroidism) means less energy used and the BMR drops.

How is BMR Measured?

BMR is measured in calories (or kilocalories, often called ‘kcal’). To get an exact number, scientists would need to measure all the heat and energy your body produces at rest. This is done in a controlled setting, usually a laboratory. The person needs to be fasted, awake but relaxed, lying still in a comfortable environment, and with controlled air temperature. This accurate testing isn’t practical for most people.

Instead, people often use estimation equations to work out BMR. These are based on measurements like age, sex, weight, and height.

Common Equations to Estimate BMR

Two well-known methods estimate BMR in adults:

• Harris-Benedict Equation
• Mifflin-St Jeor Equation

Here is what these look like:

Harris-Benedict Equation (revised)

• For men:

  BMR = 88.362 + (13.397 × weight in kg) + (4.799 × height in cm) - (5.677 × age in years)

• For women:

  BMR = 447.593 + (9.247 × weight in kg) + (3.098 × height in cm) - (4.330 × age in years)

Mifflin-St Jeor Equation

• For men:

  BMR = (10 × weight in kg) + (6.25 × height in cm) - (5 × age in years) + 5

• For women:

  BMR = (10 × weight in kg) + (6.25 × height in cm) - (5 × age in years) - 161

These equations are estimates. Actual daily energy needs may be different based on many factors, such as activity, stress, health, and genetics.

BMR and Weight Management

Your BMR is a starting point for working out how much energy (calories) you need each day. Knowing this can help people manage their diet for weight loss, maintenance, or gain.

• If you eat fewer calories than your body uses (below TDEE), you lose weight
• If you eat more calories than your body uses, you gain weight
• If you eat the same as your body uses, your weight stays the same

But eating fewer calories than your BMR is not safe. Your body needs enough energy for essential functions. Prolonged very low calorie intake can slow your metabolism further and damage organs.

People who want to lose or gain weight should consider the BMR and add or subtract calories based on physical activity and overall lifestyle—not just stop at the BMR.

Boosting BMR Naturally

There are some habits known to support a higher BMR in most people:

• Build muscle with regular strength or resistance exercise
• Keep physically active throughout the day
• Avoid extreme calorie restriction
• Get enough protein, which can slightly increase metabolic rate during digestion

There are many myths about foods or supplements that ‘boost’ metabolism significantly. Scientific studies do not support most of these claims. Staying active and keeping muscle mass are more reliable ways to increase BMR.

How BMR Changes Through Life

People’s BMR is highest during periods of rapid growth (infancy, childhood, adolescence, pregnancy). BMR tends to decline as people get older. This is linked to loss of muscle mass and other age-related changes.

Health problems, stress, sleep deprivation, and some medicines can also reduce BMR. Regular physical activity and healthy food choices can help slow down the natural decline where possible.

Health Problems Linked with BMR

Abnormal BMR can cause health issues or signal other problems. High BMR may be a sign of overactive thyroid, infection, or fever. Low BMR is often linked to underactive thyroid, extreme starvation, or ageing.

Low BMR can mean fatigue, weight gain, feeling cold, and sometimes depression or sluggishness. High BMR can cause weight loss, feeling too warm, or rapid heart rate. If someone notices big changes in weight or energy levels without clear reasons, they should talk to a health professional.

Health problems that affect BMR include:

• Thyroid disorders (hyperthyroidism or hypothyroidism)
• Cushing’s syndrome
• Muscle wasting diseases
• Infections

Getting medical help and following a balanced diet is important if BMR is unusually high or low due to illness.

BMR and Different Population Groups

Certain groups have unique BMR patterns due to life stage or health status. Here are a few examples:

• Pregnant women: Higher BMR for the growth of the baby
• Infants and children: High BMR due to rapid growth
• Elderly: Lower BMR due to loss of muscle mass
• People with disabilities or serious illness: Might have different BMR needs

Healthcare workers and carers should watch for signs that someone might not get enough calories for their BMR. Signs can include unexplained weight loss, tiredness, or coldness.

Common Misunderstandings About BMR

People can mix up BMR with related terms. It may help to clear up some confusion:

• BMR is the calorie need at rest, not including movement or digestion.
• Resting Metabolic Rate (RMR) is almost the same as BMR but is a little higher, due to less strict testing conditions.
• Total Daily Energy Expenditure (TDEE) is the sum of BMR, calories from physical activity, and calories used to digest food.

Many think a ‘slow metabolism’ is the only reason for weight gain. In reality, big differences in BMR between individuals are rare, barring illness. Weight gain more commonly links to extra calories eaten and lack of movement.

Final Thoughts

Basal metabolic rate (BMR) is a basic but important function in human health. It shows the minimum energy your body needs to keep alive and running, even while you are resting or sleeping. Many factors shape your own BMR, such as age, sex, muscle mass, genetics, and overall health. While some people are born with a slightly faster or slower metabolism, most changes to BMR are linked to body composition and health.

Knowing your BMR can guide better choices for nutrition and physical activity. Remember, most of your daily calorie needs support basic body functions, not just exercise. For anyone wishing to adjust weight or support their health, understanding BMR gives a helpful starting point. Safe and steady changes to diet and activity levels usually work best.

Be careful not to go below the calorie needs set by your BMR, as the body relies on this energy for vital processes. If you have any concerns about your metabolism or unexplained changes in weight, it is wise to speak to a healthcare professional. They can help identify if there is an underlying health issue that affects BMR.

Using information about BMR helps you make educated choices for yourself or supports others in health and nutrition. It underpins many other areas in nutrition, from meal planning to special diet support. By looking after your BMR through good habits, you can help maintain overall health for the long term.