What Is Basal Metabolic Rate (BMR)?

When you’re relaxing, on your rest days and even when you’re sitting or sleeping, your body continues to burn energy to keep your vital processes fully functioning. When you think about your daily energy requirements, you don’t only need to fuel your body for exercise - powering your brain, heart, lungs and digestion alone takes serious energy (not to mention everything else your body is constantly doing in the background!).

The amount of energy used by your body at complete rest is known as your basal metabolic rate or resting energy expenditure. 

It’s common to think your basal metabolic rate (BMR) wouldn’t account for very much when it comes to energy burn, but it might surprise you to know that it makes up about 65-70% of total energy expenditure in adults, and it can vary due to many factors. BMR can be used as a tool to help determine your total energy requirements and help develop a daily nutrition plan to best support your body, factoring in your daily activities and exercise.

Find out: 

• What is basal metabolic rate (BMR)?

• How to calculate BMR

What is basal metabolic rate?

Basal metabolic rate (BMR) is the amount of energy your body uses to stay alive, at complete rest. 

Another way of thinking of this is your “baseline metabolism” — the amount of energy, in calories, that your body uses to keep you breathing and keep your organs functioning, if you were to do nothing but lie down, in a controlled environment, for a 24 hour period. 

How to calculate BMR

The only way to accurately calculate your BMR is to directly measure it using specific laboratory equipment, however, BMR is commonly worked out using an approximate formula. What’s now a widely adopted equation was first proposed in 1990 in a study by the University of Nevada, which explained that the BMR for women within a healthy normal-weight and moderately overweight range is:

BMR (calories) = 10 x weight (kg) + 6.25 x height (cm) - 5 x age (y) - 161

It’s important to remember this is only an approximation and should be used as a rough guide only - BMR varies from individual to individual due to a vast range of bodily differences and genetics. One 2015 literature review by the University of North Carolina found that resting metabolic rate can vary by as much as 20-30% between different demographics. 

Factors affecting your BMR

Your basal metabolic rate is influenced by a variety of factors,  meaning it may change over time in response to lifestyle, environmental, emotional or life-stage changes. 

Temperature of the environment

When the temperature of the environment is very high or very low, the body requires more energy to maintain the ideal core body temperature. 

Emotional changes

When the body is under stress, cortisol and other hormones are released. These hormones activate your body’s “fight-or-flight” response by increasing your heart rate, directing blood flow to your muscles and tensing muscle tissue — burning additional energy in the process.  

Crash dieting or significant weight loss 

Research has highlighted that eating too few calories can cause the resting metabolic rate to decrease, as can significant weight loss. 

Caffeine

Studies have shown that caffeine can increase your basal metabolic rate, and it has been said that it can have a significant influence on energy balance. 

Disease, injury or infection

There’s a reason you feel so tired when you’re unwell! When you have an injury or illness, the body uses more energy during certain immune responses, and fatigue is also the body’s clever way of forcing you to slow down and rest to let your body heal. 

Body size

Larger adult bodies have more tissue, and the more cells you have, the greater the energy required to sustain your body, resulting in a higher basal metabolic rate. This also explains why a decreased BMR is a normal adaptation that occurs when you lose weight. 

Body composition

Lean muscle tissue burns energy at a faster rate than body fat, even at rest. Adipose (fat) tissue uses much less energy than other organs and tissues of the body. 

This means that strength training exercises can help support or increase your BMR by increasing your level of lean muscle mass. 

Gender and age

Men typically have a higher basal metabolic rates than women due to naturally having a higher level of muscle mass. BMR will also decline as you age, partly due to a decline in muscle mass, but a significant decline doesn’t occur as early as you may expect. According to Harvard Health, total and basal energy expenditure remain stable for most of adulthood and only begin to decline around age 60! 

What you eat

It takes energy to digest meals and snacks, but the macronutrient makeup of your meals can affect your BMR.  Eating plenty of protein is not only important for the repair and building of muscle tissue - it can also raise your BMR by 20-30%, in comparison to carbohydrates which only raise it by 5-10%. Spicy foods can also cause increased energy expenditure, while nutritional deficiencies can slow down your metabolism. 

If you’re wondering about how when you eat might affect your metabolism, recent research suggests it’s not worth worrying about. When it comes to gaining or losing weight, what has the most significant effect is the number and size of your meals.

Does exercise increase metabolism? 

While exercise obviously impacts your total daily energy expenditure, what we’re talking about here is whether exercise has an effect on your resting metabolic rate. 

Research from 2018 showed that BMR increased 22 hours following both moderate and high-intensity exercise, which may be caused by factors such as exercise-induced muscle damage and repair. 

Resistance or strength training may also have an effect. One 2000 study published by Colorado State University found that resting metabolic rate for women aged 22-35 increased after intense resistance training, and remained elevated for 16 hours after exercising. 

Another randomised controlled trial from 2014 had a group of healthy adults complete 96 resistance training workouts over nine months and their results indicated a significant increase in BMR. Does that mean everyone will experience a significantly increased BMR from strength training? Not necessarily, as the researchers also highlighted the wide variability between individuals.

Should you eat differently on rest days?

If strength training and high-intensity workouts can increase your metabolism, the question naturally follows about whether your nutrition should change on days you’re not training. If you have a regular workout schedule, your nutrition should remain largely the same as your body is still working hard beneath the surface to recover and grow stronger. Even if you are aiming to lose weight, any reduction in food consumption on rest days should be small, otherwise, you’re putting your recovery, muscle growth and energy levels at risk.

BMR and weight loss (or gain!)

According to Mayo Clinic, rarely does the metabolism slow enough to cause a lot of weight gain, and a 2016 study published in the American Journal Of Clinical Nutrition also found BMR alone was not a predictive indicator of weight gain. 

The researchers found that adults with a low BMR did not gain more weight than adults with a high BMR, and concluded that habitual differences in food intake and activity levels of individuals counterbalance variations in BMR. 

On the other hand, if it’s a goal of yours to lose weight (and it certainly isn’t for everyone), understanding your BMR can help you plan a balanced diet and exercise routine to help you lose weight at a gradual and sustainable pace.

Knowing your basal metabolic rate can help you understand your body

Besides helping you work out your daily nutrition requirements and how to adequately fuel your body, understanding BMR can also help you understand the reasons behind changes in energy or appetite, both for yourself and your loved ones!