Basal Metabolic Rate (BMR) for Weight Loss and Health
BMR is the largest source of energy expenditure in your body, and it can change. Learn how to calculate it, and make it work in your favour.
- BMR is the largest part of your daily energy expenditure. But it varies and can change with diet, body composition, and health.
- “Adaptive thermogenesis” makes fat loss harder: extreme calorie-restriction diets can lower your BMR, explaining why they often fail.
- Protecting muscle mass and avoiding extreme deficits helps maintain your BMR and makes losing (or maintaining) body fat easier.
Your body needs energy to maintain its basic functions, even while you rest or sleep. Just keeping your body going is actually the most energy-demanding part of your daily energy expenditure. It’s commonly referred to as your “Basal Metabolic Rate” (BMR) or sometimes “Resting Metabolic Rate” (RMR).
Your BMR runs 24 hours a day, 7 days a week, from conception until death. It burns energy whilst you sleep. It burns energy whilst you’re awake. It can adapt, change, and be manipulated to support and promote sustainable fat loss.
How BMR Affects Weight Loss
The formal definition of BMR is the energy required to maintain vital physiological functions in a:
- Fasted “post-absorptive" state,
- At rest
- In a normal temperature environment
- With no (or minimal) outside "stress" (e.g., noise, threats, any acute injury or illness).
It is usually expressed in kilocalories per day (kcal/day), and it is basically the energy used by all your cells and the organs they make: your heart, your lungs, and muscles, your kidneys, liver, brain, and everything else.
For most people, BMR accounts for 60-75% of their “total daily energy expenditure” (TDEE). And as far as losing (or maintaining) body fat, it's really the metabolic “baseline”.
The calculation of BMR can be used to predict your calorie needs, set appropriate targets, and understanding how it works can explain why weight loss isn’t always as easy as it sounds.
How to Calculate Your BMR
BMR is typically calculated using “prediction equations”. There are several equations available; all have their pros and cons, and different levels of complexity.
Simply multiplying your body mass in kilograms by 24 (for men) or 22 (for women) is the most basic formula. So a 100 kg man would have a BMR of ~2,400 kcal a day. But other prediction equations can take into account your age, sex, height, and the amount of fat and muscle you have (i.e., your “body composition”).
If you want a slightly more accurate BMR estimation, you can use the Henry-Oxford calculations:
Men
- Ages 18–30: BMR = (15.057 × Weight, kg) + (6.923 × Height, cm) - 381
- Ages 30–60: BMR = (11.472 × Weight, kg) + (8.696 × Height, cm) - 577
- Ages 60+: BMR = (11.711 × Weight, kg) + (6.756 × Height, cm) - 581
Women
- Ages 18–30: BMR = (14.818 × Weight, kg) + (4.866 × Height, cm) - 390
- Ages 30–60: BMR = (8.182 × Weight, kg) + (5.023 × Height, cm) - 11.6
- Ages 60+: BMR = (8.520 × Weight, kg) + (4.208 × Height, cm) + 10.7
So a 70 kg female who's 160cm tall and 35 years old, would have her BMR calculated as 1,364 kcal a day = [(8.182×70)+(5.023×160) - 11.6].
The more complicated formulas generally provide a more accurate estimation of your “true” BMR. However, even the most complex equations can still have large errors (~10 % or 200-300 kcal/day). So these equations should just be used as a guide, or “educated guess”, rather than an exact number.
What Determines or Affects Your BMR
How Adaptive Thermogenesis Affects Weight Loss
“Adaptive thermogenesis” is the technical term for the body’s response to sudden decreases in your energy intake. It does this to defend against any further weight loss. This is often overlooked when you're doing your calorie counting and calorie-deficient diet.
It is an error to assume that your BMR doesn’t change; it does!
Your body doesn’t like sudden and drastic changes to its environment, even if it technically needs it (for your health). Therefore, it will sometimes resist change by decreasing your BMR. This is great for survival purposes, but not so much when you want to lose a significant amount of body fat.
In response to severe calorie restriction, your body can reduce its BMR by 10–20%, and up to ~30% in extreme circumstances. For our 100 kg person, that could be around 500 kcal a day.
It does this by dialling down the energy use of your organs and altering the hormone levels that regulate BMR (e.g., thyroid hormone, adrenaline, and noradrenaline). This means your body starts working more efficiently, as it has fewer resources.
This response can occur within the first few days of a crash diet, and can actually persist for months or even years after stopping it. It also explains why weight regain is common amongst dieters (i.e., “yo-yo dieting”), making long-term weight loss and maintenance challenging.
Not everyone’s BMR responds in the same way to calorie restriction. Some people can actually only have small decreases in BMR, meaning they’ll lose more weight than other people, even if they're eating the same calories.
There’s also a genetic component that determines whether you maintain (or possibly increase) your BMR while you lose weight. However, there are also things you can do yourself.
