The UK is a nation of dieters, says market research outfit Mintel, with almost half of the people it surveyed admitting to having been on a weight-loss diet during the previous yeari. And of those who’d tried losing weight, almost two thirds said they were on a diet most or all of the time.
Yet losing weight can be a complicated affair. There are so many different diets to choose from these days, it’s hard to know which one to follow. But no matter how elaborate and fanciful some modern-day diets are, if they’re effective it’s because they follow one simple rule: to lose weight you must create a calorie deficit. This means restricting your calorie intake so your body’s getting less energy from food and drink than it actually needs.
However, to do this you first need to know how many calories you need to function normally. According to the NHS, men need around 2,500 calories a day to maintain a healthy body weight, while women need around 2,000 a dayii. Then if you’re looking to lose weight, eating and drinking 500 calories fewer each day will see you losing a steady 1lb a week until you reach your goal.
It’s good advice, and many people do lose weight successfully by following this method. However, its one-size-fits-all approach is just a guide, not just because we’re not all the same shape, size, weight or age but also because some of us are far more physically active than others (and vice versa).
So if you find that restricting yourself to 2,000 calories a day if you’re a man or 1,500 a day if you’re a woman isn’t working for you, perhaps it’s time to work out a more effective strategy. And that’s where BMR and TDEE come in.
BMR vs TDEE
While both tell you the number of calories your body needs, BMR and TDEE aren’t exactly the same.
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BMR is short for basal metabolic rate (basal means forming or belonging to a bottom layer or base).
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TDEE stands for total daily energy expenditure.
BMR is the the basic number of calories you need to sustain life. It’s the energy your body needs to keep your heart pumping, your circulation, lungs, brain and other vital organs working, and your body temperature regulated, and so on.
TDEE, on the other hand, is the number of calories you burn every day, not just to sustain life but to go about your daily activities (including exercise). And to calculate your TDEE you first need to find out your BMR. Then once you know your TDEE, you can set an effective calorie deficit goal.
How to work out your BMR
The term BMR is used interchangeably with RMR (which stands for resting metabolic rate). And while the two are similar, the’re not identical. Strictly speaking BMR is a highly accurate measurement that requires testing in a laboratory or clinical setting. It’s measured by using a process called indirect calorimetry – that is, how much oxygen you use and carbon dioxide you produce. And the measurement is typically taken after you’ve been asleep for eight hours and fasted for 12 hours to determine the most accurate result possible.
RMR, however, isn’t such a stringent measurement and is usually calculated using an equation (though since it’s less complicated, it’s not quite as accurate as BMR). So when most people talk about their BMR, what they probably mean is RMR. Since most sources use the term BMR rather than RMR, however, we’ll stick with BMR here.
Popular BMR equations
Several equations have been developed to roughly calculate your BMR, the most popular of which are:
Harris-Benedict equation
This was originally published in a 1918 study by the biometrician James Arthur Harris and nutritionist Francis Benedict. Then in 1984 it was revised for better accuracy – this version is known as the Revised Harris-Benedict equationiii.
The revised equation is as follows:
Men: BMR = 88.362 + (13.397 x weight in kg) + (2.799 x height in cm) - (5.677 x age in years).
Women: BMR = 447.593 + (9.247 x weight in kg) + (3.098 x height in cm) - (4.330 x age in years).
Mifflin-St Jeor equation
Another revision of the Harris-Benedict equation was published in 1990, named after two of the study’s authorsiv. This was updated to be in line with modern lifestyles, and is often thought to be more accurate than either of the Harris-Benedict equations. Indeed, in a comparison of various BMR equations, the American Dietetic Association found the Mifflin-St Jeor the most reliablev.
Here’s the equation:
Men: BMR = (10 x weight in kg) + (6.25 x height in cm) - (5 x age in years) + 5
Women: BMR = (10 x weight in kg) + (6.25 x height in cm) - (5 x age in years) - 161
Katch-McArdle equation
Newer equations for calculating BMR have also been developed for better accuracy in people who have a higher amount of lean muscle than average – athletes, for instance – such as the Katch-McArdle equation. However, to use this equation, you have to know your body fat percentage:
BMR = 370 + (21.6 x lean body mass)
The equation is the same for men and women, and you can calculate your lean body mass by multiplying your body weight by your body fat percentage.
If you’re not that handy with a calculator, you can find numerous BMR calculator tools online. Not all of them state which equation they use, but this one lets you select either the Harris-Benedict, Mifflin-St Jeor or Katch-McCardle method (select from the advanced options menu) if you want to give all of them a try: www.freedieting.com/calorie-calculator.
Working out your TDEE
Armed with your BMR, the next step is to work out your TDEE. While it may not be a precise science, this gives you a reasonable estimate of how many calories you need every day for your current level of physical activity. And it’s easier than calculating BMR, since all you have to do is multiply your BMR by one of the following numbers, depending on how active you are:
BMR x 1.2 (sedentary, little to no exercise)
BMR x 1.3 (light exercise one to three days a week)
BMR x 1.55 (moderate exercise three to five days a week)
BMR x 1.725 (heavy, hard exercise six or seven days a week)
BMR x 1.9 (having a highly physical job or doing hard exercise six or seven days a week)
When you’ve worked out your TDEE, the number represents how many calories you need each day to maintain your weight at its current level. So if you want to lose weight, you need to have fewer calories (or more if your aim is to gain weight).
Using TDEE for weight loss
Looking at weight loss, your next question should be: how low should you go? Theoretically, any number of calories lower than your TDEE will lead to weight loss, though obviously the wider your calorie deficit, the faster you’ll lose weight.
One method that’s often recommended is the 500-calorie-a-day deficit (TDEE minus 500), which – as mentioned above – should see you losing a steady 1lb a week. However, many experts recommend reducing your calories by 10 or 20 per cent of your TDEE instead. That’s because, if you have a relatively low BMR and TDEE, subtracting 500 calories from your TDEE could mean your calorie goal will be lower than your BMR.
This may not be such a good idea, since eating too few calories in the long term may put your body into what some experts call starvation mode (or nutrient deficient mode). In other words, when it doesn’t get what it needs to function normally, your body will try to conserve energy by reducing the number of calories it burns – a process called adaptive thermogenesisvi. And if your body is burning fewer calories than it would normally, it can be even harder to lose weight.
But reducing your TDEE by 10 or 20 per cent rather than a set number may be more sustainable for many of us in the long run, even if it means we don’t drop a lot of weight quickly (though slow weight loss is actually thought to be more effective, especially for keeping it off in the long run(vii).
Not sure which diet to try? Get the lowdown on two popular approaches by reading Low carb or low fat: which is best for weight loss? Also take a look at our other sports articles for lots of information to help keep you in your best shape ever.
References:
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Available online: https://www.nhs.uk/live-well/eat-well/cut-down-on-your-calories
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Roza. A.M., Shizgal. H.M. (1984). The Harris Benedict Equation Reevaluated: Resting Energy Requirements and the Body Cell Masse. Am J Clin Nutr. 40 (1), 168-82. Available online: https://pubmed.ncbi.nlm.nih.gov/6741850-the-harris-benedict-equation-reevaluated-resting-energy-requirements-and-the-body-cell-mass
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Mifflin. A.D., St Jeof. S.T., (1984). A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 50 (2), 241-47.
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Frankenfield. M.S., Roth-Yousey. L., Compher. C. (2005). Comparison of Predictive Equations for Resting Metabolic Rate in Healthy Nonobese and Obese Adults: A Systematic Review. J Acad Nutr and Diet. 105 (5), 775-789. Available online: https://jandonline.org/article/S0002-8223(05)00149-5/abstract
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Rosenbaum. M., Leibel. R.L. (2010 Oct). Adaptive thermogenesis in humans. Int J Obes (Lond). 340(0 1):S47-S55. Available online: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3673773
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Available online: https://www.nhs.uk/live-well/healthy-weight/should-you-lose-weight-fast
Christine
Christine Morgan has been a freelance health and wellbeing journalist for almost 20 years, having written for numerous publications including the Daily Mirror, S Magazine, Top Sante, Healthy, Woman & Home, Zest, Allergy, Healthy Times and Pregnancy & Birth; she has also edited several titles such as Women’ Health, Shine’s Real Health & Beauty and All About Health.
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