People tend to get confused between the calories burnt between activity and calories burnt during rest. When I explain most calories are burnt at rest, I get asked “How many calories does my body burn at rest?”.
A lot of people don’t realise they actually burn calories during rest. A lot of new clients I work with assume you only burn calories when you exercise. For instance, I remember getting one message asking:
“Why am I eating 2000 calories everyday if my watch tells me I’ve only burnt 700”
Therefore I had to explain how the body burns calories at rest. So I thought it’d be a good idea to explain so in this article.
I always say to imagine if you were to be in a coma for 24 hours, your body would burn a certain number of calories just to keep you alive.
This is what’s known as “Basal Metabolic Rate“. This is what people refer to when they are talking about their metabolisms. This accounts for about 60-70% of calories burned per day.
For your organs to function, your heart to beat and your brain to work, all these functions require calories. In fact, you burn far more calories from your BMR alone than you do from exercise. Unless you’re an athlete.
Your BMR is determined by several components. Most of which you cannot control. These tend to be:
- Body composition
By the end of this post, you’ll know exactly how to determine calories burned at rest. Also, you’ll be able to guesstimate how many calories you also burn at rest.
Below I have added a basal metabolic rate calculator. This will determine your calorie burn at rest, so you know how many calories your body burns at rest.
(credit to mealpro.net)
Once, you know your BMR, you could go one step ahead. You could then work out the total daily calories you burn. But that’s an article for later. But if you’re curious check out my calorie calculator.
If you want to calculate the equation by hand, below is the equation. The equation is different based upon your sex.
For men: BMR = 10 x weight (kg) + 6.25 x height (cm) – 5 x age (years) + 5
For women: BMR = 10 x weight (kg) + 6.25 x height (cm) – 5 x age (years) – 161(credit to: www.omnicalculator.com)
You’ve probably heard the saying:
“my metabolism has slowed down, now I’m older”
There is a tiny amount of truth to this. As you get older your BMR will slightly decrease. But the number of calories your BMR will decrease by is tiny.
If I we had a 5”10, 185lb man and we took his age at 18, 28, 38 and 58. You’ll see his BMR slightly decreases each decade with age, but the difference’ aren’t anything to write home about.
- 18: 1984 kcal
- 28: 1922 kcal
- 38: 1854 kcal
- 48: 1787 kcal
- 58: 1719 kcal
As you can see, the metabolism does slightly slow down with age. However, it’s hardly crashing like a lot of people claim it does. An 18-year-olds metabolism will decrease by 130 calories in 20 years time. That’s less than 1 and a half slices of bread each day, it’s really not much.
But why does BMR decrease with age?
As we get older, our muscles start to deteriorate. On average after the age of 30, our muscle mass decreases 3-8% every 10 years. Then after the age of 60, it decreases even more. (1)
As this muscle mass decreases, it tends to get replaced by fat mass. (1)
A decrease in muscle mass is also accompanied by a progressive increase in fat mass and consequently changes in body compositionVolpi, et al 2004
As fat burns fewer calories compared to muscle, having more fat and less muscle over time, means you will burn fewer calories at rest.
How can I prevent muscle loss?
Regular resistance training and a high protein diet. These 2 combined will ensure your body holds on to as much muscle as possible. In some cases in the right conditions, build more. Which might increase your BMR ever so slightly.
When a male and female of exact same height, age and weight are compared, the male will still have a higher resting metabolism. So why is this? This is also down to body composition.
On average women carry more body fat compared to men. (3) As well as having less fat, men tend to have more muscle mass compared to women. (4) These 2 elements combined together mean, that on average men will have a higher resting metabolic rate than women.
Men have more testosterone than women, hence why they have more muscle mass. Women have more oestrogen than men, thus why they carry more body fat.
This has been explained mostly above. More muscle and less fat tend to mean a faster metabolic rate. But in general, the more you weigh the more calories you burn at rest. When there is more of you, you have more metabolic tissue and that tissue requires calories.
This is why when people lose weight, their BMR numbers decrease. This is why people claim, losing weight slows down your metabolism. This is nothing to worry about, it’s perfectly normal and to be expected. If you weigh less your body has fewer cells to sustain, fewer calories that are needed.
There are some conditions, which will slightly lower someones resting metabolic rate. These include such things as:
- PCOS (5)
- Underactive Thyroid
Women with PCOS have a significantly lower BMR than women without. One study showed on average their BMR was around 22% lower than women without PCOS. (6)
If you have PCOS and want to work out your BMR, I have linked a PCOS calculator.
As for an underachieve thyroid, yes it will lower your metabolic rate. However, that can be fixed with medication. I am not a doctor, therefore I have linked the page from the NHS about underachieve thyroids.
If you ever ask yourself, “How many calories does my body burn at rest?”.
I hope this answers your question as well as giving you some insight as to why. The main components are age, body composition and sex. Though age and sex could arguably also be down to body composition too.
Though you can’t do much to control your BMR. You can do what you can to build and sustain as much lean muscle tissue as possible, by resistance training often and sustaining a high protein diet.
Josh is a Registered Associate Nutritionist, with the Association for Nutrition (AFN). He completed his degree in Nutrition at the University of Roehampton in 2021. He passed with a First Class with Honours.
Josh is also a tutor for up-and-coming Personal Trainers, where he teaches a Level 4 Advanced Nutrition course. This is for Personal Trainers looking to upskill their nutrition knowledge. This is done at Norfolk Health & Fitness.