What is metabolism?
It’s how your body turns food into energy. The biggest share keeps you alive at rest. When two people burn different amounts at rest, one thing explained most of the gap in a large study.
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What is metabolism?
It’s how your body turns food into energy. Your gut breaks food down into sugars and acids, your body’s fuel. You use that fuel straight away or store it.
When people talk about metabolism, they usually mean how many calories you burn in a day. That burn has three parts.
| Part | What it pays for | How big |
|---|---|---|
| Resting burn | Your heart, lungs, brain and warmth while you do nothing | The biggest part |
| Digesting food | Breaking down what you eat and drink | About 10% of what you eat, in healthy people on a mixed diet |
| Movement | Everything from a walk to getting out of a chair | Grows with how active you are |
Sources: Westerterp 2013; Westerterp 2004
Think of a phone that drains its battery with the screen off. Resting burn is that drain, and it’s the largest part of your daily total.
As a guide, the NHS says an average woman needs 2,000 calories a day and an average man 2,500. Yours could be different, based on your age, weight, height and exercise.
Use those numbers as a starting point. Want to lose weight? Our guide to how many calories to eat to lose weight takes it from there.
What changes your metabolism?
Mostly your size. And the one thing that explained most of the gap in resting burn between people? That’s your lean mass.
Lean mass is everything in you that isn’t fat: muscle, organs and bone. A Scottish study measured the resting burn of 150 adults. Lean mass explained 63% of the differences between them.
| What differed between people | Share of the gap it explained |
|---|---|
| Lean mass (muscle, organs, bone) | 63% |
| Fat mass | 6% |
| Age | 2% |
| Left unexplained | 26% |
Source: Johnstone 2005
Being a man or a woman made no clear difference once lean mass, fat mass and age were counted.
Age matters less than people think. For your body size, daily burn stays steady from 20 to 60, then falls in older adults.
Of these, lean mass is the one you can build. Our guide on how to increase your metabolism shows what works.
Do some people have a slow metabolism?
A little. In the Scottish study, 26% of the gap in resting burn stayed unexplained. So two people of the same build can burn slightly different amounts.
But a bigger body doesn’t usually burn less. Both lean mass and fat mass added to resting burn.
Heavier people also tend to move less. Yet they burned the same or more on activity, because moving a bigger body costs more.
Your thyroid plays a part too. In men, the thyroid hormone T4 explained a quarter of the unexplained gap. It didn’t in women.
An underactive thyroid can cause weight gain, extreme tiredness and feeling cold. A blood test can check it. See underactive thyroid symptoms.
How does your body burn fat?
Fat is stored fuel. Eat more calories than you use and your body stores the extra energy as fat. Eat fewer and you’re likely to lose weight, because it uses its stored fat for energy.
Where the energy in your food goes
- 1
You eat
Food is made of protein, carbs and fat.
- 2
You digest it
Your gut breaks it into sugars and acids.
- 3
You use or store it
Fuel is used now or stored in liver, muscle and body fat.
- 4
You draw on stores
Eat less than you use and your body uses stored fat.
Sources: MedlinePlus; NHS: understanding calories
Body fat works like savings. Eat more than you burn and the savings grow. Burn more than you eat and you draw them down.
So to use stored fat, eat less than you burn. Our calorie deficit calculator works out how much less.
Does your metabolism slow down when you lose weight?
Yes, more than your smaller size explains. In a US lab study, people kept their weight 10% or more below where they started.
| Who | Weight kept 10% or more lower | Weight kept 10% higher |
|---|---|---|
| Never had obesity | Down about 6 calories a day per kilo of lean mass | Up about 9 |
| Had obesity | Down about 8 calories a day per kilo of lean mass | Up about 8 |
Source: Leibel 1995
Say you have 50 kg of lean mass. Our sum: 6 to 8 calories times 50 is about 300 to 400 fewer calories a day.
The researchers say this may help explain why lost weight is hard to keep off. It was a small study of 41 people.
How to work with your metabolism
- Use the NHS guide of 2,000 calories a day for women and 2,500 for men as a starting point.
- To lose weight, eat and drink fewer calories than your body uses.
- Lose weight steadily: aim for 0.5 to 1 kg a week.
- Do strengthening activities that work all the major muscle groups on at least 2 days a week.
- If you’re trying to lose weight, get active for 45 to 60 minutes a day. You can break this up into shorter sessions.
Sources: NHS: understanding calories; NHS: obesity; NHS: activity guidelines
See how much weight you can lose in a month at that pace, and how to keep your muscle while you lose weight.
When to speak to a GP
Some changes put down to metabolism can come from your thyroid, and some need extra help. See a GP if:
- You’re putting on weight and feel extremely tired or cold more than usual.
- You’re losing weight, often despite a bigger appetite.
- Your heartbeat is unusually fast or irregular, or you’re sensitive to heat.
- Your BMI is in the overweight or obese range and you want help to lose weight.
- You have, or think you may have, an eating disorder such as binge eating.
Haven’t exercised for some time, or have a medical condition? The NHS says speak to your GP first.
Losing weight while eating more? See overactive thyroid symptoms and the NHS overactive thyroid page.