How to speed up your metabolism: what really works, what's a myth and why diets slow it down
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One kilo of muscle burns about 13 kcal a day, and metabolism is stable between the ages of 20 and 60. What really increases your energy expenditure, which tricks are myths and how to stop a harsh diet from slowing it down for years.
One kilo of muscle burns about 13 kilocalories a day at rest. One kilo of liver, about 200; one of brain, about 240. These are the values researchers use to calculate what each tissue burns, and they dismantle two ideas at once: that gaining a couple of kilos of muscle sends your metabolism soaring, and that muscle is irrelevant here. The truth lies somewhere in between, and it's far more useful.
Here's what metabolism is, whether it really slows down with age, what works to burn more energy, what's a myth, and why very harsh diets slow it down.
What your metabolism is made of
What we call metabolism is your total daily energy expenditure, and it has three main components:
| Component | What it is | How much it can change |
|---|---|---|
| Basal metabolism | What you burn at rest to keep your body running. It's usually the largest share of expenditure | Little, and mainly through lean mass |
| Thermic effect of food | What you burn digesting and processing what you eat: 5-15% of daily expenditure on a mixed diet | Somewhat, by eating more protein |
| Physical activity | Exercise and all the movement in your day: walking, standing, moving around (NEAT) | A lot: it's the most variable component |
You can estimate your basal metabolism and your total expenditure with the maintenance calorie calculator, which uses the Mifflin-St Jeor equation and shows your expenditure at each activity level.
Does metabolism slow down with age?
Less than people think, and later. In 2021, Pontzer and a large international consortium analysed energy expenditure measured with doubly labelled water, the gold-standard method, in people aged from 8 days to 95 years. Once adjusted for fat-free mass, expenditure remained stable between the ages of 20 and 60, even during pregnancy, and only began to decline after 60.
In other words: if you gain weight more easily at 40 than at 25, the most likely reason isn't that your metabolism "is broken", but that you move less, have lost some muscle or eat differently. These are all things you can change. We go into it in the workout routine for over-40s.
Which tissues burn the most energy
The reference values collected in the work of Wang and colleagues (2012) show how much each tissue burns at rest per kilo per day:
| Tissue | Kcal per kilo per day |
|---|---|
| Heart and kidneys | 440 |
| Brain | 240 |
| Liver | 200 |
| Skeletal muscle | 13 |
| Other tissues | 12 |
| Fat tissue | 4.5 |
Gaining 2 kilos of muscle adds about 26 kcal a day at rest: real, but small. Some reviews, such as the one by Westcott (2012), cite increases in resting metabolism of around 7% after ten weeks of strength training, something the new muscle alone doesn't fully explain. The strongest reason to train for strength lies elsewhere: muscle is hard to win back once you lose it, it is what lets you move more and better for decades, and when you diet it helps make sure that what you lose is fat.
What really increases your energy expenditure
1. Moving more outside the gym (NEAT)
The study by Levine, Eberhardt and Jensen (1999) is the most revealing. They overfed 16 volunteers with an extra 1,000 kcal a day for eight weeks. Two thirds of the increase in their expenditure came from NEAT, the activity that isn't exercise: moving around, standing, gesturing, walking. Differences in NEAT explained up to tenfold differences in the fat each person gained. Those who moved the most gained the least.
It's the easiest component to increase on purpose: walk more, take the stairs, get up every hour. We put numbers on it in how many steps a day you need to lose weight.
2. Strength training
It adds expenditure during the session, preserves or increases muscle and, in the long run, slows the loss of lean mass that comes with a sedentary lifestyle and with age. You'll find how many calories each type of session burns in calories burned at the gym.
3. Eating enough protein
According to the review by Westerterp (2004), the thermic effect of food accounts for 5-15% of daily expenditure on a mixed diet, and it's higher the more protein the diet contains: of the macronutrients, protein costs the most energy to process, followed by carbohydrates and, last of all, fats. It's also more filling. Work out how much you need with the protein calculator and choose your sources from the highest-protein foods.
What moves the needle most, in order: moving more throughout the day, strength training to preserve your muscle and eating enough protein. Everything else is secondary.
Myths about how to speed up your metabolism
| Promise | How much truth there is |
|---|---|
| "Eating every 3 hours speeds up your metabolism" | The thermic effect depends mainly on how much you eat and the proportion of protein, not on how many times a day you eat (Westerterp, 2004) |
| "Green tea burns fat" | A meta-analysis of 11 studies found a small effect: about 1.3 kg on average (Hursel, 2009) |
| "After 30 your metabolism plummets" | Expenditure adjusted for lean mass is stable between the ages of 20 and 60 (Pontzer, 2021) |
| "Fat burners speed up your metabolism" | Many are based on caffeine and other stimulants; none of them replaces a calorie deficit |
| "A couple of kilos of muscle will make you burn hundreds more kcal" | One kilo of muscle burns about 13 kcal a day at rest |
Why very harsh diets slow down your metabolism
When you lose weight, you burn less: there's less body to maintain. But the body usually cuts its expenditure by more than the weight lost explains. This is what's known as adaptive thermogenesis, which Rosenbaum and Leibel (2010) describe as a set of metabolic, hormonal and behavioural responses that defend fat stores, and which helps explain why most people who lose weight regain it.
The most extreme case was measured by Fothergill and colleagues (2016) in 14 contestants on the show "The Biggest Loser". After 30 weeks of extreme dieting and exercise, they lost an average of 58 kilos and their resting metabolism dropped by about 610 kcal a day. Six years later they had regained 41 kilos, but their resting metabolism was still about 700 kcal below where it started, and about 500 kcal of that drop wasn't explained by changes in weight or body composition.
| Time point | Weight | Resting metabolism compared with the start |
|---|---|---|
| End of the competition (30 weeks) | 58 kg lost on average | About 610 kcal/day less |
| Six years later | 41 kg regained | About 700 kcal/day less (about 500 not explained by weight) |
This isn't what happens to everyone who diets: it was an extreme and very rapid loss. But it points in the direction you want to avoid. A moderate deficit, enough protein, strength training and keeping up your daily movement reduce the slowdown and protect your muscle, as we explain in how to lose fat without losing muscle.
A practical plan to "speed up" your metabolism
- Move every day: rack up steps and break up long stretches of sitting. It's the biggest lever you have.
- Do strength training 2-4 days a week to gain or preserve muscle.
- Eat enough protein and spread it over several meals in whatever way suits you.
- If you diet, keep it moderate: the calorie deficit calculator gives you a figure that never drops below a safe minimum.
- Sleep well: sleeping too little usually means more hunger and less desire to move; we cover it in sleep and performance.
- Forget the shortcuts: teas, spicy food and fat burners add little or nothing compared with everything above.
The Progrevia generator creates a strength routine tailored to your level, the days you have available and your goal, which is the foundation of everything above.
- Pontzer H, Yamada Y, Sagayama H, Ainslie PN, Andersen LF, Anderson LJ, et al. Daily energy expenditure through the human life course. Science. 2021;373(6556):808-812.
- Wang Z, Ying Z, Bosy-Westphal A, Zhang J, Heller M, Later W, et al. Evaluation of specific metabolic rates of major organs and tissues: comparison between nonobese and obese women. Obesity. 2012;20(1):95-100.
- Levine JA, Eberhardt NL, Jensen MD. Role of nonexercise activity thermogenesis in resistance to fat gain in humans. Science. 1999;283(5399):212-214.
- Westerterp KR. Diet induced thermogenesis. Nutrition and Metabolism. 2004;1(1):5.
- Fothergill E, Guo J, Howard L, Kerns JC, Knuth ND, Brychta R, et al. Persistent metabolic adaptation 6 years after "The Biggest Loser" competition. Obesity. 2016;24(8):1612-1619.
- Rosenbaum M, Leibel RL. Adaptive thermogenesis in humans. International Journal of Obesity. 2010;34(Suppl 1):S47-S55.
- Hursel R, Viechtbauer W, Westerterp-Plantenga MS. The effects of green tea on weight loss and weight maintenance: a meta-analysis. International Journal of Obesity. 2009;33(9):956-961.
- Westcott WL. Resistance training is medicine: effects of strength training on health. Current Sports Medicine Reports. 2012;11(4):209-216.
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Frequently asked questions
Can you speed up your metabolism?
Yes, but mainly by increasing your daily movement (NEAT), doing strength training to preserve muscle and eating enough protein. In the study by Levine (1999), two thirds of the increase in expenditure in overfed people came from moving more.
Does metabolism slow down after 30 or 40?
Not noticeably. According to Pontzer (2021), energy expenditure adjusted for fat-free mass remains stable between the ages of 20 and 60 and starts to decline after 60.
How many calories does a kilo of muscle burn?
About 13 kcal a day at rest, according to Elia's reference values collected by Wang (2012). Fat tissue burns about 4.5 kcal per kilo per day.
Do diets damage your metabolism?
Very harsh ones can reduce expenditure by more than the weight lost explains. In The Biggest Loser contestants, resting metabolism was still about 700 kcal below its starting level six years later (Fothergill, 2016). A moderate deficit with enough protein and strength training reduces this effect.
Does green tea speed up your metabolism?
Its effect is small. A meta-analysis by Hursel (2009) found an average loss of about 1.3 kg with green tea catechins compared with placebo.
Informational content based on the available scientific evidence: it does not replace advice from a healthcare professional. If you have an injury or a medical condition, or are pregnant, check with one before changing your training or diet.
