Metabolism gets dropped into almost every conversation about nutrition, but few people could say exactly what it means. In simple terms, it's every chemical reaction that keeps you alive, and for most people the biggest share of its energy is used while they're resting and think they're doing nothing at all.
Jump to:
- What is metabolism?
- What is your metabolism made up of?
- Can you increase your metabolism?
- Frequently asked questions
What is metabolism?
Metabolism is the combination of all the chemical reactions going on in your body at any one time. To understand why, it helps to remember that you, and almost everything you know, are made of chemicals. The skin on your hands, the blood flowing through your veins and the DNA in your cells that makes you unique are all chemicals. Even the nerve impulses that let you see, hear and process information are chemicals, plus a little electricity.
Just keeping you alive takes a huge number of carefully coordinated reactions, all the time. You use chemical reactions to keep your body at a constant temperature, and to break down the protein bar you ate earlier into amino acids, sugars and fatty acids. You need more of them to transport those amino acids to your muscles, pass them across your cell membranes and turn them into new muscle protein, and more again to stand up, sit down and lift weights.
Many of these reactions need energy. Your body is essentially a chemical machine, and the energy you eat in the form of food goes to supply the different parts of your metabolism.
What is your metabolism made up of?
Your metabolism is made up of four main parts:
- Basal metabolic rate (BMR)
- Non-exercise activity thermogenesis (NEAT)
- Thermic effect of food (TEF), sometimes called diet-induced thermogenesis
- Exercise activity thermogenesis (EAT)
Basal metabolic rate (BMR)
Your BMR is the energy your body uses to maintain all the essential metabolic processes that keep you alive, even when you're at rest. For most people, BMR uses the most energy by far of all the parts of metabolism.
Every part of your body needs a different amount of energy to work. One kilogram of adipose (fat) tissue uses about 4.5 calories a day, while the same weight of muscle needs around 13 calories a day to function properly. That's why people rightly say muscle burns more calories than fat.1
Muscle has nothing on some of your organs, though. A kilogram of brain tissue uses about 240 calories a day, and the same amount of heart tissue uses about 440 calories a day just carrying out its basic functions.1 Your body usually has a lot more muscle and fat than it has of these organs, so overall, muscle and fat contribute far more to your daily calorie burn.
Non-exercise activity thermogenesis (NEAT)
NEAT is all the energy you use for physical activity and movement that isn't specifically exercise, and for most people it's the next biggest user of energy after BMR. It includes walking from your car to the office, carrying your shopping, scratching your head, fidgeting in your chair, bobbing your head to music and even moving your eyes from side to side while you eat.
Many people don't realise how important NEAT is to their metabolism. Some people have very adaptive metabolisms, meaning their metabolism speeds up and slows down a lot depending on how much they eat. Give them a lot of extra calories and they often move and fidget more, which helps them burn off some of the surplus.2
Restrict their calories and their NEAT often drops: they become much less active, with less fidgeting, less head bobbing and even fewer facial expressions, moving only for the most essential tasks.3
Thermic effect of food (TEF)
TEF is the energy you burn just processing your food, and it varies with the macronutrients you eat:
- Fat: about 1-3% of the energy it contains is used to digest it
- Carbohydrates: about 5-10% of their energy content
- Protein: by far the highest, at about 20-30% of its energy content4
According to Richie, this is a small part of the reason high-protein diets tend to work well for fat loss: some of the calories in the protein are used up just by eating it.
Exercise activity thermogenesis (EAT)
EAT is all the energy you spend when you intentionally exercise. It varies a lot from person to person, and in athletes who train for several hours every day, it can be enormous.
That partly explains how some professional athletes can eat thousands of calories a day while training and still stay lean. Only partly, because so many other factors are involved.
Total daily energy expenditure (TDEE)
Together, BMR, NEAT, TEF and EAT make up your total daily energy expenditure (TDEE): all the energy you need to support the different parts of your metabolism. This is what many people simply call their metabolic rate.
Can you increase your metabolism?
Yes, you can increase your metabolic rate, but most of the ways to do it have only a small effect, and the most effective one is exercise.
Build more muscle. Technically, being bigger increases your metabolic rate, and the most realistic way to do that is by building muscle. The amount of muscle most people can realistically build won't increase their metabolism a great deal, but it does have a small effect.
Eat more protein. If you replace some of the fat and carbohydrate calories in your diet with protein, your TEF will be higher. Again, it's not a huge amount. There's also some evidence that meals higher in whole, unprocessed foods have a slightly higher TEF than the same number of calories from processed food, but this effect is very small too.5
Move more. You can't really ask someone to fidget more, so NEAT is hard to change on purpose. That leaves the last thing anyone wants to hear and probably the best way to increase your metabolism: exercise more.
Because not many people want to do a lot more exercise, when it comes to weight loss it makes more sense to focus on the calories-in side of the equation, which means reducing how many calories you eat overall.
Frequently asked questions
What is metabolism in simple terms?
Metabolism is the combination of all the chemical reactions going on in your body at any one time. It covers everything from keeping your body at a constant temperature to digesting food, building muscle protein and moving, and many of these reactions are powered by the energy in the food you eat.
What uses the most calories in your metabolism?
For most people, basal metabolic rate (BMR) uses the most energy by far. This is the energy your body uses to keep you alive at rest. The next biggest user is usually non-exercise activity thermogenesis (NEAT), the energy you spend on everyday movement that isn't exercise.
Does muscle burn more calories than fat?
Yes. One kilogram of muscle uses about 13 calories a day, compared with about 4.5 calories a day for the same weight of fat tissue. Even so, the amount of muscle most people can realistically build only has a small effect on their metabolism.
Does protein affect your metabolism?
Protein has the highest thermic effect of any macronutrient, with about 20-30% of its energy used to digest and process it, compared with 5-10% for carbs and 1-3% for fat. Swapping some fat and carbohydrate calories for protein raises the thermic effect of your diet, but not by a huge amount.
Why do some people move less when they cut calories?
In people with very adaptive metabolisms, cutting calories often causes non-exercise activity (NEAT) to drop. They fidget less, move less and even make fewer facial expressions, which reduces the calories they burn. When they eat a lot of extra calories, the opposite tends to happen.
What's the best way to increase your metabolism?
Exercise is probably the best way to increase your metabolic rate. Building muscle and eating more protein can raise your metabolic rate slightly, but the effect is small. For weight loss, it usually makes more sense to focus on reducing the calories you eat overall.
Take home message
Your metabolism is the sum of every chemical reaction in your body, and most of its energy goes on simply keeping you alive at rest. Building muscle, eating more protein and choosing whole foods can nudge your metabolic rate up slightly, but exercise is the most effective option. For weight loss, managing how many calories you eat overall tends to make more of a difference than trying to speed up your metabolism.
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Dr Richie Kirwan is a Senior Lecturer at Liverpool John Moores University, where his research on muscle mass and cardiometabolic health has influenced health policy across the UK, Ireland and the USA. A leading authority on high-protein diets and resistance training, Richie translates complex science into actionable fitness advice. When off duty, you’ll find him in the gym, on the hurling pitch, or the salsa dance floor.
References
- Wang, Z., Ying, Z., Bosy-Westphal, A., Zhang, J., Schautz, B., Later, W., Heymsfield, S. B., & Müller, M. J. (2010). Specific metabolic rates of major organs and tissues across adulthood: evaluation by mechanistic model of resting energy expenditure. American Journal of Clinical Nutrition, 92(6), 1369-1377. https://doi.org/10.3945/ajcn.2010.29885
- Levine, J. A., Eberhardt, N. L., & Jensen, M. D. (1999). Role of nonexercise activity thermogenesis in resistance to fat gain in humans. Science, 283(5399), 212-214. https://doi.org/10.1126/science.283.5399.212
- Trexler, E. T., Smith-Ryan, A. E., & Norton, L. E. (2014). Metabolic adaptation to weight loss: implications for the athlete. Journal of the International Society of Sports Nutrition, 11(1), 7. https://doi.org/10.1186/1550-2783-11-7
- Westerterp, K. R. (2004). Diet induced thermogenesis. Nutrition & Metabolism, 1(1), 5. https://doi.org/10.1186/1743-7075-1-5
- Barr, S. B., & Wright, J. C. (2010). Postprandial energy expenditure in whole-food and processed-food meals: implications for daily energy expenditure. Food & Nutrition Research, 54(1), 5144. https://doi.org/10.3402/fnr.v54i0.5144
