What Is Metabolism, and Can You Really Speed It Up?

Metabolism is the process by which your body turns food and drink into energy. In everyday talk, a fast metabolism means you burn many calories in a day and a slow one means few. Resting needs take 60 to 70 percent of that burn, digestion about 10 percent, and movement the rest. The tricks measured so far add roughly 60 to 190 kcal a day. A half-hour walk adds about 100 kcal at 70 kg.

What is metabolism, and what does it burn?

The energy it uses falls into three parts: resting, digestion and movement. The National Academies’ 2023 energy report puts resting energy at 60 to 70 percent of the day’s total. The thermic effect of food, the cost of absorbing and processing a meal, is about 10 percent. Physical activity swings the most. Depending on the person, it is 15 to 50 percent.

Take a day that burns 2,400 kcal. I picked that round figure for easy arithmetic. Resting energy is 1,440 to 1,680 kcal and food costs about 240 kcal, which leaves 480 to 720 kcal for movement. The total has a name, TDEE, and what is TDEE covers it. The lab version of resting energy comes in two flavors, basal and resting, and BMR vs TDEE explains the small gap between them.

Not every tissue burns alike. The per-kilogram rates below are Elia’s 1992 figures, as listed by Wang and colleagues. Their 2010 test in 131 healthy adults, with MRI scans of organ size, found the values held for people under 50:

Tissue	Resting burn (kcal per kg a day)
Heart and kidneys	440
Brain	240
Liver	200
Skeletal muscle	13
Fat tissue	4.5

A kilogram of heart or kidney burns about 34 times what a kilogram of muscle does (440 ÷ 13). Muscle burns about 2.9 times what fat does (13 ÷ 4.5). Small organs with high rates burn a lot for their size. Lean mass predicts resting energy better than body weight does. The National Academies report that it accounts for 60 to 80 percent of the differences between people. Body weight alone accounts for only about 50 percent.

Does metabolism slow down in your 30s and 40s?

Not in a large 2021 analysis. Herman Pontzer and colleagues pooled doubly labeled water measurements on 6,421 people in 29 countries, aged 8 days to 95 years. Doubly labeled water is a lab method. You drink water tagged with two isotopes, and the rate each one leaves your body shows how much energy you used in ordinary life.

The team adjusted every person’s burn for lean mass and fat mass, then looked at age. Four stages came out:

• Infancy: energy use per unit of tissue climbs to about 46 percent above adult levels near age 1.
• Childhood to age 20: it falls slowly, about 2.8 percent a year, until it reaches adult levels.
• Ages 20 to 60: it stays flat, even in pregnancy. Sex made no difference once body composition was counted.
• After 60: it declines by about 0.7 percent a year.

That overturns a popular story, the one where metabolism fades in your 30s and explains middle-aged weight gain. In this dataset nothing fades between 20 and 60. After 60, a straight-line sum on a 2,000 kcal day gives about 14 kcal less each year. After ten years the day’s burn sits about 140 kcal lower. By their 90s, adjusted energy use was about 26 percent below middle age.

One limit matters. The study adjusts for body size, so it can’t say what happens if you lose muscle in your 40s. Your burn would drop with it, and the next section sizes that. If your weight is climbing at 45, I’d check the intake side of the diary and your weekly steps before blaming age.

Does muscle burn more calories than fat?

Yes, kilogram for kilogram, but the gap is small. Take the rates above. Adding 3 kg of muscle raises your resting burn by 3 × 13 = 39 kcal a day. Swapping 3 kg of fat for 3 kg of muscle raises it by 3 × (13 − 4.5) = 25.5 kcal.

Lifting programs report more, though the evidence is shakier. A 2020 meta-analysis pooled 18 exercise studies in adults, leaving out people over 65 and pregnant or post-menopausal women. In the pooled result, resistance training raised resting metabolic rate by about 96 kcal a day against controls (95% CI 45 to 147). Aerobic exercise showed no clear effect: 82 kcal, with an interval from −58 to 221 that includes zero. Both together gave 75 kcal, interval −13 to 161, which crosses zero too. The abstract doesn’t say how much muscle the trainees gained, so I can’t split the 96 kcal into new tissue and anything else. Its authors flag different methods and a high risk of bias, so I read 96 as the top of a range, not a promise.

Why does one person burn more than another?

Mostly because of how much lean tissue they carry. A sizable leftover stays unexplained. In 150 adults from northeast Scotland, lean mass explained 63 percent of the variation in basal metabolic rate between people. Fat mass explained 6 percent and age 2 percent. Twenty-six percent stayed unexplained.

Pontzer’s team saw the same spread. Even after matching on lean mass, fat mass, sex and age, total energy use varied by more than 20 percent either way. On a 2,400 kcal day that is 480 kcal or more.

This is why formulas miss. The Mifflin-St Jeor equation and its relatives see your weight, height and age, but not your leftover. Even the National Academies’ adult equations carry a root mean squared error of 246 kcal a day for women and 339 for men.

People who seem to have a fast metabolism are often just moving more. Mayo Clinic says some of them are probably just more active, and maybe fidget more. Movement outside exercise, such as housework or gardening, accounts for about 100 to 800 calories used daily. That range is wider than any trick in the next section.

Medical causes exist, but they are rare. The same page says medical conditions rarely slow metabolism enough to cause a lot of weight gain. The ones that can, such as an underactive thyroid, are uncommon. If you suspect one, ask your doctor, because an article can’t rule it in or out.

How can you boost your metabolism, and by how many calories?

All of these levers were measured in people, and all are small. I take them in this order: protein, caffeine, spice, cold.

Does eating more protein speed it up?

By a small amount. Klaas Westerterp’s review of diet-induced thermogenesis gives theoretical costs for digesting each nutrient. Fat costs 0 to 3 percent of the calories eaten, carbohydrate 5 to 10 percent and protein 20 to 30 percent. Say 400 kcal of your day comes as protein instead of carbohydrate. The cost is 80 to 120 kcal against 20 to 40 kcal, a difference of 40 to 100 kcal.

One study in the review’s table lands inside that range. It ran 24 hours in a chamber with 8 people. A diet with 30 percent of energy from protein cost 14.6 percent of intake to process, and one with 10 percent protein cost 10.5 percent. At the 8.9 MJ (about 2,127 kcal) they ate, the gap is 4.1 points, or about 87 kcal. Fat fell from 60 to 10 percent of energy in the same swap, and carbohydrate rose from 30 to 60. Protein can’t take all the credit. Westerterp notes that the main effect of protein on energy balance is thought to be fullness linked to that heat output.

Do coffee and green tea speed it up?

A little, and the dose sets how much. A meta-analysis of 6 respiration-chamber papers (18 test conditions) found that caffeine raised 24-hour energy use by 429.1 kJ (4.8 percent), about 103 kcal. Catechin-caffeine mixtures gave a similar 428.0 kJ, about 102 kcal. Dose matters, though. The authors report a slope of 0.44 kJ per mg of caffeine. That puts 100 mg at about 10 kcal and 400 mg at about 42 kcal. The abstract doesn’t list the doses behind the 103 kcal average, so I can’t tie it to a number of cups.

An older trial points the same way. In Dulloo’s 1989 study, one 100 mg dose lifted resting rate 3 to 4 percent for 150 minutes. Further 100 mg doses every 2 hours across a 12-hour day added about 150 kcal to daily energy use in lean volunteers. The European Food Safety Authority sees no safety concern for habitual intakes up to 400 mg a day in non-pregnant adults.

Green tea extract made a similar showing in 10 men in a respiration chamber. The extract held 90 mg of EGCG and 50 mg of caffeine at each of three meals. It raised 24-hour energy use by 4 percent. The same caffeine alone did nothing. Four percent of a 2,400 kcal day is about 96 kcal. Remember that this was an extract, not cups of tea.

Does spicy food help?

Barely. A meta-analysis screened 627 trials and could use 9. After capsaicin or its mild cousins, the capsinoids, energy use rose 245 kJ a day, about 59 kcal. Studies whose participants had a lower average BMI reported no effect. A separate review of the same compounds calls the effects small. High doses did better.

Does a cold room help?

It does, for as long as you’re cold. A meta-analysis of randomized trials compared acute exposure at 16 to 19 °C with a 24 °C room. Energy use rose by about 188 kcal a day (95% CI 140 to 237). Eight of its 10 trials reported energy data. The trial table lists exposures of 30 minutes to 12 hours. I could not find where the paper explains how those sessions become a daily figure. They show what a chilly room costs for a while, not what a cold house does over a winter. I wouldn’t build a plan on them.

What do the numbers add up to?

Lever	Measured change (kcal a day)	Evidence
Resistance training	about 96	meta-analysis of exercise studies, high risk of bias
Protein in place of carbohydrate	40 to 100 in theory, about 87 in one chamber study	review, 8 people
Caffeine	about 103	meta-analysis, 6 chamber papers
Green tea extract	about 96	10 men, one chamber study
Hot pepper compounds	about 59	meta-analysis, 9 trials
Cold room, 16 to 19 °C	about 188	meta-analysis, 8 trials, 30 minutes to 12 hours

None of these studies measured the levers together, and effects from different studies don’t simply add. Supplements sold as metabolism boosters fare no better. The Mayo Clinic page says such products usually don’t live up to their claims. Eating very little doesn’t switch metabolism off either, and starvation mode measures the small slowdown that does happen.

What should you do about your metabolism today?

Move more, and start with walking. Of the levers here, it is the sizable, well-measured one. The 2024 Compendium rates a level walk at 2.8 to 3.4 mph at 3.8 METs. One MET is about 1 kcal per kg per hour. Resting is 1 MET. So the extra is (3.8 − 1) × kilograms × hours. I worked it for half an hour:

Body weight	Extra kcal in 30 minutes of walking
55 kg (121 lb)	77
70 kg (154 lb)	98
90 kg (198 lb)	126

At 70 kg a 98 kcal walk matches the 96 kcal lifting and green tea figures. It tops the 87 kcal protein result and the 59 kcal pepper figure. It needs no supplement and no cold room. To run your own steps and weight through the same rule, use the steps to calories calculator. It takes its METs from the Compendium and shows the walk’s total beside the part that is extra to resting.

Your resting burn is already inside any daily total, and that includes the hours you sleep, which how many calories you burn sleeping covers. For the full daily picture, how many calories you burn a day shows where your own number sits.

Here is a step for today. Write down your weight in kilograms, multiply it by 2.8, then by the hours you walked yesterday. That is your extra burn from one day of walking, and you can set it beside the table above. Do the same sum for a week and log it as its own line, so the walk never gets mixed up with the food.