Where Do Your Resting Calories Go? An Organ-by-Organ Breakdown of BMR
October 21, 2026
Basal metabolic rate, the energy your body uses at complete rest, accounts for roughly 60% to 70% of the calories most people burn each day. But what is it spent on? Studies using imaging and calorimetry have estimated how much each organ and tissue uses at rest. The pattern is surprising: a few small, metabolically active organs account for more than half of the total, while the bulk of body weight, such as fat and bone, uses very little.
For a person with a BMR of 1,700 calories, approximate shares are: skeletal muscle 22% (about 375 calories), liver 21% (about 360), brain 20% (about 340), heart 9% (about 150), kidneys 8% (about 135), adipose tissue 4% (about 70), and everything else, including lungs, digestive tract, skin, and bones, about 16% (about 270).
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The percentages come from detailed studies such as those by Wang and colleagues and the classic work of Elia, and they vary between individuals. The weight column shows typical organ weights for a 70 kg adult. Notice that the brain, at only about 2% of body weight, uses 20% of resting energy, while muscle at 40% of weight uses 22%.
On a per-kilogram basis, the heart and kidneys are the most expensive tissues at about 440 calories per kilogram per day, the brain and liver 240 and 200, muscle about 13, and fat about 4.5.
| Tissue | Share of resting energy | Calories at 1,700 BMR | Typical weight share |
|---|---|---|---|
| Skeletal muscle | 22% | 375 | 40% |
| Liver | 21% | 360 | 2.6% |
| Brain | 20% | 340 | 2% |
| Heart | 9% | 150 | 0.5% |
| Kidneys | 8% | 135 | 0.5% |
| Adipose tissue (fat) | 4% | 70 | 20% |
| Other (lungs, gut, skin, bone) | 16% | 270 | 34% |
What Powers It
Resting energy supports basic cellular work: maintaining ion gradients across cell membranes (such as the sodium-potassium pump, which uses about a quarter of resting energy in many cells), protein turnover, nerve signaling, heartbeat, breathing, and body temperature. The brain's high use reflects continuous neural activity; its consumption barely changes between hard thinking and rest, so thinking hard does not burn many extra calories, only a few per hour. The liver runs the body's chemical processing: glucose regulation, detoxification, and protein synthesis.
Implications for Weight Management
Because muscle contributes about 22% of resting energy, gaining or losing muscle has a modest effect on BMR: each kilogram of muscle burns about 13 calories a day. Gaining 5 kg of muscle raises resting burn by about 65 calories, equivalent to a small snack. This is smaller than the figure often quoted, though muscle also raises activity energy by increasing strength and exercise capacity. Fat burns very little at rest, 4 to 5 calories per kilogram a day, but large amounts of fat require more energy to carry and move.
Organ size changes with weight loss and gain: organs shrink as you lose weight, which is part of why metabolism falls.
Why Individuals Differ
Two people of the same weight and height can differ in BMR by 200 to 300 calories because of differences in organ mass, muscle mass, thyroid function, age, and genetics. Larger organ sizes mean greater energy use. Hormones such as thyroid hormone act as the metabolic regulator: an underactive thyroid can lower BMR by 10% to 30%, and an overactive one can raise it. Pregnancy, illness, and fever raise resting energy; fasting and prolonged calorie restriction reduce it. Climate and body temperature regulation add a few percent.
Putting It in Perspective
Given these biological limits, the main controllable ways to change total daily energy use are diet, daily movement, and exercise, not trying to manipulate organ metabolism. Equations for BMR are averages; use your own weight changes to calibrate calorie targets.
Frequently Asked Questions
Which organs use the most energy at rest?
Skeletal muscle, liver, and brain each account for roughly 20% of resting energy, with heart and kidneys using about 17% together.
Does thinking burn many calories?
Very few extra. The brain uses nearly the same at rest and during hard mental work.
How many calories does a kilogram of muscle burn at rest?
About 13 calories per day, versus about 4.5 for fat.
What fraction of daily energy is BMR?
About 60% to 70% for most people.
Why do people of the same size have different BMRs?
Differences in organ mass, muscle mass, hormones, genetics, and age.