You eat food. You breathe air. Somewhere between those two things and actually feeling able to get off the sofa, something has to turn one into the other.

That job belongs to tiny structures inside nearly every cell in your body. They are called mitochondria (say it: my-toe-KON-dree-uh), and one on its own is a mitochondrion. People often call them the batteries of the cell, which is close enough to be useful — though they are really more like little engines, because they burn fuel rather than store it.

They make about nine-tenths of all the energy your body uses (National Institutes of Health, 2025). Every heartbeat. Every thought. Every time you climb the stairs, heal a cut, or fight off a cold. All of it is paid for out of what they produce.

The parts of you that work hardest have the most of them. A single muscle cell or brain cell can hold thousands. Your body puts the engines where the work is.

And here is the part worth staying for: they are not fixed. What you do each day genuinely changes how many you have and how well they run. That is what the rest of this page is about.

It is easy to treat energy as a mood or a personality trait — some people are just energetic, some just are not. But a great deal of what we call energy is a physical supply question, and mitochondria are where that supply is made.

Their job is broader than fuel, too. Beyond energy production, mitochondria participate in cell signalling, calcium balance, and the everyday housekeeping that keeps tissue healthy (Magalhães et al., 2014). Researchers have described them as a hub where sleep and body-clock rhythms, the gut, and the immune system all intersect (Casanova et al., 2023).

Which is why supporting them tends to show up in ordinary, noticeable ways: getting through the afternoon without a slump, having something left for your evening, recovering from a hard day in a day rather than three.

This is the finding that changes how people think about their own energy. You are not issued a fixed number of mitochondria at birth and left to spend them down. Your body makes new ones throughout life, through a process called mitochondrial biogenesis.

The strongest and best-established trigger is physical activity. Endurance exercise reliably increases the signalling that drives mitochondrial building in muscle, a finding confirmed across randomized trials (Khoramipour et al., 2025). The response involves a coordinated switching-on of hundreds of genes across two separate genomes at once (Fealy et al., 2021).

Building new mitochondria is only half of it. Your body also has a system for identifying worn-out ones and recycling them, so that capacity is not diluted by parts that no longer pull their weight. This clearing process is called mitophagy, and it works alongside biogenesis rather than in competition with it (Fealy et al., 2021).

The practical upshot is reassuring: quality matters as much as quantity, and the same everyday inputs that prompt building — movement, sleep, periods without constant eating — also prompt the clearing out. You do not have to manage the two separately. Living in a rhythm of effort and recovery does both.