Does muscle turn
into fat as we age?

The full narration of the video.

Muscle never turns into fat. Not even a little. Yet people in their nineties, after eight weeks of training, more than doubled their leg strength. So what is really happening inside an older person's leg?

Slice a thigh across, and watch it from age thirty to eighty. At thirty, the slice looks like prime lean meat: a big red patch, a bone in the middle, a thin ring of fat outside. After thirty, that red patch starts shrinking, bit by bit.

By around seventy-five, about one percent goes every year. Strength drops even faster: two to five times faster than the muscle itself. The odd part: many people weigh exactly the same.

The scale stays put, but the slice changes. The red shrinks, and the outer ring of fat grows thicker. And inside the red, thin white streaks of fat appear.

Like lean meat slowly turning into marbled beef. It really looks as if muscle has turned into fat. But muscle cells and fat cells are two different kinds of cells. Neither can become the other.

So where do those streaks come from? Muscle has always housed a kind of standby cell. When muscle gets hurt, they step in to help repair it, then step back.

With age, and little movement, the repair signals get muddled. Those standby cells can take a wrong turn and become fat cells, squeezing in between muscle fibres. Meanwhile, the muscle itself is shrinking.

The nerve wires that command muscle drop out one by one, and fibres left without orders slowly waste away. So muscle doesn't become fat. Muscle slowly leaves, and fat slowly moves in. Doctors call losing muscle and strength sarcopenia.

On a thigh MRI, muscle shows up grey, and the fat streaks show up as bright white lines. The white marbling in the beef is those bright lines on the scan. The good news: this can be pulled back.

Those people in their nineties trained their legs. Eight weeks later, their thighs were thicker, and they walked faster. Another study followed older adults who stayed active for a year: the fat streaks in their legs stopped growing. Every body is different, so check with a doctor before you start.

You can test it today: no armrests, stand up from a chair five times, and count the seconds. Doctors often use fifteen seconds as a line for leg strength. Take a guess first: who in your family is the fastest?