Older people have lower levels of a key molecule in their muscles that helps keep cells healthy, especially if they're not active — but those who stay active, like endurance athletes, tend to keep their levels much higher.

Older adults with more NAD+ in their muscles tend to have better-functioning mitochondria, which are the energy factories in cells.

Older adults who walk more steps each day tend to have higher levels of a key molecule in their muscles that's linked to youth and energy — especially if they hit over 13,000 steps, their levels look like those of young people. But if they take fewer than 7,500 steps, their levels are much lower.

As we age, levels of a molecule called NAD+ go down, and this might mess up how our cells' powerhouses work, especially in the brain and liver, possibly leading to diseases like Alzheimer's or diabetes.

Regular physical activity that keeps energy use high may help maintain a higher resting metabolic rate and lower the risk of losing metabolic function and gaining weight as people age.

As people get older, their muscles don't send signals as quickly when they try to move, which might be why they lose strength over time.