You know exercise is good for you, but if you're over 65, which is best? A study from Mayo Clinic found that high-intensity aerobic exercise can actually reverse aging on a cellular level.

High-intensity aerobic exercise — or cardio — includes running.

Mayo researchers compared high-intensity interval training (HIIT), resistance training and combined training in a 12-week study. They monitored molecular and metabolic changes in adults divided into age groups of between 18 and 30 and between 65 and 80.

All types of training improved lean body mass and insulin sensitivity, but only high-intensity and combined training improved aerobic capacity and mitochondrial function in skeletal muscle.

Mitochondria are tiny energy-producing structures inside cells. They change with age and activity, and tend to decrease, both in content and function, as we grow older. One result is we have less energy.

In the study, high-intensity interval training also improved muscle protein content that not only allowed cells to create more energy, but to also cause muscles to get bigger, especially in older adults.

The ability of the mitrochondria to generate energy was increased by 69 percent among the seniors and by 49 percent in the younger group.

"We encourage everyone to exercise regularly, but the take-home message for aging adults that supervised high-intensity training is probably best, because, both metabolically and at the molecular level, it confers the most benefits," says Dr. K. Sreekumaran Nair, a Mayo Clinic endocrinologist and senior researcher on the study.

Study results are published in Cell Metabolism.