High-intensity interval training (HIIT), where short bursts of activity are mixed with rest periods, is your best pick when it comes to using exercise to combat the cellular signs of ageing, according to new research.

In the study, HIIT beat weight training for boosting your cells' mitochondrial activity – the chemical reactions that release energy and fuels cell growth. This activity usually declines with age, but HIIT was shown to actually reverse it in some cases.

That could help us understand more about limiting disabilities and diseases such as diabetes as people get older, say researchers from the Mayo Clinic in Minnesota, even if it can't help you live forever just yet.

And although the study only involved a relatively small sample size of 72 volunteers, the team says the results are impressive enough to be significant.

"These things we are seeing cannot be done by any medicine," says one of the researchers, Sreekumaran Nair.

"Based on everything we know, there's no substitute for these exercise programs when it comes to delaying the ageing process."

Researchers enlisted the help of 36 men and 36 women split across two age groups; a 'young' group aged 18 to 30, and an 'old' group aged 65 to 80.

These volunteers were put into three mixed-age groups: one group did high-intensity interval training on bikes; one group did weight training; and one group did a combination of the two, for a period of 12 weeks.

The HIIT program involved three days a week of cycling, with high-intensity spells of pedalling split up by less intensive periods, plus two days a week of treadmill work.

Muscle cell make-up, muscle mass, and insulin sensitivity were then analysed and compared with a control group that did no exercise at all. 

At the molecular level, HIIT produced the biggest benefits, with the younger participants on HIIT showing a 49 percent increase in mitochondrial capacity, and the older participants on HIIT reaching a 69 increase on average.

The cells of the older volunteers started generating energy at a rate comparable to cells from much younger bodies, in other words.

Mitochondria act like energy factories for our cells, converting glucose into power for our bodies at the lowest levels, but as we get older this process starts to slow down. That leads to the cell damage and dysfunction we associate with ageing.

While scientists still don't fully understand how this all works, kickstarting more mitochondrial activity through exercise could be one way of keeping many of the signs of ageing at bay.

And while it's been well-established that exercise is good for the body, scientists are still figuring out the changes it makes at the molecular level.

What's more, the researchers say the regeneration of muscle protein seen in this study could also be replicated in the heart and brain, two other areas of the body where cells wear out more easily as we get older.

HIIT improved insulin sensitivity levels too, which reduces the risk of diabetes, though it wasn't as effective as weight training at building up muscle mass. As you might expect, any kind of exercise was shown to be better than doing nothing at all.

"If people have to pick one exercise, I would recommend high-intensity interval training, but I think it would be more beneficial if they could do three to four days of interval training and then a couple days of strength training," says Nair.

The findings have been published in Cell Metabolism.