The age on your passport doesn't always reflect how we feel. But what if there was a number that could tell you the status of your ageing from a biological standpoint? That's your biological age.

Professor Rose Anne Kenny, head of medical gerontology at Trinity College Dublin and founder of the Irish Longitudinal study on Ageing (TILDA), talked to Today with Claire Byrne about the science behind our biological age and how it might differ from our "chronological age". (This piece includes excerpts from the conversation which have been edited for length and clarity - you can hear the discussion in full above.)

"Chronological age is the number of candles on a birthday cake, or on your driving license. Biological age is the age that your cells and organs have. It generally varies to a degree from your chronological age," says Kenny. "It's well known from lots of different types of studies, both in humans and in animals, that we age at different paces. Because all ageing is, is an accumulation of different diseases and disorders, and we know that not everybody dies at the same time, at the same age."

"So the stronger our muscles are the better our muscle strength is and generally speaking that's a good predictor for longer living" Photo: Getty Images

"We know for a long time that things like sleep, exercise and diet influence, or appear to influence, why some people live longer than others, and have less diseases therefore, which cause end of life. But most recently we've been able to drill down behind that original science into what our cells look like in the context of people who are living longer and those who aren't."

Biological age and epigenetics

There are trillions of cells in the body and there are about nine major hallmarks of ageing that we can see in a cell, says Kenny. "There's a big a big race on now to actually determine which of those hallmarks is most important for measuring our biological age... But this is an evolving science and at the moment there are a number of companies which are purporting to be able to measure your biological ageing, but any of us in the field know that none of the measures are accurate in terms of predicting how long you're going to live."

You can get a sense of how healthy you are when you start to examine your biological age. "It's important to remember that some of the earlier measures of biological age have been around for an awful long time; your cholesterol or the ratio of LDL/HDL cholesterol, our blood pressure, glucose and and other measures of blood sugar, even your waist/hip ratio. They are all also biological age markers that we know an awful lot about because they've been around for a long time."

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But you can’t definitively predict how long someone will live because there are so many other factors which additionally influence the ageing process and our biological age. There is evidence to show that "you are as young as you feel," says Kenny. "Social engagement, laughter, friendship, taking down-time and less stress etc, all of those factors influence the ageing process at a biological level."

A study published in Frontiers of Neuroscience last week examining the flow of blood in the brain, showed how important exercise is for it. "They examined athletes aged 50 to 80 and then asked them not to do their usual exercise for 10 days. They measured blood-brain flow beforehand, when they were running and very fit etc, and then after the 10 day period. They showed a significant reduction in flow in the areas of the brain that we know are important for Alzheimer’s dementia: The memory areas, concentration areas, and the executive planning areas."

Kenny explains that as we get older we shouldn't stop or slow down. "We should actually make an attempt to do a little bit more every year, within our capacity, not less."

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The importance of balance

One important measure of your health is your balance. Your balance is a good "marker" because it's a measure of our musculoskeletal health, says Kenny. "Our muscles particularly are really important building blocks for the body. So the stronger our muscles are the better our muscle strength is and generally speaking that's a good predictor for longer living. But of course that doesn't happen by chance, it happens because we work on muscle strengthening etc."

If you want to test your balance at home, there are two different exercises to use. The first is standing on one leg for 30 seconds with your eyes open. "If you can do that without any problem you're free to go," says Kenny. The next exercise is standing on one leg with your eyes closed for 10 seconds, repeated on both legs. "Don't despair if you're there trying this in the kitchen and thinking 'oh my God, I'm all over the place here, particularly when I close my eyes'. As we get older it's really common for unsteadiness to creep in with eyes closed, but you can rehearse this standing in front of the sink on one leg for 30 seconds per day, pulling in the buttock muscles really tightly and you will ultimately train yourself to do this."

Some places you can pay privately to test for your biological age, but Kenny emphasises that at the moment we don’t know enough about which of the measures is the best. "With respect to the future, this is one of the fastest moving areas in medicine and I have no doubt that we will be able to do a mouth swab or a small finger prick for blood, to test our biological age going forward. The important thing is that all of these factors are modifiable. No question, the earlier you start with better sleep, better diet, certainly exercise, the better. But it's never too late.

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"No point in people listening thinking, ‘I’ve been hopeless for the last 20 years, I'm done now there's no point’. There is a point. Even in nursing home patients, if you introduce new healthy diet and improved sleep and regular exercise programs, there's an improvement in the measures that we currently use for biological age, like muscle strength, grip and balance."

Gender

"Generally speaking women live longer than men and that has been to-date attributed to the fact that men experience more stress in their working life, and their young and middle adult years, than women. That gap is now narrowing. I think that’s because there are more working women and the stress we're all experiencing isn't that gender-specific anymore."

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"[Stress] doesn't just cause vascular disease, it also upsets our hormonal systems and neuroendocrine systems, which are important for healthy brain function and also our responses to stress. In other words, the more we're stressed, bit by bit the less capable our body is of adjusting to stress. Then we take on the whole inflammatory cascades that you get as a result."

Kenny advises people to know their weight and their biomarkers, like blood pressure, blood glucose and cholesterol. "There isn't a whole lot of evidence for supplements. A more vegetarian type diet, with fish, seems to be the consensus [from the studies]."