It's never too late to take up exercise to rid yourself of middle-age spread and live longer, according to a new study.

Middle-aged couch potatoes who start exercising had the same risk of death as those who have exercised throughout their lives from their teens.

And although they are late starters to the gym, they got the same health benefits as life-long gym bunnies.

Those aged 40 to 61 who started doing the recommended weekly amount of activities had a 16 to 43 per cent reduced risk of death, according to the findings published in JAMA Network Open.

But those who were keen exercisers in early adulthood but gave up lost all the health benefits they had gained.

Dr Pedro Saint-Maurice, of the National Cancer Institute in the US, said: "National guidelines for aerobic physical activity recommend adults should participate in at least 150 minutes per week of moderate intensity aerobic activity or 75 minutes per week of vigorous intensity activity or an equivalent combination of both.

"Levels of physical activity equivalent to meeting this guideline have been associated with substantial health benefits, including reductions in all-cause, cardiovascular disease-related, and cancer-related mortality.

"However, most of the evidence on the mortality benefits of physical activity comes from studies that measure leisure-time physical activity (LTPA) at only one point, usually during midlife, for example 40 to 60 years of age.

"To date, no study to our knowledge has taken a life course approach to examine how participation in physical activity throughout the different stages of adulthood may be associated with mortality risk.

"Little is known about how long-term participation in LTPA from adolescence to early adulthood and into middle age may affect mortality "We hypothesised that participants who maintained the highest levels of activity in all age groups would have the lowest risk for mortality."

So the prospective cohort study used data from the National Institutes of Health-AARP (formerly American Association of Retired Persons) Diet and Health Study established in 1995 to 1996.

Of more than 315,000 participants, just under three-fifths (58.2 per cent) were men, and the participants were 50 to 71 years of age at enrollment.

It identified 10 trajectories of LTPA that were labelled based on the last end point for LTPA - 40 to 61 years of age - relative to the first LTPA end point - 15 to18 years of age.

The scientists classified trajectories into three categories.

The "maintainers" were those who had consistently higher or stable LTPA over time and made up 56.1 per cent of the total or 176 654.

The "increasers" were those that saw an increased LTPA from adolescence or later in adulthood making up 13.1 per cent or 41,193.

The "decreasers" were those with patterns of higher early adulthood LTPA but reduced activity later in adulthood who made up 30.8 per cent or 97,212 .

For example, participants in trajectory 10 accumulated similar amounts of LTPA when they were 15 to 18 years old and when they were 40 to 61 years, and in the middle years maintained some level of activity; hence, they were labelled maintainers.

There were more than 71,000 deaths, of which 22,219 deaths were due to CVD, and 16,388 deaths due to cancer occurred.

Dr Saint-Maurice said: "This large prospective study of adults examined participation in LTPA across the adult life course and found that compared with adults consistently inactive, increasing LTPA in adulthood after being inactive during adolescence was associated with reduced risk for all-cause and cause-specific mortality.

"Adults who engaged in LTPA only later, by age 40 to 61, had a risk for mortality that was comparable to those who engaged in LTPA consistently from adolescence throughout adulthood.

"By contrast, being active in adolescence but decreasing LTPA across the adult life course was associated with smaller benefits.

"These findings showed that adults who became physically active later in life had mortality rates similar to those of lifelong exercisers and that most of the benefits of activity performed earlier in life (adolescence or early adulthood) were lost if activity was not maintained.

"We had anticipated that participants who maintained the highest levels of activity throughout adulthood would be at lowest risk and were thus surprised to find that increasing activity early or late in adulthood was associated with comparable benefits.

"These benefits held similarly for men and women and were independent of changes in BMI over time.

"We specifically found that being inactive across early adulthood but increasing LTPA later at 40 to 61 was associated with 16 per cent to 43 per cent risk reduction in mortality.

"These mortality benefits were comparable to those associated with maintaining LTPA in all age groups from adolescence and into adulthood.

"These findings are consistent with previous studies demonstrating that increasing activity in midlife is associated with health benefits for all-cause mortality He concluded: "Increasing LTPA later in adulthood was associated with mortality benefits that were similar to those associated with maintaining higher levels of LTPA across the adult life course.

"Our findings suggest that it is not too late for adults to become active.

"These findings are particularly informative for health care professionals advising individuals who have been physically inactive throughout much of their adulthood that substantial health benefits can still be gained by improving their physical activity habits."