'It's one thing that keeps us going back to our partners for more.'Dr Jackson says sex could form a part of an overall, varied exercise regimen - if you can make it last long enough.

For most long-married couples, sex sessions last around 15 minutes rather than the 30 minutes achieved by the couples in the Canadian study.

3 underused secrets for adult sex dating-20

One theory is that vaginal absorption of hormones in semen such as prostaglandins, testosterone and luteinizing hormone could help improve the mood of women, says Stuart Brody, professor of psychology specialising in sexual behaviour at the University of the West of Scotland.

orgasm also releases feel-good brain chemicals such as serotonin, adds Dr Ghosh.

In a typical session lasting 25 minutes, the men burned an average of 100 calories, the women 69.

The intensity of the activity was measured in METs (the Metabolic Equivalent of a Task); for men, the average reading was six METs, for women it was 6.6.

Indeed, research is now showing that sex provides a 'triple-whammy' of benefits by combining a workout for the heart and lungs, the release of hormones that could lower stress and the production of new brain cells.

And - for women - the added plus is a toning effect on the muscles in the pelvic floor.

However, the Nottingham University research, which was published in 2009 in The British Journal of Urology International, also found that ejaculating more than 20 times a month in their 20s and 30s could increase prostate cancer risk later in life.

This is possibly because higher levels of sex hormones in some men, which may be responsible for a high sex drive, may also be linked to the development of prostate cancer later.

'Anthropological research has found that for humans, quite aside from the pleasure we glean from sex, one of the main drivers behind our need for sexual activity is to bond with other humans,' says Dr Ghosh.