Experts say that high levels of sedentary time – more than seven hours a day – increases the risk of these conditions even if people are physically active at other times of the day
The research, which highlights the potential risks of continual sitting, overturns previous studies suggesting that older people were the most sedentary group in the UK.
The study found 45 to 54-year-olds spend on average 7.8 hours per weekday sitting down, compared with 7.4 hours for the over-75s.
Sedentary work - defined as time spent in any waking activity done while sitting or reclined, including working, eating, reading, watching TV, or spending time on a computer - is the main reason for this inactivity.
Only the youngest group surveyed – 16 to 24-year-olds – are less sedentary than the over 75s on weekdays.
Lead researcher Tessa Strain said that the workplace routine of middle-aged office workers needed to be tackled.
“Large parts of the population are dangerously sedentary, something we have underestimated.
“We need to tackle high levels of sedentary time in early and middle age, when patterns may develop. Our findings suggest that changing habits in the workplace could be an appropriate place to start, given how much time we spend sitting there every day.”
The findings from the university’s physical activity for health research centre are published in the Journal of Sports Sciences.
Researchers drew upon data from more than 14,000 people in Scotland, taken from the 2012-14 Scottish Health Survey.
At the weekend, the weekday situation reverses. Those aged 25 to 54 were the least sedentary, sitting for between 5.2 and 5.7 hours a day. The over 75s were the most sedentary, at 7.3 to 7.4 hours a day.
For the youngest group, most of the time sitting down is in front of a TV or screen. Men spend less time in front of a screen as they get older, with women peaking in middle-age.