Nearly two-thirds of the least-fit individuals were not meeting the minimum recommended amount of physical activity (at least 150 minutes per week, or 30 minutes per day, five days per week). Yet, this group achieves the greatest health benefits from increasing fitness.

'Given the considerable survival benefit associated with improving fitness in the least-fit group, increasing fitness through regular physical activity should be a priority in unfit individuals,' Mandic said.

'Health professionals should consider a sedentary lifestyle and poor fitness as treatable and major risk factors,' she added.

These findings were published in Medicine & Science in Sports & Exercise, the official journal of the ACSM.