"This research could help us improve the design of our weight loss programs and suggests that adults with very poor fitness may benefit from additional exercise support during a weight loss program to achieve higher levels of moderate-to-vigorous physical activity and improve long-term weight loss," said study lead researcher Adnin Zaman, from the University of Colorado in the US.
For the findings, published in the journal Journal of the Endocrine Society, researchers examined the relationship between a person's level of fitness at the beginning of the study and weight loss at the end of an 18-month behavioural weight loss programme.
The programme combined a calorie-restricted diet, group-based behavioural support, and six months of supervised exercise.
The study included 60 adults with obesity or overweight. Participants exercised under supervision for the first six months of the study.
The participants wore an armband that measured their activity levels over one week at the beginning of the study, and then for a week during months 6, 12 and 18.
Their cardiovascular fitness was measured during a graded exercise test on a motorised treadmill. Participants were divided into two groups: "very poor fitness" and "poor or better fitness."
Twenty (33 per cent) of the adults who completed the 18-month study were classified as having very poor fitness at the beginning of the study, while 40 (67 per cent) were categorised as having poor or better fitness, according to the study.
Body mass index--a measure of body fat based on height and weight--was higher in those with very poor fitness at the beginning of the study compared with those in the poor or better fitness group.
There were no significant differences between the two groups in weight change at six or 12 months.
At 18 months, however, those in the poor or better category lost nearly twice as much weight as those who had very poor fitness at the start of the study--an average of 18 pounds versus 9.5 pounds.
"Future studies are needed to evaluate whether providing additional exercise support or focusing specifically on improving fitness in adults with low levels of fitness would improve weight loss," Zaman said.