People may not be as active as they once were after they retire, according to a new UK study.

Men and women living in eastern England reported a significant decline in physical activity after they stopped working, researchers found. The size of that drop varied based on the person's gender and former job.

"It is quite interesting to see the overall net decrease in physical activity," Stephen Kritchevsky said. He is the director of the Sticht Center on Aging at Wake Forest Baptist Medical Center in Winston-Salem, North Carolina.

"What I'd be wondering and concerned about is that the people - when they retire - may not adjust their energy intake to balance their reduction in expenditure," Kritchevsky, who was not involved with the new study, said.

That could lead to weight gain and related health problems, he added.

Recent research has found older people tend to spend most of their time sitting. But in other studies, retirement was tied to an increase in recreational activity.

For their analysis, Inka Barnett and fellow researchers from the University of Cambridge analyzed data from a study of 3,334 people ages 45 to 79. Participants all lived in Norfolk, UK, and filled out questionnaires about their physical activity at multiple time points.

When people first entered the study between 1997 and 2000, they were all employed. By the time researchers checked in with them again between 2002 and 2006, 785 had retired.

Barnett's team examined changes in participants' physical activity between the start of the study and 2006 to 2007.

Physical activity was measured in metabolic equivalents or METs, which reflect the amount of energy consumed by the body. For example, one MET is equal to the amount of energy it takes to sit still for one hour. Mowing the lawn equals about five METs per hour and running equals about 13 METs per hour.

Not surprisingly, the researchers found being retired was tied to a drop in the amount of energy people exerted working or getting places. But it was linked to an increase in the amount of energy used to do housework and other recreational activities.