The reasons why it's so hard can vary from person to person, particularly due to the role of genetics in weight gain.
A big reason for that age-related weight gain may be that your joints can no longer handle those long bike rides, daily jogs, or weekly pick-up basketball games. Sarcopenia, the loss of muscle tissue that occurs as a natural part of the aging process, starts around age 40. That process contributes to slower metabolism since muscle burns more calories than fat.
Reduced metabolism in older age is a major factor, as are changes in hormones. And lifestyle-related factors play a role as well. Older men and women may be more stressed and less physically able to exercise the way they used to when they were younger.
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