By Barbara Ehrenreich, from Natural Causes, which will be published next month by Twelve. Ehrenreich is the author of more than a dozen books, including Nickel and Dimed (Henry Holt). She holds a PhD in cellular immunology.

The pressure to remain fit, slim, and in control of one’s body does not subside with the end of youth — it grows only more insistent as one grows older. Friends, family members, and doctors start nagging the aging person to join a gym, “eat healthy,” or at the very least go for a daily walk. You may have imagined a reclining chair…