The science of why your body takes longer to bounce back after 40

Researchers call our ability to bounce back from health stress “biological resilience." Evidence suggests that it declines with age, driven by biological and other factors, including parenting, work stress, changes in exercise habits and menopause.

Often, these stresses pile up from early life and can reach a tipping point in our 30s and 40s.

“There are these moments where the whole system seems to undergo like a vibe shift," says Dr. Heather Whitson, a geriatrician and clinical investigator who directs the Duke University Aging Center.

These midlife declines in resilience parallel emerging science suggesting that aging itself doesn’t happen in a linear way, doctors and researchers say. A small study out of Stanford that looked at biomolecular shifts in the body found two aging “waves" appear to occur around ages 44 and 60.

While the Stanford study’s findings are difficult to generalize to the broader adult population, family-medicine doctors report seeing similar age-related changes in their patients. The first shift often happens for patients in their late 30s and early 40s, says Dr. Benjamin Missick, family medicine doctor at Novant Health in North Carolina.

That’s when they begin to question why their cholesterol is suddenly rising, why controlling their blood pressure requires more medication, and why they’re gaining weight despite maintaining their diet.

“This decline is not steady," he says. “There are times in our life where there will be rapid changes."

‘40 and falling apart’

In the days before her 40th birthday, Christina Goldpaint, a data analyst in Long Beach, Calif., proclaimed to a friend that it was going to be her best decade yet.

Then, in quick succession, she developed tendinitis in her foot, a urinary tract infection, and blepharitis, an inflammation of the eyelid. She spent the first weeks of her 40s shuttling between primary care, urgent care and specialist visits.

“That’s something that should affect somebody who’s 60, not somebody who just turned 40 and fabulous," she says. “It became 40 and falling apart."

Research shows we lose roughly 3% to 8% of muscle mass per decade after age 30, with a faster decline after 60, while fat mass increases. This can lead to less mobility and a greater risk of falls and injury, plus other long-term effects.

The changes mean we need to eat progressively fewer calories as we age to maintain our weight, says Dr. Sarah Nosal, a family-medicine doctor in New York and president-elect of the American Academy of Family Physicians.

Muscle tissue stores more water than fat mass does, so we are more susceptible to dehydration as we age, says Nosal. That, plus changes in our enzymes that process alcohol differently and at a lower level, help explain why it often feels harder to recover from a night of drinking the older we get.

We also tend to take more medications. Roughly one in five adults aged 40 to 79 take at least five prescription drugs, according to Centers for Disease Control and Prevention data. Some have effects that make us more susceptible to getting sick, plus other side effects.

Additionally, hormonal changes are at play, including the gradual decline in sex hormones and growth hormones for men, and more dramatically for women during perimenopause and menopause, says Elissa Epel, who studies aging and stress in the department of psychiatry and behavioral sciences at the University of California, San Francisco.

Emotional toll

Beyond the physical changes, the psychological toll of being a member of the so-called sandwich generation can have an impact on our biological resilience, doctors and researchers say. Many middle-aged people are caring for children and aging parents, tending to a busy career, and enduring major life events such as a significant injury or death of a close loved one.

“Your actual mental health and wellness impacts how likely you are to have poorer health outcomes," says Nosal, citing factors such as changes in blood pressure to weight gain. Stress can hurt your sleep, which can, among other things, hurt your ability to build muscle, and form and store memories, she says.

There are things we can do to age more resiliently, these health experts say, and they largely come down to good health habits: giving priority to sleep, exercising and minimizing stress.

Some evidence suggests exposing the body to low-dose stressors like exercise, a phenomenon called “hormesis," primes the system to be able to recover better down the line.

“The healthier you are before the stressor, the more likely you are to be able to respond well to the stressor," says Whitson, the Duke researcher.

Drew Dyck, a 47-year-old author and book publishing editor, used to bounce back immediately from a vigorous workout. Now, he says, it’s more of a slog than a bounce. A former high school and college basketball player, Dyck regularly spent three intense hours practicing and felt nothing the next day.

“If I do that now, I’m not exaggerating, I’ll be sore for a week," he says. “Not just my legs, but my back, my toes, my fingers, like everything you can feel."

Today, due to some tendinitis in his left knee, he gives priority to daily 2-mile walks with his wife over high-intensity exercise. It also takes him longer to recover from a bad night of sleep than it used to, he adds, so he sometimes sneaks 20-minute naps into his afternoons.

“Aging sucks," he says. “But it beats the alternative."

Write to Alex Janin at alex.janin@wsj.com