Health

The best medicine may be free

Forest therapy guide explains what a daily dose of nature does for the body

Rebecca Coleman

Harvard Staff Writer


6 min read

Susan Abookire.

Photos by Stephanie Mitchell/Harvard Staff Photographer; Illustrations by Liz Zonarich/Harvard Staff

If you’ve ever taken a walk outside on a hard day to shed stress, you may be onto something. A growing body of scientific evidence confirms what many of us know innately — spending time in nature is good for us.

It not only lowers stress and depression, it strengthens immunity, boosts cognitive performance, promotes better sleep, fights inflammation, and helps the lungs and heart, research suggests.

That’s why Susan Abookire, a Harvard Medical School assistant professor of medicine at Brigham and Women’s Hospital, guides monthly forest therapy walks for medical residents and faculty — a group at high risk of burnout. Her two-hour sessions in the Arnold Arboretum, though just a few miles from the Longwood Medical Area, seek to melt away the stressful hospital environment by immersing participants in nature using techniques from the Japanese practice of shinrin-yoku, or “forest bathing.”

Forest therapy guides help you “really slow down” into deep awareness of your senses through nature, said Abookire. For those trying it without a guide, she advises tuning in to your surroundings — notice colors, shapes, sounds, movement, scents. Also, avoid distractions. That means taking out your earbuds and turning off your phone. You can reap benefits spending as little as 20 mindful minutes in nature three times a week, research suggests, but “the more the better,” said Abookire.

Another consideration: Higher tree density seems to deliver a more potent dose of nature’s medicine.

That’s because trees release essential wood oils called phytoncides that studies show boost immunity when inhaled. Natural killer cells — the body’s first line of defense against viruses and cancer — increase when we breathe in the compound, one Japanese study showed, and the effects lasted more than a month. Another study found exposure to tree oils helps lift depression, lower blood pressure, and reduce anxiety. Others suggest wooded environments aid respiratory health.

Plants also emit fragrant compounds called terpenes that have garnered attention from scientists for their potential to stave off cancer and dementia. Studies have demonstrated the anti-inflammatory properties of a terpene called d-limonene, found in rosemary, coniferous trees, and citrus rinds, and the cancer-fighting potential of a terpene called carvacrol, found in oregano and thyme. Borneol, a terpene in ginger and some cannabis strains, has been studied for its anti-inflammatory and neuroprotective effects and as a potential Alzheimer’s treatment.

Although research into nature’s effect on cancer outcomes is only recently starting to emerge, said Abookire, plenty of studies have established that it improves cancer patients’ quality of life.

Studies measuring effects on the brain are also burgeoning, but according to Abookire there’s a reason many of us instinctively go outside for a walk to clear our heads when grappling with a thorny decision or creative puzzle.

“We evolved in the trees” said Abookire. “We had 6 million years of evolution noticing what’s around us and then hundreds of years of living in cities — a few seconds on the clock of human evolution — and just 20 years of being glued to screens.”

It’s no wonder, she said, nature acts as a kind of reset button, helping us to restore focus, creativity, and memory when we are fried.

“When you’re busy working all day on a screen and have emails and problems to solve and using your frontal cortex, basically, that is subject to fatigue,” Abookire said. “You are exhausted even though you haven’t moved out of your chair all day and that kind of exhaustion can be restored when you go outside and pay attention and just sit and smell the grass.”

Other effects on mental health are well-documented, said Abookire. Research has shown engaging with nature activates our parasympathetic nervous system — or “rest and digest” state. This counterbalances the sympathetic nervous system responsible for the body’s “fight or flight” response — the system that gets turned on when “you’re being chased by a tiger or being chased by an email,” said Abookire. An active parasympathetic nervous system promotes relaxation and recovery.

Chronic stress takes a toll on your body over time by triggering hormones like cortisol, raising the risk of high blood pressure, anxiety, depression, and addiction. But just 20 minutes connecting with nature was shown to lower levels of cortisol in a 2013 study. Multiple studies measuring salivary cortisol have shown significant drops after a walk in the forest.

Medical professionals consider getting deep restorative sleep one of the pillars of health alongside nutrition and exercise. Several studies have found forest bathing boosts serotonin, improving sleep duration and quality — and consequently mood, energy, and focus.

The heart also benefits from time in nature. Studies have shown improved metrics in heart health, including blood pressure, pulse rate, and Heart Rate Variability.

Breathing in forest aerosols seems to confer many benefits but research has also pointed to soil bacteria. Injecting mice with Mycobacterium vaccae in one study made them more resilient to stress.

Other senses are at work as well. Sounds, of birdsong and water for example, unlock health perks, including activating the parasympathetic nervous system, said Abookire.

Then there are fractals, infinitely complex patterns that are self-similar across different scales and found abundantly nature. Abookire offers snowflakes, crystals, and the veining of leaves as just a few examples. It turns out simply viewing nature can be good for you.

“If I showed you a picture of a cube and ask you, ‘Is this nature?’ you would say no because nature doesn’t have those sharp square edges,” said Abookire. “But when we are experiencing fractals, whether it’s sound or sight patterns, our bodies resonate with that.”

This means if you’re stuck indoors or have raging allergies there are ways to tap nature’s benefits until you can experience the real thing. One study found patients viewing nature from a hospital window versus a brick wall recovered faster. Another found participants viewing a green roof exhibited higher attention spans than those looking at concrete.

Abookire also runs a six-month program accredited by Mass General Brigham that trains medical professions to be Nature as Medicine Practitioners. In addition to her monthly walks guiding medical residents and faculty, she occasionally leads public forest bathing walks at the Arnold Arboretum.

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