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Eating colorful fruits and veggies is great—but only half the story. There’s surprising evidence that our mental and physical health depends on growing them ourselves.

Psychology Today Magazine, Sep/Oct 2008

By: Daniel A. Marano

Consider the earthy pleasures of gardening. You’re outdoors. The sun warms your muscles. You’re doing something both productive and edifying. And the activity places you back into the natural world, where exercise and relaxation are organically linked.

Edible gardening is better still; even the smallest of backyard or container gardens can put a surprising amount of food on your table in ways that make great nutritional and economic sense. Not to mention the juicy satisfaction of plucking that robust tomato or those tart raspberries.

But there is an even bigger yield. Call it soil-borne wellness, and here is where science is plowing totally new ground. Researchers are discovering that growing your own food — however much or little you can do — is better for your health than anyone ever suspected. And the nutritional value of what you harvest is almost the least of it. Growing your own food by messing around in your own garden proves to be nature’s fruitful way of cultivating your health — physically and psychologically.

The soil is a rich repository of microbes and other organisms with which we’ve coexisted from the beginning. As science digs deeper into understanding the effects of bacteria on human health, and especially on the immune system, it looks increasingly like ingesting components of the soil itself might be as critical to human health as the very finest fruits and veggies grown in it. In 2007, University of Colorado neuroscientist Christopher Lowry, then working at Bristol University in England, made a startling discovery. He found that certain strains of soil-borne mycobacteria sharply stimulated the human immune system. The very same bacteria also boosted serotonin levels in the brains of mice.

Mycobacteria are a broad genus of durable and waxy bugs that include many harmless critters and a few nasty pathogens, like M. leprae and M. tuberculosis. Lowry’s group focused on the immune response to Mycobacterium vaccae, a benign strain ubiquitous in soil. It is most abundant in peat bogs and other earth rich in moist organic matter.

M. vaccae thrives in typical backyard garden environments or anywhere soil is enriched with organic matter. You — yes, you — are exposed to it through inhalation and ingestion of small particles any time you dig outside or eat lettuce plucked from your garden. And that’s a good thing.

M. vaccae triggers a complex cascade of neurosignaling, says Lowry, who is trying to decipher the exact mechanism. But in the process, the bacteria stimulate immune cells to raise the threshold for inflammatory processes. Mycobacteria also interact with the nervous system to rev up production of serotonin.

Lowry is now conducting oral trials of the bacteria in mice to mimic our own exposure to mycobacteria. “At the moment, mycobacteria is a black box” — but in its effect it is “basically no different from all the SSRIs.”

It’s known that the SSRI antidepressants such as Prozac inhibit the reuptake of serotonin, increasing its availability to receptors. But exactly how these drugs work against depression, and their many undesirable effects on the body, are poorly understood. Many scientists regard the SSRIs as particularly blunt instruments for treating mood disorders.

By contrast, Lowry finds, mycobacteria “are very selective and specific.” They excite small subsets of serotonin-releasing neurons and pathways in the prefrontal cortex and hippocampus, where they directly impact cognition and mood regulation. Exposure cleanly increases the ability of test animals to cope effectively with stress and anxiety.

The possibility that mycobacteria might have anything to do with mood arose serendipitously in earlier clinical studies of the bacteria as a treatment for cancer and inflammatory disorders. Researchers observed a boost in the quality of life of their human subjects.

Collectively this work has yielded “the hygiene hypothesis.” Accumulating evidence suggests that lack of exposure to M. vaccae and other common dirt-borne pathogens early in life—resulting from deliberate attempts to sanitize our environments—might explain the sharp rise in chronic inflammatory, allergic, and immune disorders (such as asthma and inflammatory bowel disease) in the industrialized world.

The hygiene hypothesis suggests that there are real costs to displacement from the natural world. It disrupts a deep and direct connection we have to the soil and its resident organisms, a connection that our own immune and nervous systems have relied on for well-being all along.

Getting out in the garden plants us back in what now appears to be our optimal habitat. Eating fruits and vegetables — even antioxidant-rich tomatoes, melons, beets, cabbage, and berries — turns out to be only half of a newly evolving story of health. Our bodies and brains depend on the whole experience of growing our own. Our mental and physical health seem to be deeply rooted in the dirt. —Daniel A. Marano

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