In the September/October 2008 issue of Psychology Today, a study done by a neuroscientist, Christopher Lowry, at Bristol University in England found that certain strains of soil-borne mycobacteria raised serotonin levels in mice and "directly impact cognition and mood regulation." (SSRIs like Prozac increase the availability of serotonin in humans.) The same mycobacteria sharply stimulates the human immune system.
According to the article:
"Getting out in the garden plants us back in what 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."
We already knew this: Digging in the dirt makes you happy, and gardening keeps you healthy!